# 25 HP 3Ø Motor



## Kevin (Feb 10, 2016)

Just found one for $350 - used in a no-load application in a testing facility with low cycling & low hours. Gonna replace my 10HP rotary converter with this 25 so I can run my 5Hp saw and 7.5 blower motor. I'll have to get bigger caps but pretty much all the other guts will work. Also gonna add a remote start. 


I just hope it's still there tomorrow when I go to pick it up. Will save me over a grand buying a new already setup converter. I like that.

Reactions: Like 2


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## woodtickgreg (Feb 10, 2016)

I think you should do a tutorial on the conversion in the classroom.

Reactions: Agree 1 | Useful 1


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## Kevin (Feb 10, 2016)

I will.


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## Kevin (Feb 10, 2016)

Greg I just did a quick search for 25 HP rotary converters -- I think I'll be saving a lot more than $1000!


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## Schroedc (Feb 10, 2016)

I'll be interested in seeing how this whole rotary converter thing works, I've passed up a few machines because I don't have 3 phase in my building....


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## Kevin (Feb 10, 2016)

Start looking for the idler motor now. That's the big expense. Buy it right and you can't get hurt - I could flip this one if I wanted to at this price. To size my motor, I called one of the converter companies and talked to one of the techs. I had always thought that to size your motor, you double the HP of all the machines running simultaneously. This guy told me the same thing so I guess it is right. the largest two loads that will ever run at the same time are the 5HP saw and the 7.5HP blower. You don't size them by amperage as I would have thought. So that's 12.5 HP x 2 = 25HP.

Figure your needed motor size Colin and start looking now. I found mine on CL he was asking $450 and I offered $350 and he said okay. I love using 3 phase machinery. You can't hardly bog it down.

Reactions: Way Cool 1


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## woodtickgreg (Feb 11, 2016)

I think maybe for big hp motors like you are running that rotor phase converters are the way to go. But for 1 or 2 hp ish motors vfd's are the way to go. But I am really excited to see this conversion and all that it entails.


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## Brink (Feb 11, 2016)

Kevin said:


> Start looking for the idler motor now. That's the big expense. Buy it right and you can't get hurt - I could flip this one if I wanted to at this price. To size my motor, I called one of the converter companies and talked to one of the techs. I had always thought that to size your motor, you double the HP of all the machines running simultaneously. This guy told me the same thing so I guess it is right. the largest two loads that will ever run at the same time are the 5HP saw and the 7.5HP blower. You don't size them by amperage as I would have thought. So that's 12.5 HP x 2 = 25HP.
> 
> Figure your needed motor size Colin and start looking now. I found mine on CL he was asking $450 and I offered $350 and he said okay. I love using 3 phase machinery. You can't hardly bog it down.



so, this means you'll need 12.5-25 hp motor spinning the idler motor. watts in = slightly less watts out.


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## Kevin (Feb 11, 2016)

Brink said:


> so, this means you'll need 12.5-25 hp motor spinning the idler motor. watts in = slightly less watts out.



I'm not sure what this means based on your choice of words. In a RPC the 3Ø motor is being used as a generator, it is the 3Ø power supply and it is referred to as the idler motor (because it does not have the load on it directly). There are several ways to start the idler motor in a RPC and one is by use of a smaller motor called a "pony" motor or "starter" motor and there's probably other terms for it. I don't know what you mean when you say my 25HP idler motor will _"need 12.5-25 hp motor spinning the idler motor"_. If you're referring to a pony motor should I choose that route, it can be very small y comparison. I have a 1.5HP laying here that would work just fine. I just haven't decided if I want to do that or use start caps. I'm sure not going the lawn mower rope route. I'll be cycling it quite a bit when I use it so start caps are almost certainly the way I will go. 

As to the power loss factor RPC are doubled partially for this reason (but also others such as the massive amperage spike on startup of some configurations). My 25 HP idler motor has an efficiency stamp of 94.6% so power loss isn't going to be a concern.


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## Kevin (Feb 11, 2016)

woodtickgreg said:


> I think maybe for big hp motors like you are running that rotor phase converters are the way to go. But for 1 or 2 hp ish motors vfd's are the way to go. But I am really excited to see this conversion and all that it entails.



I agree with that for sure. I did a lot more study on vfd's since we discussed them last and that field of study alone is massive. One thing that was clear though, since vfd's are not power supplies, and for a bevy of other reasons, they are not suited as a means of a single vfd for full shop 3Ø conversion for multiple machines. It wouldn't be *impossible* to pull it off but it would be like building a swiss watch when a simple sundial will fill the need.

Reactions: Agree 1


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## woodtickgreg (Feb 11, 2016)

I agree Kevin, for smaller hp motors the vfd's are cheap enough that you can put them on individual machines. And some of the things you can do with them like variable speed, motor braking, ramp up or down speed, etc. Is pretty cool.
My first experience with one sold me on the use of vfd's, now manufacturers are installing them from the factory on new machines. Again, pretty cool. But I am still very interested to watch this rotor build.

Reactions: Like 1


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## Brink (Feb 11, 2016)

you're planning on using a single phase motor to spin the 25hp three phase motor. The three phase motor will act as a generator. Correct?


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## Kevin (Feb 11, 2016)

Brink said:


> you're planning on using a single phase motor to spin the 25hp three phase motor. The three phase motor will act as a generator. Correct?


You can use a single phase motor to start the 3 phase, or you can use a rope to manually start it spinning, or you can use start caps to start it spinning. 

As you knkw a 3 ph motor will not start on 1 ph without some oitside influence to get it turning. But all those methods only serve to get the motor spinning and then are taken out of the loop once the big idler/generator is spinning and power is then applied to the idler which then generates the 3 ph power. Start caps need a timer or centrifugal switch etc to take them out of the circuit quickly or something is going to lose all its smoke. 

Try not to even think about DC power when looking at this.


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## Kevin (Feb 11, 2016)

It just dawned on me why you were asking the questions that you are. You were trying to figure out how to three-phase motor runs on single phase. It will not do so without the run capacitors. The run capacitors separate the phases so that the windings in the idler motor all get fed properly At the proper times in the rotation. There's even a way to start and run the idler with only run capacitors. 

But the run capacitors are the 'missing link' in your understanding.

Reactions: Informative 1


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## Kevin (Feb 11, 2016)



Reactions: Like 2


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## woodtickgreg (Feb 11, 2016)

Looks like you scored a nice little cross slide vise with it?

Reactions: Agree 1 | Sincere 1


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## Kevin (Feb 11, 2016)

Greg I'll be on the road for a couple more hours but I took some pictures of some of the metalworking machinery in there for you and I will post them in the machinery section tonight I think you will like them.

Reactions: Way Cool 1


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## woodtickgreg (Feb 11, 2016)

Kevin said:


> Greg I'll be on the road for a couple more hours but I took some pictures of some of the metalworking machinery in there for you and I will post them in the machinery section tonight I think you will like them.


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## F.W.von (Apr 20, 2016)

My buddy talked about how to use a motor to generate that 3rd leg of power to run 3 phase stuff.
I'm still Baffled by electricity.
Maybe if you felt like drawing it out on a napkin I could upload it into my brain.
It sounds simple but I have an allergy to wires.

All I know is that if I get a rotary phase converter large enough to run the 3phase 10hp 24" planer.... Then I will be able to run any 3 phase unit off the same RPC assuming it's less than 10hp.

Still, the RPC I need is about 1500$ or less if I get my buddies Grainger discount.

From your description, I'm thinking I could find a 20hp 3phse motor, a smaller starter motor and then generate that 3rd leg for 500$ ish?
That figure depends on what you charge for drawing on napkins


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