Motor Expert Needed 3 Phase to Single

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David40

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I am in need of some expert advice on a motor sizing question. I am trying to replace a 2HP 3 phase motor on a meat cutting band saw with an equivalent Single Phase motor. I've done a bit of research on the subject and there seems to be conflicting opinions.
On one hand sources have suggested that I would need to increase the Single phase Horsepower from 30% to 50% in order to do the same job the 3 phase motor does.
On the other hand my motor supply house tech support tells me 2HP is 2 HP no matter if it's a 3 phase or single phase. The only caveat he mentioned is that the 3 phase motor would have an advantage in applications where high starting torque is required, and the band saw does not start under load.
So who is right?
Any input would be greatly appreciated.
Thanks
 
2 HP is 2 HP. The voltage does not matter, nor does whether it is a single phase or 3 phase motor.

There is some truth to the torque argument. Just compare the torque curves of the motors. But it's not an issue for your application.
 
Meat Saws

Meat Saws

I'm curious. I am assuming that a saw like that has to withstand washing down with hot water. Is a single phase motor built to handle that more expensive and harder to find?
 
Thanks, that's reassuring. I really would rather use a 2 HP because of the limited space, and I won't need to buy a new pulley either.:)
 
BJ Conner said:
I'm curious. I am assuming that a saw like that has to withstand washing down with hot water. Is a single phase motor built to handle that more expensive and harder to find?
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The existing motor is only a drip proof design. It's enclosed inside a separate compartment and the cover I had to remove has a gasket. Normally I would not expect it to get wet, but even if there were some dripping water it should not be a problem. I was more concerned with temperature, but the existing motor has a Class A rating while the replacement I am looking at has Class F insulation which is a much higher temperature rating.
 
petersonra said:
2 HP is 2 HP. The voltage does not matter, nor does whether it is a single phase or 3 phase motor.

There is some truth to the torque argument. Just compare the torque curves of the motors. But it's not an issue for your application.
Click to expand...
That's right; HP is a relationship between torque and speed, nothing to do with phases, or voltage, or anything else for that matter. And starting torque depends on what type of single phase motor you select. A Shaded Pole motor has a lot less starting torque, a PSC (Permanent Split capacitor) is going to have a little less starting torque than a 3 phase, but a Cap-start may actually have a little more.
 
norcal said:
Put a VFD on the saw, 240V 1? input models are cheap.
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That is a good idea for special size, frame, other characteristics motors, if replacing a NEMA standard frame sized three phase with a single phase and have no need for speed control or soft starting then just replacing the motor with single phase of same size is more cost effective.
 
Yeah, I was going to say.... cheap or not, this guy is already having a heart attack over the cost of the motor, he would freak if I tried to sell him an additional item. He already pissed away the cost of a service call and a contactor from Hobart who's service man mistakenly thought that was the problem. After it still did not work he gave up. (I can't believe he didn't ohm out the motor windings.) :roll:
 
As an aside, '2hp is 2hp' is not quite true.

The horsepower is a unit of power, equal to 746 watts. No matter where you get that horsepower from, it is still 746 watts. So in terms of continuous power output, a '2hp' single phase motor will be the equivalent of a '2hp' three phase motors.

However the name '2hp' is used to describe the motor as a whole, not just its continuous power output. Two motors with exactly the same rated speed and rated torque, with exactly the same name (2hp) could have very different starting characteristics or torque versus speed characteristics.

You will need to be concerned with starting torque, number of starts per hour, and stall torque...but I suspect that you won't have any problem. Like jaref said above, with some single phase motor designs you will have _greater_ starting torque than common comparable 3 phase designs.

-Jon
 
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