# Control transformer opinion



## jaymh (Aug 14, 2009)

I bumped up the pressure in a hydraulic sytem to book values and the next day I came in to find the fuse blown on the control side of the control transformer. I assume a faulty transformer. Why would a possible spike on the motor side (480V) blow the fuse on the control side? 
I'm much more versed in D.C. so your opinions will do much to educate me.


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## McClary’s Electrical (Feb 21, 2009)

Did anything else trip?? If wired properly,,,doing what you did should not affect the control fuse. If you overload the motor,,,it should have simply tripped the overload. But if the control fuse was border lined being maxxed out, the split second as the overload opens,,,with control power on coil, could create a small surge blowing the fuse,,,,,but that's a SSTTRREETTCCHH,,,,,and I think (actually) it's a mere coinsedence.


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## wwilson174 (Apr 25, 2008)

Control transformers of the voltage regulating variety are interesting little beasts, I had a puzzling experience with one on a large project which had about 40 motor control centers each of which had one of these transformers. This mcc was down because of a short circuit which brought the voltage down to practically zero . It turned out that the transformer could not deliver enough current to blow the fuse! Everything was hot as a firecracker and the fuse did not blow. I had seen this once before and the solution was to replace the transformer with one that did not have the voltage regulating feature. In this instance the panels all had data processors which reportedly required this feature. I reported the incident to the engineers who designed the system. No solution was found before I finished the job and moved on. BillW.


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## McClary’s Electrical (Feb 21, 2009)

I've seen this same problem in several locations,,,,,however, I don't see it as a problem. It makes troublshooting easy,,,instead of blowing fuses everytime you wanna try it again,,,,,you can just simply go the control cabinet and start lifting 24 volt outputs,,,,,when you see the power supply come from 4-6 volts output, back up to 24,,,,,you konw exactly what 24v circuit the short is in!!,,,,It works perfect. In those situations, I'm glad the fuse doesn't blow.


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## Richard Rowe (May 25, 2009)

I would agree, the control fuse should not be affected by the load of the pump motor at all. The control voltage whatever it may be job is to engage the coil that should not have changed. I know when you make an adjustment and something happens soon after you look at he work you did, but I don't see how what you done would blow a control fuse. I am assuming that all the heaters and load on the pump motor were correct.


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## Toronto Sparky (Apr 12, 2009)

Only thing that may affect it is if the contactor was chattering. 
That tends to take out control fuses if they're close to the limit.


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