DC Motors - Motion Control Tips

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.

How can a universal motor operate on either DC or AC supply?

May 15, 2018 By Danielle Collins Leave a Comment

A universal motor is a series-wound motor — meaning that the field and armature windings are connected in series — and is mechanically commutated with brushes and a commutator. Although its construction is very similar to a series-wound DC motor, a universal motor incorporates several modifications that allow it operate properly on either DC or AC power supply.

For proper operation with AC supply, a universal motor incorporates a compensation winding, in series with the armature and field winding.
Image credit: circuitglobe.com

When the motor runs on AC voltage, the alternating flux causes a reactance voltage, which limits the current to a much lower level than would be produced with DC voltage. To limit the effects of this armature reaction, the universal motor uses a compensation winding, which is installed in the slots of the stator, 90 degrees electrical from the main field winding and connected in series with the armature and field winding. (This arrangement is referred to as “conductively compensated.”) The current flowing in each coil of the compensating winding is in a direction opposite to the current in the corresponding armature loop near it. The winding of equal coils in opposite directions produces a cancelling effect that reduces inductance, and therefore, reactance.

AC supply also induces more significant eddy currents than are produced when the motor operates on DC supply. To curtail eddy current losses, universal motors use laminated cores (rather than solid iron), which increases their resistance and reduces eddy currents.

Armature reaction is the interference of the main field flux caused by the armature flux. This distortion and weakening of the main field flux changes the location of the magnetic neutral plane, which is the axis along which the brushes should be placed.

Armature reaction changes the location of the magnetic neutral plane, which is the axis along which the brushes should be mounted.
Image credit: Precision Microdrives Limited

It is because the armature and field windings are connected in series that a universal motor can operate with either DC or AC supply. Being connected in series means that both windings are supplied by the same source, so if the voltage source changes polarity, as it does with AC supply, both the armature and field currents also change polarity and the direction of torque does not change. (The direction of rotation is reversed by reversing the current in the field circuit.)

To reduce inductance in the armature, the current flowing in each coil of the compensating winding is in a direction opposite to the current in the corresponding armature loop near it.
Image credit: Festo Didactic

One of the primary benefits of a universal motor is its ability to achieve (theoretically) unlimited speed, with speeds up to 20,000 rpm in real-world applications. The speed-torque curve is essentially a straight line between stall torque (zero speed) and no-load speed (zero torque), meaning that as the load (torque) increases, the speed decreases. In fact, like a series-wound DC motor, running a universal motor with no load (zero torque) could lead to a runaway condition, where the speed increases until the motor begins to break apart. The converse of this phenomenon is that universal motors produce very good starting torque (high torque at low speeds).

A universal motor can produce extremely high speed at zero torque, but speed drops rapidly as torque increases.
Image credit: Groschopp, Inc.

This ability to run at very high speeds means that universal motors are good for applications that involve rapidly spinning components, such as fans, blow dryers, and vacuum cleaners. And their ability to produce high torque at low speeds makes them suitable for applications such as blenders and portable drills. Because they’re mechanically commutated, universal motors generally aren’t suitable for continuous-duty applications, due to brush maintenance and wear. The brushes also make them relatively noisy, so their primary areas of application are small consumer appliances.

Feature image credit: Groschopp, Inc.

New autoclavable BLDC motor controller from Portescap

March 5, 2018 By Lisa Eitel Leave a Comment

Portescap now sells an autoclavable Brushless DC motor controller — the CNT1530 — for driving large bone orthopedic handpiece motors. The controller leverages Portescap’s market leading autoclavable design capability and surgical application expertise to provide a reliable, customizable setup to the orthopedic market. This new controller will help improve surgical hand tool’s performance and reliability, and take application to market sooner.

The CNT1530 controller uses on-board hall effect sensors; the magnetic actuation of speed and direction provided by this design delivers exceptional reliability due to a lack of mechanical components. In addition, the electronic components are encapsulated in a silicone potting material within an aluminum housing, which provides protection of the circuit board from the harsh environments of sterilization washers and autoclaves. The controller is compatible with a range of input voltages allowing for use with multiple battery types. A small form factor enables designers to house the controller within a tool’s pistol grip to create exceptionally ergonomic designs.

The new BLDC controller is compatible with Portescap’s Large Bone Orthopedic motors. Customization is available for partners needing tailored setups.

Portescap offers the broadest miniature and specialty motor products in the industry, encompassing coreless brush DC, brushless DC, stepper can stack, gearheads, digital linear actuators, and disc magnet technologies. Portescap products have been serving diverse motion control needs in wide spectrum of medical and industrial applications – medical, life science, instrumentation, automation, aerospace and commercial applications, for more than 70 years.

Electric motors: Trends in stepper, battery-powered, and integrated motion designs

March 2, 2018 By Lisa Eitel Leave a Comment

From induction motors in electric vehicles to tiny coreless dc motors in drug-delivery equipment, electric motors continue to proliferate. Last year we saw consumer products, implantable medical devices, and automation of previously manual tasks spur the most innovation in motor design and use. This year those trends continue — and now we see increased use of servo and stepper motors in robotics.

maxon precision motors EC 4 brushless dc motors (which can be fitted with GP 4 planetary gearheads) are 4 mm in diameter with an ironless winding; ISO 13485 certified; and suitable for use in laboratory robots, micropumps, and diagnostic devices. Two lengths deliver power to 0.5 and 1 W. “We also want to offer more value-add services, as more companies than ever in the U.S. want finished motor and motion systems — so-called drop-in designs,” said McGrath of maxon. “So to that end, our expanding U.S. factory will have the capacity to supply custom assemblies and PCBs.”

In fact, whether it’s basic tin-can motors for low-cost custom actuation or integrated motors that output servo-like performance, overall use of stepper motors is growing. On the latter, a study by P&S Market Research projects a CAGR of 6.3% through 2023 for the global closed-loop stepper motor market. P&S cites the well-known benefits of control-feedback signals and power-adjusted current control as key reasons for the technology’s steady adoption.

Low-current dc motors from ISL Products Intl. Ltd. come in myriad mechanical and electrical configurations. Sometimes gearmotor modifications help the dc motors meet exacting OEM application specifications. In other cases, the motor must have special construction to avoid inducing EMI and RFI interference (Here, raw materials in the motor components are a main consideration, said to Kawaller of ISL.) Designs with reduced power consumption benefit from motors with specialized internal construction and nontraditional component materials.

Warren Osak, CEO of Toronto-based distributor Electromate Inc., sees this trend for integrated stepper motors, even while the middle of the stepper-motor market (actuated by standalone 1.8° hybrid stepper motors) recedes. Technology is either migrating upward (and taking the form of integrated motors or smart motors) or migrating downward — to lower-cost motors with less sophisticated structures, according to Osak.

“The use of traditional hybrid stepper motors has been yielding to integrated products … while in lower-end applications, we see more use of less-sophisticated motors with customer-provided integrated chips (ICs) to drive them,” he added.

Full interview with Osak: Motion-control trends distributor insight

Some closed-loop stepper manufacturers we surveyed for the Design World Trends issue include Schneider Electric Motion USA, Applied Motion Products Inc., Lin Engineering, MICROMO of Faulhaber, Oriental Motor Co., and Nippon Pulse America Inc. — with commentary from the latter in our actuator trends coverage. All detail pre-integrated or application-specific motor offerings. Case in point: Automation in the oil and gas industry has created numerous opportunities for motion control products, but stringent requirements for hazardous location ratings rule out most standard motor and drive solutions.

Use of tin-can stepper motors is on the rise, and now some new tools let engineers check their programming on the little motors more easily. Consider the Motion Checker 5 controller from Nippon Pulse America Inc. — a standalone handheld controller with an integrated driver circuit. Using it is as simple as plugging it into a normal outlet and connecting a stepper motor to run through motion sequences.

“To serve engineers in this industry, we’ve released the world’s only ATEX-rated stepper drive for hazardous locations … and this year we’ll introduce our first explosion-proof step motor to pair with it,” said Eric Rice of Applied Motion Products. “The drive is already being used on rigs in the oil field for applications where accurate braking and speed control of drill strings are required.”

Motors in (proliferating) battery-powered designs

Battery-powered designs such as ATVs, mobile robotics (including AGVs), scooters, ebikes, and drones present unique motion-system challenges. Motors that integrate into these designs might need to accept 48, 24, or 12-V input, and nearly all of them must meet tough efficiency requirements … NEXT PAGE →

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ISL modification capabilities expands dc motor customization for OEM applications

January 14, 2018 By Lisa Eitel Leave a Comment

ISL Products Intl. has announced the expansion of its brush and brushless dc motor and gearmotor modification capabilities for dc motor customization — to meet the exact specifications of virtually every specialized OEM application.

Newly enhanced engineering capabilities target a broad range of low-energy designs requiring high efficiency and extremely low current. In addition to now meeting the demands of low energy budget products, ISL’s comprehensive range of value-added services ensure that all design challenges are met whenever off-the-shelf motors are unsuitable. Unparalleled customer support for dc motor customization is provided throughout each stage of the manufacturing process to help OEMs cost-effectively speed time-to-market.

ISL low-current dc motors come in an array of mechanical and electrical configurations.

Built for durability, efficiency, and long life, the low-current dc motors and gear motors uniquely meet the needs of battery powered equipment, and are suitable for use in dispensing machines (paper towel, soap, and sanitizers), cordless power tools, medical tools, robotics, and portable instrumentation/test equipment. Plus ISL’s value-added dc motors and gear motors work in an extensive range of specialized OEM applications — including vending machines, industrial packaging machines (such as labeling and cutting), automated locks and doors, medical equipment, security cameras, and pool chair lifts.

Besides the ability to specify precise current limitations, customers can order brush and brushless dc motors and gearmotors with exact values for essentially all electrical and mechanical parameters, including voltage, speed, torque, output power, shaft configuration, overall size, and encoder type. Motors are also optimized to meet every applications’ special operating conditions and come in a variety of configurations to facilitate optimal design flexibility.

Various lead wires, connectors, pulley’s, external gears, and subassemblies address the needs of any specific assembly requirements.

To learn more about ISL’s dc motor customization and value-added dc motors and gearmotors, component modification capabilities, and assembly services, visit islproducts.com/motors-mechanical-components.

ISL is a global leader in value-added engineering and manufacturing services for electronic and electromechanical components. The company additionally offers a comprehensive portfolio of supply chain and logistics management services. For over 40 years, design engineers and OEM’s have relied on ISL’s worldwide engineering sales and support network to deliver innovative and cost-effective setups to help accelerate time to market.

ISL’s products and services are widely employed in audio/visual, healthcare, security, industrial machinery, robotics, and food service equipment. Products include audio components, motor and mechanical components, ballasts, and power supplies, potentiometers/transformers, cable assemblies and connectors. The company maintains a network of US sales representatives and is headquartered in Syosset, N.Y.