Why does my motor run hotter on a VFD?

Two reasons. The PWM output adds current ripple, which typically adds a few percent up to around 10 percent extra heating compared with a pure sine supply. And at low speed a self-cooled motor's shaft fan moves less air, so at full torque below about 25 to 30 Hz it needs derating or a forced cooling fan.

Pulse width modulation. The VFD switches its DC bus on and off thousands of times a second, making square voltage pulses whose width follows a sine wave reference. The motor's inductance smooths these pulses so the current is close to a sine wave. The output voltage itself is a train of pulses, not a sine wave.

What carrier frequency should I use?

Start with the drive's default, often 4 kHz. A higher carrier (8 to 16 kHz) makes the motor quieter but increases drive losses, so the drive may need derating. A lower carrier (2 to 4 kHz) runs the drive cooler but the motor is noisier. Long motor cables usually call for a lower carrier.

Do I need an inverter duty motor?

For most modern IEC motors up to 500 V supplies, standard insulation copes with VFD voltage spikes if the cable is not too long. Use a motor rated for inverter use (IEC TS 60034-17 / IEC 60034-25) for higher voltages, long cables, low speed at full torque or 87 Hz operation, and fit output reactors or dV/dt filters on long cables.

Do self-cooled motors need derating at low speed?

Yes. The shaft fan slows with the motor, so a self-cooled motor at full torque typically needs derating below about 25 to 30 Hz. A separately powered forced cooling fan removes this limit.

How a VFD Works: PWM Output, Motor Heating and Derating

A VFD (variable frequency drive) turns the fixed mains supply into a variable frequency, variable voltage output made of fast PWM (pulse width modulation) pulses. Here is how it works, what it does to the motor, and when the motor needs derating.

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Terms used on this page

VFD, PWM, AC / DC, DC bus …

VFD: Variable frequency drive. An electronic box between the supply and the motor that controls motor speed by changing the frequency (and voltage) it feeds the motor. Also called an inverter, AC drive or VSD (variable speed drive).
PWM: Pulse width modulation. Switching a fixed DC voltage fully on and off in very short pulses, making each pulse wider or narrower so that, on average, the output follows a sine wave.
AC / DC: Alternating current (the mains supply, which reverses direction 50 or 60 times a second) and direct current (flows one way only, used inside the drive).
DC bus: The DC link in the middle of the drive, where large capacitors store and smooth the rectified voltage.
IGBT: Insulated gate bipolar transistor. The fast electronic switch in the inverter stage. A typical drive has six, two per motor phase.
Carrier frequency: How many times per second the IGBTs switch, typically 2 to 16 kHz (kilohertz: thousand times per second).
V/f: Volts per hertz. Keeping the voltage in proportion to the frequency keeps the motor's magnetic flux, and so its torque, constant.
Back-EMF: Back electromotive force. The voltage a spinning motor generates itself, which opposes the supply. The current is set by the difference between the two.
dV/dt: How fast the voltage rises at each switching edge. Very fast edges, especially with long cables, can cause voltage spikes that stress motor insulation.
Common mode voltage: A voltage between the motor windings and earth caused by PWM switching. It can push small currents through the bearings and damage them.
Field weakening: Running above the base frequency, where the drive cannot raise the voltage any further. The magnetic flux drops, so torque falls.
PTC thermistor: Positive temperature coefficient sensor built into the winding. Its resistance jumps when the winding gets too hot, so the VFD can trip and protect the motor.
IEC: International Electrotechnical Commission. Publishes motor standards such as IEC 60034 (rotating machines) and its parts on inverter-fed motors.
Class F / Class B: Insulation temperature classes: Class F is rated to 155 °C and Class B to 130 °C. A Class F motor running at a Class B temperature has spare thermal margin.

The three stages: rectifier, DC bus, inverter

Rectifier: diodes turn the 3-phase AC (alternating current) supply into DC (direct current).
DC bus: capacitors store and smooth that DC, about 565 V on a 400 V supply.
Inverter: six IGBTs (insulated gate bipolar transistors, fast electronic switches) turn the DC bus on and off very fast to build the output for the motor.

PWM (pulse width modulation): how the pulses make a sine wave current

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The drive compares a sine wave reference with a high frequency triangle carrier (typically 2 to 16 kHz).
Where the sine is above the triangle, the IGBT switches on. The result is a train of square voltage pulses of varying width.

The motor's inductance smooths these pulses, so the current is close to a sine wave.
Changing the reference frequency changes speed; changing its amplitude changes voltage. Keeping the two in proportion is called V/f (volts per hertz) control.

The output voltage is a train of PWM pulses, not a sine wave and not a sawtooth. Only the current is close to a sine wave.

Output frequency 50 Hz Carrier frequency 4 kHz

Effects of PWM on the motor

Harmonic losses: the current ripple adds extra heating, typically a few percent up to around 10% compared with a pure sine supply.
Voltage spikes: fast switching (high dV/dt, the rate at which voltage rises) and long motor cables can cause voltage peaks above the supply voltage, stressing the winding insulation. Use inverter rated motors (to IEC 60034-25 / IEC TS 60034-17, the International Electrotechnical Commission standards for inverter-fed motors), and output reactors or dV/dt filters for long cables.
Bearing currents: common mode voltage (a voltage between the windings and earth caused by PWM switching) can drive current through the bearings and damage them, mainly on larger motors. Insulated bearings or shaft grounding rings fix this.
Carrier frequency trade-off: a higher carrier is quieter but adds drive losses and derating; a lower carrier is louder but runs the drive cooler.

Motor thermal rating on a VFD

Self-cooled motors lose cooling at low speed because the shaft fan slows down. At full torque they need derating below about 25 to 30 Hz, or a separately powered forced cooling fan.
Most motors use Class F insulation (rated 155 °C) with a Class B temperature rise (130 °C), which leaves thermal margin for VFD use.
Fit PTC thermistors (positive temperature coefficient sensors in the winding) or another temperature sensor wired to the VFD, so the drive can trip before the motor overheats.

Running a 230/400 V motor in delta for more power has its own thermal checks: see 87 Hz motor operation.

Typical allowed continuous torque on a VFD (self-cooled motor)

Typical values, check manufacturer data.

Frequently asked questions

Why does my motor run hotter on a VFD?: Two reasons. The PWM output adds current ripple, which typically adds a few percent up to around 10 percent extra heating compared with a pure sine supply. And at low speed a self-cooled motor's shaft fan moves less air, so at full torque below about 25 to 30 Hz it needs derating or a forced cooling fan.
What is PWM?: Pulse width modulation. The VFD switches its DC bus on and off thousands of times a second, making square voltage pulses whose width follows a sine wave reference. The motor's inductance smooths these pulses so the current is close to a sine wave. The output voltage itself is a train of pulses, not a sine wave.
What carrier frequency should I use?: Start with the drive's default, often 4 kHz. A higher carrier (8 to 16 kHz) makes the motor quieter but increases drive losses, so the drive may need derating. A lower carrier (2 to 4 kHz) runs the drive cooler but the motor is noisier. Long motor cables usually call for a lower carrier.
Do I need an inverter duty motor?: For most modern IEC motors up to 500 V supplies, standard insulation copes with VFD voltage spikes if the cable is not too long. Use a motor rated for inverter use (IEC TS 60034-17 / IEC 60034-25) for higher voltages, long cables, low speed at full torque or 87 Hz operation, and fit output reactors or dV/dt filters on long cables.
Do self-cooled motors need derating at low speed?: Yes. The shaft fan slows with the motor, so a self-cooled motor at full torque typically needs derating below about 25 to 30 Hz. A separately powered forced cooling fan removes this limit.

Disclaimer: general guidance only. Always confirm current, speed and thermal limits with the motor and VFD manufacturers' data.

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