Effect of Sag or undervoltage in Motors

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physis

Senior Member
Re: Effect of Sag or undervoltage in Motors

I once went searching for under voltage information and curves and found almost nothing. :(

I don't understand why there's so little infomation (at least that I can find) when it's such a concern.
 

tom baker

First Chief Moderator
Staff member
Re: Effect of Sag or undervoltage in Motors

"I once went searching for under voltage information and curves and found almost nothing. [Frown]

I don't understand why there's so little infomation (at least that I can find) when it's such a concern."

NEMA has some very good information, including curves. Did you known that overvoltage can cause a current increase? I found a series of white papers at the Baldor web site called the Cowern Papers from a retired motor engineer.
 

physis

Senior Member
Re: Effect of Sag or undervoltage in Motors

Did you known that overvoltage can cause a current increase?
Well yeah, but I don't remember having trouble finding information on that. (I probably havn't looked either)

I'll take a look at what you've mentioned Tom. :)
 

charlie tuna

Senior Member
Location
Florida
Re: Effect of Sag or undervoltage in Motors

tom,
it has been my expieiance that as you increase the voltage at the power source the current will decrease--not increase.. i used to operate a nuclear plant with large steam turbine generators and also oil fired generators--- as far as current it will decrease as you increase the voltage --- the kw meter will remain constant...
 

tom baker

First Chief Moderator
Staff member
Re: Effect of Sag or undervoltage in Motors

Charlie Tuna:
Both Charlie B and my self are ex navy Nucs. I was a mechanic before I wised up and become an electrician. What nuclear plant did you work at?
When the voltage is increased on a motor, above 10% of the nominal, the magnetic fields saturate, and the current increases.

[ August 14, 2005, 06:40 PM: Message edited by: tom baker ]
 

charlie tuna

Senior Member
Location
Florida
Re: Effect of Sag or undervoltage in Motors

tom,
i agree--once you increase the voltage to a certain point the current will stop decreasing but i believe it has something to do with capacitance and the output's power factor. i was on the comissioning crew of the first 640 class fbm --the ben franklin. we were the first boat out after the loss of the threasher and went though additional testing of systems while in construction. most of this testing was in connection with the threasher's investigation!
 

physis

Senior Member
Re: Effect of Sag or undervoltage in Motors

Tom, you and Charlie B. are both nuclear sub guys?

I been around here for a pretty good while, I had no idea.

Charlie sure is sharp as the nastie end of a locker room towel.

You keep a lower profile though Tom. It would be cool to hear more of your past on the forum.

Well, anyway, :cool: :cool: :cool:
 

ronj

New member
Re: Effect of Sag or undervoltage in Motors

What about when you use a VFD the voltage goes down and also the Freq. What happens to the current then?
 

rattus

Senior Member
Re: Effect of Sag or undervoltage in Motors

The induction motor is a wondrous thing invented by a true genius over 100 years ago. Now I do not claim to be an expert, but I do know that there are equivalent circuits which are useful in understanding the effects of low drive voltage.

As soon as I get my ducks in order, I will post this diagram. It is very similar to the exact equivalent circuit of a transformer.

[ August 16, 2005, 04:26 PM: Message edited by: rattus ]
 

charlie tuna

Senior Member
Location
Florida
Re: Effect of Sag or undervoltage in Motors

ron,
most of the drives have a constant voltage and produce a variable frequency to control the motor's speed..............
 

physis

Senior Member
Re: Effect of Sag or undervoltage in Motors

Rattus, I agree that motors are a very interesting circuit elements. I don't however know of any accurate equivelent. Motors can store energy inertially, I don't know of any possible configuration of capacitors coils and resistors that can do that.

But if you have an image or something you're interested in posting send me a PM or an e-mail and put it up for you.
 

physis

Senior Member
Re: Effect of Sag or undervoltage in Motors

Because I'm such a fantastic individual, I'm just kidding,



I've posted this image for Rattus.


Rattus, I don't quite understand what the problem is but if I try to launch the image directly from it's web space I get the web site's logo or something instead of the image. I was able to copy it to a file on my machine and the put it on the web space that I use.

try http://photobucket.com/
 

rattus

Senior Member
Re: Effect of Sag or undervoltage in Motors

OK Sam, I finally tried Photobucket and made it work.



This equivalent is similar to that of the exact transformer equivalent circuit. As the drive voltage is lowered, the slip increases, and the frequency of E2 increases which increases the rotor reactance. This increased reactance impedes the increase in rotor current making it harder and harder to maintain torque, but the motor tries and eventually smokes the windings.

With "s" = 0, we have a synchronous motor, but that never happens in an induction motor. As a load is applied, "s" increases to provide the needed torque.

When "s" reaches unity, we have a locked rotor, and the motor is indeed a transformer with the rotor conductors being the secondary.

BTW, I ran an unloaded, 1/4hp, 208-230V motor on 240V and 120V. The currents were 1.8A and 0.4A respectively. With a fan for a load, I would expect similar results.
 

physis

Senior Member
Re: Effect of Sag or undervoltage in Motors

Now you just have to take advantage of there "edit" feature and select 50% to "resize" your image.

If you do that the image size should change in your post without having to make any changes to it.
 

physis

Senior Member
Re: Effect of Sag or undervoltage in Motors

And isn't this the same circuit that you once told me can't accurately describe a motor for the same reason I posted?
 

rattus

Senior Member
Re: Effect of Sag or undervoltage in Motors

Sam, I don't think I ever mentioned this circuit because I just found it recently. I believe it to fairly represent a motor at its rated voltage and up to its maximum loading. But, if the iron saturates, this equivalent is no longer equivalent.

BTW, I measured the 1/4hp, 208-230V motor again.

120V, no load: 0.4A
120V, with fan blade: 1.8A and running slow

240V, no load: 1.8A
240V, with fan blade: 1.6A and running full bore
 

physis

Senior Member
Re: Effect of Sag or undervoltage in Motors

I think I found the circuit when I was out trying to find out how induction motors work on the internet.

I'm not sure if I'm remembering correctly though.
 
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