Four smaller motors to make a RPC, use nameplate amps or kw?

[Brian Campbell]

I am building a rotary phase converter using four smaller (2.3kw) wye motors to convert the two legs and neutral I have coming into the property into three legs and a neutral.



I live in New Zealand, on some rural properties we have two legs and a neutral coming in but are missing the third leg, I am building a rpc to generate the third leg. Our power supply is 230v phase to neutral and 415v between phases.



I need to run a 7.5kw motor on a lathe that has a full current draw of 14a



The question I have is if the rpc has enough capacity to do this using four 2.3kw motors.



The total of the four idler motors in kw is around 9.2kw, looking at it this way it seems undersized.



However, If my math’s is correct the two legs I have coming in will need to provide 21 amps each to supply the motor (3 phases x14amps = 42) (42 amps /2 phases =21 amps)



If I look at the full load amperage of all four motors combined, it is around 23amps. i.e the four motors can handle the amp draw needed by the big lathe motor.



What I’m unsure of is how the lower efficiency of using smaller motors comes into this, the motors can handle the amps, but does this translate to being able to provide the larger lathe motor with what it needs?

 

[rons]

When I was trouble-shooting some RPC and motor stuff, I noticed that motors always made different sounds when placed on end versus horizontal.
It did work out that a problem in the horizontal position was made better by setting the motor vertical. In those cases I could adjust the spring washer
thickness and get the quietest sound from a horizontal position.

What you have will probably work. See how man yes votes you get.
When you look at all the controls you have there, easier just to get a 15Hp motor and recycle 2 out of the 4 others.

 

[rons]

When I was trouble-shooting some RPC and motor stuff, I noticed that motors always made different sounds when placed on end versus horizontal.
It did work out that a problem in the horizontal position was made better by setting the motor vertical. In those cases I could adjust the spring washer
thickness and get the quietest sound from a horizontal position.

What you have will probably work. See how man yes votes you get.
When you look at all the controls you have there, easier just to get a 15Hp motor for the RPC and recycle 2 out of the 4 others. You still have 2 spares.

Or, considering your good work already. Take advantage of what you have. Only turn on enough little motors that your need in the day.
Design a circuit that detects current and controls motor activity.

 

[jim rozen]

The FLA numbers for the idlers do not mean much. You want the total idler HP rating to be at least 1.5 times the load motor horsepower. Like most things in life, bigger's better.

 

[JST]

The 1.5x oversize in HP is really just a simple way to assure getting more amps capability, and lower series impedance (less voltage drop).

You can figure the amps, or use the HP (kW rating). Using the kW is simpler, and works fine.

If you think the motor might be hard to start, use a 2x factor instead of 1.5x.

 

[Brian Campbell]

Does the x 1.5 thing still apply to my situation as i already have 2 out of the three phases and a neutral? i.e im already 2/3 the way there

The way I have been looking at it is the lathe motor needs to draw 21 amps through the two phases I do have, the smaller motors combined can handle this amp draw, im just not sure how efficiency comes in to this.

Smaller motors are less efficient than larger motors i.e put a certain amount of amps in and get a certain kw of power out.

But are they actually less efficient when used as phase converters? i.e put a certain amount of amps in one phase and get it out the other phase?

 

[JST]

That's where it DOES apply.......

Yes, efficiency works both ways, although it normally is not a huge deal for RPCs. But most RPCs are one bigger motor.

You can turn on only as many as needed, that helps efficiency.

 

[jim rozen]

"Does the x 1.5 thing still apply to my situation as i already have 2 out of the three phases and a neutral?"

Yes.