Motor Insulation Systems (6)

Carrier Frequency
As the inverter’s carrier frequency is increased, the output current waveform supplied to the motor becomes more sinusoidal. This improved output current waveform decreases motor heating thus improving motor insulation life.
At this higher carrier frequency, however, more individual voltage pulses are output and for a given cable length, rise time and motor surge impedance, the potential for voltage overshoot increases. The power generated during this overshoot will be dissipated in the motor’s windings, and insulation life will be decreased.

Figure 5, below, shows insulation life of a generic motor, when both cable length and inverter carrier frequency (fc) are varied. Note that with a 150 ft. cable length, insulation life drops from 100,000 hours to 25,000 hours, when carrier frequency is increased from 3 to 12 Khz. The longest life occurs with short cable lengths and low carrier frequencies.

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