control wiring problem

[mickeyrench]

I was ask to look at a wiring problem that consist of many control wires for a 15 hp motor . Anytime the system is started it trips one or both of the control fuse on the primary side of the control trans instantly.. It is a 480v - 120v trans and the 120v fuse never blows it is always the primary fuse. At first i though it was a bad trans but i test the three push to test lights and they all work . As soon as i put the switch in start position it blows a fuse. I disconnected the motor from the starter when trying to start motor. Also i pushed in the starter manually and it still blew a fuse. It has a electrical remote reset and it works when pushed , that is before fuse blows. Any ideas? thanks for the help

 

[Buck Parrish]

It could be a lot of things. Are you useing dual element fuses?
It may be the motor is just worn out.
Or it is working harder then designed to.

 

[mdshunk]

On the primary side? Weird. And these control fuses serve only the primary of the control transformer and nothing else? If that's the case, I'd only conclude that you have a bum control transformer on your hands.

 

[don_resqcapt19]

What are the fuse sizes?...you could have too big of a fuse on the secondary and a bad coil or a short on what is normally the #3 wire.

 

[LarryFine]

I disconnected the motor from the starter when trying to start motor.

That certainly should have no effect.

Also i pushed in the starter manually and it still blew a fuse.

To me, that's the biggest clue. The holding contact is completing the circuit, so that leaves the contactor coil itself.

I'd try disconnecting its wires and do some resistance readings. You could also temporarily place a 120v bulb in line with (or in place of) the 120v fuse while wiggling some wires.

 

[mdshunk]

To me, that's the biggest clue. The holding contact is completing the circuit, so that leaves the contactor coil itself.

Yeah, but the primary fuse? I'm assuming that the secondary fuses are propery sized and of the factory spec type, which is why I'm stuck on a bad transformer.

 

[Volta]

Is this a new installation or a new problem on existing equipment?
Could it be an inrush (to the trasformer) problem? It would be less than always if so, I suppose.

 

[jim dungar]

Yeah, but the primary fuse? I'm assuming that the secondary fuses are propery sized and of the factory spec type, which is why I'm stuck on a bad transformer.

Transformers are protected by their primary OCPD not the one on the secondary. Proper coordination is not an NEC requirement. It would be perfectly acceptable to the NEC to have a .5A primary fuse, as long as it was not more than 125% FLA and a secondary device of 20A protecting #12 wire.

In general if your transformer was bad, it would blow the fuse even without a load on the secondary. What is your open circuit output voltage?

 

[hillbilly1]

Doubt it's the transformer, since you say the fuse blows when the starter is pushed in manually, It leaves two possible problems, a short in the control wiring on the load side of the start button or control, or a bad coil. Most likely the coil. Take the wires off the coil and insulate them, then manually push in the starter, If the fuse does not blow you have found your problem, If it does blow then the problem is in the control circuit. This is assuming that nothing had been changed before this happened, same transformer, same coil, same size fuses ect. All was working before.

 

[charlietuna]

Bad coil--primary fuse would blow as soon as you changed the fuse without pushing the start button if it was the control transformer.

 

[mickeyrench]

Thanks for all the tips to check out, i 'll be able to check them monday. I never even look to see what the secondary size fuse was just that it did'nt blow. also when checking the control wires i engaged the contacts on the starter one at a time and the fuse did not blow. It had 3 sets of contacts. Thanks again

 

[quogueelectric]

Thanks for all the tips to check out, i 'll be able to check them monday. I never even look to see what the secondary size fuse was just that it did'nt blow. also when checking the control wires i engaged the contacts on the starter one at a time and the fuse did not blow. It had 3 sets of contacts. Thanks again

Check to see that the control secondary is the same voltage as the coil voltage.

 

[steelersman]

It would be less than always if so, I suppose.

what would be less than always?

 

[Volta]

what would be less than always?

The instances of the fuse blowing. I think the OP did say something like every time, which would thenmake my question irrelevant.

If the inrush current was the problem, it would be worse when the transformer is connected at the top of the voltage sinewave if the current lags by 90 degrees. It wouldn't always happen though. I have to keep saying I think, because it is a problem I've been trying to grasp for one of my customers, very similar situation, and I haven't figured it out yet.

Might be a thread soon . . .

 

[quogueelectric]

If it is in a sq d motor bucket the primary control fuses are in a bear of a spot to get at too.

 

[mickeyrench]

yes it is a square d bucket.

 

[bjp_ne_elec]

Thanks for all the tips to check out, i 'll be able to check them monday. I never even look to see what the secondary size fuse was just that it did'nt blow. also when checking the control wires i engaged the contacts on the starter one at a time and the fuse did not blow. It had 3 sets of contacts. Thanks again

Mickey - does the controller have a schematic inside that shows the proper fuse size? I'm also thinking faulty transformer or bad coil. Curious what you find. If the primary fuse was undersized, than maybe it's just a case of improper fuse size.

 

[mickeyrench]

no drawing at all . the primary 480 v fuses was i think 6/10 amp and at one point used a 1 amp fuses and they still blew. i did get a drawing when it was time to go and it does not id the fuse size or trans.

 

[bjp_ne_elec]

Do you have the controller's part number? You might be able to go on line - if not a call to the manufacturer's Tech Support line should get you the proper fusing. Let us know how you solve this one.

 

[bth0mas20]

bad coil in starter.