Motor De-rating with Rotary Phase Converter?

TwoBlocked

TwoBlocked

Senior Member
Location
Bradford County, PA
Occupation
Industrial Electrician
Never dealt with a RPC before. Got a customer that picked up a 3-ph wide belt 15 HP sander and a RPC rated for starting a 15 HP motor. From what I understand, the voltage of the two utility legs will continue to be 180 deg out of phase from each other, and the manufactured leg voltage will be somewhere around 90 deg from each utility leg.

In my way of thinking, the motor would have to be derated because the voltage phases will not be symmetrical. But I concede that perhaps the current phases will be symmetrical, and de-rating is not required.

Any gurus out there know the real answer? I did some searching, but am not sure I understand this.
 
The RPC's output resembles a high-leg delta, with roughly 120-deg. timing, like a real delts.

The 180-deg. L-L 1ph voltage and the 120-deg. L-L-L 3ph voltages occur simultaneously.
 
What Larry said.

The single phase legs are 180° apart and the high leg is at 90° with respect to the grounded conductor of the supply. But all 3 legs are at 120° relative to the (virtual) neutral of the three phase system.

ROCs are not perfect at synthesizing that high leg, so I'd expect some imbalance and increased heating, but not much.
 
LarryFine said:
The RPC's output resembles a high-leg delta, with roughly 120-deg. timing, like a real delts.

The 180-deg. L-L 1ph voltage and the 120-deg. L-L-L 3ph voltages occur simultaneously.
Click to expand...
Here is how I understand it and how a pic of the wiring I saw also is. So I don't understand how the L1 and L2 voltages can be 180 deg apart and 120 deg apart at the same time:

1751233355316.png
 
TwoBlocked said:
Here is how I understand it and how a pic of the wiring I saw also is. So I don't understand how the L1 and L2 voltages can be 180 deg apart and 120 deg apart at the same time:
Click to expand...

Maybe this picture will help. The key is that all voltage measurements (including phase angle measurements) are relative to your selection of reference point.RPC phase angles.png
 
TwoBlocked said:
Here is how I understand it and how a pic of the wiring I saw also is. So I don't understand how the L1 and L2 voltages can be 180 deg apart and 120 deg apart at the same time:
Click to expand...
Think of a rotary phase converter as if it is a rotary Tee connected transformer that gives you a wild leg output.
There is no wye connection.
 
TwoBlocked said:
Really? Can anyone confirm this?
Click to expand...

RPCs are just ordinary induction motors with electronics to get them to start on a single phase supply. Induction motors act as generators, and if you give a _spinning_ motor single phase power in, it will synthesize/generate the 3rd leg.

I believe some RPCs are rated by the HP rating of the idler motor used. Some others might be rated for the size of the motor they can run. I've not bought one so I can't confirm if one or the other type of rating is more common.
 
TwoBlocked said:
Really? Can anyone confirm this?
Click to expand...
www.americanrotary.com

Sizing Chart - American Rotary

The following phase converter wizard has been used by American Rotary Engineers to successfully size tens of thousands of phase converters. For additional sizing assistance call 1-888-743-6832. *If your power rating is different than horsepower (HP), use the below calculations (or see video...
www.americanrotary.com www.americanrotary.com
 
hillbilly1 said:
www.americanrotary.com

Sizing Chart - American Rotary

The following phase converter wizard has been used by American Rotary Engineers to successfully size tens of thousands of phase converters. For additional sizing assistance call 1-888-743-6832. *If your power rating is different than horsepower (HP), use the below calculations (or see video...
www.americanrotary.com www.americanrotary.com
Click to expand...
Hmmm ... Fortunately, the nameplate on the customer's yet-to-arrive RPC says it will start a 15 HP motor, and 45 hp total load. (Or was that additional?)
 
TwoBlocked said:
Found this video that explains it as you fine folks do. I think I see a light at the end of the tunnel. Hope it ain't a train...

Click to expand...
OK, I got it. Since the manufactured leg is 1.73 times the voltage of the average voltage (neutral) of the utility legs, and the peak voltage of the manufactured leg coincides with the zero potential difference of the utility legs, then the zero crossing of the voltage curves between the manufactured legs and the utility legs is 120 deg instead of 90 deg.

I hadn't realized the manufactured leg was at a higher voltage than the utility legs, with respect to neutral.

It reminds me of using an RC circuit to trigger an SCR.
 
A 1ph L-L load's voltage reference is the neutral, whereas a 3ph load's voltage reference is the center of the triangle, regardless of whether an actual neutral exists or its location.

By that, I mean it doesn't matter whether the source is a grounded or ungrounded wye, a high-leg, corner-grounded, or ungrounded delta, etc.
 
TwoBlocked said:
Since the manufactured leg is 1.73 times the voltage of the average voltage (neutral) of the utility legs . . .

I hadn't realized the manufactured leg was at a higher voltage than the utility legs, with respect to neutral.
Click to expand...
That's what I meant in post#2 about it resembling a high-leg delta, and it's only because of the choice of which conductor of the source is grounded.
 
TwoBlocked said:
Hmmm ... Fortunately, the nameplate on the customer's yet-to-arrive RPC says it will start a 15 HP motor, and 45 hp total load. (Or was that additional?)
Click to expand...
That’s what Grainger told me on my first one, they were wrong! Hopefully what they are telling you is correct! The machine manufacturer came back, and said, yeah, it has to be double. The one they sold me was cheaper anyway. 60 will probably be too small, don’t forget the single phase current will be more than a third more than the three phase current. The 100 may handle it, if it’s not too far from the service when allowing for voltage drop.
 
TwoBlocked said:
I figure on powering this with a 60 amp breaker as it will only be running the 15 HP motor. I think he has 100amp service in his shop. He won't be running the whole shop at any one time.
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You would need at least a 60 amp breaker for a 15hp motor if your supply was 3 phase. Since its single phase your probably going to need at least a 100 amp breaker.
 
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