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Technical Bulletin

Document No. 155-743

TB255

August 29, 2018

Motor Performance on

PWM/IGBT AC Drives

Siemens Industry, Inc.

Abstract

Use of PWM (Pulse Width Modulation) drives in HVAC systems has proven both efficient

and cost effective. However, their effect on motor insulation must be understood to

ensure consistent, long, and trouble free operation. This bulletin will explain PWM

fundamentals, their effects on motors, and appropriate field actions to ensure consistent,

trouble-free motor operation.

Conventional PWM

Drive Fundamentals

(Figure 1)

Conventional PWM drives consist of:

• Six-pulse diode rectifier to convert AC line voltage to a constant fixed level DC

voltage.

• DC link capacitor that acts as a filter to smooth the DC link voltage and help keep it

constant. The DC link voltage level is approximately 1.4 times the AC line voltage.

• Isolated Gate Bipolar Transistor (IGBT) inverter to convert the DC link voltage to a

variable voltage, variable frequency 3-phase output for controlling the speed and

torque of an induction motor and for providing overload capabilities necessary for

high dynamic motor performance.

• Micro-processor based controller to supervise the operation of the inverter as well as

to implement power control algorithms to obtain optimum dynamic performance from

the induction motor.

Figure 1. PWM Drive Single Line Diagram.

Rectifier

D C Link

Inverter

Three Phase

60 Hz. Power

Motor

Summary of content (4 pages)

PAGE 1
Technical Bulletin Document No. 155-743 TB255 August 29, 2018 Motor Performance on PWM/IGBT AC Drives Abstract Use of PWM (Pulse Width Modulation) drives in HVAC systems has proven both efficient and cost effective. However, their effect on motor insulation must be understood to ensure consistent, long, and trouble free operation. This bulletin will explain PWM fundamentals, their effects on motors, and appropriate field actions to ensure consistent, trouble-free motor operation.
PAGE 2
Technical Bulletin – TB255 Document Number 155-743 August 29, 2018 Effects of PWM Drives on Motors (Figure 2) Motor Performance on PWM/IGBT AC Drives 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.
PAGE 3
Motor Performance on PWM/IGBT AC Drives Minimizing Motor Failures Technical Bulletin TB255 Document Number 155-743 August 29, 2018 To offset the high dV/dT and the high-peak voltage effects on motors there are several precautions that can be taken: • Use inverter-rated motors It is best practice to always use inverter duty motors when the application is controlled by a drive. NEMA Standard MG-1 indicates that inverter duty motors be designed to withstand 1600V peak and a rise time >0.1 microsecond.
PAGE 4
Motor Performance on PWM/IGBT AC Drives Retrofit Considerations Summary Technical Bulletin – TB255 Document Number 155-743 August 29, 2018 • Installation location Select a location for the VFD that will minimize the motor cable length. The appropriate distance is best determined by the ability of the motor to operate on IGBT drives, as determined by the motor manufacturer.