Electrical Questions - RPC Controls

[dalmatiangirl61]

I'm looking at a Hammond 1000va buck boost transformer, I think its what I need, could someone please verify that this is the size needed?
Buck Boost transformer 120- 240used to drop power from 240V to 208V 803423033842 | eBay

 

[JST]

240 to 208 is a 32V drop, so the 1000VA suggests a current rating of 1000/32 = 31A. But the link says 24V. So the 1000VA should be about 40A.

If that is a suitable current for your loads, with a margin of 25% to 50% over the actual draw, then the VA is acceptable.

How are you planning to connect this transformer?

Since you have just one high line, connecting one transformer in "semi-wye" from the generated leg to input neutral (not the same as 3 phase neutral, which you do not have), and taking the tap to your output would be correct.

That, assuming the transformer is OK with the 15% high voltage, should give you 248V on the output. That's still a little high, but not bad. Since the actual winding as connected will be for 240+24= 264V, the 276 is not much over that. I'd not expect an issue.

Typical connection is to connect H2 and H3, X2 and X3, and finally, connect H1 to X4

Then connect H4 to "input neutral", X1 to the RPC generated leg output, and take the actual output from the H1/X4 connection.

Having the transformer connected may foul up the start, So I suggest that some means be used to not connect it until the idler has started (switch, contactor, etc).

 

[dalmatiangirl61]

240 to 208 is a 32V drop, so the 1000VA suggests a current rating of 1000/32 = 31A. But the link says 24V. So the 1000VA should be about 40A.

If that is a suitable current for your loads, with a margin of 25% to 50% over the actual draw, then the VA is acceptable.

How are you planning to connect this transformer?

Since you have just one high line, connecting one transformer in "semi-wye" from the generated leg to input neutral (not the same as 3 phase neutral, which you do not have), and taking the tap to your output would be correct.

That, assuming the transformer is OK with the 15% high voltage, should give you 248V on the output. That's still a little high, but not bad. Since the actual winding as connected will be for 240+24= 264V, the 276 is not much over that. I'd not expect an issue.

Typical connection is to connect H2 and H3, X2 and X3, and finally, connect H1 to X4

Then connect H4 to "input neutral", X1 to the RPC generated leg output, and take the actual output from the H1/X4 connection.

Having the transformer connected may foul up the start, So I suggest that some means be used to not connect it until the idler has started (switch, contactor, etc).

With the limited instructions on M-C I was thinking 1kva would work, but then looking at amp rating on the Hammond unit I had second thoughts, thank you for clarifying that.

I'm still not 100% clear on connections yet, I was hoping you knew how. If it is not abundantly clear yet, I'm playing out of my league on this one.

If the transformer is only on the generated leg I don't think it will affect start-up.

I have ordered a 1.5kva transformer, 63 amp, pretty sure that will cover my needs with a safety margin. With any luck it will be here Friday, but might be next week, I will go over your connections recommendations, but until there are wires to physically touch I have a hard time visualizing these things. Thank you for your help!

 

[JST]

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If the transformer is only on the generated leg I don't think it will affect start-up.

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The generated leg is the one the start capacitors are applied to!

So an extra load of any kind there can affect the start. It can also affect the response in cases where the start capacitors are removed from the circuit by a voltage sensing relay.

Whether "that" transformer in the case of "your" converter will cause any trouble, I do not know. Larger transformers intended to step up to 480 etc can and have.

 

[dalmatiangirl61]

Capacitors are between 2 and 3, but how that affects the price of tea in china is beyond me...

I found a pic of the transformer that PAM sells, searching the web I'm not finding anything like it, might be special wind, but I'd bet its an off the shelf variety. Anyone recognize it? Only 3 taps on one side of it.

 

[JST]

The capacitors put current (phase shifted) into the terminal for the winding that produces the generated leg, which produces the starting torque. If that terminal also has a transformer on it, some current will be diverted, and the phase shift will be somewhat different.

Result being that the capacitors connected may need to be different. Just not connecting anything there (via a contactor that closes after the thing is up to speed), lets the capacitors start the idler unmolested by any other loads.

The three terminal transformer is just a buck/boost that has the low voltage (auxiliary) winding permanently connected. So it only has three terminals. One low, one the junction of main and aux winding, and one to other end of the aux winding.

It's the same setup as a buck/boost, with somewhat fewer options for connection.