Data Sheet for Product

Technical Bulletin – TB255 Motor Performance on PWM/IGBT AC Drives

Document Number 155-743

August 29, 2018

Page 2 Siemens Industry, Inc.

Effects of PWM

Drives on Motors

(Figure 2)

PWM is the method by which the IGBTs are modulated to produce the fundamental

voltage and frequency to the motor. The IGBTs are pulsed on and off rapidly (100 to 200

nanoseconds) to produce a carrier frequency. The carrier frequency travels over the

motor cables, which are acting as transmission lines, to “carry” the appropriate voltage

and frequency to the motor.

VOLTAGE CURRENT

TIME

Figure 2. Switching frequencies of 2, 4, 8…16 kHz are Common.

When the IGBT is switched, a voltage edge (leading edge) is created that also travels

along the motor cables. Because of an impedance mismatch between the inverter end

and the motor end of the cable, some portion of the waveform is reflected back toward

the drive. If this reflection coincides with another leading edge, the two will be added

together. In some cases this “reflective wave” phenomenon can cause high voltage levels

at the motor terminals, as high as 2.5 times the DC bus voltage of the drive.

Table 1. Motor Voltages.

Supply Voltage

Typical Peak Motor Voltage Levels

Maximum Peak Voltage *

230

575

810

460

1150

1610

600

1500

2100

* Actual cable length, cable type, motor inductance and switching frequency can all have

an effect on peak voltages.

Motor insulation stress can be caused by both the high voltage change with respect to

time (dV/dT), and the high peak voltage caused by the reflective wave. This insulation

stress can result in a phase-to-phase or phase-to-stator short in the first windings of the

motor, due to the motor’s high inductance and winding capacitance causing the voltage to

dissipate rapidly. These types of voltage-induced insulation failures may occur very

quickly, sometimes in as little as a few weeks of operation.