D.C. motor failure, Control problem?

[induction]

I inspected a D.C. motor 150 H.P. . It was shiped in december and has been runing since then. My qeustion is how clean is the D.C. coming from the power supply sappose to be. It is a A.C. rectified into D.C. unit. I have been told it is common for this type of unit not to have pure D.C.. To me that sounds fishy. If a drive has a problem I could see it doing this. The motor was totaly rebuilt and ran fine during testing. Now the comutator is burnt and pitted as if it were in service for a long time. It also cogs, the plant owner said it has allways done this. It would seem to me that any impure D.C. could have this affect. Is there any guide lines for this rectified D.C. ? Is it common for this to happen? Has anyone else seen this condition. Thanks

 

[cbarn24050]

Hi, the dc current waveform is load dependent.

 

[Farkel]

First, I would check the grade of your brushes and the tension on the springs. How old is the com, and has it been reworked? I don't think you have a problem with the DC. What type of DC motor is it?, such as shunt wound, compound wound, series, etc. What type of control are you using? What is the application? Sounds like it may see some overload. What is amp pull and duty cycle? What does the nameplate show?

 

[GusD]

Hello Induction

Other than Chemically produced electricity (batteries),
everything else is only an approximation of DC current.
Generators come fairly close to a pure DC and below it comes the Rectified DC.Good filtering can make it quite good,good enough to fool DC motors to think they are getting th real thing.
Your question is a valid one "how bad can the DC be" before it starts affecting your motor performance.I beliieve ,it would very much depend on the application.
Rectified DC is pretty common in most industries and unless something is terribly wrong with your DC bus filtering,that problem shouldn't happen.
INduction,you mention that motor "cogged" but the customer said it always did that.
When you tested your motor,most likely it was a no load test.Specially with that type of power supply,unless you had a good load on the drive ,not too much would happen.
You mention that motor was rebuilt in December.How long had the motor been used in this application?Was the previous failure similar to what's happening now?
The present condition may not be related to the power supply at all.Severe Commutation problems, for whatever the reason, could make a just repaired motor look like an old one.
A scope check of the power supply,should reveal something abnormal,if in fact ,there is a problem.Better yet ,may be a waveform recorder hooked up to the power supply.You could leave it there,until you have some answers.




GusD

 

[jbartos]

Suggestion: If there are strange circumstances in the dc motor and drive, it is better to have it troubleshooted by specialists. There may be various phenomena involved that even the specialists have difficult time to pinpoint it. However, systematic troubleshooting can find the cause of the problem.

 

[kirit]

When motor tested, was it no load test?
There might be problem of neutrel axis shifting when rewinded or if your motor has interpoles then it might be interpole problem (eiether wrong connection or damage in interpole winding) In case of interpole or neutral axis problem excessive sparking will occur on commutator when full load current is drawn from brushes.

 

[induction]

We are going to test it tommorrow. THe comutator is not that bad it whiped out the brushes though. It was not tested under a load. It has been in service since december Gusd. Cbarn and Farkel they told me that the gear box was just rebuilt. We do have a dino I will see if we can load it down. The brush holders were reset on site. I will get the more information tommorrow. Thanks guys allways helps have other input.

 

[mdonner]

1. Increased brush wear can derive from continuous low load, brush improper material/size, and brush holder spring.
2. Drive problem that may cause such phenomena is at least one SCR firing improperly (SCR or firing control problem). This is easily detected by using scope.

 

[jbartos]

Suggestion: It is good to have enough of voltmeters and ammeters. Oscilloscopes are very useful since they show waveform and can memorize them.
Also, good variable resistors, rheostats, and sufficient voltage source are very helpful for the DC motor testing.
Finally, money should not be saved on good expertise that may be temporarily on welfare, unemployment, student summer vacation, etc.

 

[induction]

Ok I tested the power there is very little A.C. looked more like niose on the scope almost impossible to even see. I ran a test at work also the solid state has some A.C. coming though. We have a generated D.C. that we use also it has a clean output we are going to look into this. The gear head is the answer, this the gear head was rebuilt but it ran with this gear head going bad for almost a month "overloaded". Also the endbell was turned to mount a new blower unit changing neutral,to correct this it was changed by hand on site. During the second time though we reset it on the bench. No more cogging. Gusd thanks. I thank everyone for there input on this matter. Input from this forum is allways helpful.