# motor blowing transformer fuses



## Jlarson (Jun 28, 2009)

Have a motor company surge test it.


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## MDShunk (Jan 7, 2007)

Different types of fuses, even though they have the same basic amp rating, have different interrupting characteristics. 

Sure sounds like you could benefit from a soft start, and scrap your starter. Either that, or use your starter to bring in the soft start. Sort of a belt and suspenders approach.


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## varmit (Apr 19, 2009)

A 100 HP motor will have a considerable inrush current on starting. Depending on the load/starting time, this is probably what is blowing the fuses. Most POCO will not allow more than a 50 HP motor, without a soft start or wye-delta start, on their system. At start-up, the high motor current causes a voltage drop which increases the current seen by the transformer even more. You need a soft start.

If this motor is a new load at this facility, the utility transformer may be too small for the additional load. Generally for reliability, the transformer rating needs to be 200% of the largest motor FLA. In this case that would be about 225 kva. If this is the typical commercial lease space, it could now be a 100 kva xfmr.


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## JohnR (Apr 12, 2010)

Perhaps the transformers were too highly loaded in your area, I know that the POCO here thinks it is just fine to load up to 180% of rated KVA. 
Over time this could happen......... unless you are the only one on those transfomers?


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## bubbajack (Sep 21, 2010)

Thank you all for your reply, the motor had been working fine for some time on the existing xfmr's which are just for this location only.


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## John Valdes (May 17, 2007)

bubbajack said:


> I have a 480v 100hp motor that when started has blown the power company's fuse twice, but different xfmr fuse each time. The motor is fed from old nema style contactor with newer overload set at 135amps. The starter is also fused at 150amps. Other loads though minimal are on this same service and have no issues. Motor and conductors all the way back to the main service have been megged and no issues noted. Not understanding if the motor is bad why don't overload or fuses in starter blow first. Does anyone have suggestions?


A 100 HP motor really should not be started across the line like you are doing. That is the first thing I would address. Like Varmit said a soft start or wye/delta start system should be employed like the one pictured below. They are less expensive than the contactor arrrangement required for wye/delta starting. I would not use a wye/delta start system as they are out dated and very expensive. Also your electricians will have problems with connecting the motor to that type of set up. (most electricians).

Second. You are way under fused for your motor. Using 135 amps. (I cannot see your nameplate FLA). 350 amps is the maximum fuse allowed on this motor circuit. I cannot see how this motor is not blowing those 150's every single time it starts. Are you sure the motor is fused at 150 amp? and will still start? This concerns me.

Taking this motor out and having a motor shop test it would be expensive and time consuming. If you have a good motor shop and have a good relationship with them, they can bring the surge tester to your facility. Like Marc suggested.

What were the values on the megger testing. Post them for further review.

Lastly. POCO could be your issue. Sounds like it to me from your description of the problem. Ask them what size fuses they are using.


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## RIVETER (Sep 26, 2009)

Touchy issue, but have you tried placing a capacitor on the load side of the starter. The motor current will stay the same but the service current may decrease. Just a thought. Now, I'll go get my helmet on.:thumbsup:


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## John Valdes (May 17, 2007)

RIVETER said:


> Touchy issue, but have you tried placing a capacitor on the load side of the starter. The motor current will stay the same but the service current may decrease. Just a thought. Now, I'll go get my helmet on.:thumbsup:


Purple Helmet. I know who you are now. You have been outed.


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## Toronto Sparky (Apr 12, 2009)

100HP 480v motor can easily pull 350+ amps in-rush even without a physical load.


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## RIVETER (Sep 26, 2009)

John Valdes said:


> Purple Helmet. I know who you are now. You have been outed.


Now, I've got to go back and get my flack jacket.:thumbsup:


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## bubbajack (Sep 21, 2010)

I talked to motor shop owner and he seems to think its a poco issue, so i met with poco at location and megged lines again with them there. again found nothing, so set up 3 peak reading amp clamps and turned starter on and motor ran like a champ. Each meter read appx 1040 amps on start up on 3 different occasions. nameplate says 116fla and running amps was 111, and yes they have had this motor fused at 150 for I dont know how long. poco keeps saying it wasnt there equipment but the motor runs now. I even asked poco if they have any requirements for needing soft starts and such and he said no?!? If the issue was poco... good luck getting them to admit to it i guess.


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## varmit (Apr 19, 2009)

I deal with equipment that has "always worked before" very often. Almost always there are items that were not installed properly initially. Sometimes there is some change: process change, mechanical equipment wear/failure, the original less than ideal installation begins to cause problems, the utility load has increased; or any number of other factors or combinations of factors that causes the electrical equipment to not work as "it's always worked before".

My example:
I checked out a "machine will not run" at a small plant. The motor windings were burnt out. I found that this was a 380 volt motor wired to a 240 volt circuit. It had "sort of ran" for 6 years. The plant manager told me that they would take their foot AND SPIN THE BULL WHEEL TO START THE MOTOR. I had to ask"Why would you do this?" Answer: "It would run after we spun it to get it going." The motor was an ancient commutator motor, so I had to replace the motor and install a new drive and wire everything on the proper voltage.


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## John Valdes (May 17, 2007)

Toronto Sparky said:


> 100HP 480v motor can easily pull 350+ amps in-rush even without a physical load.


True. But the NEC only allows for 350 amp maximum. Edit: 300 amp, for 116 FLA.



bubbajack said:


> I talked to motor shop owner and he seems to think its a poco issue, so i met with poco at location and megged lines again with them there. again found nothing, so set up 3 peak reading amp clamps and turned starter on and motor ran like a champ. Each meter read appx 1040 amps on start up on 3 different occasions. nameplate says 116fla and running amps was 111, and yes they have had this motor fused at 150 for I dont know how long. poco keeps saying it wasnt there equipment but the motor runs now. I even asked poco if they have any requirements for needing soft starts and such and he said no?!? If the issue was poco... good luck getting them to admit to it i guess.


Varmit makes some very good points regarding past and present. I also have heard the infamous "Its always worked before". Many times. That's when I know its time to start checking everything. No offense.

What size wire feeds this motor? Please post that. I already know you have the wrong size fuses and fuse blocks installed. I am fairly confident the fuse blocks you have will not accept 350 amp fuses? Check. Edit: 300 amp
You posted the starting currents. Why are 150 amp fuses not blowing. Are they SUPER DUPER slow acting motor fuses?

Has there always been a 100 HP motor in this same exact spot? If the answer is yes, what about RPM? Same RPM at this exact same spot? Is this a straight run in conduit to the motor with no terminations of the conductors? Have you opened the peckerhead to be sure the connections are insulated well? Just a pinhole would trip the breaker or blow fuses. Check every connection from the primary on the breaker to the motor terminal connections.
Where does the fuse holder and or breaker get its power from? Go there and double check everything too.

Lastly. POCO could have put in larger fuses without your knowledge. And all may be fine now. They may have even fixed something and they don't tell you. The motor shop guy is the one I would trust, not them.

But it your responsibility to make sure your end is up to snuff. I can see two things just from reading your post that are not.


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## Toronto Sparky (Apr 12, 2009)

The fuses time delay? I'm sure the ones one the transformers primary are not..


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## Mr.C (Dec 20, 2009)

What are you using to start the motor?


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## Introyble (Jul 10, 2010)

bubbajack said:


> I have a 480v 100hp motor that when started has blown the power company's fuse twice, but different xfmr fuse each time. The motor is fed from old nema style contactor with newer overload set at 135amps. The starter is also fused at 150amps. Other loads though minimal are on this same service and have no issues. Motor and conductors all the way back to the main service have been megged and no issues noted. Not understanding if the motor is bad why don't overload or fuses in starter blow first. Does anyone have suggestions?


Been there seen it, you sure you have the windings wired properly because it sure sounds like you don't


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

I had the same problem with a 400 HP. Except it always blew "c" phase. The power company eventually changed the entire 750 kva xfmr because of internal problems


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