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Originally Posted by oil pan 4
I have been studying and testing soft starting single phase motors.
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Thanks for sharing - I am just making assumptions so far
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The only way to soft start a single phase motor with a load attached using constant frequency power is to reduce starting voltage down to around 220v with a transformer then quickly ramp up to full 240+ volts.
Using this sequence on an air compressor with an auto transformer I was able reduce starting amps by about 20%.
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My load is a centrifugal pump, so it does not have a lot of load on start. There is not much water moving until you hit 30 - 35 Hz. I'm more concerned with the startup current that the motor wants, with a slip of 1800 rpm to begin with instead of maybe 60 rpm at running speed.
I am hoping that I can start at about 0.5 Hz .. since that seems to be where all of my VFDs start for some reason .. and ramp up to 60 Hz in the 2 - 5 second range. The capacitor start that is part of the sump pump will still shift the phase, I hope, and give some starting torque. But the speed switch (I think that is what deactivates the capacitor start) should switch the circuit to 'normal' single phase induction motor after maybe 20 Hz?
At a buss voltage of only 48V on a lead acid pack, I may not be able to run the sump pump at 60 Hz. But I need to get at least high enough to disengage the capacitor start, and to actually move some water. And of course I need to limit the current on the motor to the full load amps. It will be interesting, and I will no doubt learn (or perhaps re-learn) some stuff. And I will be using a sump pump that has bad seals and that overheats from previous abuse ... so if I mess it up, it's not much of a loss ....