Re: Motor operating voltage range?
jkim780: I am not much with formulas and such any more, but there are plenty of people on here that can work any of the math out. But I will answer your questions with this.
When you operate a motor at a lower voltage the current goes up. The motor will draw the same HP (many previous discussions on this issue, but current does go up)
Effect on efficiency-Efficiency is effected by Power Factor, if Current is higher motor is acting as a more inductive load and Power Factor will be lower, efficiency will be lower.
Effect on torque-Torque will basically be the same (can probably prove me otherwise with the formulas down to a fractional value difference). With the current increase this is what will provide the flux that will maintain the torque in the motor. (There is another quit lengthy discussion on this topic on this site). I'm going to say this but there will probably be a few on here that will jump all over me. Voltage itself does not affect torque. Torque is produced by the interaction of the stator magnetic field and the magnetic field produced in the rotor. You have to have two magnetic fields to produce torque. The magnetic fields are effected by the current that flows through the stator coils and the rotor bars/windings.
Effect on Windings-The voltage rating is determined by the insulation characteristics. A lower voltage will not break down the insulation however moving onto the next question.....
With the reduced Voltage you have an increased current of course this will effect the heating characteristic of the motor. You could be operating in an overloaded condition and not be over the HP rating of the motor. The overloads should protect you, but if you continue to operate this way, overloads also fail and your motor may just burn up. I have also seen where that midnight electrician just say's Oh we'll just stick a little higher overload in, it won't hurt and as Charlie said before, yes it will work and will run, but for how long.
You have to do all the math to figure out exact numbers. And yes I will be the first to admit I have put a 230V rated motor onto 208 and not had a problem at all. But as I said before, I was ready to stand there and say, it was my decision you can yell at me but I took into account the size of the motor, what it was used for, possible damage from failure. And then I did do all the math and the current increase didn't even require the overloads to be changed.
Re: Motor operating voltage range?
Originally posted by jkim780:That's good point although I was expecting answers from the technical point of view (putting aside legal issues). I guess what I am wondering is. . . .
Even if I could remember all the math and equations, I would be disinclined to answer your set of four questions. That is because I believe you are heading down an unsafe path. It is one thing to ask, ?What would happen if . . . ,? when the intent is to gain knowledge so as to become better at your job. But it seems to me that you are asking, ?What would happen if . . . ,? in the hopes that when the answers arrive you can say, ?Well that?s not too bad, I can live with that.? I will not walk with you down that path.
The National Electrical Code is not a design manual. It cannot answer your questions. Its one and only function is related to safety. It tells us two things that are related to your situation, and both are safety issues. (Someone will have to help me with the exact wording and the reference articles, for my NEC books have not yet been unpacked from my recent move.)
- Equipment that is listed or labeled must be installed and operated in accordance with the listing or labeling instructions.
- Equipment shall not be operated at a nominal voltage that is different than that given by the manufacturer.
Re: Motor operating voltage range?
Even if I could remember all the math and equations, I would be disinclined to answer your set of four questions. That is because I believe you are heading down an unsafe path. It is one thing to ask, ?What would happen if . . . ,? when the intent is to gain knowledge so as to become better at your job. But it seems to me that you are asking, ?What would happen if . . . ,? in the hopes that when the answers arrive you can say, ?Well that?s not too bad, I can live with that.? I will not walk with you down that path.
Charlie b,
Don't get me wrong. My intent is to gain knowledge so as to become better at my job. I never was trying to head down an unsafe path. I had a client complains.. "whine, whine, but it ran fine for a year, why are you telling the voltage is too low?, whine, whine". It's a hassle because the motors run about a year... instead of 7 years.
I am trying to give him a better answer rather than telling him it's because the code says so, more importantly for my own education. In refrigeration, I have seen a lot of the repairs are replacing 230V motors that even the name plates may say are 208V rated. As 69boss302 stated, some are running fine and some aren't. I just wanted to know why.
The National Electrical Code is not a design manual. It cannot answer your questions. Its one and only function is related to safety. It tells us two things that are related to your situation, and both are safety issues.
Like I said, I didn't ask for the code requirements or any legal issues because I am fully aware of those consequences. I was just asking for the engineering aspect of it.
I have also seen where that midnight electrician just say's Oh we'll just stick a little higher overload in,..
That was the exact response that I've got from the electrician when this issue was brought up.
Thanks for the link and all the replies. It did help me a lot understanding this matter.