three phase on single phase line

[fixbear]

To answer your question, Yes.  They need more than a set of capacitors. unless it was built to match the motor exactly. And the load is constant.  Be aware that motor load has to be downrated 20%  

 

If you have a varying load or multiple motors you would need a Roto-phase.  Home made ones are very common.  Usually there will also be a idling motor and some way to to start it spinning.  Like a smaller single phase motor or a rope.It does not make true three phase. The idling motor has to be equal in size or larger that the largest one on the system.  

 

It can be done with today’s VFD drives.  And it will be a true square wave to boot.  I actually used one to stabilize a whole  de-humidification kiln plant due to one phase being off 4 degrees.  My local power company paid for it.

[izzygreen]

I’ve seen the roto phase and I’ve used a VFD but the capacitor box is new to me. I need more information about them before insisting to my customer that he needs to have a proper 3Ø line installed to prevent his new grinder from failing the way his old one did.

 

Thanks

[fixbear]

The capacitor box has to be designed and used with a very specific motor. Pre 1970’s motors were no big deal as they were built with a wider range, thicker winding’s,  and a higher power factor. Then there was again a spec change in the late 80’s   Today we have draw presses that can make finer wire.  And motors are built closer to load spec due to better engineering. ie; computers.  And energy efficiency requirements.   You can not have any auxiliary loads like control on the cap system.  A motor replacement would involve a redesign of the cap system.  It does not make true 3 phase power,  nor does it support full load of design. The load will also pulse and make vibration with the cap system or Rotophase. It will also void any manufacturers warranty by using one. 

 

I used to see finish marks from the phase pulse with many of my machine tools.  Especially my Bullard lathe.

 

If this is a new replacement machine,  Install a VFD with it as the controller. .If you didn’t, you would still have to replace te motor controller for proper overload protection. And that’s more than half of today’s cost of a VFD. 

[izzygreen]

Thanks

[fixbear]

Maybe I should explain what is happening in the motor. 3 phase motors have a basic set of 3 or multiples of 3.  L1, L2, L3 inputs. Now the motor can be wound two ways.  Wye or Delta. Delta is the normal way. So L1 would connect to winding one and three.  L2 would connect to winding one and two. And L3 would connect to winding  two and three. Multi voltage motors split each set of winding’s to either parallel them for low voltage or series them for high voltage. .The incoming phase voltage has a angle of 120 degrees. So at any given time during the cycle you have a plus voltage, a mid or zero and a minus voltage. and they continue to go up and down with time. 60 cycles per second in the USA.  Often 400 cycle on high end electronics like cellular control systems.  Now if one phase has a voltage difference or phase angle difference the motor changes speed as it is running and one leg will show higher amps than the other two. And will eventually cause a winding overheat, shake, and insulation brake-down.  So anything driven by the motor will have a vibration.  I’ve seen it actually break the fins loose in a squirrel cage blower. 

 

It wasn’t till I bough a triple oscilloscope that i got to actually see the problem in front of me.  I had been fighting with my customers power company for 3 years.  And had gotten to know their meter and test tech quite well. He also had been trying to find the problem with me.  Eventually NIMO brought a recording antalizer out and put it on the system for a week. I got him to give me a set of readings with phase angle.  Power company kept insisting that it was us creating the problem.  Yet, when I went on generator it went away.  We finally got a meeting with the engineers,  division brass, and meter and test.  I went over there whole grid from the substation to us. Being the only 3 phase customer 15 miles from the sub-station, they didn’t have equal transformers and capacitors on the phases. (Inductance moves phase angle back,  capacitance moves it forward)   This amplified it to me.  But I said to them that it was phase angle.  And they continued to deny for 3/4 of a hour till I showed them on a graph the phase 2 was 4 degrees behind where it belonged.  Meter and test stopped the arguing and said I was right.  .After 2 weeks they determined that when they replaced one of the sub-station transformers, it was built different and causing the problem. That was 20 years ago and they still haven’t replaced it.  They instead dropped off a ABB VFD capable of 400 amps 480 volt for me to install at the plant service point.  Power bill dropped from the efficiency gained.  Compressor amp’s dropped, and all motors (9) smoothed out. It was quite a education on motors and power for me.