Run capacitor for a DIY 600V rotary phase converter

[TRC]

I want to build a 600V rotary phase converter and I'm struggling with the choice for the run capacitor. Are they mandatory? If yes, any tips on how to choose the capacitor for that third leg? Do the voltage need to be around 575, or can I settle for lower? Do I need to bother with the third leg phasing?

To keep it short, I've got:

1. 1xbridgeport pancake 3ph 575V 3hp motor
2. 1x5hp 3ph delta 575V 1155rpm idler-to-become motor (I've also got a wye one, only difference is that it's 3600RPM and 50 pound lighter, in case it's what I truly need)
3. 1x10kva 3ph 208-575 transformer (to 240V 1ph to 640V 1ph for at most 6.6KVA)
4. monophase 240V outlet
5. 1/6 hp 1ph 1750 rpm motor to spin the big one

What I need:

1. run capacitor bank (exact capacitance unknown, that what I'm here for)
2. Misc. electrical stuff (mag switch, box, button, etc)
3. 10 KVA 240-208 transformer (running a 575V motor at 660V seem dangerous)

I'm planning to follow the following schematics:

from the following video:

Disclaimer: I've tried with a vfd between the transformer and the outlet, it burnt instantly. I'm not planning on settling for a 3k$ 575V VFD when I need to spend about 200$ for a RPC.

If you also have a RPC, I'd like to know how the abnormal phasing of the third leg is affecting the motor.

 

[dalmatiangirl61]

Would it not be easier to replace/rewind the existing motor?

 

[DDoug]

Keep everything at 240 vac up to the output of the RPC, and then use the 3 phase xformer to step it up to 600 vac

 

[TRC]

Would it not be easier to replace/rewind the existing motor?

Yes, but it would be more expensive

Keep everything at 240 vac up to the output of the RPC, and then use the 3 phase xformer to step it up to 600 vac

I dont have a 240V 3ph motor, only a 240 1ph 5hp. I'd like to use my 575V motor instead

 

[dalmatiangirl61]

I think you are pissing into a stiff breeze, but hey, whatever makes you happy. I see complete Bridgeport heads fairly regularly on ebay, iirc someone posted a used one here in the past week.

 

[TRC]

I think you are pissing into a stiff breeze, but hey, whatever makes you happy. I see complete Bridgeport heads fairly regularly on ebay, iirc someone posted a used one here in the past week.

I know it would be easier to just go and get a new motor, rewind it, get a new mill, have 3ph installed at my place for 200k$ and any other solution that are easier to implement. But, I almost have a completed RPC. I just need capacitors and a few component I can find to almost any surplus to get it completed. I just want to be able to choose the right capacitors and see what i do afterward. If I can pay 200$ and get a decent RPC, plus the learning experience, I'll take that over the whatever solution that is not cheaper.

Plus, once I get it, I wont need to care for finding 240V and will be able to replace my machine tool for industrial ones without any second though.

 

[dalmatiangirl61]

Well your going to need the rpc no matter what, its the odd voltage motor that is not worth fooling with. If it was me, I'd either find a used 220/440v B-port motor, or make an adapter plate for regular C-face motor, if it even needs an adapter. And at some point that machine is going to be sold, the 575v motor is going to be a big negative.

As for higher than 440v caps, considering 575v is used in other places, I would guess they are available somewhere, but 220/440 seems to be the norm here.

 

[TRC]

Well your going to need the rpc no matter what, its the odd voltage motor that is not worth fooling with. If it was me, I'd either find a used 220/440v B-port motor, or make an adapter plate for regular C-face motor. And at some point that machine is going to be sold, the 575v motor is going to be a big negative.

It's the Canadians norm to use 575V 3ph for industrial motors.

I already tried to go for a C face, but the special shaft I need to turn is a real issue here. the result was vibrating so much that I felt it in the handle of the mill. I dont have access to anything to balance that shaft.

I just want to know a way to choose the right capacitors. I already found 600V cap, I just need to know what I need, or a way to get that knowledge.

 

[Karl_T]

Read this article for the concept of how to set up run capacitors. its for 220 volt but the idea is the same.

DANG = the file is too large. PM me your email addy. I will send it that way.

 

[JST]

They are not mandatory, and they are not even the best solution to the inherent lower voltage of the idler output.

Plenty of folks use no output capacitors, or use a transformer instead.

 

[TRC]

They are not mandatory, and they are not even the best solution to the inherent lower voltage of the idler output.

Plenty of folks use no output capacitors, or use a transformer instead.

Do you happent to know what the voltage would be approximately? Not especially onto a 600V RPC, I just want to have an idea.

 

[jim rozen]

Salient fact: you are trying to run a 3 hp load motor with a 3 hp idler. The idler is undersized and it will be a tough go even if you sort out the voltage issue. And yes if it's going to work at all you will need to tune with capacitors. Not saying 'you can't get there from here' but it's close to that.

 

[TRC]

Salient fact: you are trying to run a 3 hp load motor with a 3 hp idler. The idler is undersized and it will be a tough go even if you sort out the voltage issue. And yes if it's going to work at all you will need to tune with capacitors. Not saying 'you can't get there from here' but it's close to that.

The idler is a 5hp motor, I've no idea why I wrote 3hp there. As for tuning, how do I do it? Do I go full empirical and purchase a box of various cap and end up with a capacitor bank? Someone up said the cap where not mandatory, so are the cap that important?

 

[JST]

Jim Rozen actually uses an RPC that has NO capacitors...... It IS considerably oversized for his load motors... which probably helps a little.

The capacitors are one way of getting a bit of boost for the generated output. There are others.

The entire purpose is to get the voltage up reasonably equal to the other lines, so that all three phase voltages are about equal. That is helpful, but perhaps not required. Unbalance makes the load motor work a bit harder. It is less good for some "rectified loads", such as downstream VFDs, CNC machines, etc. The use of a transformer for boosting may be better in those cases.

The basic RPC will work regardless. The output voltage will be present regardless, just at the "back EMF" voltage, and not the same as the incoming line. The voltage comes from the basic motor operation, and it is also phased correctly due to the motor structure.

You can figure the raw generated leg will be 5% to 15% low, simply because it is the back EMF of the motor (with a small added loss due to rotor resistance). Back EMF has to be lower than line voltage, or there will be no current flow into the motor.

If you use a capacitor setup for boosting, it is somewhat empirical. You want the voltage to hold up under load, but you generally do not want it much more than 10% high at no load (264V for a 240V system). A high no-load voltage can affect smaller loads as well as the control systems of some powered machines.

 

[TRC]

Jim Rozen actually uses an RPC that has NO capacitors...... It IS considerably oversized for his load motors... which probably helps a little.

The capacitors are one way of getting a bit of boost for the generated output. There are others.

The entire purpose is to get the voltage up reasonably equal to the other lines, so that all three phase voltages are about equal. That is helpful, but perhaps not required. Unbalance makes the load motor work a bit harder. It is less good for some "rectified loads", such as downstream VFDs, CNC machines, etc. The use of a transformer for boosting may be better in those cases.

The basic RPC will work regardless. The output voltage will be present regardless, just at the "back EMF" voltage, and not the same as the incoming line. The voltage comes from the basic motor operation, and it is also phased correctly due to the motor structure.

You can figure the raw generated leg will be 5% to 15% low, simply because it is the back EMF of the motor (with a small added loss due to rotor resistance). Back EMF has to be lower than line voltage, or there will be no current flow into the motor.

If you use a capacitor setup for boosting, it is somewhat empirical. You want the voltage to hold up under load, but you generally do not want it much more than 10% high at no load (264V for a 240V system). A high no-load voltage can affect smaller loads as well as the control systems of some powered machines.

Ok, and if I don't use any capacitor, do I need add anything between L3 and L2 of the idler (referring to the schematic)? Or do I just remove that part of the circuit?

 

[DDoug]

Yes, but it would be more expensive

I dont have a 240V 3ph motor, only a 240 1ph 5hp. I'd like to use my 575V motor instead

So go out and BUY a 240 vac 3 phase motor.
You'll be money ahead.

I've done it.
I make 600 vac 3 phase in my shop with a 40 hp RPc.

 

[BT Fabrication]

No need to re invent the wheel when you havent a clue. just buy the control box, they are only $500 usd complete. the 3 phase motor is where the $ is. find a good used 10hp for a few hundred bucks then buy the right size box. get the output voltage balance wrong and it starts overheating motors and costing bigger $

 

[TRC]

So go out and BUY a 240 vac 3 phase motor.
You'll be money ahead.

I've done it.
I make 600 vac 3 phase in my shop with a 40 hp RPc.

I already have the motor, I won't go and get another big motor in my small shop. I will consider this as a B plan, but I'll stick to 575V at the moment.

No need to re invent the wheel when you havent a clue. just buy the control box, they are only $500 usd complete. the 3 phase motor is where the $ is. find a good used 10hp for a few hundred bucks then buy the right size box. get the output voltage balance wrong and it starts overheating motors and costing bigger $

I know that and I purposefully choose not to do it because it would be more expensive than purchasing a few capacitors, a switch and get an electric box from the scrapyard.

Gentlemen, thank you for these recommendations. I'm glad that you are taking your time to give me advice.

However, I'd like to keep the subject of this conversation to the question asked or other information related to building a RPC from scratch. This post is to gather information about whether it's feasible or not, not to get told not to do it because it's not the easiest way.

I'll make a decision afterward, don't go think I'm 100% going this way.

 

[DDoug]

I already have the motor, I won't go and get another big motor in my small shop. I will consider this as a B plan, but I'll stick to 575V at the moment.

I know that and I purposefully choose not to do it because it would be more expensive than purchasing a few capacitors, a switch and get an electric box from the scrapyard.

Gentlemen, thank you for these recommendations. I'm glad that you are taking your time to give me advice.

However, I'd like to keep the subject of this conversation to the question asked or other information related to building a RPC from scratch. This post is to gather information about whether it's feasible or not, not to get told not to do it because it's not the easiest way.

I'll make a decision afterward, don't go think I'm 100% going this way.

Where to find a steevco relay for 600 vac ?

BTW xforming 240 vac 3 phase up to 600 vac (after the RPC) allows you to use the "fine tuning" voltage taps, another method to bring up the 3rd (generated) leg.

 

[TRC]

Where to find a steevco relay for 600 vac ?

BTW xforming 240 vac 3 phase up to 600 vac (after the RPC) allows you to use the "fine tuning" voltage taps, another method to bring up the 3rd (generated) leg.

I'm using a 1/6 hp motor to start the idler, I won't need a relay and a start cap.

And you're right, I'd be benefiting from the adjustment tap on the transformer if I was using a 240.