AC Motors - Motion Control Tips

AC induction motor inertia: What’s the difference between WK2 and WR2?

April 29, 2019 By Danielle Collins Leave a Comment

In any motor-driven system, the difference between the motor inertia and the load inertia will affect the system’s performance — especially its ability to accelerate the load quickly and efficiently.

While servo motors are the preferred choice for applications that specify a precise move profile, AC induction motors are often the better choice for applications where a rotating device (such as a fan, blower, or gearbox) needs to achieve a given speed within a certain amount of time and maintain that speed regardless of changes in the load.

In servo-driven motion control applications, we typically look at the inertia ratio — the ratio of the load inertia to the motor inertia — to give us an idea of how the motor will perform for the type of application and performance requirements (high acceleration, fast settling time, accurate positioning). Although inertia matching is less critical for induction motor applications than for servo-driven systems, inertia still plays an important role in the sizing and selection of an AC induction motor. This is because with rotating loads, the required motor torque is directly related to the inertia of the load.

T = torque required by motor

J = inertia of rotating load

α = angular acceleration of load

Note that inertia includes the inertia of the load (a fan, for example) plus the inertia of the motor and any other rotating components (such as couplings or pulleys) between the motor and the load.

Two definitions of inertia for AC induction motors

In motion control applications, inertia is generally given as J (as shown in the equation above) and is defined as “mass times radius squared.” However, in many rotating equipment applications, inertia is given as either WR² or WK² and is defined as “weight times radius squared.”

Notice that the inertia equations for induction motors use weight (which is a force) rather than mass. Also notice that the distance (radius) can be specified as either “R” (WR²) or “K” (WK²). R is the radius of the object, from its center to its outer edge. K is the radius of gyration, which is the distance from the point around which the object rotates, to the point at which the mass can be considered to be concentrated.

These expressions for inertia — WR² and WK² — are often used interchangeably, although R (radius) and K (radius of gyration) are rarely the same values. Here’s why…

If a hollow cylinder with extremely thin walls rotates around its axis, it’s reasonable to assume that the cylinder’s mass (weight) is concentrated around the outside circumference. So the radius of gyration for the thin-walled cylinder will be measured from the center to the outside edge of the cylinder. Thus the radius of gyration, K, is equal to the radius of the cylinder, R.

If a hollow cylinder has very thin walls, the mass can be assumed to be concentrated at the outer circumference of the cylinder.

However, if the cylinder is solid and uniform, its mass (weight) can be assumed to be concentrated at a distance of 1/2 the radius. In this case, the radius of gyration is ½ of the radius (K = ½R).

If the cylinder is solid and uniform, the mass is assumed to be concentrated at 1/2 the radius, so the radius of gyration, K, equals 1/2*R.

Using inertia to determine torque and acceleration

The distinction between these two inertia equations, WR² and WK², is important because in AC induction motor applications, inertia is used to determine the motor torque required to achieve a desired speed within a given time.

T = acceleration torque (lb-ft)

W = weight of load to be accelerated (lb)

R = radius (or K, radius of gyration) (ft)

N = change in speed (rpm)

t = time to accelerate (s)

Note: 308 is a constant that incorporates the conversions from lbf to lbm, and from rotations per minute to radians per second

Inertia can also be used to determine the time the motor will take to reach a given speed with a defined acceleration torque.

This is important because induction motors draw very high current during start-up and acceleration, and high current for an extended period of time can damage the motor.

What is droop control in the context of AC motors and drives?

April 17, 2019 By Danielle Collins Leave a Comment

One of the fundamental characteristics of an AC induction motor is that the rotor spins at a slower rate than that of the stator’s rotating magnetic field. This difference in speed between the rotor and the stator’s magnetic field is known as slip.

Slip allows the motor to produce torque — the higher the slip, the higher the torque production, but also, the lower the motor’s efficiency (in most cases). But in load-sharing applications, motor slip can be used to prevent one motor from taking a disproportionate share of the load.

N_s = synchronous speed (rpm)

N = rotor speed (rpm)

Load-sharing, in this context, refers to two or more AC motors connected to and driving the same load. Theoretically, each motor in a load-sharing set-up would experience an equal percentage of the load. But in real-world applications, numerous factors, such as variations in load along the length of a conveyor, or inconsistencies in the driven process, can cause the load to be distributed unevenly among the motors.

“Natural” load balancing

Because slip is inherent in AC motors, when two or more motors are connected to one drive, their natural slip will work to achieve balanced load sharing. As one motor experiences increased load, its speed (i.e. rotor speed) slows down, increasing slip between the rotor and stator. The second (and subsequent) motors, which are connected to the same drive, will also slow, allowing them to take on the additional load. While using the motors’ natural slip is a simple method of load-sharing, it is very imprecise and not reliable at high loads.

Droop control

For applications where load-sharing needs to be precise and controllable, some AC drives include a function — typically referred to as droop control — that manipulates motor slip to better ensure equal load-sharing. Here’s how it works:

Droop control effectively shifts the motor’s torque vs frequency curve so that an increase in torque causes a decrease in the drive’s output frequency.
Image credit: Schneider Electric

The drive monitors the motor torque (via current draw), and if the torque increases beyond a set value (indicating that the motor is taking on too much load), the droop control function decreases the output frequency of the drive.

The rotor speed has slowed due to the increased load, and the decreased output frequency from the drive causes the stator magnetic field to also decrease in speed. Hence, there’s a smaller difference between stator speed and rotor speed. In other words, slip is reduced, which reduces the motor’s torque production, allowing other motors to take more of the load.

Droop control can work both ways — reducing the output frequency of the drive when the load increases (to decrease the amount of load on an overloaded motor), or increasing the output frequency of the driven when the load decreases (to increase the amount of load on a non-drooping motor). This two-way method of controlling load sharing is sometimes referred to as “bipolar” droop control.

Droop control is often used to increase slip in an overloaded motor, allowing other motors connected to the same load to take a higher share of the load. One application where droop control is useful is a conveyor with multiple driven rolls. If one section of the conveyor (and, therefore, one motor) sees an increase in load, droop control prevents the motor from sustaining this disproportionate share of the load, preventing damage to the motor and, potentially, to the system.
Image credit: Rockwell International Corporation

Bison Gear and Engineering launches next-generation permanent magnet ac motor

April 9, 2019 By Miles Budimir Leave a Comment

At the Automate Trade Show, Bison Gear and Engineering announces the launch of VFsync, a next-generation permanent magnet ac motor. Designed for today’s demanding machine drive applications, the new synchronous motors run at high efficiency with advanced variable frequency drives. The IP66/IP54 platform of three-phase motors range in power from 0.25 to 1.5 hp, and are supplied with swivel connectors and shielded cables to make installation trouble free. Popular frame sizes include IEC B14, sizes 71, 80 and 90 along with NEMA 56C mounting. The product line includes the new motors, quick connect cables and a market leading, programmable and networked VFD available in an IP 20 panel style or enclosed IP66 models.

The new motors were optimized with FEA software and then tooled with highly efficient internal permanent magnet style rotors. VFsync provides a compact footprint that is 56 % smaller and 63 % lighter than common three-phase induction motors. VFsync motors were designed to work seamlessly with Bison Gear & Engineering’s line of integral horsepower gear reducers to power even the most demanding applications with precise speed control.

Each frame size is available in either 230 or 460 V construction, and either TENV, TENV w/encoder or TEFC styles.

To learn more about VFsync, visit www.bisonvfsync.com.

Motion Trends: New motor breeds are smart, connected, and compact

April 5, 2019 By Lisa Eitel Leave a Comment

This year’s survey of trends in electric motors for motion control and automation revealed several trends and emerging developments in the use of synchronous ac motors; stepper motors, especially in closed-loop offerings; brushless motors of various types; and a smattering of specialty direct-drive offerings just making inroads.

Proliferation of synchronous ac motors

With construction and operation much like those of brushless dc (bldc) motors, synchronous ac motors — sometimes billed brushless ac, permanent-magnet (PM) ac motors, and even PMACs — are now spreading from sophisticated servomotor applications into other industrial-drive applications. More specifically, they’re seeing increased use on printing and packaging equipment; conveyors; vehicle hub drives (which we’ll explore in an article on AGVs and robotics in the special motion issue of Design World coming in May); hoists and cranes; and regenerative elevator drives.

These lightweight low-inertia motors typically deliver 5 to 150 hp at high torque and efficiency. Case in point: Sinochron PMAC motors from ABM Drives maintain efficiencies 10 to 15% better than comparable induction motors.

Synchronous ac motors’ variable-speed operation necessitates control via an inverter or VFD specially designed to start and synchronize the rotor-stator interaction. Just as bldc motors, synchronous ac motors use electronics (typically Hall-effect sensors) to dictate the correct amount of current to the windings. Arrays of four or more permanent magnets on the rotor make for synchronous operation.

But unlike bldc motors with trapezoidally wound stator coils (and back-EMF output with trapezoidal waveform needing direct current input) synchronous ac motors are wound sinusoidally. That makes for sinusoidal back-EMF output (and the need for sinusoidal current input) as well as an audibly and electrically quieter motor. Visit motioncontroltips.com and search, “difference between bldc and synchronous ac motors” for more on this.

Continuous commutation avoids torque ripple and makes for a power factor that is high … and well as super-premium efficiency even when accounting for controller losses.

Unlike induction motors that exhibit low efficiency and power factors at low speeds, PM motors also maintain performance without necessitating a gearbox or gearmotor design.

These Unimotor hd brushless ac servomotors from Control Techniques, a Nidec Motor Corporation business, are low-inertia pulse-duty servomotors that accept 230-V or 460-V input; are capable of 6.3 to 757.6 lb-in. output (to 2,257 lb-in. peak); and come in frame sizes of 55 to 190 mm. These motors have a torque profile matched to Digitax HD servo drives for up to 300% peak overload for peak dynamic performance. The Unimotor hd servomotors excel in dynamic flying shear; pick and place; and cut-to-length applications requiring hard accelerations and decelerations.

Stepper motors (PM and others) in new places

Small motors up to and including NEMA 34 and 60-mm size motors (in bldc and servo as well as hybrid stepper and closed-loop stepper variations) continue to see rising use. These small motors represent up to 70% of the market (unit) volume.

Leading applications are those in 3D printers, robotic handling, medical devices, and industrial motion designs. Our industry experts had some interesting input on stepper motors for these industries.

Harris Ayaz | Applications Engineer • ISL Products International: PM stepper motors are increasingly being used in industrial applications. We’ve had customers in the automated-printing, packaging, and electronic-clock industries approach us recently. These motors are much smaller than the more popular hybrid steppers. PM motors can be manufactured with customized mounting configurations and specialty connectors.

The latest trends in the area of hybrid stepper motors include the addition (pre-integration by the motion-component supplier) of a linear screw and nut on this shaft to deliver a complete linear-motion solution. This makes steppers a more plug and play solution for customers.

The NEMA34 hybrid stepper motor from ISL Products shown here is used in industrial robotics.

Other trends are the increase in double-shafted stepper motors as well as steppers with gearboxes.

Jonas Proeger • Trinamic Motion Control: I don’t know how they build them, but the impressively small sizes of some motors is allowing for new applications. We’ve seen impressive designs for robotic-surgery applications, where there are six or more tiny motors in a fist-size end effector on a surgery robot.

Most stepper motors are still operated in open loop, and that’s going to be the case for a long time — because after all, one of the biggest benefits of stepper motors is how they can offer reliable and precise positioning at low cost without the need of a feedback system. That said, we see a rise of closed-loop systems making stepper motors full-fledged servos — and they don’t have to hide behind their bldc or dc siblings.

Ayaz | ISL Products International: The rise of closed-loop control is to mimic servos and their feedback. Stepper motors can pair with all-in-one driver solutions; this allows for closed-loop control and reduces the need of purchasing two separate components. Many customers want a turnkey operation method, and steppers with closed-loop systems provide adequately for consistent loads.

Proeger • Trinamic Motion Control: With more and more of the algorithms implemented in microelectronics, it’s my view that ultimately that there will be no difference between interfacing with a bldc servo or a stepper servo … or even an open loop stepper. When that time comes, the decision on which motor type to choose will only be a mechanical decision: While stepper motors have their sweet spot at low speed and high torque, bldc motors perform better at higher speed.

Shown here are Portescap 20DBM captive and non-captive stepper-based linear actuators.

Closed-loop steppers and encoder innovation cross-pollination

Closed-loop stepper motors leverage high-resolution feedback and DSPs — both increasingly affordable thanks to Moore’s law. Globally, the use of these closed-loop stepper systems will grow at 6.3% CAGR through 2023, according to the Prescient & Strategic Intelligence. Where once closed-loop control was overkill, now such setups boost efficiency on positioning or motion tasks that need load-responsive control.

Many uses are in robotics, packaging, and semiconductor manufacturing. These leverage the strengths of stepper-motor construction even while outperforming traditional open-loop offerings. In many cases, closed-loop stepper offerings take the form of integrated motors that include power, drive and controller boards; a feedback device; communication and I/O electronics; and cables and connectors.

Eric Rice • Applied Motion Products: Closed-loop stepper control continues to gain market share in positioning and velocity-control applications. This growth in use is primarily driven by new integrated motor designs that capitalize on the benefits of closed-loop control (such as greater torque output, higher acceleration rates, quieter operation, and greater efficiency) … but it’s also driven by the benefits of the integrated motor design itself. These benefits include fewer cables, no external drive or amplifier, simplified parts lists, and cost savings.

Furthermore, the benefits of both closed-loop control and integrated motor designs are easy to observe — which helps end users feel confident when adopting these technologies for the first time.

Need for encoders in these closed-loop steppers and other motors has meant mounting demand for higher encoder capabilities (to complement today’s high-performance networking and connectivity options).

Proeger • Trinamic Motion Control: The rise of stepper servos will spur a new generation of encoders. Typically, in embedded drives, the best choice is a magnetic encoder. That’s because such drives are compact and rugged and work with a simpler mechanical setup. The disadvantage here is if a design engineer wants to use it for FOC of a stepper, many magnetic encoders have limited resolution. Recall that stepper motors have a typical pole count of 50 compared to 1 to 16 for a typical bldc motor. Another drawback is their internal-processing latency. That’s prompted demand for high-resolution magnetic encoders with low latency.

Mike Maroney | Senior product manager • Applied Motion Products: The unquestioned dominance of optical encoders is over. Magnetic encoders now offer prices an order of magnitude cheaper (per component cost) than competitive offerings. Plus they offer easier unit assembly — thanks to an airgap controlled by the PCB as well as fixturing tolerances tighter than those of their optical counterparts.

These factors combined with the rapidly technological advances in magnetic encoders are helping to shrink the performance advantage of the optical encoder … which means magnetic encoders will only continue to take more market share … and will even help encoders enter markets where they have not historically been feasible.

Patrick Wheeler | Product manager • Aerotech: Another trend in encoder design and application in motors is that towards absolute encoders and other serial protocols. The challenge is the latency created by serialization of signals when high-level of process control must be combined with actual position of linear actuators or stages.

Need for speed (and torque): Brushless-motor trends

Electronically commutated (EC) motors or ECMs — also known as bldc motors — are indispensable in an array of motion designs needing long life and top efficiency.

As an aside … note that the U.S. Department of Energy (DOE) specifies that ECMs are those low-horsepower bldc motors with integrated drives and controls (so common in HVAC designs). The agency distinguishes between bldc motors (brushless dc motors with separate drives and controls) and EC motors (having integrated drives and controls). But many OEMs and motor suppliers use the terms interchangeably.

What’s more, most manufacturers of EC (bldc) motors supply to both the HVAC and motion-control markets, which is accelerating the adoption of the terms EC motor and electronically commutated motor in the motion industry.

The EPOS4 product line from maxon precision motors includes two new EtherCAT Compact controllers — the 24/1.5 and 50/5. The controllers are suitable for use with both bldc and brushed dc motors. maxon is also upgrading the capabilities of this platform with dual-loop control. Such control simultaneously controls motor and load feedback for top system performance. EPOS Studio software speeds setup times with an automatic Regulation Tuning feature.

Dave Beckstoffer | Product manager • Portescap: Brushless dc motors are trending to provide multiple options to cover a broader spectrum of applications. New designs are being tailored for speed, torque, or a combination of both. Slotless motors are adapting designs to be multi-polar for additional torque or optimizing magnetic design for higher speeds. Some applications — industrial hand tools, for example — are demanding both within their operating range – free run and tightening phases. Portescap’s Ultra EC motor development has excelled in these applications, providing a faster free-run phase and higher peak torque capability.

Ayaz • ISL Products International: New trends in brushless motors are emerging in the field of medical devices and robotics. We’ve been involved with projects in various industries but in these two markets, bldc motors are most common.

Recently, we’ve seen more use of outrunner-type bldc motors in medical equipment and industrial applications. Previously outrunner-type bldc motors were most common in light recreational consumer electric vehicles. Outrunners have magnets attached to a ring or sleeve on the outside of the stator coils for output that takes the form of motor can rotation.

In smaller-scale robotics, digital servomotors commonly are used. These motors are cost effective and (with feedback) provide precise movement through PWM. We also see a lot of planetary gearheads paired with bldc motors — for speed reduction and increase in torque. With tactical robotic systems, planetary gearheads are best for high-torque and high efficiency as well as heavy load-carrying capacity.

Shigenobu Nagamori, founder and CEO of the global motor manufacturer Nidec, predicts several trends in industries employing electric motors.

“In 1973, I founded Nidec to replace all electric motors in the world with brushless dc motors … convinced of this technology’s potential. 45 years later, we’ve seen innovations related to brushless dc motors — the most impactful being expansion of the hard-disc drive (HDD) motor market in the 1990s. Now, there are four new trends of similar magnitude …”

Home appliances traditionally powered by brushed motors are increasingly using brushless dc motors — Benefits include higher power efficiency, compatibility with battery power, and longevity. Nearly all motors in room air conditioners, refrigerators, washing machines, and vacuum cleaners are (or will be) replaced by brushless dc motors, added Nagamori.

Electrification of automobiles — Migration from internal-combustion engines to electric motors is progressing rapidly … and myriad other previously engine-powered components are being replaced by versions driven by electric motors.

Proliferating robots (including collaborative robots) — Most are equipped with motors as well as speed reducers. Nidec factories that once made HDD motors are now being geared for producing robotic speed reducers instead. “Robots are expensive but increasing use (even in people’s homes) will come as prices decrease,” added Nagamori. In fact, it’s predicted the world population will reach nine billion with three service robots for every human by 2050.

Replacement of manual tasks in logistics and agriculture — The potential for drones in logistics and agriculture is especially high, though new breakthroughs and regulations are still needed. Nagamori expects these to happen within the next couple years.

Proeger • Trinamic Motion Control: Regarding brushless motors, there is an interesting trend driven by the drone market. While typically in industrial applications we’ve seen mainly seen inrunner motors — often with NEMA double-digit-style flanges — we now see more and more outrunners that generate an impressive amount of torque at really low cost. Typically, they have a higher pole count then the traditional inrunners … resulting in smaller transmission ratios for gearboxes. They haven’t yet arrived in traditional industrial machinery, but they’re used in a lot of warehouse-automation systems as well as some collaborative robot projects.

Interoperability, supplier configuration, and design-right functionality from motors

Interoperability is a motor functionality getting big play this year.

Maroney • Applied Motion Products: We strive to keep products compatible across product lines and generations. As a baseline, we adhere to standard NEMA frame sizes on stepper motors and metric sizes on servomotors — and we certify our fieldbus implementations with relevant standards organizations … such as ODVA for EtherNet/IP, for example.

Beyond that, our proprietary Q Programming language is the same for all our motors — from our smallest open-loop stepper drive to our largest servodrive. This lets engineers who use our products learn a single language and apply it to all axes of motion in all their systems — from the smallest setup axes to the largest and most complex. This reduces programming time and simplifies troubleshooting.

In fact, our focus on backwards compatibility ensures that OEMs’ machines will be serviceable for years into the future and retrofittable with our latest offerings.

Beckstoffer • Portescap: In the design-development phase of any new product, we ensure complete motor systems can be offered — including motor, gearbox, and encoder. Mechanical compatibility guarantees the new design will integrate with other products with ease. Case in point: Some new motors might include a front face designed to accept the closest diameter gearboxes. The rear flange of the motor might have geometry to accommodate relevant encoders. This lets engineers use new products in systems … complete with motion feedback.

Motor suppliers tout an expanding menu of value-add services (or in-house engineering support) for OEMs and design engineers seeing help in their motion-design work.

Ayaz • ISL Products International: During the past 12 months we’ve helped customers (OEMs) through our value-added approach. We specialize in optimizing standard dc motors and gearmotors … and tailor each motor or gearmotor to the OEM’s specific requirements.

The most common value-add we provide is customized output performance parameters (such as speed and torque) for each customer application. We are also able to offer various motor and gearhead types, shaft customizations, material selections, magnet types, lead wires and connectors, and encoder variations. We use our in-house engineering experience to guide engineers in the right direction … after all, motion control is a precise task.

James Gallant | Director of operations • ISL Products International: We aim to be design engineers’ à la carte menu when it comes to ordering dc motors and gearmotors.

This is a NEMA-23 stepper-motor-driven linear actuator from ISL Products. The stepper-based design is used in automated door locks. Read the Motion Trends piece on linear actuators for more actuator application examples.

Beckstoffer • Portescap: Innovation is our passion. We extend that passion to our customers by engaging with them early in their design to offer our expertise for the motion portion of their design. It goes beyond just the motor composite, we review the mechanical and electrical parts that engage with the motor composite to ensure the optimum efficiency of the system. A joint design effort results in a more efficient and compact final product.

Shown here is the Portescap B16 gearhead; B32 gearhead; and the R22HT, which is a power-dense mini-motor gearhead that is suitable for battery-operated tools.

Nicole Ashton • Portescap: When we engage with customers earlier in the design process, it helps reduce motor integration risk while providing breakthrough design impact. When we work with a customer from their initial ideation stage, that’s when true collaborative innovation happens, and benefits maximized.

Of course, online configuration tools and software continue to change design-engineering approaches.

Beckstoffer • Portescap: Today’s design engineers require increasingly detailed information online — to support deeper research on available options for motion in their designs. We’ve developed MotionCompass software to provide design engineers a way to leverage our expertise anytime.

By providing dynamic recommendations based on torque and speed, we’ve shortened the iteration process. That’s also because physical testing of motors has been replaced by online calculations. This lets design engineers confidently prototype with initial motor selections and ultimately reduces the time to get new products to market.

Ayaz • ISL Products International: Brush-type motors are still common in industry. Brush dc motors have the advantage of not needing a controller for operation. A new trend we’re seeing is the rise in coreless brushed motors, also known as ironless motors.

These motors are more efficient than comparable motors and exhibit less cogging torque — plus are made with magnets such as neodymium to deliver high torque. The increased use of coreless brushed motors is primarily in cordless and handheld devices in the medical industry — because the motors have relatively low current draw.

Motors prebuilt with linear devices and other mechanical elements abound to simplify assembly. Many suppliers sell gearboxes with integrated leadscrews to reduce part counts and avoid mechanical issues associated with couplings and more. Consider a specific example: One medical-device (a single-use biopsy tool) is now built with a motor and gearbox as well as an integrated leadscrew and nut in a gearbox extension.

Beckstoffer • Portescap: Gearbox design is being advanced by three design objectives — to increase efficiency, provide higher output torque, and accept higher motor input speed. This challenges design engineers to balance these three requirements as new products are developed.

As one solution to address all three objectives, we introduced the R22HT … again, to provide significantly higher output torque and allow higher input speeds … as well as boost efficiency by stage … all in planetary technology. In fact, the design gives OEMs a broader selection of gear ratios to fine tune the torque and speed output for any application.

MotionCompass is a web-based application from Portescap — accessible via motioncompass.portescap.com — that helps engineers size and select motors for disparate motion applications. The software generates recommendations from a wide array of brushless dc and brush dc coreless products and includes performance curves in the output.

Other miniature more designs include brushless dc cannulated gearmotors that allow inline driving of Kirschner wires and pins in orthopedic surgery. Other motors go still smaller.

Richard Halstead | President • Empire Magnetics: One technology using cutting-edge micromotors and microdevices is known as LIGA — from a German acronym that’s short for lithography, electroforming, and molding and indicating processes using high-energy X-rays. The micromachining and microdevices at beamline facilities require designs constructed of gears, motors, and other components that are tiny … as many LIGA structures have active areas of 0.25 to 4 mm.

What is plugging for electric motors?

February 26, 2019 By Danielle Collins Leave a Comment

There are generally three types of electrical braking for motors: regenerative braking, dynamic braking, and plugging. Of the three methods, plugging provides the fastest stop, but it can be harsh on both the electrical and mechanical components. Because of this, it’s the least commonly used method of braking, but it is appropriate for some applications.

Plugging can be more harsh on electrical and mechanical components, but it provides a faster stop than dynamic braking methods.
Image credit: secs.oakland.edu

Plugging — sometimes referred to as “reverse current braking” — is possible on both DC motors and AC induction motors. For DC motors, plugging is achieved by reversing the polarity of the armature voltage. When this happens, the back EMF voltage no longer opposes the supply voltage. Instead, the back EMF and the supply voltage work in the same direction, opposing the motor’s rotation and causing it to come to a near-instant stop. The reverse current produced by the combined supply voltage and back EMF is extremely high, so resistance is placed in the circuit to limit the current.

DC motor circuits for regular motoring operation (left) and for plugging (right). Notice that for plugging operation, the armature voltage is reversed and resistance is added to the circuit.

For AC induction motors, the stator voltage is reversed by interchanging any two of the supply leads. The field then rotates in the opposite direction and the motor’s slip (the difference between the speed of the stator’s rotating magnetic field and the speed of the rotor) becomes greater than unity (s > 1). In other words, the rotor spins faster than the rotating magnetic field in the stator. Torque is developed in the opposite direction of the motor’s rotation, which produces a strong braking effect.

Slip, which is the difference between the speed of the stator’s rotating magnetic field and the speed of the rotor, is a fundamental property of AC induction motors. In normal motoring operation, the rotor spins at a slower rate than that of the stator’s rotating magnetic field.

When the motor speed reaches zero, if it is not disconnected from the supply, it will begin to reverse, or rotate in the opposite direction. In some applications, reversal of the motor’s direction is the goal. But when plugging is used to brake the motor, a zero-speed switch or plugging contactor is used to disconnect the motor from the supply when its speed reaches zero.

One of the potential problems with plugging as a braking method (especially when the braking time is short) is that it can be difficult to brake the motor at exactly zero speed. Another drawback to plugging is that it can induce high mechanical shock loads on the motor and connected equipment, due to the abrupt stop that it causes. Plugging is also a very inefficient method of stopping and, therefore, generates significant heat.

Despite these drawbacks, plugging is used in equipment such as elevators, cranes, presses, and mills, where a rapid stop of the motor (with or without reversal) is required.

Get your motor running… efficiently

January 29, 2019 By Miles Budimir Leave a Comment

A few tips for design and selection of induction and permanent magnet ac electric motor systems will make them more energy efficient.

Gabriel Venzin
ABM Drives Inc., ABM Greiffenberger U.S. Subsidiary

Efficiency in motor designs is a more relevant concern than ever. With electric motors providing over 60% of all industrial power, efficiency is a key design parameter. Here we’ll look at some general requirements as well as more specific ones for permanent magnet ac (PMAC) motors, which hold for common industrial applications in a limited power range.

To start, efficiency is defined as the ratio of delivered mechanical power to the supplied electrical power. An electrical motor that is 85-percent efficient converts 85 percent of electrical energy into mechanical energy. The remaining 15 percent dissipates as heat.

Exploded view rendering of an ac induction motor illustrating the housing, bearings and fan components.

Energy efficient electric motors use high-quality materials and optimized designs to achieve higher efficiencies. For example, more aluminum in the rotor and better slot-filling factor in the stator reduce resistance losses. Optimized rotor configuration and rotor-to-stator air gap reduce stray-load losses. Improved cooling fan design provides motor cooling with a minimum of windage loss. Higher quality and thinner steel laminations in the rotor and stator cores allow operation with substantially lower magnetization losses. Finally, reduced friction losses result from higher-quality bearings.

Selection points for both induction and PMAC motors

Optimizing the dimensions of rotor/stator laminations and the quality of steel used is key to improving performance
Hysteresis and eddy current losses together are called core losses. Around 20 percent of total losses are caused by eddy currents and magnetic core saturation.

Eddy currents induced in laminations moving relative to a changing magnetic field cause significant power losses. Laminated stator cores reduce eddy current losses and are based on the quality, resistivity, density, thickness, frequency and flux density of the iron. Eddy current losses can be minimized with a higher number of laminations.

Hysteresis losses are caused in the magnetic circuits when the flux is continually changing. The majority of flux-carrying material used in electric motors is steel used for stator and rotor cores.

Minimize the flux density and core losses by reducing the thickness of laminations. Hysteresis losses are reduced by annealing better grades of steel for laminations to change the grain structure for easy magnetization.

Reduce eddy current losses by increasing the resistivity of the steel with silicon, but silicon content increases die wear during stamping because it increases steel hardness. Steel crystals damaged during stamping severely degrade the magnetic quality of the affected volume.

Annealing flattens the laminations and recrystallizes the crystals damaged during stamping, which extends one sheet thickness into the lamination.

The special design of ABM’s Sinochron motor rotor with internal magnets leads to an almost ideal sine distribution of the magnetic flux. It’s designed for sensor-less control mode, making it an alternative to servo drives.

Use bath impregnation process for lamination, versus drip for optimum thermal management
Impregnating stators strengthens stator winding electrical insulation, protects against chemicals or harsh environments, and enhances thermal dissipation. Thermoset plastics including epoxies, phenolics, and polyesters are used for impregnating stators. ABM uses the bathing method whereby stators are immersed in resin for an extended period of time to assure optimum penetration and protection. Another impregnation method is called vacuum-pressure, which uses a tank that is evacuated first and then pressurized to achieve penetration into the stators. Driving out air pockets from the electric winding enhances the thermal conductivity of the winding.

Design slots in the stator to maximize the volume of copper that can be inserted
Low stator winding mass causes up to 60 percent of total losses, so to reduce them the mass of stator winding must be larger which reduces electrical resistance. Electric motors that are highly efficient contain more than 20-percent extra copper compared to standard efficiency electric motors. The stator’s insulated electrical windings are placed inside the slots of the steel laminations. The cross-sectional area must be large enough for the power rating of the motor.

In general, open or semi-closed stator slots are employed in induction motors. In semi-closed slots the slot opening is much smaller than the width of the slot, and winding is more difficult and time consuming to manufacture compared to open slots. However, the air gap characteristics are better compared to the open type.

The number of stator slots must be selected at the design stage because this number affects the weight, cost, and operating characteristics. The advantages of more slots are reduced leakage reactance, reduced tooth pulsation losses, and higher overload capacity. The disadvantages of more stator slots are increased cost, increased weight, increased magnetizing current, increased iron losses, poorer cooling, increased temperature rise, and reduction in efficiency.

SINOCHRON PMAC motor efficiency is more than twice compared to standard asynchronous motors under partial-load. It’s made possible by a continuously excited synchronous motor, which operates with practically no rotor losses.

Use high grade/pure aluminum in the rotor die casting
Custom designed rotors can maximize starting torque, reduce conductor resistance and increase efficiency. Most induction motor rotors are squirrel-cage designs. They are rugged, simple in construction and less expensive, but they have a low starting torque. Copper rotors increase efficiency but are difficult and expensive to manufacture.

Tightening manufacturing tolerances allows optimum air gap between rotor and stator
The air gap is the radial distance between the rotor and stator in an electric motor for standard radial motors. An optimum air gap needs to be maintained for design efficiency. Air gap dimension involves the design of stator, rotor, motor housing, and bearings. All these affect the exact alignment of the stator and rotor axes.

Use copper windings that meet DIN EN 13601 and UL Norm standards
The European standard DIN EN 13601 specifies copper and copper alloys for general electrical purposes including dimensions and tolerances, tensile test, hardness test, bend test, electrical resistivity test. It also specifies characteristics of coppers for electrical purposes including the preference for oxygen-free or deoxidized coppers which may be heat-treated, welded or brazed without the need for special precautions to avoid hydrogen embrittlement which causes cracking.

For global compliance, UL determines that magnet wire complies with IEC 60317. Magnet or enameled wire is an electrolytically refined copper or aluminum wire that has been fully annealed and coated with one or more layers of insulation. ABM, for example, uses wires with a total of 12 insulation coats. Typical insulating films in order of increasing temperature range are polyvinyl, polyurethane, polyester, and polyimide up to 250 °C. Thicker rectangle or square magnet wire is wrapped with a high-temperature polyimide or fiberglass tape.

Use of more copper, larger conductor bars and conductors increases the cross-sectional area of stator and rotor windings. This lowers resistance of the windings and reduces losses due to current flow. A high-efficiency electric motor will usually have 20 percent more copper in the stator winding.

General selection guidelines
The motor selection process involves some basic considerations. For starters, verify the application requirements. Then, what torque and speed is needed when and how often? What is the duty cycle? What are the ambient conditions such as temperature and pressure? Even the most efficient motor will not perform to its utmost efficiency if used for the wrong application.

Many of the electric motors discussed here are used for gearmotors, a combination of a gear reducer and an electrical motor. A gearmotor delivers high torque at low speed. In short, gearmotors take motor power and reduce its speed while magnifying its torque.

Gearmotor duty cycles do not only impact the motor performance rating, e.g., a continuous duty cycle
S1 motor will provide a higher output if it’s used at an S3 intermittent duty cycle, but also especially for gearboxes used in extraordinary applications where torque peaks and system shocks are possible. For such situations, it’s critical to carefully determine what duty cycle and what peaks/shocks per a specific time period are experienced so the engineering team of the motor/gearbox OEM can select and recommend a suitable drive system.

A good start is to review data on past extreme temperature and or pressure conditions in which the application will be operating and any other unique conditions that may be relevant to the design. Always inform the motor OEM if your application is being used in extreme conditions – this can be a game changer.

Design housing for best cooling
The electrical efficiency of motors has improved so much that the power drawn by cooling fans has become a larger percentage of the total losses. Optimization of cooling fan dimensions involves using the minimum power for the fan, while providing adequate cooling. The optimal fan design may result in a 65 percent reduction in fan power requirements. An important design feature is the clearance between the blade and the housing. The space between the housing and the fan blade should be as small as possible to prevent turbulence and decrease backflow.

A cool motor runs more efficiently. To obtain the best airflow, optimize the cooling fan and fan shroud design. Assuring a tight bond between the stator and motor housing provides the best cooling performance.

Select low-friction bearings suited for operating speeds
Ball or roller bearings are used in high-efficiency electrical motors. They consist of an inner and outer race and cage containing steel or ceramic rollers or balls. The outer race is attached to the stator and the inner race to the rotor. When the shaft rotates, the element also rotates and the friction of the shaft rotation is minimized. They have a long life and maintenance costs are low. High precision application allows the smallest air gap possible. Thermal contraction and expansion can affect shaft and housing fits as well as internal bearing clearances.

Power output governs shaft size and bearing bore. Load magnitude and direction determines bearing size and type. Consider additional forces such as unsymmetrical air gaps causing magnetic pull, out-of- balance forces, pitch errors in gears, and thrust loads. For bearing-load calculations consider the shaft as a beam resting on rigid, moment-free supports. Ball bearings are more suitable for high-speed applications than roller bearings. High-speed factors include cage design, lubricant, running accuracy, clearance, resonance frequency, and balancing.

Knowing the ambient temperature range and the normal operating temperature range will help determine the most effective lubrication method for the bearing: oil or grease. Normal operating temperatures for gearmotors considered here range from -25 to 40 °C. Synthetic grease has good performance properties over a wide range of temperatures. Grease allows simplified maintenance, cleanliness, fewer leaks, and contaminant protection.

Bearings need a minimum load, so rolling elements rotate and form a lubricant film rather than skidding, which raises operating temperatures and degrades lubricants. Allow a minimum load equal to about 0.01 times the dynamic radial-load rating for ball bearings. It’s especially important when bearings are approaching 70 percent of recommended ratings.

Use a quality balancing machine, high standards and balance at the operational speeds of the motor application
Noise and vibration can result when the center of mass does not co-exist with the axis of rotation. Balancing has limited effect on efficiency but impacts operating noise and life expectancy that is also important for maximum use of resources.

Bearing vibration readings are normally taken on three planes – vertical, horizontal and axial. Vertical vibration may indicate a mounting problem. Horizontal vibration may mean a balance problem, whereas axial vibration may indicate a bearing problem. Balancing at operational speeds is important because unbalance can also be caused by centripetal forces at the bearings.

Additional design criteria for PMAC motors

PMAC motors are a growing alternative to ac induction motors, which for decades have been the workhorse of almost any electrical motor application. PMAC motors preserve the reliability and simplicity of the ac induction motor while offering higher efficiency, synchronous operation, and the opportunity to use a smaller frame size.

PMAC motors replace the magnetic field induced in the conductors in the rotor with permanent magnets, usually made from alloys of rare-earth metals, giving them much lower electric resistive losses than ac induction motors because no electric current is induced in the rotor. In place of mechanical commutation, a control system is required to determine which coils are supplied with current to produce the maximum torque. Magnetic fields generated by rare-earth PMAC motors can deliver the same torque as ac induction motors with a smaller, lighter motor.

This illustration shows the special design of the SINOCHRON PMAC motor rotor laminations and the imbedded magnets that allow a sinusoidal motor motion.

Select PMAC motor size to allow operation in partial load area for best efficiency
Energy savings from PMAC motors depend on the number of service hours, motor size, and motor loading. PMAC motors nearly always maintain higher efficiency – no matter the speed or torque compared to ac induction motors and makes them an attractive alternative to induction motors in variable-speed applications. By substituting existing line-powered three-phase electric motors with PMAC motors, energy savings of 20 to 35 percent can be expected.

Optimize design of rotor laminations to show a sinusoidal magnetic field
For example, ABM synchronous motors with high-performance permanent magnets come with a sinusoidal flux distribution as well as Electro- Motive-Force. Stator windings, for distributed windings, are normally identical to asynchronous motor windings. It results in lowered vibration, noise and maintenance cost as well as increased overall performance.

Selection of rare earth versus ferrite (ceramic) magnets
Neodymium rare earth, samarium cobalt magnets or ferrite (ceramic) magnets are used in PMAC electric motors. Rare earth magnets are two to three times stronger than ferrite or ceramic permanent magnets but are more expensive. Samarium cobalt magnets are optimal for high-temperature applications because of their high energy density, 250 to 550 °C temperature resistance, small decrease of parameters due to temperature increase, and oxidation protection.

The selection of samarium cobalt or neodymium as an electric motor magnet is
based on operating temperature, corrosion resistance and required performance. A low-grade neodymium magnet may begin to lose “strength” if heated above 80 °C. High-grade neodymium magnets operate at temperatures up to 220 °C.

Ferrite or ceramic magnets have won wide acceptance due to their strong resistance
to demagnetization, exceptional corrosion resistance, and low price. Magnetic losses occur when operating at temperatures above 250 °C, but are recovered when the magnet is brought down to a lower temperature. Cold temperatures of -40 °C may result in permanent losses of magnetic strength unless the circuit has been designed for such extremes.

PMAC motors require inverters
PMAC inverter drive units can be loss-free in no-load operation/standstill. New motor design combinations offer advantages in powering conveying equipment, escalators, spooling machines, compressors and traction drive units. By substituting existing line-powered three-phase drive units, energy savings of up to 30 percent can be expected.

The characteristic profile of PMAC drive units makes them well suited to drive pumps and fans that operate continuously. No additional components, like encoders, are needed. Up to 25 percent smaller footprint allows machine designs to be more compact. The motors have excellent control behavior and combined with a sensorless drive controller unit, have excellent true running even at low speeds and impressive dynamics at impulse load and speed variations.

Select an inverter that can provide sensorless operation
Standard PMAC drives can “self-detect” and track the rotor’s permanent magnet position. This is critical for a smooth motor start and also allows for optimum torque production, which results in optimum efficiency. Lack of position or speed sensor reduces the cost and increases reliability of a drive system.

Programming and optimization of inverter
The importance of programming the controller settings for a specific motor to attain optimum efficiency is becoming more and more important with the ever- increasing efficiency regulations.

Choosing manufacturing partners
When choosing partners during a machine build, remember that there are two methods for choosing an electric motor source. Either pick a standard motor that might or might not fit a specific application or choose a competent motor partner to engineer and manufacture a motor that fits the application exactly.

Standard motor solutions are suitable if a design engineer doesn’t have the time or engineering resources to have a custom version engineered — or if it needs a quick setup. New modular approaches to design and construction let manufacturing engineers get reasonably priced custom motors even in modest volumes.

No matter the approach to motor selection, be sure to continually improve the design/drive system by comparing predictions of performance with measurements. Then use the result of the analysis to improve the next iteration.

ABM Drives
www.abm-drives.com

Premium-efficiency, cast-iron three-phase ac motors from AutomationDirect

December 18, 2018 By Miles Budimir Leave a Comment

IronHorse MTCP2 industrial duty motors meet the premium efficiency requirements of the Energy Independence and Security Act of 2007. MTCP2 motors are offered in T-Frame (1 to 300 hp) and TC-Frame/C-Face (1 to 30 hp). C-face kits are available for motors over 30 hp.

These motors are available in 1,200, 1,800, and 3,600 RPM and have TEFC (Totally Enclosed Fan Cooled) enclosures with cast-iron frame and ribbed design for maximum cooling. The motors feature full length, cast-iron mounting feet, steel fan cover, and cast-iron junction box with rubber gasket and rubber dust cover.

MTCP2 motors have premium quality ball bearings (1 to 75 hp) or roller bearings (100 to 300 hp) with maintenance free bearings on 10 hp and below. V-ring shaft seals are used on the drive end and opposite drive end. The motors are electrically reversible and have Class F winding insulation.

MTCP2-series motors start at $180.00, have a two-year warranty, and are CSA and CE approved. Available accessories include bases, junction boxes, fans and fan shrouds.

To learn more, visit www.automationdirect.com.

The truth about VFDs and power factor

December 7, 2018 By Danielle Collins Leave a Comment

Power factor is essentially a measure of how effectively a piece of equipment (or an entire facility) uses electricity to produce useful work, such as heating, lighting, or motion. Electricity companies monitor power factor and often charge customers a penalty if their power factor falls below a specified threshold — typically 0.90 or higher.

Fortunately, most electrical loads in residential buildings are resistive (heating and lighting, for example) and have a high power factor, so consumers aren’t normally subjected to this metric. Industrial plants, on the other hand, typically have high inductive loads that significantly reduce their power factor. For example, AC induction motors — which are used for driving pumps, fans, compressors, and conveyors — have relatively low power factors, even when their capacity is put to full use. And when these motors are lightly loaded, their power factor drops even lower — sometimes approaching zero.

When an induction motor is heavily loaded (top), the voltage and current waveforms are nearly in-phase (small displacement), and power factor is high. When the motor is lightly loaded (bottom), the wave forms are out of phase (large displacement) and power factor is low.
Image credit: Yaskawa America

There are several methods of improving power factor in an industrial facility, but when the main culprits are induction motors — especially induction motors not operating at full load — applying variable frequency drives (VFDs) is often the best solution.

Recall that in an AC induction motor, power is supplied directly to the stator and a magnetic field is induced in the rotor. The power supplied to the stator is referred to as “real,” or “active” power because it produces torque. The power used to induce the magnetic field in the rotor is referred to as “reactive” power because it does not actively produce work. The combination of real power and reactive power is referred to as “apparent” power. Real, reactive, and apparent power are often depicted on a power triangle.

In an ideal system, power factor (PF_D) is the cosine of θ, which equals real power (P) divided by apparent power (S).

Note the subscript, D, in the power factor notation. In the power triangle, θ represents the displacement (phase difference) between voltage and current. Thus, this expression of power factor is often referred to specifically as the displacement power factor, PF_D.

For resistive loads, all (or nearly all) of the power used is real power that produces useful work — heat or light, for example. Therefore, current and voltage remain in phase (θ = 0), and the power factor is near unity (i.e., 1). But the reactive power required for inductive loads — to induce a magnetic field in a rotor, for example — tends to pull the current out of phase with the voltage (hence, a larger θ angle), causing a lower power factor.

The amount of real power required by an induction motor varies with the load, but the amount of reactive power (power required to generate the rotor’s magnetic field) is constant regardless of the load. Thus, when an induction motor is lightly loaded, the ratio of real power to apparent power decreases, resulting in a lower power factor.

Using VFDs to drive induction motors can improve power factor — but it’s not the panacea that some manufacturers suggest.

Variable frequency drives typically have very high PF_D values. This is because the DC bus capacitors supply the necessary reactive current to the motor for inducing the rotor’s magnetic field, and the AC supply line only has to supply real power. This means voltage and current remain almost perfectly in phase, with very little displacement, and the power factor can be at or near unity.

But VFDs also introduce harmonic current distortion. And because harmonic currents don’t produce useful work, they are reactive, thus negating some of the power factor benefits of the VFD.

To determine the true power factor, PF_T, which includes the effect of harmonic distortions, we use the equation:

THD = total harmonic current distortion

Fortunately, there are ways to reduce this VFD-induced harmonic distortion and minimize its effects on power factor. For VFDs that use a standard, diode-based rectifier, adding impedance via a line reactor or DC link choke will reduce THD.

Another option is to use an active front end (AFE) drive, which is a VFD that uses IGBTs, rather than diodes, to rectify the incoming AC power to DC. Active front end drives have significantly lower THD than standard diode-based rectifier designs. For example, an AFE drive typically has THD around 5 percent, where a standard VFD with a diode-based rectifier can have THD in the range of 45 percent.

Q: Why does the utility care how efficiently a plant or equipment uses the power it supplies?

A: Because low power factor causes higher line currents, which put more stress (primarily in the form of heat) on cables, transformers, and other equipment. Also, the lower the power factor, the more apparent power (kVA) the utility must supply to meet the real power (kW) requirements.

When do you need a soft starter for an AC motor?

November 21, 2018 By Danielle Collins Leave a Comment

Image credit: WEG Electric

The traditional method of starting an AC induction motor “across the line” results in full voltage, current, and torque being applied immediately when the motor is started, and likewise, being removed immediately when the motor is stopped. While this is the most simple starting method, the high inrush current (often 6 to 7 times the motor’s rated current) and peak starting torque can damage the motor, driven equipment, and product. Across the line starting also causes high peak power demand, which can trigger peak demand fees from the utility company.

A soft starter can eliminate these problems by gradually increasing voltage to the motor terminals during startup, providing a controlled ramp-up to full speed. This lowers inrush current and controls starting torque, reducing mechanical shocks to the system and to the product.

In a soft starter, three pairs of SCRs control voltage to the motor during startup.
Image credit: WEG Electric

Soft starters are also known as reduced voltage soft starters (RVSS).

A soft starter relies on three pairs of SCRs (silicon controlled rectifiers) — one pair for each phase of power — that are applied gradually for portion of each voltage phase, limiting the voltage provided to the motor. In turn, current is reduced proportionally to the reduction in voltage. Torque, however, is proportional to the square of the voltage, so even a small reduction in voltage results in a large reduction in torque. For example: a 50 percent reduction in voltage yields a 50 precent reduction in current and a 75 percent reduction in torque.

Where:

T₂ = Torque at reduced current/voltage

T₁ = Torque at locked rotor current

I₂ = Reduced current

I₁ = Locked rotor current

V₂ = Reduced voltage

V₁ = Full voltage

Once the motor is up to speed, the soft starter is bypassed and the motor is connected across the line, allowing full power to the motor terminals. Soft starters introduce almost no harmonics into the system and typically have efficiencies of 99 percent or better.

A soft starter (solid-state starter) allows current to be limited (top) during motor starting. The corresponding torque reduction (bottom) is proportional to the square of the reduction in voltage.
Image credit: Eaton Corporation

It’s important to note that a VFD (variable frequency drive) can provide the same controlled starting and stopping functions that a soft starter provides, albeit in a different way — by varying the frequency of the voltage, rather than by controlling the amount of voltage, supplied to the motor. VFDs offer other benefits over soft starters, the most significant being the ability to control motor speed throughout the operating range. VFDs can also provide holding torque (full torque at zero speed), which is critical in applications such as elevators and cranes.

For applications such as conveyors and fans that require speed and torque control or current limiting during starting and stopping — but run at constant speed otherwise — soft starters provide a simple, economic solution in a small footprint.

What is DC injection braking and how does it compare with other methods?

November 15, 2018 By Danielle Collins Leave a Comment

Many variable frequency drives (VFDs) include DC injection braking functionality.
Image credit: Galt Electric

AC induction motors excel at driving high-inertia loads at high speeds — in fans, saws, conveyors, and mills, for example — and variable frequency drives make it possible to match the motor speed to the application requirements by varying the frequency of the supplied voltage.

But when quick stops are needed — whether for safety reasons or to prevent wasted time and lost productivity waiting for the load to come to a standstill — high speeds and high load inertia make it difficult to brake the motor and stop the load.

In an AC induction motor, three-phase AC power is fed to the stator windings, creating a magnetic field that rotates. This magnetic field induces a current and a corresponding magnetic field in the rotor. The interacting magnetic fields of the rotor and stator cause the motor to turn.

DC injection braking works just as its name implies — by injecting DC voltage into the motor windings. Before DC injection braking can begin, the AC power to the motor stator must be disconnected — typically by the opening of a relay. In turn, a relay controlling motor braking closes, allowing DC voltage — sourced by the DC bus — to be applied to the windings. To prevent the braking current from exceeding the drive and motor ratings, circuitry in the VFD or in an external braking module controls the amount of voltage applied.

DC injection braking begins when AC voltage is removed (S1 opens) and DC voltage is injected into the windings (S2 closes).
Image credit: KB Electronics, Inc.

When DC current is applied to the motor windings, it creates a fixed (rather than rotating) magnetic field. Braking action is produced by the rotor working to align to this stationary field. The higher the DC current, the stronger the braking force.

The energy generated during braking is dissipated as heat by the motor (particularly the rotor) and the controller, so thermal limitations of these components dictate how much braking current can be applied and the amount of time for which it can be applied before overheating occurs. If DC injection braking is used frequently, the additional heating created during braking must be taken into account when sizing the motor.

Although DC injection braking can be used to hold a load stationary, it’s not generally recommended due to heat generated in the motor. And because DC injection brakes require a constant power supply, they’re not considered fail-safe devices.

DC injection braking is just one of several electrical methods of bringing an AC induction motor to a stop. Two other forms of braking — dynamic braking and regenerative braking — convert mechanical energy generated as the rotor slows down into electrical energy.

During motor starts and stops, or when the load is overhauling, an AC induction motor functions as a generator and produces electrical energy from mechanical energy. This electrical energy can be fed back to the AC power source.
Image credit: Bonitron

In a dynamic braking system, this electrical energy is released as heat through a voltage regulated resistor. In regenerative braking, this electrical energy is fed back to the power supply or to a common DC bus. Dynamic braking is best for applications that require only periodic braking without high heat dissipation demands, while regenerative braking is best for applications with frequent stops and starts or overhauling loads.

A drawback of both dynamic braking and regenerative braking is that the braking force decreases as the motor (rotor) slows down. DC injection brakes are often used in conjunction with dynamic or regenerative braking systems to provide the final braking power required to fully stop the motor and load. And because DC injection brakes can cause thermal problems for the motor and drive, the use of dynamic or regenerative braking in the beginning, together with DC injection braking at the end of the stop, reduces the risk of overheating while providing fast, complete stopping for high-inertia, high-speed AC induction motor applications.