DC Motors - Motion Control Tips

New high-speed, compact brushless motors from Portescap

March 19, 2019 By Miles Budimir Leave a Comment

Portescap has expanded its Ultra ECTM range of brushless dc slotless motors with the introduction of the new 35ECS motor. This 2-pole motor offers ultra high speed and power (up to 330 W, max continuous) without compromising on the smooth operation and long life offered by Portescap’s brushless slotless motors.

The new 35ECS motor is available in two lengths: the 35ECS60 and the 35ECS80. The 35ECS60 and 35ECS80 motors feature Portescap’s patented Ultra EC coil technology which provides unparalleled torque and power density with limited core losses over a wide range of working speeds without friction and brush wear. These motors feature a patented high-speed rotor design as well to sustain speeds up to 40,000 rpm. The 35ECS80 is unique and opens new areas for slim/long package with impressive torque capabilities at high speeds, while the 35ECS60 allows high speed capabilities at low voltages. Thanks to its rugged design, the 35ECS can sustain peak torque during 2 sec up to 1.1 Nm (35ECS60) and 2.0 Nm (35ECS80).

The 35ECS motors can be used in a range of applications, including industrial applications, enhancing the life and reliability of a device without compromising on power and machine throughput and saving energy. The new 35ECSs are an ideal choice for applications such as battery-operated hand tools (industrial tightening devices or drills), factory automation equipment (large fans or vacuum pumps), and also for applications with diameter constraints.

The new 35ECS motors feature a laser-welded front flange to ensure the strongest housing to sustain high torque reaction. A temperature probe on the coil head ensures an optimized control of motor performances in heavy duty applications. These motors are offered with hall sensors and a total of six different coils to match speed and voltage requirements. Upon request, Portescap can also provide options for customization including gearboxes, encoders, coil variations and mechanical interface modifications.

For more information, visit www.portescap.com.

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.

Bodine’s SCR-rated PMDC gearmotors — for hybrid applications

October 10, 2018 By Lisa Eitel Leave a Comment

To provide greater flexibility and compatibility with unfiltered SCR controls, Bodine Electric Co. now offers a new selection of 90-V and 180-Vdc gearmotors with a rated maximum armature speed of 2,500 rpm. These gearmotors are designed for “hybrid” applications that neither match the 90-V 1,725-rpm industry standard (for operation with unfiltered speed controls) nor Bodine’s typical voltage and rpm for filtered 130-Vdc speed control operation. These new stock models from Bodine can replace 90-V or 180-Vdc gearmotors sold by Baldor, Bison Gear, Leeson, or Grainger.

Clockwise from left: Bodine type 24A-Z, 33A-WX, 33A-5L/H, and 42A-FX SCR-rated gearmotors.

The initial product launch includes 70 standard/stock models with four of Bodine’s most popular gearhead designs.

Bodine SCR rated PMDC gearmotors provide high starting torque, adjustable speed and predictable speed/torque characteristics. All feature Class F TENV insulation. The Bodine type 24A-Z parallel shaft gearmotor features unvented gearhousing and all-steel gear trains that are permanently lubricated for maximum life and performance. It delivers up to 120 lb-in. rated torque. Speed ranges from 14 to 417 rpm, depending on the gear ratio.

The type 33A-WX parallel shaft gearmotor is constructed with all steel helical gears that meet or exceed AGMA 9 standards for maximum life and quiet operation. It delivers up to 205 lb-in. rated torque, with speeds from 8.0 to 658 rpm, depending on the gear ratio. The type 42A-FX is the largest of the three parallel shaft gearmotors. Like the type 33A-WX it features hardened steel gearing for quietness and high output. It delivers up to 350 lb-in. rated torque, and speeds from 8.3-500 rpm. The type 33A-5L/H hollow shaft gearmotor connects directly to the driven load and features versatile, universal mounting options. It delivers up to 89 lb-in. rated torque, with speeds from 33 to 500 rpm, depending on the gear ratio.

The SCR rated gearmotors and their corresponding controls and accessory kits are available through Bodine’s extensive distributor network or from the Bodine web site. Custom designs are available to qualified OEMs. Typical OEM modifications include factory installed encoders or brakes, custom wire harnesses, and application-specific mounting and shaft details.

Bodine Electric Company offers over 1,200 standard stock products, and thousands of custom designed fractional horsepower (FHP) gearmotors, motors and motion controls (fixed and variable speed AC, brushless DC, and permanent magnet DC). Bodine gearmotors and motors are designed for demanding industrial and commercial applications such as medical devices, scientific and laboratory equipment, labeling equipment, printing presses, packaging equipment, and factory automation. Bodine Electric is headquartered in Northfield, Illinois (20 miles north of Chicago) with manufacturing and assembly operations in Peosta, Iowa, U.S.A. Bodine’s quality management system is certified to ISO 9001. For motr information, visit www.bodine-electric.com.

What are coreless DC motors?

October 9, 2018 By Danielle Collins Leave a Comment

A typical brushed DC motor consists of an outer stator, typically made of either a permanent magnet or electromagnetic windings, and an inner rotor made of iron laminations with coil windings. A segmented commutator and brushes control the sequence in which the rotor windings are energized, to produce continuous rotation.

A typical DC motor consists of an outer, permanent magnet stator and an inner rotor made of iron laminations with windings.
Image credit: maxon motor ag

Coreless DC motors do away with the laminated iron core in the rotor. Instead, the rotor windings are wound in a skewed, or honeycomb, fashion to form a self-supporting hollow cylinder or “basket.” Because there is no iron core to support the windings, they are often held together with epoxy. The stator is made of a rare earth magnets, such as Neodymium, AlNiCo (aluminum-nickel-cobalt), or SmCo (samarium-cobalt), and sits inside the coreless rotor.

A coreless DC motor does away with the iron core in the rotor. Instead, the rotor windings are wound in a skewed, or honeycomb fashion to form a self-supporting hollow cylinder. The stator magnet sits inside the coreless rotor.
Image credit: maxon motor ag

Other terms for coreless DC motors include “air core,” “slotless,” and “ironless.”

The brushes used in coreless DC motors can be made of precious metal or graphite. Precious metal brushes (silver, gold, platinum, or palladium) are paired with precious metal commutators. This design has low contact resistance and is often used in low-current applications. When sintered metal graphite brushes are used, the commutator is made of copper. The copper-graphite combination is more suitable for applications requiring higher power and higher current.

Coreless DC motors have a rotor that is hollow and self-supporting, which reduces mass and inertia.
Image credit: Portescap

The construction of coreless DC motors provides several advantages over traditional, iron core DC motors. First, the elimination of iron significantly reduces the mass and inertia of the rotor, so very rapid acceleration and deceleration rates are possible. And no iron also means no iron losses, giving coreless designs significantly higher efficiencies (up to 90 percent) than traditional DC motors. The coreless design also reduces winding inductance, so sparking between the brushes and commutator is reduced, increasing motor life and reducing electromagnetic interference (EMI).

Motor cogging, which is an issue in traditional DC motors due to the magnetic interaction of the permanent magnets and the iron laminations, is also eliminated, since there are no laminations in the ironless design. And in turn, torque ripple is extremely low, which provides very smooth motor rotation with minimal vibration and noise.

Because these motors are often used for highly dynamic movements (high acceleration and deceleration), the coils in the rotor must be able to withstand high torque and dissipate significant heat generated by peak currents. Because there’s no iron core to act as a heat sink, the motor housing often contains ports to facilitate forced air cooling.

Coreless DC motors are available in both cylindrical and flat (disc) configurations.
Image credit: MICROMO

The compact design of coreless DC motors lends itself to applications that require a high power-to-size ratio, with motor sizes typically in the range of 6 mm to 75 mm (although sizes down to 1 mm are available) and power ratings of generally 250 W or less. Coreless designs are an especially good solution for battery-powered devices because they draw extremely low current at no-load conditions.

Coreless DC motors are used extensively in medical applications, including prosthetics, small pumps (such as insulin pumps), laboratory equipment, and X-ray machines. Their ability to handle fast, dynamic moves also makes them ideal for use in robotic applications.

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.

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’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.

Allied Motion debuts Globe Line of mil-spec PMDC motors, planetary gearmotors

June 13, 2018 By Miles Budimir Leave a Comment

Allied Motion Technologies announces the Globe line of mil-spec-qualified fractional horsepower PMDC motors and gear motors. These motors are available off-the-shelf or on short lead time through Allied’s QuickShip program. Modified versions – with specially machined mounting provisions, holding brakes, tachometers and/or encoders – are available within two to three weeks through Allied’s QuickMod program.

Models range in size from 0.75 to 2.25 in. (19 to 57 mm) in diameter. Rated speed ranges from 2,400 to 19,200 rpm, with rated power from 3 to 62 W. Planetary gearhead models are available with output torque rated to as high as 62 Nm (8,800 oz-in.).

Allied Motion offers free expert assistance to help customers select the right Globe line motor or gear motor for their application.

For more information, visit www.alliedmotion.com.

External rotor motor basics: Design and applications

June 6, 2018 By Danielle Collins Leave a Comment

Conventional brushless DC motors are constructed with a permanent magnet rotor located inside a wound stator. But one type of DC motor is designed with the rotor on the outside and the stator and housed inside the rotor. Permanent magnets are mounted on the inner diameter of the rotor housing (sometimes referred to as the ‘bell” or “cup”), and the rotor rotates around the internal stator with windings. This design is often referred to as an external rotor motor, but can also be called an outer rotor motor, an outrunner motor, or a cup motor.

As its name suggests, an external rotor motor is designed with the rotor on the outside and the stator housed inside the rotor.
Image credit: Nidec Corporation

The external rotor design provides several performance advantages. First, to house the stator, the rotor of an external rotor motor is by necessity larger than the rotor of a conventional DC motor. And a larger rotor means higher inertia, which helps to dampen torque ripple (a common problem in conventional DC motors) and provide smooth, stable operation, even at low speeds.

Another advantage of external rotor motors is that they can typically produce higher torque than comparably sized internal rotor designs. Recall that torque is a product of the magnetic force times the radius of the air gap (length of magnetic flux). For a given motor diameter, external rotor motors have a larger air gap area than inner rotor designs, and the larger air gap allows a higher force to build. They also have a larger air gap radius, which increases the “lever arm” for torque production. The larger diameter (and, therefore, circumference) of the rotor in external rotor designs also means the rotor can accommodate more poles, which further increases magnetic flux.

Compared to an internal rotor motor, an external rotor motor has a larger area for flux to develop, and a larger air gap radius, which acts as the “lever arm” for torque production.
Image credit: T. Reichert, Power Electronic Systems Laboratory

External rotor motors are axially shorter than inner rotor motors with similar performance characteristics. This compact size and high torque production make them ideal for directly driving the propellers of remote-controlled model airplanes and drones. In high-precision applications, such as optical drives, their smooth, consistent speed is a benefit over other motor types. And in applications with varying loads, such as industrial power tools, pumps, fans, and blowers, the high inertia of external rotor motors can help to “push through” load fluctuations and provide steady output torque.

Fan and blower applications are one of the more common uses for external rotor motors thanks to a specific design benefit: The external rotor can serve as the hub of the fan or blower impeller. This provides a compact package and allows the impeller to act as a large, rotating heat sink and assist with motor cooling.

But integrating the rotor into the impeller also increases the motor’s mechanical time constant — the amount of time required for the motor to reach 63.2 percent of its final speed for a given voltage — an important parameter for ensuring the motor doesn’t overheat.

τ_m = mechanical time constant of the motor
R = winding resistance
J = rotor inertia
K_e = back EMF constant
K_t = torque constant

As shown in the equation, the motor’s mechanical time constant depends in part on the rotor inertia. When the rotor is integrated into the impeller, the inertia of the rotor and impeller are considered together. This higher inertia results in a higher mechanical time constant, and therefore, a longer time for the motor to reach its required speed.

Brushless dc motors for industrial power tools: Comparison of options including slotless motor variations

June 5, 2018 By Lisa Eitel Leave a Comment

Industrial power tools have operating profiles that differ from those of most other motor-driven applications. That’s why motors that go into these tools need specialized designs.

By Thomas Baile, Business Development Manager | Portescap

Most electric-motor-driven designs demand torque for the entirety of their motion. In contrast, power tools for fastening, gripping, and cutting applications have a motion profile consisting of two stages.

Motors for industrial power tools nutrunner Portescap

Power tools benefit from Portescap EC slotless motors with efficiency of copper turns to minimize joule losses; compact design; and strong coil integrity. The motors can also deliver a wide range of torque constants.

Power-tool speed stage: At first (as the tool drives a fastener to thread into place or the jaws of a cutting or gripping tool approach a workpiece) there’s little resistance.

Power-tool torque stage: Then when the tool performs the more forceful work of tightening, cutting or gripping, there’s a sudden torque demand.

These alternating speed and torque cycles constantly repeat in industrial power-tool applications even while necessitating different speeds and torques for various durations. That’s why such tools — especially those that are battery-operated so running off low voltage and limited available power — benefit from special motor designs that minimize losses.

As we’ll explore, motors that also operate at faster free-run speeds for the speed stage can trim cycle times to boost productivity … and motors that deliver high peak torque during the torque stage can perform a wider range of tough jobs without excessive heating.

Unlike typical motion designs that demand torque output from the motor throughout the motion, industrial power tools (IPTs) for fastening, gripping, and cutting often have a motion profile split into two stages. This cycle continuously repeats.

Selecting and optimizing brushless motors for power tools

Consider the primary options for handheld industrial power tools — brushed dc motors and brushless dc motors. Battery-powered industrial power tools run on low voltages of 12 to 60 V … so here, brushed dc motors are typically economical but have limited life. The brushes exhibit modes of wear due to electrical influences (from current related to torque) and mechanical influences (due to the friction associated with speed).

Brushless dc motors are more reliable in power tools, as they’re less susceptible to mechanical wear (no brush friction) and can sustain high peak current (no brushes) during the tightening stage, providing far greater life in the hand tool. Brushless dc motors are better suited than brushed dc motors for industrial power tool applications because they require high speed and high peak current.

Brushless dc motors for power tools are typically one of two configurations:

Conventional inrunner motor configurations include permanent magnets on the rotor with three stator windings around the rotor.

Conventional inrunner motors include permanent magnets on the rotor; outrunner (external-rotor) configurations have it reversed. Inrunner motors excel in certain handheld industrial power tools because they have reduced inertia, lower weight, and lower losses — plus their longer length and smaller diameter complement handheld designs for ergonomics.

Outrunner (external-rotor) configurations have it reversed — so coils are innermost on the stator and the magnets are on the O.D. In other words, the stator coils form the motor center while the permanent magnets on the rotor’s inner surface spin within an overhanging rotor surrounding that core.

Inrunner motor configurations excel in handheld industrial power tools because they have reduced inertia, lower weight, and lower losses. Their longer length and smaller diameter also complement handheld designs for ergonomics. Plus a lower rotor inertia brings better tightening and gripping control.

For slotted motors, magnetic induction in the lamination is high because the airgap between the laminations (stator) and magnet is small. However, the motors are inherently rugged. Slotless motors have independently formed coils. The diameter is usually optimized for magnetic induction with a given copper volume.

Brushless dc windings can be constructed in different physical configurations:

Motors with slotted stators: Here, the coils wind through the slots around the stator. Magnetic induction in the lamination is high because the airgap between the laminations (stator) and magnet is small. So, the motors allow a small magnet diameter. The volume of the copper is limited by the slot space and the difficulty of winding within the slot. Having the coil inside the stator slots is advantageous in that it reduces coil-stator assembly thermal resistance.

Without current, the rotor has preferred magnet positions in front of the lamination. That means the motor is prone to generating a cogging or detent torque. One way to decrease the detent torque is to skew the lamination.

But again, slotted motors are inherently robust because the coil is in the lamination.

Motor with slotless stators: Slotless motors have coils wound in a dedicated operation. This coil goes into the motor airgap during motor assembly. Magnetic induction in the coil is lower than in slotted motors because the airgap is larger. So the motor diameter is usually optimized for magnetic induction with a given copper volume.

In fact, induction in slotless motors is usually less than that of a slotted brushless motor. So bigger magnets are typically used to compensate for the loss of induction. One caveat to this solution is that it can increase rotor inertia.

But slotless motor power density is a strength. R/K² — the ability to maintain speed under load, with lower values being better — is low in slotless motors because induction for a given copper volume is optimized … as evidenced by a sloped curve. Without circulating current, the rotor sees a continuous permeance. That means slotless motors don’t exhibit cogging or detent torque. Iron losses at high speed in slotless motors are also far smaller than those of comparable designs.

There is another caveat: Slotted motors can handle higher temperatures than slotless motor designs — even to 200° C compared to 150° C, to give a typical comparison. That in turn allows more torque generation. Even so, in power tools usually the limiting factor is maximum temperature over time — to an average maximum of 47° C — or so — to accommodate what’s comfortable for the typical hand-tool user. Beyond that value, heating can become uncomfortable for the operator holding the tool. Safety regulations also require maximum temperature to be held lower.

Slotted brushless dc motors

Slotless brushless dc motors
(Ultra EC)

Pros

Small thermal resistance (coil/casing)

Maximum speed in excess of 100krpm

Fully customized motors

Hipot capability (to 2,500 V)

Torque

Smooth operation and no cogging

Little iron losses at high speed … low operating temperature and easier control

Low noise and vibration

Winding flexibility

Cons

Cogging

No standard products

Autoclavable option not available

High thermal resistance

Of course, the electrical performance of a motor is defined by the magnetic circuit. The magnet has a fixed value but the second component (the copper winding) can be easily modified. By changing with the wire diameter and number of turns, the motor’s torque constant k_t and resistance R can be fine-tuned for torque and speed. Consider the modes of industrial power-tool operation:

During the speed stage, the motor must operate at high rpm with little resistance:

ω = (U – R · I ) / k_t

Where ω = Speed in rad^.sec^-1 and U = Voltage; R = Resistance in ohms and I = Current in Amps; and k_t = Torque constant in Nm/A. Because the torque constant is in the denominator of the calculation, smaller k_t values make for higher RPMs. This allows more operations in the same time period which boosts productivity.

Now consider the power tool’s torque stage — when the motor must deliver peak torque at low speeds. Torque is the product of the torque constant and current:

C = k_t · I

Where C = Torque in Nm and I = Current in Amps. Higher k_t values here make for a higher output torque at a given current. So by adjusting a motor winding’s k_t design engineers can optimize either the speed or the output torque to balance optimization of torque with that of speed operation to reduce overall working cycle time. There is no unique solution: kt has to be chosen as the best compromise for a range of working profiles. Motor design experts can support one in this coil design process based on simulations and experience.

Reviewing thermal losses of motors during operation

Consider the losses of a typical industrial power-tool operating cycle. There is a relationship between copper losses and torque. So the design engineer may opt for a motor with a low k_t value to increase speed … and then compensate for a low k_t with more current (I) to get target output torque. But higher current increases copper losses:

Copper losses = R · I²

So with higher current comes faster heating of the motor and power-tool — thus limiting maximum possible torque. That’s why power-tool motors should be designed to draw as low a current as possible … to limit heat dissipation (and keep tool temperature cool enough to handle) and conserve battery life.

Copper losses and iron losses affect overall torque and speed output.

Now consider iron losses and how they relate to speed. Eddy-current losses increase with the square of speed to heat the brushless motor even if it’s just rotating under a no-load condition. That’s why high-speed motors need special design features to limit eddy-current heating.

Increased power-tool speed promptly makes iron losses exceed copper losses. So motor windings should be tuned for each duty cycle to minimize losses. Ultra EC winding technology greatly reduces iron and copper losses — which in turn gives design engineers more flexibility.

More specifically, some new electronically commutated (EC) brushless motors on the market build on a slotless motor design with specialty coil technology. These reduce copper losses because unlike slotless motors with skewed windings, they have windings parallel to the motor axis to maximize magnetic force and power. Plus they reduce iron losses at high speed. Among other benefits, these motors optimize speed and torque for the most challenging applications — including power tools that take the form of fasteners, grippers, and cutting tools.

High-speed motors in power tools must be of a special design to limit eddy-current heating. To this end, Portescap Ultra EC brushless slotless motors have a patented U-coil technology to reduce copper losses. Unlike slotless motors with skewed windings, Portescap Ultra EC motors have windings parallel with the motor axis to maximize magnetic force and power. Plus Ultra EC motors reduce iron losses at high speed. Straight windings also give the motors shorter rotor lengths compared to those of skewed windings which lowers rotor inertia and reduces iron losses.

Two example applications for EC motors in power tools

Assume for one design we have an average duty cycle including two seconds of moderate free speed — as in a pruning shear, nutrunner, gripper, or stapler. Speed is 20,000 rpm and at 2 seconds torque demand goes to 0.84 Nm.

Assume for the other design we have a heavy-duty cycle including three seconds very fast free speed — as in an automotive nutrunner optimized for productivity. Here, speed is to 40,000 rpm and at 3 seconds torque demand goes to 0.69 Nm.

For the first design:

Brushless motor type	Ultra 64	Skewed winding	Slotted motor	Ultra 90
Ø · L (mm)	30 · 64	30 · 64	28.5 · 88.5	30 · 90
Poles	4	4	4	4

Here straight-coil EC Ultra motors are more efficient (exhibiting less losses) than skewed-winding or slotted motors.

For the second design:

Brushless motor type	Ultra 64 HS	Slotted 90	Slotted 100	Ultra 90 HS	Ultra 64	Ultra Speed 60	Ultra Speed 80
Ø · L (mm)	30 · 64	28.5 · 88.5	34 ·99	30 · 90	30 · 64	35 · 60	35 · 80
Poles	4	4	4	4	4	2	2

There are higher iron losses than in the standard-duty design because the speed is doubled. Even so, straight-coil EC Ultra motors are still more efficient than skewed-winding or slotted motors.

Portescap | www.portescap.com

Motors with slotless construction excel in most industrial power tools because they have less induction in their laminations than other motors — which equates to no iron losses. Further Portescap improvements on slotless motor construction include straight turns, inner and outer heads, special outputs to prevent loose wires, and axial and radial forming.

For more than 25 years, leading manufacturers have relied on Portescap’s products, expertise, and support to develop corded and battery-powered tools while improving quality control and flexibility. Portescap innovation has helped transitions from pneumatic to electric tools while raising industrial power-tool performance standards. In 2013, Portescap patented the first slotless motor design with its Ultra EC coil.