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Nick Hulme	29/05/2018 15:50:13
750 forum posts 37 photos	Posted by not done it yet on 29/05/2018 11:12:46: Clive, I am sure that VFDs do not change the supply voltage. I may be wrong, but don’t think so - they alter the frequency - it is in the name and that seems to say it all. Also, reducing the motor power was the requirement from the start of the thread. Some Vector Drive VFDs increase current (and thus voltage) at any speed when RPM drop is detected in order to maintain the set speed
Andrew Johnston	29/05/2018 15:50:49
7061 forum posts 719 photos	Posted by not done it yet on 29/05/2018 11:12:46: Clive, I am sure that VFDs do not change the supply voltage. I may be wrong, but don’t think so - they alter the frequency - it is in the name and that seems to say it all. Also, reducing the motor power was the requirement from the start of the thread. A VFD doesn't change the supply, ie, input, voltage but they most certainly can, and do, change the output voltage. The output of a VFD is actually a PWM waveform, not a sine wave per se. The peak to peak PWM voltage is fixed, usually the same as the rectified mains. The width of each sequential pulse varies, in order to create a waveform with a given frequency. However, a uniform percentage change of width across all pulses will leave the frequency the same, but will change the output voltage. Clive is correct to say that as the frequency reduces the VFD can also reduce the effective applied voltage. This is needed as less voltage is required to drive a fixed phase current, as the back EMF is lower as the motor is spinning more slowly. Andrew

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