Rotary Phase Converter

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
I've seen where most here suggest sizing a RPC by doubling the HP of the load. For instance, a 15 HP motor would need a 30 HP RPC. I have a customer who bought a 15 HP 3Ø table saw and needs a RPC to convert his 240V single phase power.
I talked with a local shop that sells RPC and they said they don't size them this way. They said if the motor was the only load they just go up one size, 20 HP in this case. I know little about these things and I don't want to get something that won't work for my customer.
What is the consensus here about sizing a RPC?
 
I've seen where most here suggest sizing a RPC by doubling the HP of the load. For instance, a 15 HP motor would need a 30 HP RPC. I have a customer who bought a 15 HP 3Ø table saw and needs a RPC to convert his 240V single phase power.
I talked with a local shop that sells RPC and they said they don't size them this way. They said if the motor was the only load they just go up one size, 20 HP in this case. I know little about these things and I don't want to get something that won't work for my customer.
What is the consensus here about sizing a RPC?
Maybe I am misunderstanding, but it seems to me you simply size a RPC to the size of the 3 phase motor. If a RPC says its good for a 15 HP motor wouldnt it be for just that? Perhaps you are mixing up the 2X rule to estimate the single phase current draw from the three phase value?
 

hillbilly1

Senior Member
Location
Atlanta,Ga
Occupation
Field coordinator/ technical support
The paperwork that came with one I just installed said 150%, which would have been nice to know when I ordered it. Talked to the manufacturer, and they said 200%. Cheap 7 hp Chinese vacuum pump. The thing sucks some current starting up. Also had to turn off the phase imbalance protection in the smart starter.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
Occupation
Field coordinator/ technical support
Maybe I am misunderstanding, but it seems to me you simply size a RPC to the size of the 3 phase motor. If a RPC says its good for a 15 HP motor wouldnt it be for just that? Perhaps you are mixing up the 2X rule to estimate the single phase current draw from the three phase value?
That’s what I thought too, but according to the phase converter instructions, they said 150%.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
That's kinda odd. So if I want to run a 10 HP motor, I don't buy a 10 HP phase converter? Who came up with this system?
I think it started when people first used ordinary three phase motors idling as RPCs.
If you wanted to put an ordinary three phase motor on a single phase and spin it up with a single phase starting motor to server as an RPC you would need a 50 HP idler to run a 25 HP motor.
So the metric for designation of an RPC became the HP of the idler motor it was equivalent to.
Just speculation....
 

GoldDigger

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Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I just looked at an American rotary chart, and their part numbers have a number in them (it doesn't say HP or horsepower or anything) that is twice the three phase motor HP size
To the extent that motor size correlates with HP, you might speculate on the correlation of ROTO nominal size with underlying "motor" horsepower of the ROTO operated as a motor.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
Occupation
Field coordinator/ technical support
The specs on the first one I got said it would run a motor up to x amount of amps, which was more than the motor amps of my load. It said high torque motor loads may require a larger rotophase converter. So a 5 hp converter could theoretically start a 5 hp motor, but if the load requires high starting torque, then a larger converter would be required. So it would start and run a 5 hp motor as long as it’s unloaded when it’s started. So in a round about way, it’s really false advertising.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
UPDATE & QUESTION
Seems the customer either had a communication brake-down with me or just got "antsy" to play with his new toy. He bought a RPC but it is only a 20HP. The seller/dealer says it will work, we'll see!

My question is, do I size the OCPD for the RPC or load?

The nameplate on the RPC motor says:
Volts: 230/460
HP: 20
Amps : 50/25
..............................................................................................
Nameplate on the equipment
Main motor: 15 HP 220V/440V
Feed motor: 2 HP 220V/440V
Full-Load 43A @ 220V

Surely you don't size it for both added together do you?
 

gar

Senior Member
210914-2353 EDT

What is a rotary phase converter? It is what might be called a strange transformer.

RPC are usually made with a non-synchronous induction motor. Have never done a detailed analysis of how one works, But I believe a synchronous motor would make a better rotary converter,

But let us do a simple analysis of what an RPC does. It is a 3 phase motor run as a single phase motor. For simplicity consider a wye motor as the RPC and a wye load. Two single phase wires supply one phase of the RPC. Some way is needed to start the RPC, usually a capacitor. You would start the RPC with no load. Thus, an RPC can be almost any size for starting.

Note that regular induction motors do not run at synchronous speed. A starting capacitor may not be optimum for best operation under RPC full load, So either a less than optimum capacitor is used for starting, capacitance is changed once started. You would start an RPC under no load.

When a motor load is connected to the RPC, then current for two legs of the load come directly from the single phase source, and do not have anything to do with the current the RPC has to supply the third phase of the load. So the RPC does not have to supply all the power to the load.

If your load does not require the full power capability of its motor, then you might run that motor as a single phase unit, only require a third phase for starting. Here a simple capacitor may be adequate.

This should be enough you you to think about the application.

.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
I haven't seen any mfg instructions as of yet, I only saw the nameplate. The customer asked me to stop by and look at everything. He was not there, so all I could do was look at the equipment. I was pretty sure, since the RPC was only supplying the 3rd leg and the single ph source was feeding through the RPC, that I would size it only for one of the two. But I wanted to make sure I did everything right. I have been involved in connecting a RPC but I just hooked up what was there already.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Ronk lists the size of their rotary phase converters in kVA and the kVA rating is just a bit higher than the motor horsepower they are intended to be used with. For example a 5.5kVA converter for a 5 hp motor, or a 21 kVA for a 20 hp motor.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
Occupation
Field coordinator/ technical support
I was just at a job site yesterday where I installed one, and could have posted the info so you would have a heads up on what you need, but unfortunately I will not be back out there again until next week. Sorry!
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
I was just at a job site yesterday where I installed one, and could have posted the info so you would have a heads up on what you need, but unfortunately I will not be back out there again until next week. Sorry!
I most likely won't be installing it until sometime next week, maybe later depending on how much of my work load I get done.
 
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