How do variable frequency drives change the motor speed?
Variable frequency drives (VFDs) are used to control the speed of both induction and synchronous motors. The complexity of the controller can range from a simple, manually operated control panel to a more sophisticated programmable logic controller (PLC) that can automate control based on input from system variables such as flow, level or pressure.
Three-phase induction motors operated on line voltage have relatively smooth sine waves with a 50- or 60-hertz (Hz) frequency, depending on the location. For two-pole motors, this equates to a 3,000- or 3,600-rotations per minute (rpm) synchronous pole speed. Without a VFD, the speed of the motor is relatively constant and changes minimally as a function of load due to motor slip. With a VFD, voltage and current fed to the motor are controlled by turning on and off insulated-gate bipolar transistors (IGBTs). The amplitude of the voltage and current values is controlled by the frequency of the switching and on and off time of the IGBTs. This means the longer the switches are on compared to the time they are off, the larger the voltage and current values.
Pulse width modulation (PWM) is a high-frequency waveform generated by the IGBTs to simulate a sine waveform to the motor. The frequency of the sine wave can be varied by the VFD, resulting in a change in motor speed. PWM is advantageous because the power loss in the switching devices is low, resulting in relatively high efficiency and energy benefits when considering the reduced hydraulic power at lower speeds. With the power being switched on and off rapidly, the resulting power waveform closely approximates the average current and voltage that a utility power source would normally supply a motor (sine wave). Refer to Image 3.
For more information on VFDs and how they are applied in pumping applications, refer to “Variable Frequency Drives: Guidelines for Application, Installation, and Troubleshooting” at pumps.org.