motor collapsing fields

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hlevinson

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The other day I was called out to a marquee (moving electrical sign) because of a faulting mechanical moving part. By accident, I hit a shunt switch that killed (opened the main) the power to the entire sign. Nothing major happened from activating the shunt, except the embarrassment I had to endure of having to call someone out to unlock the main disconnect enclosure, to through the switch closed, and turn the power back on to the marquee.

The following day, the company that built the sign, E-mailed me back claiming the following;

?The motor (for the sign) is a giant inductive load, and when the field collapses on a big power dump (from activating the shunt), it's possible to injure the fuses to the point where they'll pop before the drive faults out. (They're a semiconductor based 'amp trap' current limiting fuse, and the semiconducting material can be partially fried to the point where it will blow at a lower current limit level on the next high current event).?


My question is;

Is this true? If true, can some direct me in the direction, information or understanding on motor collapsing fields and possible over currents it can cause?
 
Re: motor collapsing fields

Typically when a field is collapsing, in synchronous machines, it creates high voltage at the terminals with an oposit polarity. There should be some circuits (resistor and diodes) that will short them in order to protect the windings and other devices. If the shunt trip creates 'problems', then every black out would create the same problem.
 
Re: motor collapsing fields

One of the characteristics of an inductive load is that it does not like the value of current flowing through it to change. It will create a voltage that will oppose any change in current. The faster the current tries to change, the higher the voltage. If you try to change the current instantly from full load to zero (i.e., using the shunt trip), the generated voltage can be extremely high. So I am willing to accept what the manufacturer told you.
 
Re: motor collapsing fields

Lets get serious. If suddenly stopping an induction motor caused fuses to "partially melt" and eventually fail, then why aren't there problems with every across-the-line starter?

Amp-Trap fuses, by Ferraz-Shawmut, are for use with semiconductors, they do not contain semiconductor material.
 
Re: motor collapsing fields

Semiconductors can be destroyed by back emf generated by inductive loads if they're switched off but still electrically connected to the semiconductor devices. Almost every design that uses semicondors to drive inductive loads also has protection for the back emf. That protection is not perfect and can fail. More accurately, get blown up.

Edit: And with the back emf protection out of the way the semiconductor driving the load is next.

[ August 09, 2005, 04:23 AM: Message edited by: physis ]
 
Re: motor collapsing fields

Several years ago I was called for a problem on a large irragation pivot. The complaint was every time the unit was shut down it blew the fuses to the pump. I found it had been wired off of one contactor with the fuses for seperate motors after the contactor. The collapse of the drive motor field along with the very long conductors to get to the drive motor was feeding back and when it would hit the fuses and the pump motor it would blow the fuses. Even though the contactor was de-energized, the current generated by the inductive collapse was more than enough to blow the fuses.What I found strange here was the drive motor circuit was not extremely large but the length of the feed conductors out to the pivot seemed to amplify the effect.
 
Re: motor collapsing fields

I would like more information. This sounds like a case of finger pointing or an excuse to shift blame (i.e. not honor a warranty) on any future problems.

It appears that the subject motor is fed by a drive of some type. Unless the drive has a re-gen feature, the inductive kick from the motor field will not be passed back to the line side and the AC fuses. The AC side of any electronic motor controller should remain energized until the controller has stopped, allowing the controller's protective functions to operate correctly. Controlled stops are permitted in NFPA 79.

There is no claim that the fuses have failed, only that they have been "weakened". The only fuses I have ever heard of getting weak are the old "loaded-link" design.

There would be less of an argument if the claim was the internal controller's circuits have been stressed due to overvoltage.
 
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