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VFD Driven Motor Applications

Variable Speed Applications

A variable frequency drive allows a customer to change the motor’s operating speed by changing the frequency (Hz) of the power supplied to the motor. While this is convenient for the customer, it causes several problems for the motor and special consideration must be given to the motor’s ability to operate on various frequencies.

  • Motors operated below their approved turn down ratio do not move enough air to adequately cool the motor. This will cause the motor to run hot and decrease the motor’s life.
  • Motors operated above 60Hz will experience decreased torque output. Additionally, care must be taken to ensure the motor, and all mechanical devices driven by the motor, are able to withstand these higher RPMs. Please contact your TECO-Westinghouse representative for the maximum safe operating RPM of your motor.

Effects of Harmonics on Induction Motors

Power supplied by (PWM) variable frequency drives contain harmonic content in the electrical waveforms they generate at various frequencies. The harmonic content and non-sinusoidal waveforms generated by the drive can cause the following problems:

  • High harmonics in the waveform increase the motor’s “hot-spot” causing the motor to run hotter than it would under normal conditions.
  • Voltage spikes at the motor terminals resulting from harmonics & reflective wave can exceed the rated peak voltage of the motor winding. The magnitude of the voltage spike and rise time is a function of the inverter switching speed (carrier frequency).
  • Harmonic frequencies generated by the VFD could excite resonance in the motor frame resulting in increased noise and vibration levels.
  • Corona effect causing partial discharge on the motor windings leading to insulation breakdown.
  • Capacitive discharge from the motor shaft to ground causing pitting of the bearings leading to bearing failure.

Motor Insulation System & Supply Voltage

The motor insulation system for VFD approved TECO-Westinghouse motors is based on and exceeds NEMA
Standards. As defined in NEMA MG-1 Part 31.4.4.2 the insulation system must be capable of withstanding voltage spikes of 3.1 x rated line to-line voltage for motors rated ≤600V and 2.04 x rated line-to-line voltage for motors rated >600V with a rise time of ≥ 0.1μs. TECO Westinghouse motors rated ≤600V have been designed to withstand voltage spikes up to 2000V with a rise time of ≥ 0.1μs. Voltage spikes and rise time should be measured at the motor terminals and should not exceed the above given values.

Cable Length & Type

Cable length and type connecting the motor and drive have a significant influence on the peak voltages occurring at the motor terminals.

Cable Length

The following conditions should be met when installing TECO-Westinghouse motors on an inverter: