Darrow Hanesian
Director of product development
Lenze/AC Tech
Uxbridge, Mass.

An ac motor operating with an open-loop vector drive fully recovers from the instantaneous application of its full rated load in 150 msec. Most impressive is that this dynamic response is accomplished open loop, without the benefit of feedback

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Motion engineers generally fall into two camps when it comes to ac drive technology: Those already taking advantage of its benefits — low price, compact design, simplicity — and those who will soon start. Both groups, however, face a similar challenge in that they must understand the difference between ac drive types in order to select the right one for a particular job.

In general, ac drives work (controlling ac motor speed) by varying the frequency of the current supplying the motor. Although frequency can be varied many ways, and in relation to other variables such as voltage, the most common methods in use today are "volts per hertz," open-loop vector, and closed-loop vector. How these techniques differ determines where each drive type works best.

Volts per hertz

Volts per hertz (V/Hz) technology is the most economical and easiest to apply of the three speed-control methods. Here, the drive controls shaft speed by varying the voltage and frequency of the signal powering the motor.

Now, the rotor of an ac induction motor is magnetically coupled to the stator through an induced magnetic field. The speed at which the magnetic field rotates around the stator is known as synchronous speed and is determined by:

n = 120 f/N

where n is synchronous motor speed, 120 is an electrical constant, f is the applied frequency, and N is the number of motor poles.

The equation illustrates one of the basic principles of speed control: Reducing applied frequency to an ac induction motor causes the magnetic field to turn at a proportionally slower rate, thereby reducing rotor speed.

This is only part of the story, however. Induction motors are designed to operate from line voltage at line frequency. But the whole purpose of V/Hz drives is that they don't hold systems to power line shapes. What they do instead is maintain an optimal voltage-to-frequency ratio, so that the motors they power will produce their rated torque over the widest possible speed range.

Consider a 460-Vac motor designed for 60-Hz operation. If applied frequency is reduced to 30 Hz, the shaft will slow to half its original speed. In this situation, a V/Hz drive also halves the voltage (here, to 230 Vac) in order to maintain the 7.67 V/Hz ratio, which allows the motor to continue producing its rated torque.