What is PWM?

Original OtomasyonAVM PWM lead image

What Is PWM and What Does It Do in a VFD?

Pulse-width modulation (PWM) varies how long an electrical switch stays ON during repeated fast switching cycles. In an AC variable frequency drive, this switching helps turn energy from the DC link into a suitable three-phase motor output. Pulses at the motor terminals and the fundamental motor output frequency are different things.

PWM in one sentence

While the switching period stays constant, the ON portion becomes longer or shorter, changing the width of each pulse. In a simple DC signal, the switch may give 12 V when ON and 0 V when OFF. PWM does not continuously reduce 12 V to a lower voltage.

How are duty cycle and PWM frequency calculated?

One period is T = ON time + OFF time; duty cycle is D = ON time / T; repetition frequency is f = 1 / T. If the switch completes one cycle every millisecond, frequency is 1 kHz. If it is ON for 0.25 ms, duty cycle is 25% and the simple 0–12 V signal has an average voltage of 3 V over one period. With a fixed period, changing ON time changes the duty cycle, not the frequency.

Move the slider to see how ON time changes pulse width:

Change the duty cycle (12 V · 1 kHz)

Duty cycle: 50%

ON time: 0.5 ms · Average signal voltage: 6 V

This drawing shows only a simple DC signal switched between 0 and 12 V. The slider percentage does not calculate an AC motor’s speed.

How does an AC drive use PWM for a motor?

A common AC variable frequency drive rectifies the incoming supply, feeds a DC link, then switches power transistors such as IGBTs in a coordinated three-phase pattern. Pulse widths change over time so the drive can create the required fundamental output frequency and effective motor voltage. The voltage at the motor terminals remains pulsed, while motor current can follow a smoother waveform. The single-switch drawing above explains the principle; it does not represent the entire three-phase inverter output.

In a simple sinusoidal PWM scheme, the duty cycle for one phase increases as its sine reference approaches the positive peak, decreases toward the negative peak and sits near the middle around a zero crossing. The other two phase references are shifted by 120 degrees. A real drive therefore does not send a fixed 50% pulse at all times. Motor winding inductance helps smooth the current produced by these fast voltage pulses.

Motor field frequency and switching frequency are different. The fundamental output may be 25 or 50 Hz while the power devices switch at several kHz. A four-pole induction motor has a synchronous speed of 1500 rpm at 50 Hz and 750 rpm at 25 Hz. Shaft speed is slightly lower under load because of slip. Raising the switching frequency alone does not raise motor speed.

What happens when the two frequency settings change?

In a fan drive, reducing the fundamental output frequency from 50 Hz to 25 Hz slows the motor's rotating field. Raising the carrier switching frequency from, say, 4 kHz to 8 kHz in the same application does not double the target shaft speed; it may change audible noise and drive heating. These parameters serve different purposes and should be identified separately in the parameter list.

Does PWM replace an analog speed reference?

No. A properly installed 0–10 V or 4–20 mA analog signal can provide a speed reference. A potentiometer often supplies that input; it is not a series resistor dissipating all the motor power. The control stage interprets the command, and the inverter power stage creates PWM pulses. An analog reference and PWM can work together.

Illustration of a PWM waveform in a variable frequency drive application

What do IGBTs and MOSFETs do?

IGBTs and MOSFETs are themselves types of transistor. In a suitable power stage, they switch rapidly between conducting and blocking to form the pulses. Conduction and switching losses still exist; device choice depends on power, voltage, speed and design constraints. PWM is therefore not lossless.

Does PWM itself guarantee energy savings?

Keeping a power device mainly ON or OFF can reduce its dissipation compared with linear power control in a suitable circuit. Plant-wide savings from a drive depend on load characteristics, production requirements and operating time. Reducing fan or pump speed to match demand does not have the same outcome as running a conveyor at fixed speed. Likewise, speed control and torque control address different process needs.

Frequently asked questions

Does 50% duty cycle mean an AC motor runs at 50% speed?
No. The slider percentage describes the ON portion of a simple DC switch. AC motor speed relates to the drive's fundamental output frequency, the motor's pole count and slip under load.

Is 12 V at 50% PWM the same as a constant 6 V?
The simple pulse signal has a 6 V average, but its effect depends on the load. For example, a 10 Ω resistor dissipates 7.2 W on average with 12 V pulses at 50% duty, versus 3.6 W with a constant 6 V. Average voltage and power dissipated by a load must not be equated.

Is a higher PWM frequency always a better drive?
No. Higher frequency may change audible whine, but it can increase switching loss and temperature. Cable conditions and EMC emissions also matter.

Editor's field tip

Before changing carrier frequency to address motor sound, check the drive manual for current derating, ambient temperature and motor cable limits. A quieter setting is not necessarily better for the power stage. If the aim is to change shaft speed, examine the frequency reference and the chosen control method separately.