Rotary Converters

[Darren]

Not really electronics, but here will do 


I have a comercial 3hp RPC, I might need 5 or 6hp at some point.

So my first question is can RPC's be run in parallel? If so what if one was say 3hp and the other was 2Hp, would one be fighting the other?

My last question, where can I find a big enough transformer to make a 5hp RPC (or bigger) without breaking the bank?
I presume a 240 isolating transformer will do if wired as an auto unit?

Anybody got anything or know where what and how?

PS, I know how to make an RPC and have done so in the past, I just need a proper transformer that doesn't cost me more than the machine I'm trying to run !!!

[kvom]

You would have to connect the 3 leads of the first motor to the second motor, and the two motors have the same speed in order for the phases to be in synch.  However, it would then seem that the current draw from the two motors together would be greater than the rating of either motor.  Personally I would sell your 3HP unit and buy a larger one to fit.

[Darren]

You would have to connect the 3 leads of the first motor to the second motor, and the two motors have the same speed in order for the phases to be in synch. 

Ah yes of course, that would be the same as running the mill as a second idler to boost a larger motor .... 

Personally I would sell your 3HP unit and buy a larger one to fit.

Don't really need to do that, I have a spare 3hp motor as a second idler, so it looks like all I'd need is another larger transformer. Unlike the US we tend to need 440V three phase in the UK as that is what most of the motors are. Very few are dual voltage from what I've seen.

440V 5HP Rotaries get pretty expensive, around £1K, 10HP would scare you ..

[tinkerer]

Can't you use it as a single phase and use a capacitor to start it? Would only put out 2hp though.

[John Hill]

Darren, I expect your isolation transformer would do the trick but be aware of insulation issues at the higher voltages. (Dont take my word on it but as far as I know all common stuff is rated at 1000volts anyway) I think the voltage will be a tad high?  It will be effectively twice the star voltage which will be more than the delta voltage.

However, wont 5HP be pushing the current sourcing capacity of your single phase circuit?

[Darren]

Can't you use it as a single phase and use a capacitor to start it? Would only put out 2hp though.

Wouldn't work as the motor is 440V. 

[Darren]

Darren, I expect your isolation transformer would do the trick but be aware of insulation issues at the higher voltages. (Dont take my word on it but as far as I know all common stuff is rated at 1000volts anyway) I think the voltage will be a tad high?  It will be effectively twice the star voltage which will be more than the delta voltage.

However, wont 5HP be pushing the current sourcing capacity of your single phase circuit?

I think most transformers are tested to about 4Kv

The 5HP should be fine, I have 8mm armour feeding the garage consumer unit, and 6mm T&E from there to the rotary. So could go up to 45A main fuse if needed. That should be more than enough. Even for heavy start up currents.
But if like most people the rotary was on a normal radial ring (socket circuit) then the main fuse would be 32A and if a wall socket was used further limited to 13A plug fuse.

Bear in mind the amperage of 440V is around half of 240V so really lots of scope what ever circuit is used.

A 240V/240V would theoretically give 480V, but with losses it should be spot on, I'm guessing as I've not tried it, but it will be close.

[tinkerer]

Missed the 440 part. They can be run parallel. Thinking you have to step down the voltage with the transformer.

[Darren]

This guy is making 415v 3ph from 240V single phase without a transformer using a duel voltage motor.

Anyone know if this is for real?

[John Hill]

This guy is making 415v 3ph from 240V single phase without a transformer using a duel voltage motor.

Anyone know if this is for real?

I didnt run the video, I only have narrow gauge broadband here, but it would seem feasible in that the motor windings are acting as auto transformer windings.

[Bluechip]

Darren

'Course it's feasible, I was faffin about with such stuff 50 yrs ago. Why the guy thinks it's remarkable enough to put it on Youtube is anyones guess.



This is all he's doing. As he says, there is an imbalance on the output.

'***kin understatement, that is. 

John Hill is quite right.  It is used as an autotransformer.

Not a practical solution except maybe for very low output motors. As soon as you load the thing the output droops a lot.

With todays 1ph. to 3ph. VFD's, who wants to go pratting about with this stuff ??

Dave BC

[Darren]

Thanks Dave,

I was guessing it would be something like that, but I was thinking as you would be asking the motor to generate the stepped voltage on three legs as well as the third 240V phase it would be asking a lot of it.

With no 415V input backed by a never ending mains supply I could see how the output could be limited for heavier loads.

[Bluechip]

Darren

If you look at the second part of the vid. the (apparently 1HP ie 746W) motor is unloaded !!

I doubt if that is sucking much more than 40-50W. There are only the I^2 x R losses, a bit of hysteresis loss and the bearing friction to load it.

I wonder what would happen with a 10" saw blade on  it?

As far as I know, these VFD's rectify the AC input, then squeeze it and chop it into a sort of square or pseudo-square wave 3ph.
So, the VFD is doing most of the work.   

Hmmmm .. I'll give it a miss I think ..

Why don't you have a go ... 

I believe it's Character Building   

Dave BC

[Darren]

Hi Dave I did have a go once, about 2 yrs ago. But I could not get the output when I started the lathe up. The lathe just bogged down.

I was just trying to find out if I got something wrong, but I don't think so. I now have a commercial RPC so the question was just massaging my curiosity 

Thanks for your input, I think I'll leave this one in bed where I left it ....