motor troubleshooting

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Jraef

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weressl said:
That is not exactly corect. The thermal protection model in the ASD software takes into consideration the speed and the load characteristcs. The thermistor is to protect against misapplication or rather to extend the motor application to conver all possible scenarios. I would say 95% of aplications could live without thermistors and it is an unnecessary expense. It is very useful in hazardous(classified) aplications, even though it is not required.

I don't disagree with that, but if you read those VFD manuals in detail you will likely see that the thermal protection modeling they preform is based on an assumption that you are using a motor rated for inverter applications. Such s not always the case; many people retrofit VFDs into older existing motors that were never designed to handle the excess heat created by the output harmonics and reduced cooling. Maybe not a great idea, but it happens all the time. There are also a lot of motor salesmen who, when confronted with an inverter application, seem to think the entire concept of inverter duty motor designs is a scam and not to be paid any attention. The problem is, they are usually not around when the motor later fails.
 
Jraef said:
I don't disagree with that, but if you read those VFD manuals in detail you will likely see that the thermal protection modeling they preform is based on an assumption that you are using a motor rated for inverter applications. Such s not always the case; many people retrofit VFDs into older existing motors that were never designed to handle the excess heat created by the output harmonics and reduced cooling. Maybe not a great idea, but it happens all the time. There are also a lot of motor salesmen who, when confronted with an inverter application, seem to think the entire concept of inverter duty motor designs is a scam and not to be paid any attention. The problem is, they are usually not around when the motor later fails.

Actually no.

The thermal modeling is conservative and is based on standard NEMA design B motors precisely because the manufacturer assumes the worst reasonable application conditions.

I routinely retrofit ASD's to existing non-inverter rated motor with careful application review and appropriate setting. Out of the 100+ I had one winding failure that I know of. Not unexpected though, and it was a small 5HP (3.5HP runing) semi-constant torque application. Lasted about 2 years of continuous duty.

Now of course the advice I always leave is to replace the motor with an inverter rated motor at the time of winding failure, but there is no reason - in most cases - why a TEFC severe duty rated motor could not be used on a drive for a variable torque application with 10-30% minimum speed limitation.

Again, on a case-by-case basis.
 

Ranch

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I didn't read everything in the thread but surfed through looking for the word "insulation" and could not find it. But I did see inverter duty mentioned on the motor. NEMA MG30 part 1 (I think or is it MG31?) and IEEE 841 for example - we use the latter.

Can we determine if the motor failed "in phase" (meaning not ot ground) possibly between the first and second turns of a phase winding? (Your megger may not know this, surge testing may). If so, consider insulation on your 460V motors being a minimum of 1600V.
 

hillbilly

Senior Member
One thing not yet mentioned is the drive application.

You can't just install a VFD (ASD) on a existing drive system without doing some homework and expect it to perform.

Drive loads can be defined as requiring Variable Torque, Constant Torque or Constant Horsepower, to perform the intended work.

This is determined by the characteristics of the load on the motor.....does the motor need to produce more torque at lower speed (constant horsepower), less torque at lower speeds (variable torque), or constant torque at all speeds (constant torque) to perform the work......
These factors (among others) will determine the type of drive (and motor)required.

just my opinion
steve
 
mdshunk said:
Huh? I guessed it was probably a lack of cooling, you say that's nonsense. Then Jref brings it up and you say "exactly right". You're just mean.

I did not say, nor did I mean to imply, that all of the comments were 'unqualified nonsense'.

Lack of cooling is certainly one of the reason, even though in the case of variable torque loads the 40/50% argument is wrong. The fact is that 10:1 turndown ratio is suggested by most motor manufacturers and based on load characteristics, below 30% the curve really flattens out, very difficult to control and you don't get much useful work out of it anyway. But I did not have that in mind as "unqualified nonsense'.
 
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