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Old 02-18-17, 10:31 AM   #13
oil pan 4
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Join Date: Nov 2012
Location: NM
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I will do 3 phase eventually because I can score a free good motor from work. Anything smaller than 10hp goes in the recycling bin and doesn't get sent off to be rebuilt.
The 3 phase VFD units are between $350 and $450 for what I would need.
I have added an unloader valve on a time delay to keep the compressor unloaded until it's up to speed and to possibly unload it before shut down if I chose to do so.

The key to soft starting a single phase motor is to start the motor unloaded, as in not coupled to anything and on a resistive starter.

Then engage the load swiftly but not quickly enough to drag motor RPMs down.
I have already tested engaging the stopped compressor to a motor running at full speed. If I jerk the slack out of the belt the motor will surge will far exceed soft start and running amps at shut off.
If something like a magnetic clutch is used that instant on may slow the motor enough to reengauge the start circuit, potentially making amps even higher than what my simple belt pull test showed.
If I tension the belt more gradually I can keep the motor amps less than the start surge and equal to or slightly above shutdown amps.

Also I got the temco 400-480uf start capacitor in, to replace the original 594uf start capacitor. It is replicating the results of my 2 capacitor test, where the reduced start capacitance slightly reduced, stabilized and brought the motor up to speed faster than the 594uf capacitor did.
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