AC Motors and AC Drives

Many applications require the speed of an AC motor to vary. The easiest way to vary the speed of an AC induction motor is to use an AC drive to vary the applied frequency. Operating a motor at other than the rated frequency and voltage affect both motor current and torque.

Volts per Hertz (V/Hz) Ratio

The volts per hertz (V/Hz) ratio is the ratio of applied voltage to applied frequency for a motor. 460 VAC is the most common voltage rating for an industrial AC motor manufactured for use in the United States. These motors have a frequency rating of 60Hz. This provides a V/Hz ratio of 7.67. Not every motor has a 7.67 V/Hz ratio. A 230 Volt, 60 Hz motor, for example, has a 3.8 V/Hz ratio.

Constant Torque Operation

AC motors running on an AC line operate with a constant flux because voltage and frequency are constant. Motors operated with constant flux are said to have constant torque. Actual torque produced, however, is dependent upon the load. An AC drive is capable of operating a motor with constant flux from approximately 0 Hz to the motor’s rated nameplate frequency (typically 60 Hz). This is the constant torque range. As long as a constant volts per hertz ratio is maintained the motor will have constant torque characteristics.

The following graphs shown on Picture 1 illustrate the constant volts per hertz ratio of a 460 volt, 60 Hz motor and a 230 volt, 60 Hz motor operated over the constant torque range. Keep in mind that if the applied frequency increases, stator reactance increases. In order to compensate for this, the drive must simultaneously increase voltage proportionally. Otherwise, stator current, flux, and torque would decrease.

Picture 1: Volts per Hertz ratio graphs

Constant Horsepower

Some applications require the motor to be operated above base speed. Because applied voltage must not exceed the rated nameplate voltage, torque decreases as speed increases. This is referred to as the constant horsepower range because any change in torque is compensated by the opposite change in speed.

Power[HP] = (Torque[lb-ft] x Speed[RPM]) / 5252

If the motor is operated in both the constant torque and constant horsepower ranges, constant volts per hertz and torque are maintained up to 60 Hz. Above 60 Hz, the volts per hertz ratio decreases, with a corresponding decrease in torque (Picture 2).

Picture 2: Volts per Hertz ratio decreases above the main operating frequency

Reduced Voltage and Frequency Starting

Recall that when a NEMA B motor is started at full voltage, it develops approximately 150% starting torque and 600% starting current. When the motor is controlled by an AC drive, the motor is started at reduced voltage and frequency. For example, the motor may start with approximately 150% torque, but only 150% of full load current. As the motor is brought up to speed, voltage and frequency are increased, and this has the effect of shifting the motor’s speed-torque curve to the right. The dotted lines on the following speed-torque curve represent the portion of the curve not used by the drive (Picture 3). The drive starts and accelerates the motor smoothly as frequency and voltage are gradually increased to the desired speed.

Picture 3: Speed-Torque curve


Some applications require higher than 150% starting torque. A conveyor, for example, may require 200% rated torque for starting. This is possible if the drive and motor are appropriately sized.

Selecting a Motor

AC drives often have more capability than the motor. Drives can run at higher frequencies than may be suitable for an application. In addition, drives can run at speeds too low for self-cooled motors to develop sufficient air flow. Each motor must be evaluated according to its own capability before selecting it for use on an AC drive.
Harmonics, voltage spikes, and voltage rise times of AC drives are not identical. Some AC drives have more sophisticated filters and other components designed to minimize undesirable heating and insulation damage to the motor. This must be considered when selecting an AC drive/motor combination.

Distance Between the Drive and the Motor

Distance from the drive to the motor must also be taken into consideration. All motor cables have line-to-line and line-to-ground capacitance. The longer the cable, the greater the capacitance. Some types of cables, such as shielded cable or cables in metal conduit, have greater capacitance. Spikes occur on the output of AC drives because of the charging current in the cable capacitance. Higher voltage (460 VAC) and higher capacitance (long cables) result in higher current spikes. Voltage spikes caused by long cable lengths can potentially shorten the life of the AC drive and motor.

Service Factor on AC Drives

A high efficiency motor with a 1.15 service factor is recommended when used with an AC drive. Due to heat associated with harmonics of an AC drive, the 1.15 service factor is reduced to 1.0.