AC Motors - Motion Control Tips

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

December 18, 2018 By Miles Budimir Leave a Comment

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

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

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

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

To learn more, visit www.automationdirect.com.

The truth about VFDs and power factor

December 7, 2018 By Danielle Collins Leave a Comment

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

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

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

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

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

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

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

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

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

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

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

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

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

THD = total harmonic current distortion

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

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

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

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

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

November 21, 2018 By Danielle Collins Leave a Comment

Image credit: WEG Electric

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

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

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

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

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

Where:

T₂ = Torque at reduced current/voltage

T₁ = Torque at locked rotor current

I₂ = Reduced current

I₁ = Locked rotor current

V₂ = Reduced voltage

V₁ = Full voltage

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

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

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

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

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

November 15, 2018 By Danielle Collins Leave a Comment

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

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

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

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

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

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

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

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

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

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

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

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

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

Conveyors with direct-drive motors offer key advantages to Pa. facility

November 11, 2018 By Lisa Eitel Leave a Comment

American Axle Manufacturing (AAM) is increasing the functionality of its St. Mary’s, Pa. manufacturing operations with conveyors sporting advanced drive technology. Small yet powerful direct-drive motors from Electric Torque Machines actuate Dorner conveyors that move small parts around the AAM facility.

AAM designs, engineers, validates, and manufactures an array of driveline, metal-forming, powertrain, and casting technologies — and is known for manufacturing large components such as axles and differentials. However, the AAM St. Mary’s facility manufactures many of the small and lightweight parts that go into AAM drivetrains. Transporting these parts from station to station and within workcells requires small and precise conveyors. The challenge is that the compactness of such conveyors is degraded by the bulk of traditional gearmotors. That’s what prompted AAM to specify conveyors driven by Dorner’s Universal Drive offerings — with direct-drive motors from Electric Torque Machines at their core.

Dorner Conveyor and Electric Torque Machines helped improve the material handling setup inside the Pennsylvania facility of American Axle Manufacturing.

The direct-drive motors have high continuous torque density (particularly at lower operating speeds) to eliminate the need for a bulky gearbox. Integrated into Dorner’s 2200 Universal Drive — which includes the motor, configurable motor mount, and controller — they reduce the conveyor-drive size by nearly 70% while boosting functionality and maintaining required output torque.

“We chose Dorner’s Universal Drive for its unique motor,” said AAM facilities engineer Michael Vandervort. “The motor is smaller and lighter than traditional ac motors … and can run in designated working areas unguarded, because the motor runs cool. In contrast, traditional ac motors are too hot to touch.”

ETM’s direct-drive motor imparts advanced functions to simplify automated tasks.

Vandervort also cited the wide speed and torque ranges of the Dorner Universal Drive as key benefits. “The drive lets us use the conveyors for applications such as slow pass-under vision tasks; heavy accumulation tasks; and high-speed part transfers.”

The on-conveyor inspection function is especially challenging. “At our facility, we use a lot of small conveyors to move light loads. Electric Torque Machines developed motor tuning functions to help stabilize our vision imaging on pass-under applications with very low torque requirements. We really appreciate ETM working with us and developing solutions for our specific needs,” Vandervort added.

“It was energizing to work with industry leader AAM — and encouraging to see the industry ready to adopt new technologies. AAM’s application illustrates the advantages of direct-drive motors,” added ETM V.P. of engineering Scott Reynolds.

What are leading methods for VFD control of AC motors?

October 3, 2018 By Danielle Collins Leave a Comment

AC motors are often used in equipment that runs at constant speed regardless of load, such as fans, pumps, and conveyors. But when speed control is required, an AC motor is paired with a variable frequency drive (VFD), which regulates the motor’s speed by using one of two control methods — scalar control or vector control — to vary the frequency of the supplied voltage.

A scalar is a quantity that has only magnitude, such as mass or temperature. A vector is a quantity that has both magnitude and direction, such as acceleration or force.

In a VFD, AC power is converted to DC through the rectifier. The rectified power is then filtered and stored in the DC bus (link). The inverter converts it back to AC power with the proper frequency and voltage via pulse-width modulation.
Image credit: what-when-how.com

Scalar control methods

Scalar methods for VFD control work by optimizing the motor flux and keeping the strength of the magnetic field constant, which ensures constant torque production. Often referred to as V/Hz or V/f control, scalar methods vary both the voltage (V) and frequency (f) of power to the motor in order to maintain a fixed, constant ratio between the two, so the strength of the magnetic field is constant, regardless of motor speed.

The appropriate V/Hz ratio is equal to the motor’s rated voltage divided by its rated frequency. V/Hz control is typically implemented without feedback (i.e. open-loop), although closed-loop V/Hz control — incorporating motor feedback — is possible.

V/Hz control maintains a constant ratio between voltage (V) and frequency (Hz).
Image credit: Square D

V/Hz control is simple and low-cost, although it should be noted that the closed-loop implementation increases cost and complexity. Control tuning is not required but can improve system performance.

Speed regulation with scalar control is only in the range of 2 to 3 percent of rated motor frequency, so these methods aren’t suitable for applications where precise speed control is required. Open-loop V/Hz control is unique in its ability to allow one VFD to control multiple motors and is arguably the most-commonly implemented VFD control method.

Vector control methods

Vector control — also referred to as field oriented control (FOC) — controls the speed or torque of an AC motor by controlling the stator current space vectors, in manner similar to (but more complicated than) DC control methods. Field oriented control uses complex mathematics to transform a three-phase system that depends on time and speed to a two-coordinate (d and q) time-invariant system.

The stator current in an AC motor is made up of two components: the magnetizing component (d) of the current and the torque-producing component (q). With FOC, these two current components are controlled independently (each with its own PI controller). This allows the torque-producing component, q, to be kept orthogonal to the rotor flux for maximum torque production, and therefore, optimum speed control.

Vector control, or field-oriented control, converts 3-phase currents in a stationary reference frame to a 2-phase system (consisting of a flux component, d, and torque-producing component, q) with a rotating reference frame. Here, the torque-producing current (q) can be independently controlled to ensure maximum torque production. The system is then transformed back into a 3-phase system in a stationary reference frame for output to the motor.
Image credit: Freescale Semiconductor

Like scalar methods, vector VFD control methods can be open-loop or closed-loop. Open-loop vector control (also referred to as sensorless vector control) uses a mathematic model of the motor operating parameters, rather than using a physical feedback device. The controller monitors voltage and current from the motor and compares them to the mathematical model. It then corrects any errors by adjusting the current supplied to the motor, which adjusts the motor’s torque production accordingly. With senseless vector control, it’s important to have a very accurate mathematical model of the motor, and the controller must be tuned for proper operation.

Closed-loop vector control uses an encoder to provide shaft position feedback, and this information is sent to the controller, which adjusts the supplied voltage to increase or decrease torque. This is the only method that allows direct torque control in all four quadrants of motor operation for dynamic braking or regeneration.

Vector control methods are more complex than scalar VFD control methods, but they offer significant benefits over scalar methods in some applications. For example, open-loop vector control enables the motor to produce high torque at low speeds, and closed-loop vector control allows a motor to produce up to 200 percent of its rated torque at zero speed, useful for holding loads at standstill. Closed-loop vector control also provides very accurate torque and speed control for industrial applications.

What are slip rings and why do some motors use them?

August 21, 2018 By Danielle Collins Leave a Comment

Slip rings — also referred to as rotary electrical joints, electric swivels, and collector rings — are devices that can transmit power, electrical signals, or data between a stationary component and a rotating component. The design of a slip ring will depend on its application — transmitting data, for example, requires a slip ring with higher bandwidth and better EMI (electromagnetic interference) mitigation than one that transmits power — but the basic components are a rotating ring and stationary brushes.

A complete slip ring assembly includes end caps, bearings, and other structural features. But the basic components of a slip ring are the ring and the brushes.
Image credit: Moog Inc.

If the rotation of one component involves a fixed number of revolutions, it may be possible to use spools with sufficient cable length and rotating speed to allow for the required revolutions, although cable management in this setup can be quite complex. But if one component rotates continuously, using cables to transmit signals between the rotating and stationary components isn’t practical or reliable in many cases.

Slip rings in AC motors

Image credit: brighthubengineering.com

In a version of the AC induction motor referred to as a wound rotor motor, slip rings are used not for transferring power, but for inserting resistance into the rotor windings. A wound rotor motor uses three slip rings — typically made of copper or a copper alloy — mounted to (but insulated from) the motor shaft. Each slip ring is connected to one of the three phases of rotor windings. The slip ring brushes, made of graphite, are connected to a resistive device, such as a rheostat. As the slip rings turn with the rotor, the brushes maintain constant contact with the rings and transfer the resistance to the rotor windings.

Slip rings on a wound rotor AC motor. Once the motor reaches operating speed, the brushes are lifted via the springs and the slip rings are short-circuited via the sliding contact bar.
Image credit: Wikipedia

Adding resistance to the rotor windings brings the rotor current more in-phase with the stator current. (Recall that wound rotor motors are a type of asynchronous motor, in which the rotor and stator electrical fields rotate at different speeds) The result is higher torque production with relatively low current. The slip rings are only used at start-up, however, due to their lower efficiency and drop-off of torque at full running speed. As the motor reaches its operating speed, the slip rings are shorted out and the brushes lose contact, so the motor then acts like a standard AC induction (aka “squirrel cage”) motor.

The slip rings in a wound rotor motor form a secondary, external circuit. Inserting resistance to this circuit allows the motor to produce very high torque at start-up, which is necessary for moving loads with high inertia.

Slip ring or commutator?

You may have noticed that the design and function of a slip ring sounds very similar to that of a commutator. While there are similarities between the two, there are critical distinctions between slip rings and commutators. Physically, a slip ring is a continuous ring, whereas a commutator is segmented. Functionally, slip rings provide a continuous transfer of power, signals, or data. Specifically, in AC motors, they transfer resistance to the rotor windings.

Commutators, on the other hand, are used in DC motors to reverse the polarity of current in the armature windings. The ends of each armature coil are connected to commutator bars located 180 degrees apart. As the armature spins, brushes supply current to opposing segments of the commutator and, therefore, to opposing armature coils.

Slip rings are used in virtually any application that includes a rotating base or platform, from industrial equipment such as index tables, winders, and automated welders, to wind turbines, medical imaging machines (CT, MRI), and even amusement park rides that have a turntable-style operation. Although the traditional application for slip rings was to transmit power, they can also transmit analog and digital signals from devices such as temperature sensors or strain gauges, and even data via Ethernet or other bus networks.

Feature image credit: Rotary Systems Inc.

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

August 8, 2018 By Danielle Collins Leave a Comment

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

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

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

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

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

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

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

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

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

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

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

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

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

July 12, 2018 By Danielle Collins Leave a Comment

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

Image credit: ABB

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

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

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

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

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

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

Synchronous AC motors
Permanent magnet rotor AC motors
Servo motors

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

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

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

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

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

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

Severe duty motors: What makes them suitable for harsh applications?

June 14, 2018 By Danielle Collins Leave a Comment

While there is no broad standard that defines severe duty motors, the IEEE 841-2009 standard, which addresses “premium efficiency, severe duty, totally enclosed fan-cooled (TEFC), squirrel-cage induction motors — up to and including 370 KW (500 hp),” is often used as a benchmark.

Image credit: ABB

However, this standard was written for the petroleum and chemical industries, and many of its specifications are designed to address motors used in pumps, fans, and blowers in petroleum and chemical applications.

But manufacturers often use the IEEE 841 standard as a benchmark for motors that are designed or marketed for other industries, adopting those features and specifications that are relevant to their applications and performance requirements.

In many cases, a motor labeled as “severe duty” falls between a standard TEFC motor and an IEEE 841 rated motor.

A totally enclosed fan-cooled (TEFC) motor has a frame that prevents air exchange between the inside and outside of the motor, preventing airborne contamination — and to some extent, liquids — from entering the motor. An external fan forces air over the frame to assist with cooling, and the frame is ribbed to increase the surface area for dissipating heat.

This is in contrast to an open drip-proof (ODP) motor, which has open vents that allow air to circulate directly over the motor windings. The design of an ODP motor prevents liquid from entering through the housing within a 15 degree angle from vertical, but there is no protection against airborne contamination.

According to the IEEE 841 standard, severe duty motors are used in the following service conditions:

Ambient temperatures from -25° C to 40° C (-13° F to 104° F)
Altitude to 1000 m (3281 ft)
Humid, corrosive, or salty environments
Full voltage starting
Class I, Division 2: explosive gas or vapor present during abnormal operation, but not likely to exist under normal operating conditions

To address these extreme conditions, severe duty motors are commonly made from cast iron and use higher-quality paint for better corrosion-resistance. Improved seals, such as V-rings in place of O-rings, are generally required for both the drive end and non-drive end of the motor, to provide increased protection against solid and liquid contamination. And the standard ingress protection (IP) rating for severe duty motors is IP55, which specifies limited ingress of dust and of water from a water jet nozzle.

The rotor cage of a severe duty motor is typically made of copper or aluminum, and the fan is made of bronze alloy or conductive plastic to prevent sparking. Severe duty motors should use class F insulation with a 1.15 service factor, which means their max operating temperature is up to 155° C, or 311° F, and allowable temperature rise is 115° C, or 239° F.

The IEEE 841 standard also sets out specific guidelines for the construction of the main terminal box, which must be cast iron and gasketed. Vibration allowances are 0.08 in/s (2.0 mm/s), and corrosion resistance of the main components is based on a 96-hour salt spray test.

While these specifications are specific to IEEE 841 rated motors, many of them are incorporated into severe duty motors for the general market. In addition, severe duty motors typically have a warranty of 3 years or more — an improvement over the 1 year or 18 month warranty found on most general-purpose motors.

Notice that the IEEE 841 standard applies to “premium efficiency” motors, as defined by the National Electrical Manufacturers Association (NEMA). In 2016, the Department of Energy (DOE) issued the “Integral Horsepower Motor Final Rule,” which mandates that all industrial electric motors from 1 to 500 hp, manufactured after June 1, 2016, must meet NEMA premium efficiency standards.

In compliance with this rule, the majority of severe duty motors, even if they are not fully IEEE 841 compliant, are designed to meet premium efficiency standards. The DOE rule only applies to motors manufactured or imported for sale in the U.S. However, the NEMA premium efficiency designation is generally comparable to the IEC 60034-30-1 classification of IE3.

Feature image credit: WEG