Motor operating voltage range?

jkim780 · Jun 21, 2005

Can 230 volt rated motor operate on 208V system? I understand a motor has +/- 10% tolerance but considering the voltage fluctuation on the service, is it ok to operate 240v motor on 208v system?

I see one mfgr show the unit operating voltage range 187-253v for 240v and 414-506v for 460v. What if the mfgr. only says their motor is rated at 230v? Can I still assume that this motor will operate on above voltage range? or Do I need a boost transformer?

[ June 21, 2005, 03:52 PM: Message edited by: jkim780 ]

charlie b · Jun 21, 2005

Re: Motor operating voltage range?

Can it operate at 208V? Yes it can. Will it enjoy the experience, or will it even survive the experience? Who knows? If the manufacturer does not explicitly tell you that the motor is rated for 208 volts, then I would say that that motor should not be used.

jkim780 · Jun 21, 2005

Re: Motor operating voltage range?

What would be the problem putting 240v rated motor to 208v system? I can see that it will draw more current but wouldn't it be within the motor winding tolerance?

69boss302 · Jun 21, 2005

Re: Motor operating voltage range?

The only real problem is the same as all the other things used for electricity. If it is not listed or rated for that voltage, If something happens the manufacturer is not responsible, the distributor is not responsible, the guy using it is not responsible, the person that is responsible is the one that said it was OK to use that motor on that voltage. Now what may happen, who know's, but it can just be traced back to the fact that someone made a decision to use an incorrectly rated motor so if anything remotely goes wrong it's their fault. God Bless our legal system

jkim780 · Jun 21, 2005

Re: Motor operating voltage range?

Originally posted by 69boss302:
The only real problem is the same as all the other things used for electricity. If it is not listed or rated for that voltage, If something happens the manufacturer is not responsible, the distributor is not responsible, the guy using it is not responsible, the person that is responsible is the one that said it was OK to use that motor on that voltage. Now what may happen, who know's, but it can just be traced back to the fact that someone made a decision to use an incorrectly rated motor so if anything remotely goes wrong it's their fault. God Bless our legal system

69boss302,

That's good point although I was expecting answers from the techical point of view (putting aside leagal issues). I guess what I am wondering is;
How voltge would impact the motor performance?
How does voltage relate with torque?
How does voltage variation impact motor windings?
Will there be excessive overload(heating)problem?

[ June 21, 2005, 06:10 PM: Message edited by: jkim780 ]

69boss302 · Jun 22, 2005

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.

charlie b · Jun 22, 2005

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.

bthielen · Jun 22, 2005

Re: Motor operating voltage range?

I'd like an explanation of how the current will increase simply by reducing input voltage. I believe this is a common misunderstanding. If this were true, reducing the voltage to 0 volts would increase current infinitely, and increasing voltage would reduce current and therefore not hurt the motor, would it not?

In the simplest of terms, the reason current of a dual voltage motor (480/240 for example) is inversely proportional to the voltage is because the motor windings are rewired, increasing winding resistance to maintain the power output (HP). Ohm's law still applies. Apply 480 volts to a motor wired for 240 volts and power will increase 4-fold due to the low resistance in the 240v windings, which are not designed to handle the increased current load. The smoke will be let out of the motor rather quickly I imagine.

I realize this is a simple explanation, but someone please correct me if I am wrong.

Bob

growler · Jun 22, 2005

Re: Motor operating voltage range?

I'm not an engineer and it's been a few years since I did much work with motors. But there is one thing of a technical nature I can tell you. All motors are not created equal. Are you talking about a three phase squirrel cage motor, wound rotor, induction, repulsion, cap-start, cap-run. Different motors are wound ( designed ) for different performance needs. If there is no real load then the voltage difference may not mean as much. The first things to look for in choosing a motor or frame & shaft size, voltage & number of phases, and design characteristics. Some motors don't work as well when you vary the load or try to speed control. Be sure to read all the manufacturers specs. You can start to find information in the American Electrician's Handbook and there are many motors specific books on the market.

69boss302 · Jun 22, 2005

Re: Motor operating voltage range?

This is where one of the rather lengthy discussions was on this subject, you may want to look at it.
http://www.mikeholt.com/cgi-bin/codeforum/ultimatebb.cgi?ubb=get_topic;f=3;t=001446

And an article in ECM http://ecmweb.com/mag/electric_highs_lows_motor/

It's not so much a misunderstanding. It's just that it applies to the most common of motors. No it does not apply to all motors.

[ June 22, 2005, 02:19 PM: Message edited by: 69boss302 ]

ccjersey · Jun 22, 2005

Re: Motor operating voltage range?

If you search for this topic on this forum, there are better explanations than I can give, but the best evidence I know of is to look at the amperage ratings of 208/230 motors. The nameplate amps will be higher for 208. I assume it is so that the magnetic field strength can be equal to that created by a 230 volt supply.

on the over-voltage side, the magnetic field will saturate rapidly at some point above rated voltage and no increase in power will be possible, so heating goes up, effeciency and motor life go down etc.

It may be logical, but extrapolating to either 0 volts or gross overvoltage, does not prove anything. The motor is more complex than that!

Jim

charlie b · Jun 22, 2005

Re: Motor operating voltage range?

Originally posted by bthielen: I'd like an explanation of how the current will increase simply by reducing input voltage. I believe this is a common misunderstanding. If this were true, reducing the voltage to 0 volts would increase current infinitely, and increasing voltage would reduce current and therefore not hurt the motor, would it not?

The motors we most commonly encounter are ?constant power? devices, not ?constant resistance? devices. So no, they do no obey Ohm?s Law. ?Constant power? means that V times I is constant. If you decrease V, you will increase I. But there is a physical limit to how far you can go, and it isn?t to zero volts. If you reduce V too far, the current going through the motor will create so much heat that the motor will fail. If you increase V too far, it will exceed the rating of the insulation system, and again the motor will fail.

69boss302 · Jun 22, 2005

Re: Motor operating voltage range?

Just thought I would through this out. Do you realize that on a dual voltage motor when you change the voltage applied you still keep the same amount of voltage across each winding. Going from 480V to 240V does nothing but change the windings from series to parallel inside the motor. Each coil always has the same voltage across it at the rated voltages.

jcormack · Jun 22, 2005

Re: Motor operating voltage range?

A motor's current is determined by it design ie-speed-torque curve as the load (torque) requirement causes the motor to slow the current increases until it reaches break-down (stall) The amount of current available is determined by the voltage at the terminals - the voltage/current component will be be whatever is required to maintain speed at that point of slip on the curve(determined by the # of poles and hz applied) - in very simple terms if you lower the voltage the current will increase to provide the KW (hp) required to motivate the given load.

Torque Available (in general) is related to tthe voltage squared, so a reduction of 10% voltage on a 240 volt motor (216) results in a reduction of 19% of Torque Available. Motors are typically designed to run +- 10% - but the closer you are to normal the better.

jim dungar · Jun 22, 2005

Re: Motor operating voltage range?

A motor is a magnetic device. A motor requires a specific amount of magnetic field in order to create it's torque. The magnetic field strength is related to the electrical properties of voltage, current, and frequency.

If any one property is held constant then changing a second property will require a change in the third property in order to maintain the rated magnetic field and resulting horsepower.

With a fixed frequency, current is inversely related to voltage. A reduction in voltage causes an increase in current. There will be a certain low voltage point where the magnetic field can no longer be maintained enough to spin the motor.

With a variable frequency, current can be held constant if the ratio between design and actual volts and hertz is maintained (i.e. 8:1 for 480V 60hz). This is the principal behind the current generation of adjustable speed drives.

Disclaimer: This is a simplified presentation. A detailed magnetic field explanation would not add to the general points presented.

jkim780 · Jun 23, 2005

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.

charlie b · Jun 23, 2005

Re: Motor operating voltage range?

Originally posted by jkim780: I guess what I am wondering is;
How voltage would impact the motor performance?
How does voltage relate with torque?
How does voltage variation impact motor windings?
Will there be excessive overload(heating)problem?

I apologize for mis-interpreting your intentions. Here are my answers (without math).

Unless the lower voltage is too extremely low, I don?t believe you would discern any difference in the way the motor performs. It would drive the same load at the same speed (or nearly so). That is the most likely reason that the client is telling you, ?it ran fine for a year.?

Torque is directly related to current. With a lower voltage, you get more current, and therefore more torque. But here again, I don?t think you would be able to see a difference. You hit the ?On? button, and the motor starts and runs. Did it (and is it still) delivering more torque? Unless you connect sensors and perform calculations, no one knows. All you do know is, once again, ?it ran fine for a year.?

The voltage rating of any device or component, including motor windings, is based on the ability of the insulation system to prevent leakage current (i.e., from the conductor to the outside world). If you reduce the voltage below the nominal rating, the chances of leakage current are also reduced. If you increase the voltage beyond (I should say ?way beyond?) the nominal rating, you run the risk of a ground fault that could destroy the motor.

YES! There will be an overheating problem. That is the primary reason motors should not be operated below the voltages that their manufacturers declare to be the rated voltages. It also explains why the motors last for one year, and not for seven. I have described this process in some depth elsewhere on this Forum, but a quick statement is, ?The rate at which an insulation system breaks down over time increases dramatically if the temperature is increased even slightly beyond its design value.? A rule of thumb is that a 10 degree C rise in temperature will reduce the lifetime by half. So I might guess that if a seven year motor only lasts one year, it must be running 30 degrees hotter than its design value.

Let me now address your client?s statement that ?it ran fine for a year.? I like to use the following analogy, whenever someone tells me, ?It?s never been a problem before.? I have posted this before in this Forum, and likely will again.

Suppose that just before you back your car out of the driveway each morning, you put on blinders and earmuffs.

Suppose that you wait for a random amount of time, and then just back into the street.

When you get into the street, you can take off the blinders and the earmuffs, and drive to work.

Question: If you do this ten days in a row, and if you don?t hit anything during those ten days, would you conclude that this is a safe driving habit? Or would it take 20 consecutive days without incident, to convince you it was safe? 30? How many?

Everyone is welcome to steal this analogy shamelessly.

But give me authorship credit, if you wish to use the following aphorism:

"An accident waiting for a place to happen will, given time, find that place." Charlie Beck

pierre · Jun 23, 2005

Re: Motor operating voltage range?

Charlie B
You really are full of .... qoutes

bthielen · Jun 23, 2005

Re: Motor operating voltage range?

What I'm gaining from this, along with the education, is that motors are far more complex than they are given credit for and have their own set of rules which can vary depending on the type of motor and the application.

Thanks,

Bob

growler · Jun 23, 2005

Re: Motor operating voltage range?

230 volt motor running at 208 volts. One thing that I have seen left out of calculations before is the actual voltage at the motor( voltage drop over a long run not using the higher current). A friend called me about a similar situation a few years back and when I did the voltage drop calculation. I found that they only had about 180 volts at the motor. They had replaced with a more powerful motor and had not redone the feed. Legal for the current but the voltage drop got them. Burned up a couple motors.

Motor operating voltage range?

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