Variable-Speed OperationTo maintain maximum torque and efficiency of an ac motor, a constant magnetic field strength, or flux, is desired. To maintain constant flux, the ratio of voltage to frequency or volts-per-hertz should be constant. The controls of a vfd adjust the voltage along with the frequency (see Figure 1); however, two departures from a linear volts-per-hertz relationship are required. At low frequencies, it is necessary to provide a higher level of voltage, Vb, known as boost to ensure adequate torque to start the load turning. At frequencies above 60 Hz, the voltage must be limited to the rated voltage of the motor.
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.
The principle design feature required to accommodate variable-frequency operation is low-speed cooling capability. Constant motor torque means approximately constant current and thus constant heating within the motor. Since most motors are fan-cooled, cooling effectiveness is lost as speed is reduced. Inverter-duty motors must have cooling systems designed to effectively remove full-load heat at the low end of the constant-torque operating range.
Inverter-duty motors must also be capable of operating continuously at speeds above their rated speed if required by the application. This capability varies with both horsepower rating and base speed rating, with a maximum continuous capability of twice rated speed for the smallest frame sizes in each speed, dropping off as the motors increase in size.
Output Waveform EffectsIn last month's column, we discussed the effects of fast turn-on switching devices in creating surge voltages within the motor windings. Inverter-duty motors should have winding insulation that is resistant to the effects of these surge voltages.
The second aspect of the inverter voltage waveform that impacts the design of an inverter-duty motor is the high-frequency or harmonic content. Harmonic components of the voltage cause additional heating in the stator iron of the motor by inducing circulating currents known as eddy currents, and in the windings by causing harmonic currents to flow in addition to the fundamental current required to provide the torque for the load. The result is that the total RMS current in the motor when operating from a vfd will be slightly higher than when operating directly from the ac line.
The effect of the additional heating due to harmonics is to consume the margin for overload normally designed into a standard motor. An inverter-duty motor will typically have a service factor of 1.0 when operated from a vfd. (Remember that the service factor is a multiple of rated horsepower that the motor is capable of carrying continuously when supplied with rated voltage and frequency.) The same motor, when operated from the ac line, will have a service factor of 1.15.