Balancing RPC generated legs with Variacs or buck/boost

[Aryan165]

Hi,

This is another thread on RPC balancing I know, but I've read lots of threads about it here and RPC leg balancing using transformers between individual legs isn't discussed much (sure it is mentioned from time to time). I know capacitors are usually used for balancing like the Fitch Williams design.

The Object is to boost the 2 ''generated'' voltages so they don't drop too low under load, staying closer to the ''pass through'' voltage supplied by the incoming 240V single phase. I've heard of using buck/boost transformers to boost it by say 32V, but in the UK buck/boost transformers don't seem to be as ubiquitous as they are in the US (thinking of the usual gray Acme / GE ones).


Wondering if variacs can be substituted in place of dedicated buck/boost transformers?

10 hp idler, powering 3-5 hp machines, I'm thinking 2x 3kVA variacs (on RPC output), between lines A-C and B-C (A-B being across the single phase incoming). I'm thinking larger than necessary variacs to keep impedance down.

Does this sound reasonable or am I making a gross mistake regarding number of transformers, how they are connected, transformer siz etc?

Any assistance is appreciated!

 

[JST]

UK varieties of RPC would be different, since the voltage you start with is not the line-to-line voltage. Your standard 3 phase is 400v and you are supplied 240 single phase from line to neutral.

A US RPC would really only need one transformer, because the input 240V is also the target output. It would be connected from the generated leg to the input neutral (and must be rated for the higher voltage).

Assuming your target is the 400V, relative to the same neutral that comes in, you need a pair of 240V input transformers or variacs, just as you suggest.

Rate the transformers or variacs per the current needed, and not just the power. The power needed is only due to the difference in voltage. If you needed 220V at 10A (for a simple example), and you had 200V, the boost transformer would have to handle 10A on a 20V secondary, or 200W (VA), even though the entire power is 2200W (VA).

Variacs are typically current limited by the sliding contact, which is why current is a better measure than power.

I would suggest connecting the transformers only after the RPC has started. A load can interfere with some starting methods.

 

[Aryan165]

JST,

Thanks for the reply.

My plan is pony start the idler, balance the 240V three phase (with transformers in this case) and then use a step up 3 phase transformer to go to 400V 3 phase.
I deliberately left the step-up (from 240 to 400) aspect out of this post, focusing only on balancing using transformers as if I wanted 240V 3-phase, to keep it focused on that issue. I know 2 buck/boost transformers can be used in open-delta to act as a 3 phase transformer, but that's not my intention, I think I will just find a 3 phase transformer and use it.

So my question is: can I use 2 variacs to boost the 2 generated voltages out of the RPC, before feeding the 3 voltages to the primary of my 3 phase step up transformer?

 

[JST]

Sure, they are just a transformer that happens to adjust.

If you select your 3 phase transformer cleverly, you may not need any extra adjustments. Using one that has various taps intended for voltage adjustment will potentially allow an all-in-one solution.

 

[Aryan165]

That was actually my second question, I've seen 3 phase autotransformers with say taps for 380-400-420 or something similar like that. Say I get 240-220-210V out of the RPC, can I connect them to the 230V input taps, but on the secondary take off one from the 400 tap, and the other 2 from higher voltage taps to make up for the voltage drop a little bit? I was wondering if that's possible or if it would cause problems. I've attached photos of a potential transformer just an example. This one happens to be 4kVA. Not necessarily what I'll use.

 

[TDegenhart]

Are you attempting to maintain a constant voltage on all three legs regardless of the load condition on the work motor? Actually the best is constant current.

Tom

 

[JST]

I would not necessarily suggest any adjustment system that dynamically tries to maintain voltage. It can be done, the power company does that at substations, but is complicated

A set and forget system that merely sets the weaker generated legs a bit high at no load with the idea that they will fall to be just a bit low under load is the best idea for most. The transformer is just more stable than the capacitor plan.

 

[Aryan165]

Are you attempting to maintain a constant voltage on all three legs regardless of the load condition on the work motor? Actually the best is constant current.

Tom

No, just trying to boost the two generated voltages a bit with no load so they don't drop as much under load.

 

[Aryan165]

I would not necessarily suggest any adjustment system that dynamically tries to maintain voltage. It can be done, the power company does that at substations, but is complicated

A set and forget system that merely sets the weaker generated legs a bit high at no load with the idea that they will fall to be just a bit low under load is the best idea for most. The transformer is just more stable than the capacitor plan.

Yes that's what I thought. I am not trying to dynamically compensate for voltage drop. Just set it a bit higher to start with so it doesn't drop as low as without any voltage correction.

 

[jim rozen]

I would not necessarily suggest any adjustment system that dynamically tries to maintain voltage. I....

WHAT a killjoy. All I envision is a bank of giant variacs operated in unison via bellcrank levers all run off solenoids or maybe syncro-selsyn motors. 0.1 percent votage regulation!

 

[JST]

WHAT a killjoy. All I envision is a bank of giant variacs operated in unison via bellcrank levers all run off solenoids or maybe syncro-selsyn motors. 0.1 percent votage regulation!

Go for it!

We want pics, and voltage logs!

 

[Aryan165]

Sorry about the delay, just wanted to say thank you for your replies!