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ABC’s of VFD’s Installation Need to Know (Part 2)

by Paul Figie, Application Engineer, EZAutomation

VFD’s PWM Technology

Most VFD’s today take advantage of high speed microprocessors to create the waveform output necessary to drive the motor being controlled. This is accomplished by switching outputs on and off at varying duty cycles called Pulse Width Modulation or PWM. The speed or frequency at which output is switched must be much greater than the frequency of the waveform that is being created this is called the modulation or carrier frequency. Depending on the output device configuration used in this switch process the carrier frequencies are generally between 1-20 KHz. The figure below gives an example of the microprocessor generated PWM and the waveform it creates.

Generating PWM signals to control varying output frequencies has become very practical with the advancement of today’s Microprocessors. Microprocessor Controlled VFD’s have become very sophisticated in their control capabilities but not without some drawbacks. The high speed switching to create the output to the Motor generates Electromagnetic Interference (EMI). This EMI noise is inherent to all VFD’s using PWM and can be extremely problematic in your control system if extra steps are not taken to minimize it. Proper VFD installation is extremely important not only for the VFD and motor but also all other electrical devices in you control system. Improper VFD installation can not only cause damage to your motor and drive, but could cause erratic unpredictable and potentially dangerous conditions to surrounding electrical control devices.

1. VFD - Read the VFD Manufactures Drive Installation procedures. Pay close attention and follow all of the recommendations. This should include:

2. Motor - Use Inverter Duty Motor type. Check that you are not exceeding manufactures performance recommendations.

3. Surrounding Equipment - VFD’s not only transmit electrical noise in proximity to the drive itself but also along the wiring run to the motor.