Motor Failing--Is it the Insullation?

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fifty60

Senior Member
Location
USA
I have a 3phase motor that is failing. What is happening is that it is intermittently shorting out and blowing the fuses. Would this be from failing insulation? I believe so, but I am a little confused as to why it is intermittent and not a continuous short from the failed insulation.

The motor uses type B insulation, with a 66 C temperature rise at rated load and 40C. This is kind of pushing the limits of class B insulation right?
 

fifty60

Senior Member
Location
USA
I unfortunately do not have that information, though I really wish I did. You are asking what is the resistance reading with the motor not running? Or are you asking about the temperature of the windings derived from the insulation resistance method?

I do not have the motor in front of me, all I have are the specs for the motor and the information that the motor is sporadically blowing the fuse. The motor has overload protection, but the overload is never opening. So it must be a short from the insulation right? Why would it not be a continuous short?
 

kwired

Electron manager
Location
NE Nebraska
I unfortunately do not have that information, though I really wish I did. You are asking what is the resistance reading with the motor not running? Or are you asking about the temperature of the windings derived from the insulation resistance method?

I do not have the motor in front of me, all I have are the specs for the motor and the information that the motor is sporadically blowing the fuse. The motor has overload protection, but the overload is never opening. So it must be a short from the insulation right? Why would it not be a continuous short?
Megger is your friend here. If that test passes look at fuse holder next - it may have weak connection and is producing too much heat and that is taking out the fuse.
 

fifty60

Senior Member
Location
USA
There are two motors on the same machine. One of the motors started doing this same thing, intermittently blowing the fuse, and it eventually completely failed. Now the other motor is starting to also blow the fuse, but not constantly. First thing I would do would be to use the Megger, and if that passed I would try to measure the temperature of the windings using the resistance method. I am in the process of trying to get this done.

What I would like to know is, does this make sense that it would be failing insulation. Intermittent short circuits, and then the motor stops working. Makes sense to me, but this is my first time dealing with something like this.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
What I would like to know is, does this make sense that it would be failing insulation. Intermittent short circuits, and then the motor stops working. Makes sense to me, but this is my first time dealing with something like this.

most motor failures I have run unto are about like that. Intermittent trips of the OCPD, and eventually it trips and can't be reset because the short is continuous.
 

big john

Senior Member
Location
Portland, ME
How many fuses blow during each fault?

If you are repeatedly blowing fuses in groups of two or three, and seeing how this affects multiple motors and is intermittent, then I would bet a lot of money this is not insulation failure.

I think either the driven load is mechanically binding causing the motors to pull locked rotor current or maybe you have an intermittent single phase condition causing the same serious overcurrent.

Overload relays do just that: Trip on sustained overload current that produces a lot of heat; there are a number of faults that may not trip them, even assuming they are sized properly. You may be able to rule out single-phasing if these are the electronic style OL with phase loss detection.
 
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Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I have a 3phase motor that is failing. What is happening is that it is intermittently shorting out and blowing the fuses. Would this be from failing insulation? I believe so, but I am a little confused as to why it is intermittent and not a continuous short from the failed insulation.

The motor uses type B insulation, with a 66 C temperature rise at rated load and 40C. This is kind of pushing the limits of class B insulation right?
Class B insulation, even on a motor with a 1.0SF, means 80C rise (no idea where the 66 came from) over an ambient of 40C, so that means it is good up to 120C, which is 248 degrees F. If your motor is over 248F, blowing fuses is EXACTLY what you want to see, because they are preventing a FIRE! But I doubt that's what is going on.

What size fuses and what type?
What is the motor HP or FLC?
What is the motor running?
How is it turned on and off, and more importantly, how often?
 

Tony S

Senior Member
I unfortunately do not have that information, though I really wish I did. You are asking what is the resistance reading with the motor not running? Or are you asking about the temperature of the windings derived from the insulation resistance method?

I do not have the motor in front of me, all I have are the specs for the motor and the information that the motor is sporadically blowing the fuse. The motor has overload protection, but the overload is never opening. So it must be a short from the insulation right? Why would it not be a continuous short?


I give up, you want people to guess what it may be.

No valid readings equals guesswork.

It could be the motor. It could be the cable. It could be the panel. It could be underrated OCPD. It could be the earth has tilted on its axis causing a disturbance in the magnetic flux.

Who is replacing the blown fuses?
 

kwired

Electron manager
Location
NE Nebraska
I unfortunately do not have that information, though I really wish I did. You are asking what is the resistance reading with the motor not running? Or are you asking about the temperature of the windings derived from the insulation resistance method?

I do not have the motor in front of me, all I have are the specs for the motor and the information that the motor is sporadically blowing the fuse. The motor has overload protection, but the overload is never opening. So it must be a short from the insulation right? Why would it not be a continuous short?
Does not have to be motor insulation, can be any point in conductor(s) between the fuse and motor as well. Maybe you have a bare spot on a conductor that only occasionally vibrates over some grounded object and makes contact. Megger is still your friend, don't meg the motor only meg the supply conductors as well.
 

fifty60

Senior Member
Location
USA
The motor is a 1HP motor that is rated at 575V, and a rated FLA of 1.5A. The service Factor is 1.25, it is a type 3428M and a XPFC enclosure. The fuses are 3A type CC. The overload is external, and is set to the nameplate value of 1.5A. The highest amp rating I record is 1.45A, so it is withing the FLA.

1 of the motors on the machine see's drastically different temperatures than the other. The surface temp of one motor is 87 C, while the surface temp of the other motor is 71 C. The motor with the 87 C surface temp failed first, and was replaced, and now the other motor with the surface temperature of 71 C is beginning to fail.

I hate that all I have are surface temps, and I am pulling teeth trying to get better information to work from. The 66 C temperature rise is the manufacturers tested data. I have used the 460VAC version of this motor in the past with no problems. The 575V version was custom, and uses the same frame size and insulation. The manufacturers tested at rated load temperature rise for the 460VAC version is also 66 C.

What I am thinking has happened here is that the manufacturer built a custom 575VAC motor using the 460VAC design, used the same frame size and insulation class. They probably should have used class F insulation.

I'm trying to get the Megger testing done on the motor, but the motor is several hundred miles away. Trying to decide if I should have the motor rebuilt with the class F insulation, or just use a step down transformer to 460VAC and use the 1HP 460VAC motor I know works in this application...
 

kwired

Electron manager
Location
NE Nebraska
If it is motor insulation breakdown the motor is essentially toast already, just not blackened toast yet. Just a matter of time before it won't work at all anymore.
 

matt902

Member
Location
Canada
How many fuses blow during each fault?

If you are repeatedly blowing fuses in groups of two or three, and seeing how this affects multiple motors and is intermittent, then I would bet a lot of money this is not insulation failure.

I think either the driven load is mechanically binding causing the motors to pull locked rotor current or maybe you have an intermittent single phase condition causing the same serious overcurrent.

Overload relays do just that: Trip on sustained overload current that produces a lot of heat; there are a number of faults that may not trip them, even assuming they are sized properly. You may be able to rule out single-phasing if these are the electronic style OL with phase loss detection.



I agree.This is common.A motor goes bad or trips an overload and no one investigates the problem mechanically.
 
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