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Old 01-13-16, 02:29 PM   #29
WyrTwister
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Quote:
Originally Posted by oil pan 4 View Post
You don't want to run VFD on single phase.
Typically for high starting torque motors you use capacitor start. Capacitor start uses a mechanical centrifuge to turn the start circuit on and off. During run up the start circuit does not cut out until the motor is almost at operating speed.
If the motor runs for more than 5 or 10 seconds with the start circuit powered up it will fry something in the start circuit, usually the capacitor or the windings.
If you drop the motor speed down too low the start system will reengage, frying the starter.

This is why industrial pumps are pretty much always use 3 phase inverter duty rated motors connected to VFDs. The motors and what ever they drive can be soft started and ran at variable speed, usually any speed between 20hz and 60hz. Some VFDs can produce 100hz.
You can get a 3 phase VFD that can be powered single phase 240vac power.
Not sure if single phase VFD's are made ?

You are right , some single phase motors have centrifugal switches , but not all do . The submersible pump motors do not , there is a relay in the " box " that senses when it is appropriate for the start winding to be turned off .

Will have to check , but I think single phase A/C compressors do not use centrifugal switches .

Most single phase motors have capacitors .

With motors larger than a few hp , 3 phase is almost always better . It is certainly simpler .

With a VFD , the power AC is rectified to DC . The electronics then convert it back to a semblance of AC . Usually 3 phase . The VFD could be built to be feed single phase or DC from a source like wind turbine or PV cells .

God bless
Wyr
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