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.

[bjp_ne_elec]

OK Mickey - it's Tuesday and you're leaving us in suspense. What was the root cause?

[mickeyrench]

to make it short i found a wire going to #2 on the holding circuit had a slice in it . the wire is located behind the fuse block / starter area where it is hidden.
i cleared the wire from touching any metal and with the motor leads diconnected
[ because operations won't let me start motor until today] the starter operated each time i started it. 2x
the 120 v fuse was 3a. i put in 1a. also found bucket had 24v power when breaker was off. it appears to becoming from plc cabinet, this not indicated on print.
#2 wire was not connected backup yet,[ i was taken off to do something else] and i am hoping that was the cause of the problem.
will be back today.

[don_resqcapt19]

to make it short i found a wire going to #2 on the holding circuit had a slice in it . the wire is located behind the fuse block / starter area where it is hidden.
i cleared the wire from touching any metal and with the motor leads diconnected
[ because operations won't let me start motor until today] the starter operated each time i started it. 2x
the 120 v fuse was 3a. i put in 1a. also found bucket had 24v power when breaker was off. it appears to becoming from plc cabinet, this not indicated on print.
#2 wire was not connected backup yet,[ i was taken off to do something else] and i am hoping that was the cause of the problem.
will be back today.
The #2 wire on a standard start/stop circuit should be hot before the starter is pulled in. You indicated that the fuses blew when you pushed the starter in. Is the 24 volts on an aux contact to tell the PLC that the motor is running?

[LarryFine]

The #2 wire on a standard start/stop circuit should be hot before the starter is pulled in. You indicated that the fuses blew when you pushed the starter in. Is the 24 volts on an aux contact to tell the PLC that the motor is running?Wouldn't you need to know which transformer lead is grounded to make such a conclusion?

[don_resqcapt19]

Wouldn't you need to know which transformer lead is grounded to make such a conclusion?
Larry, if the stop start control wires are on the grounded side of the control transformer, the installation is not code compliant. 430.73.

[Smart $]

Larry, if the stop start control wires are on the grounded side of the control transformer, the installation is not code compliant. 430.73.

NEC 2008 divides 430.73 of the 2005 edition into two numbered paragraphs: 430.73 and 430.74... the latter being pertinent to your point. (Additionally, what was 430.74 is now 430.75 under the 2008 code.)

[Smart $]

Larry, if the stop start control wires are on the grounded side of the control transformer, the installation is not code compliant. 430.73.

Forgot to mention that only applies to remote control and not wiring internal to a bucket.

Nonetheless, typical bucket start/stop wiring is as depicted below...

http://i145.photobucket.com/albums/r204/Smart_S/startstop-typicalsqdbucket.gif

[mickeyrench]

I went in today connected the wire to the #2 terminal on the starter and the fuse blew again , disconnected it and starter works fine . wiring seems to be a two wire control , it is operated by hand-off- auto switch . the #2 wire goes to one of the six other contactors in the bucket with the starter. i guess i will wait to get a call that something is not working and finish the job. thanks for all the help. its great to have you guys when i am stuck.

[don_resqcapt19]

I went in today connected the wire to the #2 terminal on the starter and the fuse blew again , disconnected it and starter works fine . wiring seems to be a two wire control , it is operated by hand-off- auto switch . the #2 wire goes to one of the six other contactors in the bucket with the starter. i guess i will wait to get a call that something is not working and finish the job. thanks for all the help. its great to have you guys when i am stuck.
Now that is different. If the system is two wire control, and you have a conductor on the #2 terminal, it appears that they are using what is normally the seal in contact as an aux contact to control other loads or equipment. You really need the control drawing to troubleshoot this type of problem. If there is not one available you may have to trace all of the wires and make one.

[ohm]

Remember if the starters don't pull in completely (dirty, sticking, chattering, etc) they will draw much more current than normal. I would blow down the panel just for grins.

And, I saw a bad control transformer fool an army of electricians and engineers. Load it down with a bulb, just to eliminate it.