7.5hp baldor rpc for 10ee

[pennywise]

7.5hp baldor, 184tc, 208-230/460, wired for low voltage. 3450rpm.

I am attempting to build an RPC for an older '42 MG 10ee. I am just going to manually start the motor, so no starting capacitors necessary.
I've been following the Fitch design, by way of balancing the Run capacitors, but am having an issue. I have an assortment of 7.5, 25, and 55mfd capacitors. Unlike what Fitch claims, adding caps to the a(line1)-c(generated leg) raises my Vbc more than the Vac.
My motor (with no load) shows Vab 246, vac 231, vbc 233.
At cp55mfd cs25mfd, vac 248, vbc 247.
Once I begin using different combinations of mfd on Cp and Cs, vbc seems to increase faster than vac until I'm around 265vbc.
I can get the voltage within ~4-5v difference between vac and vbc with several combinations through the range. I worry that the only way to test this with a load is to plug in my 10ee, which is a pretty expensive system to fix if something gets fried. Is this something that warrants concern, or should I just plug it in and see what happens with the voltage so I can get these balanced properly?

 

[JST]

Your MG type needs 3 phase for the motor, not electronics, and motors are pretty rugged.

In your place, I;d see if there is an issue with zero capacitors, then add enough to raise the no load maybe 10%, and leave it.

Your other option is to do the boost with a transformer and not with capacitors. That is more stable and predictable overall.

 

[pennywise]

Your MG type needs 3 phase for the motor, not electronics, and motors are pretty rugged.

In your place, I;d see if there is an issue with zero capacitors, then add enough to raise the no load maybe 10%, and leave it.

Your other option is to do the boost with a transformer and not with capacitors. That is more stable and predictable overall.

So having an unloaded voltage of 265-270 (10% over my vab line) for vac and vbc is okay for the idler and monarch motor?

Additionally, would it be advisable to further balance the lines once I do connect the lathe into the system, or should I just let it be at that point? I see that there is a potential for running too high when no load is applied if the balance is done with a load on the idler, however maybe a middle ground can be found by splitting the difference between the unloaded and loaded voltages?

 

[JST]

264VAC is the normal maximum for the 240V line. That should not bother anything. Also, the motor, when connected, will be running, so it will be drawing the voltage down.

The thing to worry about, if anything, is the control system transformer etc. That may have much less loading, and might see the entire high voltage. But, that should NOT be connected to the "generated leg". Connect the machine so the controls are served by the "pass-thru" legs.

In my opinion, if you have to have the unloaded voltage over 264V, there is a different problem. The RPC may be undersized, etc. Don't set it up to have very high unloaded voltage in an attempt to compensate.

Using a transformer to get your voltage is more stable. And you can know what the maximum voltage is.