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.

Electrical power to shaft output

Four connected stages of motor power conversionIncoming AC passes through a rectifier, the DC link and an inverter to the motor. The DC link includes capacitance. The inverter switches the motor supply; the motor produces shaft torque.Incoming three-phase AC1 RectifierAC → DC2 DC link+−3 InverterControlled switchingM4 Motor → shaft torque
  1. Rectifier. Converts the incoming AC into DC.
  2. DC link. Capacitors help support the intermediate DC supply and smooth its ripple; the link is not an unlimited energy store.
  3. Inverter. Semiconductor switches connect the motor phases to the DC rails in a controlled sequence.
  4. Motor. Winding currents produce a rotating field and torque. Some input energy becomes heat and other losses.
A common voltage-source drive during motoring. This functional diagram omits protection, filters, control circuits and braking paths. Regeneration needs a suitable energy path; a basic diode rectifier cannot return energy to the mains.

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.

Voltage pulses and winding current

Switched voltage and smoother motor currentThe upper plot shows illustrative phase-leg voltage switching between positive and negative DC midpoint levels. The lower plot shows approximately sinusoidal motor phase current with small ripple. They share the same time interval but have different vertical units and independent scales.Phase-leg voltagerelative to DC midpoint+0−Motor phase current+0−Same time interval →
  1. Upper trace: switching voltage. In this ideal two-level phase leg, the switch node alternates between +VDC/2 and −VDC/2 relative to the DC-link midpoint. Pulse widths vary through the electrical cycle.
  2. Lower trace: current through one motor phase. Motor inductance resists sudden current changes. The current is smoother than the switched voltage, although ripple and distortion remain.
  3. Read the axes separately. The horizontal time interval is shared. Voltage and current use different vertical units and scales; trace heights cannot be compared. The current’s phase depends on the operating condition.
Illustrative waveforms, not measurements from a lift. Only 14 switching cycles are drawn to keep the pulses readable; real switching frequency is usually much higher than the motor’s fundamental frequency. The upper trace is not line-to-line or phase-to-neutral motor voltage.

Control methods respond differently to load.

Frequency is a starting point

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.

Further reading.

Optional reading to explore this lesson in more detail.