vfd problem

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puckman

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ridgewood, n.j.
at work today a large motor is started and one of two vfd drops out on undervoltage . it must have a timer in the circuit because each time [ twice] it would take 20 minutes to clear the vault. anyone have any reason why only one vfd would drop out? they are the same unit and right next to each other. they are mitsubihi z200 maybe 1992. whats the best way to prevent it from happening again?
 
I found a place in Virginia called Precision Electronics. Very helpful and can test and rebuild almost and VFD and get it back to you in a couple of days.
PM me and Ill send you the contact info.
 
puckman said:
at work today a large motor is started and one of two vfd drops out on undervoltage . it must have a timer in the circuit because each time [ twice] it would take 20 minutes to clear the vault. anyone have any reason why only one vfd would drop out? they are the same unit and right next to each other. they are mitsubihi z200 maybe 1992. whats the best way to prevent it from happening again?
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I suppose, for various reasons, there could be marginal differences in the point at which the VFD detects and trips on undervoltage. If one trips that would reduce supply loading and reduce voltage drop thus reducing the probability that the other will.
Prevention?
Maybe the first thing to look at is just how much drops supply voltage drops when the large motor is started.
 
I agree with Besoeker, could be slight differences in the the tolerance of the the undervoltage relay. Check the drop, if it's not too great the VFD may be adjustable to not pick up the small drop. The VFD's in question are not tied to the motor that is started, is it?
 
Then i would just check the severity of the drop and go from there. See if the VFD undervoltage limits can be adjusted. I'm assuming putting the motor that causes the issue on a VFD would be impractical or to costly?
 
On a VFD, the UV trip function is based on the DC bus voltage. If a motor is running and maybe overhauling at the moment the larger motor causes the sag, then it may keep that VFD from tripping when another does. Then, as mentioned above, when the other one trips, it lessens the sag and that same one never trips. Just a scenario to help explain why there would not necessarily be consistent between two drives.

But to chime in, the real solution is to solve the sag.
 
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