Effect of Sag or undervoltage in Motors

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Any study on the effect of sag or undervoltage incidence frequency to the life of induction motors?

Have there been a study on the effect of sag similar to the CBEMA curve for induction motors :eek:
 

coulter

Senior Member
Re: Effect of Sag or undervoltage in Motors

Under voltage wouldn't matter to a transformer. It might even be happier - lower magnetizeing current.

I'm probably the only one out here that doesn't know what a CBMEA curve is.

carl
 

charlie

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

Carl, I doubt that you are the only one. I have not heard it referred to until today on this forum. The acronym stands for "Computer Business Equipment Manufacturers Association guidelines for operation of electronic devices. It is essentially a damage curve in reverse. Basically, if you build your equipment within these parameters of a voltage supply, you will have no major problems with your equipment. The supply voltage should stay within the highs and lows of the curve and not have any dropouts longer that shown on the curves. :D
 

charlie

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

Pwrtech, yes, some studies have been done by different manufacturers and curves have been published. I am not aware of which ones have done that but I do remember seeing a set of curves with the under and over voltages plotted with the efficiencies. :D
 
Re: Effect of Sag or undervoltage in Motors

hello carl and charles

Thanks.

Need the technical explanation on the effect of voltage fluctuation on induction motors.

poweringtech
 

pqtest

Member
Re: Effect of Sag or undervoltage in Motors

Hello poweringtech-

For Carl and Charles, CBEMA changed their name recently to the Information Technology Industry Council, and I believe that their web site is www.ITIC.org. The CBEMA (now ITIC) curve is available for free download if you would like more information. Click on "reports".

poweringtech, I did a power quality study for the Department of Transportation recording and looking at the effects of voltage sags. Almost 47% of all equipment failures could be attributed directly to sags, including damage to several pan/tilt motor bases for traffic cameras. We did not look at motors with any real horsepower, but I suspect that most can safely handle the typical sag, which is just a few cycles in duration.

Undevoltages, if low enough, can cause overheating of motors and transformers and might be of greater concern.

If you PM me, I'll send a copy of our paper.

Thanks!

Mark
 
Re: Effect of Sag or undervoltage in Motors

Hello Mark,

Thank you very much but what is PM and how will I do it.

Thanks for your help

I removed you e-mail address. Charlie

[ November 23, 2004, 05:44 AM: Message edited by: charlie ]
 

charlie tuna

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

i always used a rule of thumb---an induction motor running at "full load" will almost double the amperage through it's windings with a reduction of only ten per cent of the rated voltage.
 
Re: Effect of Sag or undervoltage in Motors

Hello Charlie Tuna,

Thanks for that wonderful rule of thumb.

Will it be safe to say that when the voltage is 20% below ampereage will be four times the rated current? :roll:
 

charlie tuna

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

the key words are "running at full load" --most motors do not run at their full rated horse power.
i have played with test equipment while controlling generator voltage and the ten per cent reduction in voltage near doubles the amperage drawn by the motor! i would think going below ten per cent would smoke the motor?
 
Re: Effect of Sag or undervoltage in Motors

What is the exact relation between undervoltage and current drawn by the motor. Can someone explain what exactly happens when the voltage at motor terminals drop?
 

charlie tuna

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

re,
the way i look at is this:
the motor is connected to a load that requires say-5 horsepower to lift a weight. it will require 5 horsepower's worth of watts to lift this weight. as you decrease the voltage the amperage will increase to maintain 5 horsepower's
worth of energy.

i was involved in some testing of 125 horsepower motors on a nuclear plant and the test equipment readings showed that these particular pump motor's current would double as we reached a ten per cent reduction in rated voltage.
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: Effect of Sag or undervoltage in Motors

Originally posted by charlie tuna:
re,
the way i look at is this:
the motor is connected to a load that requires say-5 horsepower to lift a weight. it will require 5 horsepower's worth of watts to lift this weight. as you decrease the voltage the amperage will increase to maintain 5 horsepower's
worth of energy.
Charlie that is an important part of this question.

Your example of a motor lifting a weight is a great example of the mechanical load staying constant as the voltage drops.

Now change the mechanical load from lifting a weight to driving a fan, as the voltage drops the motor will slow, the fan will move less air, the mechanical load will be less and the current drawn will not go up it may even go down, that is until we drop the voltage to low.

Would you agree?
 

charlie

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

Bob, I don't agree. A motor is set up to rotate with a set percentage of slip and is driven by the supplied Hz. If the voltage on you fan drops, the motor will still try to keep up and draw more current to do so. :D
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: Effect of Sag or undervoltage in Motors

Originally posted by charlie:
Bob, I don't agree. A motor is set up to rotate with a set percentage of slip and is driven by the supplied Hz. If the voltage on you fan drops, the motor will still try to keep up and draw more current to do so. :) I even used the same augment about design speed.

I have since changed my mind.
 

physis

Senior Member
Re: Effect of Sag or undervoltage in Motors

Thats cool, I felt the same way until about a two years ago. I even used the same augment about design speed.

I have since changed my mind.
I've noticed that, I think it was less than a year ago.

Imagine my surprise when I thought we agreed and one day, from out of nowhere, you wouldn't believe me. :D
 

charlie tuna

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

the design of the induction motor causes it to try to maintain the same speed which is a function of frequency. it will slow down because the slip will increase somewhat as the voltage goes down.
 
Re: Effect of Sag or undervoltage in Motors

For now let's assume that the motor must maintain a constant mechanical torque.
When voltage drops (at full load) the input electrical power decreases and this results
in lower revolution speed. When rotor revolves slower, slip increases and this will cause
the motor to draw more current and this latter will increase the input electrical power and hence
there will be equilibrium between mechanical and electrical powers again.

Is this explanation exact/correct?
 

charlie tuna

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

ree,
remember how an induction motor works! the power required to create a magnetic field in the rotor is induced from the coils in the stator. the magnetic field of the rotor is (near) locked onto the rotating field of the stator. this is a function of frequency. the motor's "slip" will begin to increase as the strength of the field weakens -- but this will cause more induced current into the rotor due to the difference in field rotation. as long as the load remains constant the motor is trying to produce the horsepower necessary to do the job. it doesn't care if it's voltage or current -- it needs so many watts to perform the task of the load. but we know that high current will produce high heat that will distroy the motor's insulation and end it's life!
 
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