Equipment Voltage Change - Amp Draw?

[Br4nd0n]

If a multi-motor piece of equipment is originally rated at 460V/30A (nameplate values), but somewhere down the line, many years ago, someone "converted" it for use on 230 - does simple ohm's law apply here? I would think this equipment would need 60A. Am I off base here?

 

[Besoeker]

If a multi-motor piece of equipment is originally rated at 460V/30A (nameplate values), but somewhere down the line, many years ago, someone "converted" it for use on 230 - does simple ohm's law apply here? I would think this equipment would need 60A. Am I off base here?

If the motors were changed to 230V motors of the same power rating as the original motors, then probably so.
But it isn't a definite. You need to consider the power factor and it might not be the same for the lower voltage motors.

 

[steve66]

Yes, the motors should need approx. twice as much current, or about 60 amps.

But its not ohms law, its the formula for power P= E X I or current times voltage.

So to keep the same motor power, if you cut the voltage in half, you have to double the current to keep the power constant.

A motor is typically a constant power load.

That's in contrast to a constant resistance load where you would use ohms law. In that case, if you cut the voltage in half, the current would be cut in half also.

Steve

 

[charlie b]

I would want to know how this "conversion" took place. What did they physically do to the motor? Was there any tear down and rebuild involved, or did they simply swap wires in the pothead? We are assuming that the conversion process did not change the horsepower rating of the motor, but is that really a safe assumption? I can't say.

 

[Br4nd0n]

steve666 - Sorry if my terminology is incorrect. Throughout my apprenticeship we called the relationships E=IxR, P=IxE, and ALL of the transpositions "Ohm's Law". I understand that's technically incorrect...

Charlie b - I'm relocating this equipment for a customer. It is fairly old and all they can tell me is that it was converted long ago. They've been in the old building something like 30 years and that building does not have 480V (nor does the new one) and I've only been there once to do a quick look. I plan on returning tomorrow and will poke around inside the control cabinet to see if I can make heads or tails of anything. Unfortunately I was told, and I quote, "a crazy old guy did all the wiring".

Here's the messed up part! This equipment is currenty being fed with #10 and is fused inside the cabinet at 30A! I didn't track down the circuit breaker feeding it, but I can imagine it's a 30. Seeing this, I asked an employee if they had problems with it tripping the breaker or blowing fuses and he claimed no... but the fuses looked very new and clean and there was an older dirty set, probably blown, laying in the bottom. I've already ran #6 to a J-Box above the new location in the new building with the intent of providing a 60A circuit. I'm concerned that 60A might be over-sized. I want to provide the proper protection for this equipment.

 

[Jraef]

... I want to provide the proper protection for this equipment.

Good man, somebody has to finally step up to the plate.

Keep in mind that just because the motor nameplate says 30A/460V and now it will be 60A 230V, that does not necessarily mean that the motor WILL draw that much current. The motor current is determined by the actual load, i.e. work being performed. It could very well be that this motor was grossly over sized to start with, it used to be very common to do that. So they may have just been lucky all these years because the motor wasn't working very hard at all. The evidence of clean fuses (good eye there by the way) means that OCCASIONALLY someone probably pops them. But knowing operators, they likely think they did something wrong so they quietly replace the fuses and don't admit to anything.