How to match ac motors and variable-speed drives

Careful consideration must be given when selecting a drive/motor package to ensure you get the desired performance. This article presents guidelines for selecting and sizing an adjustable frequency drive (AFD) and ac motor package for a given variable speed application. AC motor selection Two general categories of ac induction motors are suitable for operation with ac drives: NEMA has defined f...


Key concepts

Inverter-duty motors produce full-load torque without exceeding the temperature rise of the insulation.

Consider the application, load characteristics, speed range, environment, and drive requirements.

The available torque for starting and peak loads is different using an AFD.

AC motor selection
Sizing the ac motor
Sizing the ac drive
General sizing method for use with multiple induction motors
Reflected wave considerations

Speed range explained
Drive technologies explained

Careful consideration must be given when selecting a drive/motor package to ensure you get the desired performance. This article presents guidelines for selecting and sizing an adjustable frequency drive (AFD) and ac motor package for a given variable speed application.

AC motor selection

Two general categories of ac induction motors are suitable for operation with ac drives:

  • Fixed speed NEMA design B motors with insulation systems designed for pulse-width modulated (PWM) inverter power

  • Inverter-duty motors designed specifically for inverter variable speed operation.

    • NEMA has defined four standard classes (A, B, C, and D) of squirrel cage motors. The NEMA design B motor is the most common type of three-phase industrial ac motor in use today. It is used in general industrial applications and has normal across-the-line characteristics (starting torque and current, breakdown torque, and full-load slip).

      Generally, inverter-duty motors are designed to produce full-load torque from zero to base speed without exceeding the temperature rise of the insulation system. Inverter-duty motors are often described as motors that have a 1000:1 constant torque capability (see the sidebar on "Speed range explained"). This description is just another way to say the motor is designed to operate from zero to base speed and produce full-load torque over the entire speed range. For applications that require the motor to operate at, or near, zero speed the motor usually has a motor-mounted encoder for feedback to the AFD. Motors that operate at a speed range of 10:1 or less usually do not require encoder feedback, unless the application requires very precise speed regulation of the motor or high response from the AFD and motor. High response means that the output of the inverter changes very quickly with the slightest change of encoder feedback.

      Before selecting a motor for use with an AFD, it is important to understand the nature of the application in terms of load (torque) characteristics, speed range, environment, and drive requirements. Regarding torque characteristics, the majority of variable-speed ac drive applications fall into either variable torque or constant torque applications. Centrifugal pumps and fans are variable torque applications where most fixed-speed, energy-efficient ac motors can be used without concern of overheating. The horsepower required to operate centrifugal pumps and fans decreases with the cube of the speed. If you reduce the speed of the ac motor to one-half of base speed, the horsepower required is only one-eighth of rated horsepower. On variable torque applications, the motor insulation system must be designed for PWM power. Most of this article addresses constant-torque applications.

      The exact motor performance curve for a given fixed-speed NEMA design B motor for operation on an AFD varies and should be supplied by the motor manufacturer. The motor speed/torque performance graph shown in Figure 1 is typical for most standard fixed-speed, totally enclosed fan-cooled (TEFC), premium-efficiency motors. The graph shows that these motors can produce constant full-load torque over a 4:1 speed range, 100% of base speed down to 25% of base speed. Many standard TEFC motors can be modified for AFD operation by installing a larger fan to provide a wider constant torque speed range.

      Fig. 1. This motor speed/torque performance graph is typical for most standard fixed-speed TEFC premium-efficiency motors. These motors can produce constant full-load torque from 100% of base speed down to 25% of base speed, after which the full-load torque must start to decrease.

      Most standard ac motors are designed to operate at a fixed, rated frequency and speed, such as 460 V ac, 60 Hz, 1800 rpm. At this fixed speed, the cooling system, which is usually a shaft-mounted fan, will keep the motor from overheating. However, when operated as an adjustable speed device at slower speeds, the motor shaft-mounted fan produces very little cooling action. The full-load torque must start to decrease once the AFD reduces the motor speed to 25% of base speed.

      Figure 2 shows the capability of an inverter-duty motor designed specifically for AFD variable speed applications. Inverter-duty motors not only have an insulation system designed for PWM power, but they are also designed to operate at zero speed with full-load torque and not exceed the temperature rise of the insulation system. Traditionally, these motors were totally enclosed nonventilated (TENV), totally enclosed air-over blower-cooled (TEAO-BC), or dripproof-guarded force-ventilated (DPG-FV) enclosures (Fig. 3.). These enclosures provide a cooling system that does not depend on motor speed. Therefore, these motors can provide full-load torque at any speed below base speed — including zero speed.

      Fig. 2. Graph indicates the capabilities of inverter-duty motors that are designed specifically for AFD variable-speed applications. These inverter-duty motors have insulation systems designed for PWM power, and are designed to operate at zero speed with full-load torque without exceeding the temperature rise of the insulation system.

      Recently, some motor manufacturers have developed inverter-duty, (TEFC) motors that can provide full-load torque from zero to base speed without overheating. These are special inverter-duty electrical designs that are not designed to meet NEMA across-the-line starting torque or current specifications. The designs provide reduced heat loss at low to zero speeds and higher losses near base speed where the fan provides maximum cooling. These special inverter-duty TEFC motors also provide the performance shown in Fig. 2 in a standard NEMA TEFC frame and enclosure.

      AFDs and motors may also operate in the constant horsepower range, shown above base speed in Fig. 1 and 2. Center winders and machine tool applications are ideal for using the constant horsepower range. Constant horsepower operation of most standard TEFC motors is limited to 150% of base speed. One of the limiting factors is the amount of noise that the shaft-mounted fan produces at speeds above base speed. Inverter-duty motors, TENV, or blower-cooled motors have a much wider constant horsepower operating range because the cooling is independent of motor speed. For a constant horsepower range greater than 150% of base speed, get application assistance from the AFD/motor supplier.

      Fig. 3. Surrounding air is blown through this drip-proof guarded force-ventilated (DPG-FV) ac motor, along the rotor, and out of the drip-proof covers on the drive shaft end. A

      Sizing the ac motor

      Motor users are responsible for ensuring that drive train mechanisms, the driven machine, and process materials are capable of safe operation at the maximum speed of the machine. Failure to observe these precautions could result in bodily injury or equipment damage.

      The following procedure provides a conservative, engineering-based approach for sizing and selecting various ac motors for use with an ac drive.

      • Determine the required drive/motor output horsepower, starting torque, constant-torque speed range, constant horsepower range (if any), and total speed range.

      • Select the type of motor required: standard, fixed-speed, TEFC, energy efficient, or inverter-duty motor (TENV, TEAO-BC, DPG-FV, or TEFC).

      • Using graphs from the motor manufacturer, such as the ones shown in Fig. 1 and 2, confirm that the required torque falls within the "acceptable" region of the graphs.

        • Sizing the ac drive

          The capabilities of the ac drive are determined by its output current rating. The chosen drive must have a continuous current rating equal to or more than the maximum motor load current. Be sure to consider all loads including startup acceleration.

          Fig. 4. The multipurpose industrial ac drive shown

          Also, when applying an AFD with a motor, keep in mind that the available torque for starting and peak loads is different using an AFD compared to across-the-line motor operation. Typically, AFDs limit current to 150%, providing about 150% torque for starting and peak loads. An across-the-line motor can have starting torque and peak load capabilities that exceed 150%. The AFD and motor combination must be sized to produce the appropriate amount of torque that the load will demand.

          General sizing method for use with multiple induction motors

          To size the AFD for multiple motor applications or for any application from 6 to -120-Hz operation, use the following procedure:

          • Determine the motor full-load amperes at rated line voltage for each motor to be driven.

          • Add the full-load current requirements for each motor to determine the total full-load current.

          • Add the high currents of any overloads which may exist, such as acceleration, peak load, etc., to the full-load currents of all the motors being controlled by the AFD, and determine maximum short-term load at line voltage. (Note that motor acceleration is by linear timed-rate acceleration control. Therefore, locked-rotor amperes normally associated with across-the-line starting of ac motors are not encountered.)

          • Select the ac drive rating with a current capacity that will support the required currents as calculated in the previous steps.

            • Once the appropriate drive has been selected based on current rating, verify that the drive will provide the appropriate speed regulation over the desired speed range of operation. Most manufacturers indicate speed range and speed regulation capabilities of their drives. The desired speed regulation and speed range will also dictate the type of drive technology required: V/Hz, sensorless vector, or flux vector (see sidebar "Drive technologies explained").

              Reflected wave considerations

              With increased market demand for AFDs and increased use of insulated gate bipolar transistor (IGBT) technology, awareness has been raised regarding certain application issues surrounding the motor and AFD. Under certain installation conditions, there exists potential for motor insulation damage when operated with PWM inverters. In some cases, high voltage spikes at the motor terminals can produce destructive stress of the motor insulation. Awareness of this issue along with the proper selection and application of the motor and AFD together will greatly reduce the risk of this type of failure.

              Peak voltages seen at the motor input terminals depend on IGBT rise times (dv/dt). Typically IGBTs have rise times in the 50-400 nsec range. This, in conjunction with the short duration of pulses to the motor (50 nsec to 1 msec), can result in excessive overvoltage transients at the motor. The charts in Fig. 5 show a typical PWM output waveform from the drive terminals.

              The cable installed between the drive and the motor is impedance to the PWM voltage pulses. These cables contain significant values of inductance (L) and capacitance (C). These L and C values are directly proportional to the length of the cable run. When this cable-surge impedance does not match the surge impedance of the motor, a reflected wave occurs.

              Solutions to reflected wave problems include:

              • Specify motors with insulation systems designed for PWM power. Inverter-duty-rated motors have insulation systems designed to withstand the anticipated magnitude and rate-of-rise of the voltage spikes at the motor terminals.

              • On low-horsepower applications, use 240-V ac AFDs and motors, the reflective wave impact is fundamentally reduced to half that of a 460-V ac system.

              • Limit motor cable lengths to that specified by the manufacturer. Limiting motor cable lengths to the manufacturer-specified limits ensures that the cable impedance matches the impedance of the motor.

              • Install a drive output line reactor or filter. The inductance in the reactor interacts with the fast output rise times of the drive to slow down rise time and voltage magnitude, thus reducing any reflected waves or ringing. The specialized filter eliminates voltage reflection by closely matching cable impedance with the passive elements in the filter.

              • Select a matching drive/motor package. Matching drive/motor packages offer superior design and proven performance because these combinations have been tested for dynamic stability. When applied properly, motor stress effects and high peak voltage are minimal.

                • — Edited by Jack Smith, Senior Editor, 630-288-8783,

                  Speed range explained

                  Most standard general-purpose ac motors are designed for operation at a fixed speed, such as 1800 rpm. For example, the size of the fan (TEFC) is sized to provide proper cooling at the designed speed. When motors are powered with an inverter, the motors now operate over a speed range, depending on the application. Most energy efficient motors can operate with full-load torque (nameplate amps) over a 4:1 speed range (450 to 1800 rpm). By installing a larger diameter fan, many of these motors can operate over a 10:1 speed range (180 to 1800 rpm) with full load torque.

                  Traditionally, motors that needed to operate over a 1000:1 speed range (1.8 to 1800 rpm) required an external blower motor to cool the motor. Some motor manufacturers have developed special TEFC motors specifically for inverter operation that can operate over a 1000:1 speed range without a separate blower motor. If a motor can operate with full load torque over a 1000:1 speed range, it can operate at zero speed with full-load torque.

                  Drive technologies explained

                  The following list explains different types of drive technology. Speed regulation and range dictate the type of drive required or an application.

                  V/Hz — an open loop means of controlling a motor. The drive varies the voltage and frequency to the motor to control the speed without any feedback.

                  Flux vector — an encoder is added to the motor shaft. It provides actual speed feedback in a closed loop system, which allows the drive to precisely control the motor speed.

                  Sensorless vector — a way to approach flux vector speed regulation without the need for adding an encoder to the motor. The drive calculates motor speed instead of having an actual speed measurement from an encoder.

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