# three phase induction motor



## John Valdes (May 17, 2007)

If they are new just install them. If they are used, megger the motor to ground and across phases to see if the motors are good. Old used motors should be reconditioned. That means bearings and cleaning. Especially if they have been sitting for extended periods of time. Have your machinist check all the bearing tolerances on reconditioned motors.


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## souranka (Apr 29, 2010)

*burning out of induction motors*

PLease let me know what are the thigns I need 2b careful about to stop preventing the burning out of any motor.What the factors that leads to burning out or damage of the motor.
Regards 
Souranka
09561825708


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## ACDC (Apr 15, 2010)

souranka said:


> PLease let me know what are the thigns I need 2b careful about to stop preventing the burning out of any motor.What the factors that leads to burning out or damage of the motor.
> Regards
> Souranka
> 09561825708


Hi there,
Just wondering, are you a qualified Electrician? 
What is your motor voltage 400v or 525v.......?

Greetings


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## souranka (Apr 29, 2010)

*kind attention:ACDC*

Mr ACDC...
as you have missinterpreted my question this goes for you....
I and every electrical engg know the basic ABCD of motor burn outs..but i wanted to know if there s something else that anybody could add...this s big world and even highly qualified professional sometimes learn from ground level labours and workers...If know smthng other than normal insulation tests,preventing from moistur or dirt or Back EMF concept then post your concept.or else please dont try to measure who knows how much and who has what qualification.
regards 
Souranka


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## ACDC (Apr 15, 2010)

souranka said:


> Mr ACDC...
> as you have missinterpreted my question this goes for you....
> I and every electrical engg know the basic ABCD of motor burn outs..but i wanted to know if there s something else that anybody could add...this s big world and even highly qualified professional sometimes learn from ground level labours and workers...If know smthng other than normal insulation tests,preventing from moistur or dirt or Back EMF concept then post your concept.or else please dont try to measure who knows how much and who has what qualification.
> regards
> Souranka


I actually just wanted to know because i was going to use technical terms that only an Electrician would understand! Not trying to compare/ measure anyone's knowledge!!!. 

Maybe better that i don't advise you though.......


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## John Valdes (May 17, 2007)

Motors burn up for several reasons. Most good motor people will tell you many times its mechanical issues and not electrical. Mechanical issues lead to electrical issues. So if you keep a good preventative maintenance program in your facility, you should fare very well as far as motor life expectancy and efficiency.

Bearings. Always replace bearings whenever a motor is disassembled. At this point along with a fresh new set of bearings from their plastic protective covers and in their original boxes, all machine surfaces should be measured for the tolerance determined by your maintenance body. Tolerances differ from one bearing manufacturer to another. So, pick a close tolerance, but not one that is so tight you cannot realize it. A good hands on machinist is a must in this area. If not, you must use a motor shop or machine shop for measuring and repairing
End bells (outer races) and shaft (inner races) must be included. With small motors a metal receptacle (trash) is in order. Installing bearings without tolerance considerations is plain ignorant. Never store bearings outside of their original sealed containers. (boxes)

Loads and Service Factor. While it is believed that motors can run in their service factor, it is in your best interest never to run them in their service factor unless its an emergency. Get to the bottom of the issue and correct it.
Overload. Set overload relays to motor nameplate FLA, not SF. Like I said above, only run in the service factor in emergencies. 
Under load. While this is a maintenance dream, this is not efficient. Do not increase motor size just because it fits on the same frame. I have seen motors 3 times the size they were when they left a machine production line. This is a waste of money for all concerned.
Keep electrical panels locked and only available to qualified technicians. Operators are notorious for turning adjustable OLR's up to keep production running. 
Starting and Stopping. Motor starting is the most critical period the motor and equipment will endure. Soft starting whether mechanical or electronically can save on machine and motor wear. I have included some pictures of soft starts. These have bypass included. Controlled stops can have a big influence on machinery. Only you can determine when and where to apply specialized starting and stopping.

Finally. 3 phase induction motors are simple machines. Just a little care is require to keep them running efficiently for many years to come. Always look at new cost vs repair cost. Today we replace 100HP motors as repair is just not economical, unless its just bearings and cleaning. This is not true for everyone.
Find a motor shop you can trust and get to know them. Go to their shop and visit. See what types of equipment they have available and what types of chemicals are used. Do they have VIP? Do they have 24 hour service? Things like this make the difference between a guy with a varnish tank and true professional.


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## souranka (Apr 29, 2010)

ACDC said:


> I actually just wanted to know because i was going to use technical terms that only an Electrician would understand! Not trying to compare/ measure anyone's knowledge!!!.
> 
> Maybe better that i don't advise you though.......



Dear ACDC,
If thats the thing,then I apologize .I missunderstood your comment.Please dont mind,Im sorry.:001_huh:
Expecting your views about the topic.
Regards
Souranka


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## ACDC (Apr 15, 2010)

souranka said:


> Dear ACDC,
> If thats the thing,then I apologize .I missunderstood your comment.Please dont mind,Im sorry.:001_huh:
> Expecting your views about the topic.
> Regards
> Souranka


Hi there, no problem 
I guess John Valdes said it all.
Good luck!


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## John Valdes (May 17, 2007)

I was in the mood to talk motors. I know I missed alot of good points as usual.


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## Reece (Nov 24, 2009)

If a motor is running at sycn. speed and the line voltage drops to half the voltage for 19 cycles, will the motor be able to recover from this? I am asking this question, because the power company is telling me they have ways of keeping machines running during this kind of outage or brownout.


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## John Valdes (May 17, 2007)

Reece said:


> If a motor is running at sycn. speed and the line voltage drops to half the voltage for 19 cycles, will the motor be able to recover from this? I am asking this question, because the power company is telling me they have ways of keeping machines running during this kind of outage or brownout.


Your question makes no sense. What is sync? Hz? 
If the voltage drops to half voltage and the Hz do not follow, the motor is toast.
19 cycles? No such thing, unless a VFD is being used. Even then, that would be to slow for most motors. Formula - Volts/Hertz proportional = speed.

Where are you located?


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## micromind (Aug 11, 2007)

If a motor is running at full speed, and driving a load, and the input voltage drops to 1/2 of normal for 19 cycles (about 1/3 of a second), its inertia and that of the load will keep it turning until normal voltage is restored. 

19 cycles on a 60HZ system is not a very long time, even if the voltage goes to zero for 19 cycles, the speed will drop somewhat, but not stop altogether. Even a hermetic compressor motor can ride through 1/3 of a second. 

Rob


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## Electric_Light (Apr 6, 2010)

John Valdes said:


> Your question makes no sense. What is sync? Hz?
> If the voltage drops to half voltage and the Hz do not follow, the motor is toast.
> 19 cycles? No such thing, unless a VFD is being used. Even then, that would be to slow for most motors. Formula - Volts/Hertz proportional = speed.
> 
> Where are you located?



Synchronous speed means speed corresponding to line frequency. For 60Hz 2pole motor, it is 3,600RPM less slip. 

When he said voltage drops for 19 cycles, he meant 19/50sec or half a second (India is 50Hz, IIRC). 

Motor is highly inefficient while it is accelerating to speed, so repeated starting will cause it to burn out, especially with high inertia load such as a large cage type ventilation fan. 

Some motors are specifically designed for repeatedly accelerating high inertia loads, such as elevator duty motors.

For large motors that see a lot of hours, buying new is usually better than getting it rewound. Search around and you will see that high efficiency will no longer maintain the original efficiency after being rewound.


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## Reece (Nov 24, 2009)

60 HZ is .016667 seconds. If the Utility voltage drops for 19 cycles, then the voltage will be down for 19 x .016667 = .31667 seconds. My question is will the motor be able to recover from this if it is 50-80 percent loaded. The load will reduce the speed, because the flux in the motor will drop. I think it will pull high current and trip the overload. What do you think?


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## Reece (Nov 24, 2009)

This is for John Valdes. Here is a picture of one cycle of a *sine wave. Google Electrical sine wave and you will see what I am referring to. *

*I am located in Sanford,NC.*


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## ElectricArcher (Feb 25, 2010)

I don't think the O.L.'s would have the time needed to build up the heat required to trip the O.L.'s in 1/3 of a second!


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## John Valdes (May 17, 2007)

Electric_Light said:


> Synchronous speed means speed corresponding to line frequency. For 60Hz 2pole motor, it is 3,600RPM less slip.
> 
> When he said voltage drops for 19 cycles, he meant 19/50sec or half a second (India is 50Hz, IIRC).
> 
> ...


Once again my fingers outpaced my brain. I did not read the question properly. Thanks for the wake up call.


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## micromind (Aug 11, 2007)

Just about any motor could ride through a 1/2 second loss of power and recover with no ill effects. Even a very lightweight one, like a submersible pump motor. 

Here's an example; years ago, I ran a farm in Alfalfa Oregon. The hogs and cows outnumbered the people by a wide margin. lol.

This farm was 160 acres, and was sprinkler irrigated. It had a 40HP close-coupled pump drawing water from a pond. The control scheme consisted of a hand-off-auto switch, a start button, and a reverse-action pressure switch. The pressure switch would stop the pump in the event of low pressure (like when the pond went dry, and it lost its prime), but only if the switch was in the auto position.

The startup procedure was to have the switch in hand, prime the pump, then press the start button. The pump would start, and the pressure would go way up. Then, the discharge valve was opened. The pressure would go way down. Once the pressure was stable, the switch was turned to auto. 

When the switch passed through the off position, the starter would drop out, but it'd pick back up again when the switch went to auto. This drop-out time was about 1/2 second. 

In the hundreds of times I started this pump, not once did anything ever trip when they were switched to auto. 

If the system were to experience many 1/2 second power losses in rapid succession, the O/Ls would eventually trip. But not with just one, or even several.

Rob


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## Reece (Nov 24, 2009)

Thanks to all that have replied. The PE from the local power company has suggested to install a Constant Voltage Transformer on the control circuit. He thinks this would allow the machines to ride through a 1/2 second power dip.

Has anyone ever used these devices?


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## Electric_Light (Apr 6, 2010)

Reece said:


> Thanks to all that have replied. The PE from the local power company has suggested to install a Constant Voltage Transformer on the control circuit. He thinks this would allow the machines to ride through a 1/2 second power dip.
> 
> Has anyone ever used these devices?


Ferroresonant transformer is not very efficient and it won't ride through for half a second. If the motor has next to no inertia in load, but working against a heavy load, like stirring very viscous material, a flywheel would provide a cushion against shock from sudden change in motor output. 

Trains sometimes ride through non-electrified transition point using momentum and it rides through. Of course, if it makes an emergency stop and gets stuck there, it'll have to be towed out.


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