Variable frequency drive. - VFDs can be used to control the speed of a machine. Variable frequency drive. - VFDs can be used to control the speed of a machine. There can be significant energy reduction if the machine is slowed down when it does not need to operate at full speed. For example, pumps operated so that they run at a speed dependent on the flow requirements will reduce the cavitation losses in the pump. If the pump is required to run at constant full speed, the addition of a VFD will actually waste another 5% of energy.
The major source of problems in the real world, is conducted emissions and these are typically in the region of 20kHz to 10MHz.
The source of the emissions, is the output of the VFD which comprises 6 solid state switches that switch each output between the DC bus rails. These switchews are typically high speed IGBTs which can switch in less than 100nS.
The output waveform is a PWM waveform designed to create a sinusoidal current flow into the motor.
A typical 40 HP VFD has an efficiency in the order of 97% overall. Your comment about the rectifier being on 80% efficient is not correct.
So, at full load, we may have a VFD at 97% and the motor at say 92% (depending on size and design) and the pump (or machine) may have an efficiency of say 80% or even less, so the wasted energy in the variable frequency drive is small, The wasted energy in the motor is also relatively small. The major losses are in the driven load an that is why we target the driven load.
If our driven load has an efficiency of 80% and the losses (20% of rating) are essentially frictional, then reducing the load to half load would not alter the frictional losses, so the efficiency would drop to 66%. If we slow the speed of the machine to 70% speed, then the frictional losses are halved, so are now 10% of rating. If the same work can be done at 70% load, then the efficiency is back up to 80%.
Keywords: 40 HP VFD
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