dc motor

[richerdick]

A dc motor at work is driving a lineshaft that drives 4 seperate levels of machinery via mechanical clutches. we have a problem at work whereby it is throwing out the clutches whilst accelerating. This is happening on a motor drive combination that has been in and running for several years so it is a fault rather than a design issue. It is throwing the clucthes, out say once every couple of days no regular pattern. From past experience i might have looked into the drives reference being at fault but on one occasion the drive stopped due to the blowing of the field fuses. The fuses were changed and the drive ran happily for the next couple of days then the drive again threw out its clutches. I have meggerd the field it seems ok. at the time of writing i have just changed the whole drive to see if this resolves the issue. a couple of days ago i changed the field controlled rectifier but this didnt resolve it
not really sure that changing the drive will fix the problem. starting to think that it might be an intermittent fault of the motors field (shunt). any thoughts?

 

[itsmoked]

Sounds like the drive may be commanding instant rapid speed changes. Do you have feedback of some sort that could be going intermittent?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[richerdick]

hi yes encoder feedback but would that blow the field fuses?

 

[Skogsgurra]

If the field fuses are on the DC side. And if they are close to rated field current, then the fast armature current change _may_ induce enough extra current in the field circuit so that the fuses blow.

I would put a recorder (current AND voltage) in the field circuit to see what is happening. Use a fast one, at least 1 kSa/second. That will show you if there is a short, an intermittent or armature reaction that blows the fuses. Or, if the field rectifier is controlled, if the control does things. How is your excitation current limit set?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[richerdick]

here is what i know about the field.
dc voltage 280v
set to control to 5250mA and when running normally seems to do this +-20ma. the field is controlled by a conrolled rectifier, two phases with diodes on one half of the bridge and thyristors on the other half. the fuses are 20A and on the ac side of the rectifier.
could I trouble you to elaborate on the armature reaction comment. this perhaps seems relevant as we do have a standing issue with these motors where the brush tags come of and arc against the side of the motor. I had disguarded this as being a possible problem because It was the field fuses that blew. In my mind i was discounting the armature as it was the field fuses that blew is that naive.
thanks for the help

 

[Skogsgurra]

OK. As you know, there is a magnetic interaction between armature and field coils. In the best of all worlds, this interaction is very small. But, as load torque rises, the field is moved away from the pole center and saturates the pole edge so that total flux is reduced. That is the so-called armature reaction.

When these things happen very quickly, the flux change induces an extra voltage in the field winding and can cause more field current than normal to flow for a short moment.

But arcing from brush to end bell is a very violent thing that you need to do something about. My guess is that your field fuse problem may go away with the arcing.

Is the fuse a semi-conductor type ("Silized", perhaps) or an ordinary slow-blow fuse?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[richerdick]

Hi it is a semi conductor fuse.
Do you think that I can rule out any reference, feedback issues, from causing the fault with the veiw that the field fuses have blown. Although the drive is throwing out clutches my thoughts were that if the field were for some reason weakening that the motor may accelerate rapidly and torque out the clucthes, I can see a reference or feedback issue causing the clutches to go but not the field fuses to blow. Am I correct.
I will go and check the motor out again and get hold of some sort of recorder
thanks again

 

[richerdick]

Sorry it is not a semiconductor fuse just a hrc

 

[edison123]

Looks like

Field Failure => Overspeed => Clutch throwing.

Cause for field failure ? Ground flash of the brush holders.

Recently, I had to redesign brush holder assembly in two different dc motors due to insufficient creep distance. But in oth case cases, the client did not inform of any field fuse failure.