Variable speed
Asynchronous motors - Choice and related issues

Although asynchronous motors, like synchronous motors, do not have a strictly constant speed, since [relation (1)] N = N_s (1 – g ), they can hardly be considered variable-speed motors. Their speed is limited, on the one hand, by the operating speed at maximum torque and, on the other hand, by the synchronous speed, giving a variation range of less than 10%. What's more, there are no simple, low-cost electrical means (such as the field rheostats of DC motors) for adjusting their speed.

However, motor speed variation has two fundamental benefits.

—The first, and most obvious,...