Motors operating from variable frequency supplies are often referred to as inverter-duty motors, named for the circuit within the vfd that creates variable-frequency ac voltage. They differ from standard motors for two reasons; the voltage waveform produced by an inverter differs from the (relatively) pure sinusoid provided by the ac line in ways that affect the motor; and the performance of the motor and load combination at variable speed differs from that at rated speed. Let's look at the variation of motor performance with speed first.
The resultant torque capability of a typical motor in variable-frequency operation is shown in Figure 2. Over most of the range below 60 Hz, the vfd and motor are capable of delivering constant torque to the load; typically the constant-torque turndown is 1:6, for a low end frequency of 10 Hz. In variable-torque application, where the torque requirement of the load drops off as speed is reduced, the available turndown is 1:10, for a low end frequency of 6 Hz. Above 60 Hz, the torque capability drops off due to reduced volts-per-hertz, and the motor is said to be in constant-horsepower operation.