Reliance SP500 AC Motor Drive Question

You will have to run it on ~480 v. If you have 240 v available you will need to obtain a 1:2 step-up transformer. Look for one that is rated for at least 7500 va which will accommodate 7.5 HP throughput. A friend recently built a rotary phase converter with a 480 v motor and transformer. He was able to use ordinary (surplus) 370 v capacitors by placing 2 of them in series to achieve a highter voltage rating. The formula for total capacitance of units in series is: (C1 x C2) / (C1 + C2). Tip: Less start and run capacitance in required for 480v than for 240v - on the order of 1/2 as much.

Bob Swinney

Series connected capacitors work fine and the value of the start and run capacitors needed is even less than half the 240v values.

If you change the windings on a motor to enable it to operate from twice the supply voltage, the start and run capacitors each still need to store the same amount of energy. Since energy stored is 1/2 C x V squared the value of each capacitor should be divided by 4.

This is shown clearly if you consider a 240v motor that needs a

500uF start capacitor. This could be provided by parallel connecting two 250uF capacitors.

The same motor rewound for 480v operation would need these same two capacitors but this time series connected to yield 125uF at twice the working voltage.

Jim

Jim sez: Jim sez:

"Since energy stored is 1/2 C x V squared the value of each capacitor should be divided by 4."

Sorry, Jim. I was playing by memory, somehow my brain dragged up "Q = CE". Guess I was wrong.

Bob Swinney

Either you did not answer the question I asked, Bob, or I did not understand your answer. My question is whether it is possible to run a Reliance SP500 AC variable frequency motor inverter drive with a single phase input. At least some smaller inverters will work this way. I just do not know if this particular 575 volt drive will work with a single phase input. Anybody have any more experience with Reliance variable frequency motor drives than I have?

I'd be using a transformer to step up from 230 to 575 volts single phase to feed the drive. Some loss of drive current rating is acceptable. Question is whether it will work. Potential problems I can imagine include some sort of single phasing protection that cannot be over-ridden or a larger than normal DC link voltage ripple causing some sort of problems.

Obvious way to answer question is to try it, but I'd prefer to know if it will work before buying the drive.

Thanks, Mill

Sorry MP. I did not engage reading device before activating keyboard! I guess all I saw was "Reliance" and "7.5 HP" so I thought you were talking about a motor. As per usual, I must excuse myself from discussing VFD devices.

Sorry again for my confusion.

Bob Swinney

Why not ask Reliance? I'm sure your local Reliance Sales Office can connect you with drives tech support.

Randy