Danielle Collins - Motion Control Tips

How does stiffness affect piezo output force and displacement?

August 15, 2018 By Danielle Collins Leave a Comment

Piezo actuators operate on the inverse piezoelectric effect: when voltage is applied, the actuator contracts or expands. But when the actuator is blocked from moving (by a load applied in the direction of travel), it generates a force. This relationship between displacement and force in piezo materials is an inverse relationship. In other words, the more force an actuator must generate against a load, the less travel it can produce. A “free” actuator — one that experiences no resistance to movement — will produce its maximum displacement, often referred to as “free stroke,” and generate zero force.

Conversely, when an actuator is blocked from moving, it will produce its maximum force, which is referred to as the blocked, or blocking, force. Theoretically, when the actuator is blocked, it is working against a load with infinitely high stiffness. But materials with infinite stiffness don’t occur in the real world, so the blocking force is determined by applying a voltage to the actuator, with no load applied. This “free” operation, as described above, generates the actuator’s maximum displacement, or free stroke. A force is then applied to return the actuator to its original length. This force is measured and recorded as the blocking force.

The stiffness of the actuator is the ratio of blocking force to free stroke.

k_p = stiffness of piezo actuator

F_block = blocking force

ΔL_fs = free stroke (nominal displacement) with no external load

In piezo actuators, force and displacement are inversely related. Maximum, or blocked, force (F_block) occurs when there is no displacement. Likewise, at maximum displacement, or free stroke, (ΔL_fs) no force is generated. When an external load is applied (A), the stiffness of the load (k_e) determines the displacement (ΔL_A) and force (ΔF_A) that can be produced.
Image credit: APC International, Ltd.

There are two types of load that can be applied to a piezo actuator: a spring load, which increases as the actuator moves, or a constant load, which does not vary. The actuator’s ability to produce displacement and force depends on the type of load.

Piezo output force and displacement with a non-constant (spring) load

Many piezo applications, such as valve operation, introduce loads that vary with the piezo’s stroke — in other words, spring loads. When the stiffness of the applied spring load is low compared to the piezo stiffness, the piezo output force (F_eff) is small. In other words, a spring (load) with low stiffness provides very little “blocking” for the piezo to work against in order to generate a force. Therefore, the piezo output force is greatly reduced from its maximum, or blocking, force.

F_eff = force produced with spring load

k_e = stiffness of applied load

On the other hand, the spring reduces the actuator’s displacement only slightly:

ΔL = displacement 0f actuator with spring load

Performance of a piezo actuator with no load (A) versus an actuator with a low-stiffness spring load (B). Note that the displacement (ΔL) with the spring load is slightly less than the maximum displacement, or free stroke, (ΔL_fs), but the force produced (F_eff) is much less than the maximum, or blocked, force (F_block). The stiffness of the spring load (k_e) is shown by the blue line.
Image credit: PI Ceramic GmbH

A low-stiffness spring load — typically no more than 10 percent of the piezo stiffness — is generally recommended for preloading a piezo actuator to ensure the displacement is not severely reduced.

Piezo output force and displacement with constant load

When a constant load is applied, there is an initial deflection, or compression, of the actuatordue to the load. This means the zero point of the working curve is offset, or shifted, but the actuator’s stroke is unaffected.

ΔL_N = offset of the zero point with constant load

F = applied load

Some force is generated when the load is applied, causing the initial deflection, but once the actuator responds to the application of the load, its remaining work goes into generating the displacement. The zero point of the blocking force (maximum force) is also shifted, but is magnitude is not affected.

When a constant load is applied, the zero point of the actuator’s displacement is shifted, but its total displacement, or free stroke, is not affected: ΔL’_fs = ΔL_fs
Image credit: PI Ceramic GmbH

Feature image credit: Noliac

What are the benefits of an active front end (AFE) drive?

August 8, 2018 By Danielle Collins Leave a Comment

A variable frequency drive, or VFD, delivers power from the source — typically the utility — to a motor in three basic steps:

A rectifier converts the AC power to DC
A DC bus receives, smooths, and stores the power
An inverter converts the DC power back into AC with the necessary frequency and voltage via pulse width modulation (PWM)

In a VFD, AC power is converted to DC through the rectifier (shown here as a standard 6-pulse version, consisting of six diodes). The rectified power is then filtered and stored in the DC bus. The inverter converts it back to AC power with the proper frequency and voltage for the motor, via pulse-width modulation.
Image credit: what-when-how.com

The rectifier is typically a 6-pulse type, so named because it consists of six diodes: one for when the voltage is positive and one for when the voltage is negative, on each of the three power phases. But while diodes are simple and cost-effective, they produce harmonics that are transferred back into the power system, resulting in additional heat and losses, and even causing erratic behavior of connected equipment.

Multiple rectifiers (i.e. 12-pulse, 18-pulse, or 24-pulse) can reduce harmonics, but as rectifier sections are added, footprint and cost go up. Another solution to reduce harmonics is to add passive filters, which introduce a low impedance to absorb harmonic frequencies. But passive filters are useful only in limited conditions, and they have a large footprint and generally use more energy than other alternatives. An active front end (AFE) not only reduces harmonics, but also provides other benefits that can reduce costs for the end user.

Rather than using diodes in the rectifier to convert the incoming AC power to DC, an active front end uses insulated gate bipolar resistors (IGBTs). IGBTs are devices whose switching is controlled electronically — hence the term “active” front end. The active front end monitors the input current waveform and shapes it to be sinusoidal, reducing total harmonic distortion (THD) to 5 percent or less. (Note that THD is only measured for lower-order harmonics. An LCL filter is necessary to reduce higher-order harmonics caused by the switching frequency of the IGBTs.)

An active front end (AFE) drive replaces the diodes in the rectifier with IGBTs, which significantly reduce harmonics and allow regenerated power to be fed back to the supply.
Image credit: Mesta Electronics, Inc.

Harmonic distortion is also the primary contributor to the system’s power factor in VFD applications. Power factor is determined by two variables. The first is the displacement, or phase angle, between the applied voltage and resulting current. For VFD operation, the voltage and current remain in phase, and the displacement between them (cos θ) is kept near unity (1).

The other factor is total harmonic distortion, which has an inverse relationship to the power factor — the higher the THD, the lower the power factor. So an active front end, which reduces harmonic distortion (for example from 45 percent to 5 percent) significantly improves the system’s power factor.

Power factor is essentially a measure of how efficiently the electrical power is being used to perform useful work. Utility companies often penalize industrial customers if their power factor falls below a set threshold, often in the range of 0.85 to 0.95.

Another benefit of active front end drives is their ability to handle regenerative power. The IGBT rectifier design allows four-quadrant operation, meaning the motor can produce either positive or negative torque (as occurs during braking or back driving), during either the forward or reverse motor rotation.

When torque and motor rotation are in different directions (i.e. clockwise torque and counterclockwise motor rotation, or vice-versa), the motor acts like a generator, and the power produced by the motor can be fed back into the electrical supply. This eliminates the need for large resistor banks to dissipate the regenerated energy and reduces costs by feeding the energy back to the line, where it can be reused.

New CVK-SC Stepper Motor Speed Control System from Oriental Motor

August 6, 2018 By Danielle Collins Leave a Comment

CVK-SC series — 0.72º, 24 VDC input stepper motor speed control

Oriental Motor is pleased to introduce the new CVK-SC series stepper motor speed control system. Specifically designed for common speed control applications with constant duty, the CVK-SC series features ease of use, holding torque when stopped, and increased repeat positioning accuracy. This speed control system offers a simple configuration consisting of a stepper motor, driver, and programmable controller.

The CVK-SC series’ operating speed, acceleration and deceleration time, and running current can be set via the driver switches, and simply turning the FWD (RVS) input to ON or OFF allows for easy control. With a speed range of 0.02 to 600 r/min, two speed settings, and no additional controller required, the CVK-SC series is ideal for speed control applications that demand more precise performance but require simple control at the same time.

The CVK-SC series speed control stepper motors utilize our latest 5-phase stepper motor technology, allowing for more torque, increased overhung loads, significantly reduced vibration and noise technology. And there is no need for a separate pulse generator or controller as they are both built-in. The CVK-SC series is available in three motor sizes and two driver types with 24 VDC power input.

Oriental Motor

What are wound field motors and where are they applied?

August 2, 2018 By Danielle Collins Leave a Comment

Brushed DC motors are generally available in two types, depending on the construction of the stator: permanent magnet or wound field. Both motor types use current and windings to produce a magnetic field in the rotor, but they differ in how the stator magnetic field is produced: via permanent magnets inside the stator or with electromagnetic windings.

Image credit: Electronics Tutorials

Wound field motors are further categorized by how the armature (rotor) and field (stator) windings are connected: series wound, shunt wound, or compound wound. Although performance characteristics vary among the three wound field motor designs, these motors generally have higher torque and speed capabilities than permanent magnet types.

Series wound DC motors

When the armature windings and field windings are connected in series, the motor is referred to as a “series wound” DC motor. Being connected in series means the current through the armature and the field windings is equal (I_total= I_a= I_f), which allows the motor to draw a significant amount of current. And for series wound motors, torque is proportional to the square of current, so these motors are able to produce very high torque, especially at startup.

Series wound DC motors are best for applications that require high startup torque, without the need for speed regulation.
Image credit: National Instruments

On the other hand, series wound motors are not good for speed control. Here’s why: As the motor is loaded, its speed decreases, which causes the back EMF to decrease and net voltage to increase. This increased voltage causes armature current and field current to increase. But the current eventually becomes high enough to cause saturation of the magnetic field, and the flux between the armature and the stator will increase at a slower rate than the rate of current increase. Thus, the motor isn’t able to produce enough torque to bring the speed back to its pre-loaded value.

Voltage equation for DC motor:

E_net = E – E_b

E_net = net voltage

E = supply voltage

E_b = back EMF voltage

Based on these characteristics — high starting torque but poor speed control — series wound DC motors are often used as starter motors for large equipment with high inertia loads, such as cranes and elevators. They’re also found in consumer products that require only coarse speed control, such as blenders and hand tools.

The universal motor is a special design of the series wound motor that can operate on either DC or AC power.

Shunt wound DC motors

When the armature and field windings are connected in parallel, the motor is referred to as a “shunt wound” DC motor. (In electrical parlance, a parallel circuit is referred to as a shunt.) The parallel connection between the windings means the current supplied to the motor is divided between the armature and the field (I_total= I_a+ I_f). The shunt (field) windings have a high resistance, preventing them from drawing high current at startup. But unlike series motors, shunt motors provide very good speed regulation.

Shunt wound DC motors are used in applications where the required starting torque is low, but good speed regulation is important.
Image credit: National Instruments

The initial effect of an increased load on a shunt motor is the same as for a series wound motor: speed decreases, reducing back EMF and increasing net voltage. But in a shunt wound motor, the increased net voltage causes the armature current to increase. In a shunt motor, torque is proportional to armature current, so torque increases. This additional torque increases the motor speed to compensate for decrease that occurred when the load was applied. All of this happens instantaneously, making DC shunt motors essentially constant speed devices, regardless of load.

With low starting torque and constant speed, shunt wound DC motors are used in applications where good speed regulation is needed with varying loads, such as grinding machines and lathes. Another common use for shunt wound motors are processes that require constant tension, such as printing and winding.

Compound wound DC motors

A hybrid between the series wound and shunt wound designs, a compound wound DC motor has a field winding that is connected in series with the armature winding and another field winding connected in parallel (shunt) with the armature winding. There are several sub-types of compound wound DC motors, depending on whether the shunt field winding is connected only across the armature winding (referred to as a “short shunt” design, or whether the shunt field winding is connected across the series combination of the armature and the field winding (referred to as a “long shunt” design).

Compound wound DC motors can be short shunt design, with the shunt field connected across only the armature, or long shunt design, with the shunt field connected across both the armature and the field windings.

In the short shunt design, if the polarity of the shunt field matches the polarity of the series field and the armature, it is referred to as a cumulative compound motor and has the combined characteristics of series wound and shunt wound motors: high starting torque and good speed regulation. Conversely, if the polarity of the shunt field is opposite that of the series field and armature, it is referred to as a differential compound motor.

Cumulative compound motors are used in a wide variety of applications, from conveyors to heavy equipment such as ball mills. Differential compound wound motors have few practical applications, since they tend to overspeed when the load is reduced and to drop speed significantly when the load is increased.

Omron releases NX1 controller for IIoT applications with OPC UA and SQL connectivity

August 1, 2018 By Danielle Collins Leave a Comment

The newest iteration of Omron’s NX1 machine automation controller series is designed to improve productivity through integration with information utilization, quality management, and safety. The high degree of integration helps manufacturers collect and utilize data more effectively and improve overall productivity. Its compact design helps bring the control of compact machines to a whole new level.

High-speed data processing that won’t hamper control performance

Manufacturers need to visualize data on machine and process status, predictive maintenance, and other things that impact productivity, but high-speed processing of large amounts of data tends to increase control cycle time. The NX1 collects synchronized data from sensors, servomotors, and other devices within the same fixed cycle time thanks to a unique multicore technology that executes high-speed data processing tasks and machine control tasks simultaneously. The controller then sends all collected data to the host IT system while keeping control performance at the ideal level.

Three industrial Ethernet ports — EtherNet/IP and EtherCAT — along with an OPC UA server interface for industrial automation and information technology are housed in a compact design just 66 mm wide. This connectivity enables data usage at production sites and provides an intuitive, secure connection to host IT systems. The database connection model can directly store production information in databases without using PCs or middleware, ensuring traceability of all products in real time and improving quality control.

Integration of control and information

The NX1 provides synchronized control of I/O and motion within a 1 ms cycle time and motion control of up to 12 axes to enable high-speed, high-precision control. The built-in EtherNet/IP, EtherCAT, OPC UA and database connection functionality allows information usage within and between machines, as well as visualization of production status with MES/SCADA, traceability using an SQL database and other applications.

Integration of control and quality management

When connected with the NX-HAD4 high-speed analog input unit, the NX1 simultaneously obtains analog data from four insulated channels at the industry’s fastest sampling speed of 5 µs. Rather than requiring special inspection machines with a built-in PC, this standard controller can be used for inspections such as vibration inspection of high-speed rotators. The inspection process can therefore be seamlessly incorporated into production lines.

Integration of control and safety

The NX1 series, alongside the NX-SL5 safety CPU, integrates control and safety features previously unavailable in the market. With the NX1 and the NX-SL5, CIP safety and EtherCAT safety can be realized together on the same control system, giving robotics and factory automation users the option to integrate with varied safety networks. Safety control systems can be built within and between machines, and control and safety can be managed as a module, simplifying the creation of flexible production lines.

With improved communication between advanced machine control functionality and vital tasks related to information utilization, quality management, and safety, the new NX1 helps manufacturers stay on top of predictive maintenance and productivity monitoring without slowing down production. Better integration provides the foundation for implementing process improvements and creating a more efficient production line.

Omron Automation

How do rotary voice coil actuators work?

July 17, 2018 By Danielle Collins Leave a Comment

Most linear motion devices have a rotary equivalent, and voice coil actuators are no exception. While the versions depicted in most product images and videos are linear, rotary voice coil actuators fill a need for smooth, arc-shaped motion and torque production with fast response times. But unlike the design relationship between rotary and linear motors, developing a rotary voice coil actuator isn’t simply a matter of taking the linear version and rolling it into a cylinder. While their components are the same, the configurations of linear and rotary voice coil actuators are quite different.

Voice coil actuators work on the principle of the Lorentz force, which is the force generated when a current-carrying conductor is placed in a magnetic field. In the case of a voice coil actuator, the magnetic field is created by permanent magnets and the conductor is a coil of wire. When current is applied to the coil, a force is generated, and the magnitude of the force is proportional to the applied current. (All other variables in the force production, such as magnetic flux and length of wire, are fixed for a given voice coil design.)

F = k*B*L*I*N

F = force (N)

k = force constant

B = magnetic flux density (tesla)

L = length of wire (m)

I = current (amps)

N = number of conductors

The most common linear voice coil actuator design features a moving coil, although the actuator can be constructed so that either the coil or the magnet is the moving part. In the moving coil design, a stationary cylindrical permanent magnet produces a radial magnetic field. Housed inside the permanent magnet cylinder is a coil that is wrapped around a non-conducting structure, or holder. The permanent magnet is housed in a ferromagnetic cylinder that is open at one end and has a “core” through its middle, which is necessary for completing the magnetic circuit. The coil assembly rides in the air gap between this core and the interior surfaces of the permanent magnet. Although it sounds complicated, the construction is actually quite simple, as shown below.

The construction of a linear voice coil actuator is relatively simple. In this example, the magnet is stationary and the coil moves.

A rotary voice coil actuator is sometimes referred to as a “flattened” version of the linear design described above. In other words, to transform a linear voice coil actuator into a rotary version, place the linear voice coil actuator lengthwise on a table (as it would sit if it were operating horizontally) and flatten it into an essentially 2-D object. Then bend the ends so that it forms an arc (like a piece of macaroni). Now you have a rotary voice coil actuator.

In this rotary voice coil actuator, the permanent magnet segments are on the top and bottom (white) and the coil (gold) rides in the air gap between them.
Image credit: BEI Kimco

Rotary versions provide torque rather than force. Similar to their linear counterparts, the amount of torque generated is proportional to the applied current, and the direction of the current determines the direction of motion. Excursion angles are typically less than 180 degrees, and commonly fall between 15 and 60 degrees.

Voice coil actuators are, technically, non-commutated DC motors. The term “voice coil” is often used because one of their first applications was to vibrate the paper cones in loudspeakers. Rotary voice coil actuators are also referred to as “arc segment” actuators or “limited angle torque motors” because they produce arc-shaped motion.

One of the strengths of voice coil actuators is their extremely smooth, cog-free motion with no torque ripple. This makes rotary versions ideal for applications that require high torque and quick response, such as gimbal positioners. They are also well-suited for moving optics and for controlling stabilization devices.

This gimbal assembly design uses two rotary voice coil actuators. The inner axis moves 40 degrees in 32 milliseconds, and the outer axis moves 20 degrees in 140 milliseconds.
Image credit: BEI Kimco

Feature image credit: H2W Technologies, Inc.

What is the Department of Energy’s Integral Horsepower Rule?

July 12, 2018 By Danielle Collins Leave a Comment

The U.S. Department of Energy (DOE) has been regulating energy efficiency for electric motors since 1997, when the Energy Policy Act went into effect, requiring most 1 to 200 hp general-purpose motors to meet NEMA Energy Efficiency standards. Since then, the DOE energy efficiency regulations have evolved, with the Energy Independence and Security Act (EISA) of 2007 upping the efficiency requirements for 1-200 hp general-purpose motors to the new NEMA Premium Efficiency levels and adding the requirement for certain 201 to 500 hp motors to meet NEMA Energy Efficiency standards.

Image credit: ABB

However, the 2007 EISA standard — which went into effect in December of 2010 — left significant gaps in the motors that were covered and made it relatively easy for manufacturers to work around the rules by producing motors with slight deviations from those covered by the standard — such as special mounting designs not specified in the regulations.

In 2010, a motor coalition was formed in order to, “Determine and document a plan to improve the efficiency of the greatest number of units, providing the greatest savings impact while reducing potential enforcement issues, while maintaining full product utility for American industry.” As a result of this coalition’s work, the Amended Integral HP Rule was released in May of 2014 and was implemented in June of 2016 as the Integral HP Motor Final Rule.

This Integral Horsepower Rule supersedes the EISA standard and specifies that nearly all motors covered must meet NEMA Premium Efficiency levels. But more importantly, it closes the gaps and potential loopholes in the EISA standard and expands the scope of covered motors to those that meet the following criteria:

Is a single speed induction motor
Is rated for continuous duty (MG 1) operation or for duty type S1 (IEC)
Contains a squirrel-cage (MG 1) or cage (IEC) rotor
Operates on polyphase alternating current (AC) 60-hertz sinusoidal line power
Has 2-, 4-, 6-, or 8-pole configuration
Is rated 600 volts or less
Has a three or four digit NEMA frame size (or IEC metric equivalent), including those designs between two consecutive NEMA frame sizes (or IEC metric equivalent) or an enclosed 56 NEMA frame size (or IEC metric equivalent)
Has no more than 500 horsepower, but greater than or equal to 1 horsepower (or kilowatt equivalent)
Meets all the performance requirements of a NEMA design A, B or C electric motor or an IEC design N or H electric motor

Annual energy and cost savings that can be achieved by replacing a standard efficiency motor with a premium efficiency motor, versus repairing the standard efficiency version, as outlined in the DOE’s Integral Horsepower Rule.
Image credit: U.S. Department of Energy

Still, the Integral Horsepower Rule does not include all motors used in industrial applications. The most significant omissions from the rule, relevant to industrial motion and motion control include:

Synchronous AC motors
Permanent magnet rotor AC motors
Servo motors

But there’s a good reason these motors aren’t covered by the Integral Horsepower Rule. The DOE considers permanent magnet (PM) motors, switched reluctance (SR) motors, and synchronous reluctance (SynRM) motors to be “newly emerging” or “advanced” technologies that can achieve significantly higher efficiency levels than NEMA Premium Efficiency. In fact, these motors may meet or exceed “super efficiency” standards, which have been explored, but not finalized or published, by NEMA.

Specifically, NEMA recognizes that permanent magnet and switched-reluctance motors have both high power density (torque-to-weight ratio) and high efficiency and recommends that they be considered for many types of industrial applications, including:

When the application requires speed control (i.e., when an adjustable speed drive is required for speed regulation)
When driven equipment is in operation for over 2,000 hours per year
When an old standard efficiency motor is driving a centrifugal load with throttled or damper flow control and can be replaced with a variable speed PM or SR motor and controller
When operations involve frequent starts and stops (this is a good application due to the low inertia of PM and SR motors)
When small motors operate at partial load a good deal of the time
When the PM or SR motor can be used in a direct drive configuration to displace a single or two-speed motor with gearbox (e.g., a cooling tower fan drive motor), a gear motor, or a belted power transmission system
In vertical pump-mount applications where resonance frequencies must be avoided.

The chart below demonstrates the efficiency of permanent magnet motors as compared to NEMA Energy Efficient and Premium Efficiency motors, as well as to IEEE 841 motors.

Full-load efficiency values for Permanent Magnet versus NEMA Premium Efficiency, IEEE 841, and NEMA Energy Efficient motors.
Image credit: Baldor Electric

NEMA Premium Efficiency sets forth full-load efficiency standards for 1 to 500 hp, three-phase, low-voltage NEMA Design A and B general, special, and definite purpose induction motors. Because the Premium Efficiency levels are provided in Table 12-12 of the NEMA MG1 Motors and Generators standard, it is sometimes referred to as “12-12 efficiency.” For motors not produced in the United States, the IE3 classification, as defined in the IEC 60034-30-1 standard, is generally compatible with the NEMA Premium Efficiency standard.

What’s the difference between an EC motor and a BLDC motor?

July 5, 2018 By Danielle Collins Leave a Comment

Deciphering motor terminology can be frustrating, especially when comparing the operation and performance of various motor types. For example, some motors are characterized by their speed (synchronous or asynchronous) while others are characterized by their commutation method (brushed or brushless). And then there’s the issue of whether the motor is operated with AC or DC power — but even this seemingly simple distinction can be misleading. Case in point: “DC” motors use a DC power supply, but the current is switched to AC in order to feed the motor windings, so some experts argue that these motors should, technically, be placed in the same category as motors that run on AC power. And just when you think you have the terminology and specifications all figured out, along comes another name or acronym, added to the mix.

In the motion control world, a relatively new motor distinction that is becoming more widespread is the term “electronically commutated,” or “EC,” motor. So how does this “new” EC motor fit into the existing hierarchy?

The short answer is, EC motors are not a new technology. They are simply permanent magnet brushless DC (BLDC) motors that are being distinguished by their method of commutation (i.e. electronic) rather than by their physical characteristic of lacking brushes. These motors, whether referred to as “brushless DC” or “electronically commutated,” have a permanent magnet rotor with a wound stator. Electronics determine the sequence for commutation, or energizing of the stator windings, based on the rotor position, which is most often provided by either three Hall sensors or a rotary encoder (although there are methods of determining rotor position without additional feedback devices).

The brushless DC (aka EC) motor on the right uses electronics to determine the sequence for energizing the stator windings, based on rotor position provided by Hall sensors or an encoder. The brushed DC motor on the left is mechanically commutated via brushes and a commutator.
Image credit: Think RC/JP Commerce, LLC

A quick google search of “EC motor” or “electronically commutated motor” reveals that this terminology is used extensively in the HVAC and refrigeration markets, especially for equipment such as fans, blowers, and compressors — all of which have traditionally been powered by AC induction motors.

Recall that AC induction motors operate via AC current provided to the stator, which creates a rotating magnetic field. This field induces a current, and therefore a magnetic field, in the rotor. The stator and rotor magnetic fields rotate at different speeds (aka “asynchronous”) to generate torque. While inexpensive and easy to operate, AC motors’ two main operating characteristics result in reduced efficiency when compared to BLDC, aka EC, motors. First, inducing current in the rotor windings increases power consumption, and second, the difference in speed between the magnetic fields — known as “slip” — causes heat generation. This inefficiency has not gone unnoticed by the United States Department of Energy.

EC (aka BLDC) motors have higher efficiencies than AC induction motors, in part because EC motors don’t experience slip between the rotor and stator rotating fields.
Image credit: H.C. Nye Co.

BLDC, or EC, motors have high efficiency over a range of speeds, rather than a high peak efficiency at a single speed.

Stricter, and more specific, energy efficiency rules from the DOE have made EC motors the preferred choice in HVAC and refrigeration applications.
Image credit: ebm-papst

This report, published in 2013, details the Department of Energy’s analysis of power consumption in the U.S. The report shows that residential and commercial end uses account for approximately 39 percent of annual energy consumption, with the industrial sector accounting for approximately 32 percent. (Transportation makes up the remaining 28 percent.)

The report then goes on to discuss opportunities and barriers to reducing overall energy consumption — primarily by increasing the efficiency of motor-driven systems and components in commercial and residential applications. One of the key ways to achieve this is by replacing the motors traditionally used in these applications — primarily AC induction motors — with higher efficiency motors — namely, EC designs.

This statement from the report explains…

For almost all equipment types in both the residential and commercial sectors, the market is transitioning to permanent magnet motor technology for the highest-efficiency models within each category. Permanent magnet motors are becoming increasingly cost-effective based purely on simple payback period. They also offer other non-energy benefits such as reduced noise and the ability to reach higher rotational speeds.

In fact, the DOE’s most recent Small Motor Efficiency Rule and Integral Horsepower Motor Rule, together with various DOE rules specific to HVAC and refrigeration equipment (think blowers and compressors) as well as to pumps and fans, have compelled motor manufacturers and equipment OEMs to transition from AC induction motors to EC motors in many types of applications and equipment.

So, if an EC motor is really just a BLDC motor by a different name, why add another term to the already complex hierarchy of motor nomenclature? One reason may be to avoid confusion for those who are most familiar with AC motors and might associate any DC motor with the brushed designs that dominated the DC motor market for many years. Using the term “electronically commutated,” rather than “brushless DC,” makes it less likely that users will confuse the modern, high-efficiency, long-life motors with brushed designs, which suffered from poor efficiency, frequent maintenance, and reduced service life.

Another reason can be found in the DOE publication cited earlier. There, the agency specifies (emphasis added):

In general, ECMs refer to low horsepower BLDC motors that have integrated drives and controls and are commonly found in HVAC applications.

Here, the agency makes a distinction between permanent magnet, brushless DC motors with separate drives and controls (BLDC motors) and those that have integrated drives and controls (EC motors). However, some manufacturers and OEMs clearly use the terms “EC motor” and “BLDC motor” interchangeably when describing a permanent magnet, brushless DC motor, regardless of whether the drives and controls are integrated into or separate from the motor.

Since numerous manufacturers of EC (BLDC) motors supply not only to the HVAC and refrigeration markets, but also to the motion control market, it’s no surprise that the terms “electronically commutated motor” and “EC motor” are permeating the motion control lexicon. And if it helps reduce the confusion between BLDC, PMSM, BLAC, and the plethora of other motor names and acronyms out there, that can only be a good thing for those of us tasked with decoding the jargon to evaluate and specify the best design for our application.

What is a Halbach array and how is it used in electric motors?

June 28, 2018 By Danielle Collins Leave a Comment

A Halbach array is arrangement of permanent magnets that creates a stronger field on one side while reducing the field on the other side to near zero. This is accomplished by orienting the magnets so that their poles are out of phase, typically by 90 degrees. This orientation essentially re-routes the magnetic field below the structure (the “non-working” surface) to the plane above the structure (the “working” surface), strengthening the magnetic field of the working surface and reducing the field of the non-working surface to nearly zero.

In a normal magnet (left) the field strength is equal on the top and bottom of the magnet. In a Halbach array (right), the field strength is concentrated on the top side, with a very weak field on the bottom.
Image credit: K&J Magnetics

The effect of rotating permanent magnets to create a strong field on one side and a virtually zero field on the other is sometimes referred to as “one-sided flux,” and was considered a curiosity by its discoverer, John Mallinson, in the 1970’s. But in the 1980’s, Klaus Halbach of Berkeley Labs rediscovered this phenomenon and created what are now known as Halbach arrays, using them to focus and steer the beams in particle accelerators.

In addition to linear, or planar, configurations, Halbach arrays can also be configured in circular, or cylindrical, arrangements, with the magnetic field concentrated at either the outer diameter or the inner diameter. An electric motor can be constructed using a cylindrical Halbach array, with the magnetic field directed inside the cylinder and windings inserted into the field. The Halbach array can either be stationary, with the windings rotating, or the Halbach array can serve as the rotating element, with the windings stationary. In the latter configuration – sometimes referred to as an “inside out” design, the stator windings would be electrically commutated, and the motor could be operated as a brushless DC (BLDC motor) or as a brushless AC (BLAC) motor (also referred to as a permanent magnet synchronous motor, or PMSM).

A Halbach cylinder can be used in an electric motor, as in this example, with the magnetic field concentrated at the inner diameter and the Halbach array rotating around the windings.
Image credit: Lawrence Livermore National Laboratory

Although not yet mainstream, electric motors based on the Halbach array offer measurable benefits over conventional designs, including high power density and high efficiency. One of the enablers of these benefits is that a Halbach array motor does not require laminations or back iron, so the motor is essentially ironless. This significantly reduces eddy current losses and hysteresis losses – often referred to as “iron losses” or “core losses.” Eddy currents, and therefore eddy current losses, do, however remain in the windings, due to the relative motion between the windings and the magnetic field. These can be reduced by using a type of wire, referred to as “Litz wire.” The elimination of back iron or laminations also reduces weight and inertia, allowing the motor to start and stop faster and to reach higher top speeds (10,000 rpm or greater) for highly dynamic applications.

Litz wire consists of individually insulated strands that are braided or woven together in a manner that minimizes losses.

The primary downside to Halbach array motors is cost – specifically, the cost of manufacturing the Halbach array. Because the magnets are arranged with each one repelling its neighbor, the processes of assembling the magnets and ensuring the adhesive or fixing method is sufficient is relatively labor-intensive.

What’s the difference between cogging torque and torque ripple?

June 25, 2018 By Danielle Collins Leave a Comment

The basic operation of an electric motor relies on the interaction between the rotor’s permanent magnets and the stator’s energized windings. But even when the motor is not powered and no current is flowing though the windings, there is a magnetic attraction between the permanent magnets of the rotor and the ferromagnetic teeth of the stator. The magnetic attraction varies depending on the flux density, or strength, of the magnetic fields, and this variation causes uneven torque production, which is referred to as “cogging torque” or “torque ripple.”

While these two terms are sometimes used interchangeably, cogging torque generally refers to the phenomenon that causes torque variations, and torque ripple is the effect these variations have on motor performance.

Cogging torque

Cogging torque is a product of the magnetic interaction between the poles of the rotor’s permanent magnets and the steel laminations of the stator’s teeth. In other words, when the poles of the rotor line up with the teeth of the stator, a force is required to break the attraction, and this force is referred to as cogging torque. Cogging torque is position-dependent, according to the location of the stator teeth relative to the permanent magnets, as the magnets constantly search for a position of minimum reluctance.

A motor’s cogging torque profile depends on the number of permanent magnets in the rotor and the number of teeth in the stator and can be minimized through mechanical means by optimizing the number of magnetic poles and teeth, or by skewing or shaping the permanent magnets to make their transition between stator teeth more gradual.

Cogging torque occurs in permanent magnet motors, including brushed and brushless DC motors and synchronous AC motors. Because it occurs in an un-energized motor, it is sometimes referred to as “open-circuit torque.”

Torque ripple

The result of cogging torque is a variation in torque production — referred to as torque ripple — that occurs when the motor is energized with constant current. Due to the nature of the interaction between the magnetic fields of the rotor and the stator (described above), torque ripple varies sinusoidally. At high speeds, it is often filtered out by the inertia of the system, but at lower speeds, torque ripple can cause unwanted speed fluctuations, vibrations, and audible noise.

Torque ripple (green) varies sinusoidally due to the interaction between magnetic fields of the stator and rotor.
Image credit: Precision Microdrives Limited

Cogging in stepper motors

Stepper motors also exhibit cogging torque, but in stepper motor discussions, it is often referred to as “detent torque.” Like cogging torque, detent torque is a result of magnetic equilibrium in a non-energized motor. This magnetic equilibrium must be overcome before the motor will rotate, which means that the detent torque reduces the amount of running torque the motor can produce. The amount of detent torque a stepper motor experiences is proportional to the motor’s speed, so the effect of detent torque on running torque is more substantial at higher motor speeds.

Permanent magnet and hybrid stepper motors, which use a permanent magnet rotor, exhibit detent torque; but variable reluctance steppers, which use a non-magnetized, soft iron rotor, do not. Of permanent magnet and hybrid types, hybrid steppers have higher detent torque due to their toothed rotors, which allows them to better manage the magnetic flux between the stator and the rotor.

Detent torque reduces the ideal torque (and power) that the motor could produce, with the effect becoming larger as speed increases.
Image credit: Geckodrive Motor Controls

Detent torque is typically 5 to 20 percent of the motor’s holding torque, which is the amount of torque the motor can produce when the windings are energized but the rotor is stationary. But detent torque is not always a liability: When the motor is decelerating, it counters the motor’s momentum and helps it to stop more quickly.