Pulse Width Modulation (PWM) is the core switching technology used inside modern Variable Frequency Drives (VFDs). It enables an inverter to synthesize a 3-phase AC voltage output from a constant DC voltage source, allowing smooth speed and torque control over induction motors.

While a VFD appears to deliver a standard AC sine wave to the motor, it actually generates a high-speed train of square DC voltage pulses.

This guide breaks down the internal physics of PWM, how duty cycle dictates voltage, the trade-offs of setting carrier frequency, and how PWM drives motor efficiency.

1. How PWM Works Inside a VFD

To understand PWM, you must look at the three internal power stages of a frequency converter:

  [ Mains 3-Phase AC Input ]
              │
              ▼
  ┌───────────────────────┐
  │ Rectifier Bridge      │  <-- Converts AC to DC
  └───────────┬───────────┘
              │
              ▼
  ┌───────────────────────┐
  │ DC Bus (Capacitors)   │  <-- Filters and stores smooth DC voltage (e.g., 560V DC)
  └───────────┬───────────┘
              │
              ▼
  ┌───────────────────────┐
  │ IGBT Inverter (PWM)   │  <-- Switches DC on/off thousands of times per second
  └───────────┬───────────┘
              │
              ▼
  [ Synthesized AC Current to Motor ]

The Pulse Width Principle

Instead of altering the amplitude (height) of the voltage, the VFD’s Insulated Gate Bipolar Transistors (IGBTs) switch the full DC bus voltage on and off at high frequencies.

Duty Cycle: The ratio of “ON time” versus “OFF time” within a single switching period.
Low Speed / Low Voltage: The IGBTs stay ON for short periods and OFF for longer periods (low duty cycle).
High Speed / High Voltage: The IGBTs stay ON for longer periods and OFF for shorter periods (high duty cycle).

Because the motor’s stator windings possess high inductive reactance, the motor filters out the high-frequency voltage spikes and draws a smooth, sinusoidal AC current wave.

2. Key PWM Parameters & Performance Trade-Offs

When commissioning a VFD, adjusting PWM settings directly impacts system efficiency, thermal loss, and audible motor noise.

Carrier Frequency (Switching Frequency)

The carrier frequency (typically adjustable between 2 kHz and 16 kHz) determines how many times per second the IGBTs switch.

Carrier Frequency Setting	Advantages	Disadvantages
Low Carrier Frequency (2 kHz – 4 kHz)	Minimal thermal loss in VFD IGBTs, lower EMI noise, longer allowable motor cable runs	Loud audible high-pitched motor “hum”, slightly higher motor harmonic heating
High Carrier Frequency (8 kHz – 16 kHz)	Completely silent motor operation (above human hearing range), smoother low-speed torque	Higher VFD heat generation (requires current derating), severe dv/dt insulation stress on motor

3. How PWM Optimizes Energy Efficiency

VFDs equipped with PWM technology deliver energy savings on industrial machinery compared to traditional direct-on-line (DOL) or mechanical throttling methods:

A. Quadratic Torque Savings on Pumps and Fans

According to the Affinity Laws, flow rate is directly proportional to motor speed, while power consumption scales with the cube of the speed (P proportional to N cubed). By using PWM to drop fan or pump speed by just 20%, power consumption drops by nearly 50%.

B. Dynamic Voltage-to-Frequency (V/f) Matching

PWM continuously maintains an optimal V/f ratio across the operating range:

At 50 Hz, the VFD outputs 100% voltage (e.g., 400V).
At 25 Hz, PWM automatically scales the average effective voltage down to 50% (200V), preventing core saturation and reducing iron losses in the motor.

4. Engineering Checklist for PWM Setup

When configuring VFD PWM parameters in the field:

Audible Noise vs. Drive Heating: If the motor is located in an unoccupied plant room, keep the carrier frequency low (2 kHz–4 kHz) to keep the VFD cool. Increase it to 8 kHz–12 kHz only if operating in quiet commercial environments (e.g., HVAC in office buildings).
Check Cable Distance Derating: If your motor cable exceeds 50 meters, lower the carrier frequency to reduce high-frequency capacitive leakage current and prevent drive overcurrent trips.
Enable Over-Modulation with Care: Over-modulation allows the VFD to output higher effective AC voltage near top speed, but it introduces extra low-order harmonics into the motor.