A drive controls the motor supply.
A variable-frequency drive, or VFD, converts incoming electrical power into a controlled output for an AC motor. Changing output frequency changes the speed of its rotating magnetic field.
Voltage and current must also suit the motor and load. A drive is more than a device that simply changes frequency.
Three main power stages.
- RectifierConverts the incoming AC supply to DC.
- DC linkProvides an intermediate energy store and supply.
- InverterSwitches the DC to create a controlled motor output.
The output is switched, not a smooth mains waveform.
Electronic switches apply rapidly changing voltage pulses. Modulation controls the effective motor supply; motor inductance helps shape current.
The output’s fundamental frequency and the much higher switching frequency are different. Neither should be confused with the fixed frequency of the incoming mains.
Control methods respond differently to load.
A four-pole motor field rotates at 900 rpm at 30 Hz. An induction rotor’s actual speed also depends on slip; a synchronous rotor follows the field during synchronous operation.
Simple voltage/frequency control and more advanced vector control have different capabilities. Some drives use an encoder; others estimate relevant motor conditions from electrical measurements.
The lift controller and drive share the task.
The controller requests motion and coordinates the journey. The drive controls the motor according to its command and feedback. Brake sequencing and other permissions belong to the complete control arrangement.
A VFD does not replace the mechanical brake or the lift’s safety systems. Suitable operation depends on the motor, load, feedback and configured control method.
A closer look
Above a motor’s base-speed region, voltage limits can require field weakening and reduce available torque. A frequency increase does not promise constant torque, unlimited speed or suitability for the connected machinery.
Check your understanding
Is a drive’s switching frequency the same as the motor’s commanded electrical frequency?
Reveal the explanation
No. The inverter can switch many times within each cycle of the motor’s fundamental output waveform.