Basic Operation of AC Induction Motors (5)

Constant Power Operation

The prior discussions regarding voltage boost and field oriented control as a means to maintain motor flux have been presented in regard to "constant torque" operation. This can also he thought of as operation "below base speed" (Figure 10).

Above the speed at which the output voltage of the controller is maximum, the controller can no longer maintain constant flux as speed is increased further (since the voltage cannot be increased to keep pace with the frequency). This is equivalent to where a DC motor begins to be "field weakened" to achieve higher speeds. Both for AC as well as DC machines, voltage (armature voltage for DC) remains constant, so for constant load current, constant output power
is available.
As the frequency supplied to an AC induction motor is increased (with voltage held constant), the resultant "field weakening" causes a reduction in the motor peak torque capability as seen in Figure 11.
This family of curves can alternatively be drawn as speed - power, rather than speed - torque curves (Figure 12). The fact that the peak power decreases as speed is increased by field weakening is the most "inherent" limitation to the "constant power speed range" of an AC induction motor drive.

A technique which is commonly employed to achieve wider speed ranges above base speed (constant power) is to utilize some of the “constant flux” speed range to augment the inherent constant power capability. By selecting a motor winding which does not require full voltage until some speed already into the desired constant power speed range, the constant power speed range can be extended as seen in Figure 13. The plots of Figure 13 show an example where the application demands a constant 100 HP from 650 RPM to 3200 RPM. By utilizing this technique, the motor size does not have to be increased in order to satisfy the wide constant power speed range.

This same technique is also used to extend the constant power speed range of DC systems as well. In the case of DC, it is to avoid commutation limits to the top speed at which constant power can be provided. For both AC and DC systems, the “price” which is paid to use this technique is an oversized source of power (higher kVA inverter or DC supply). Wind and unwind applications, along with machine tool spindles employ this technique quite commonly.