We are a small, remote site doing solution mining. There are only a handful of us here-so we wear many hats out of necessity.
My question, which I am sure is simple for most all here, is a bit beyond me to answer with certainty.

Our problem is: we have a 3 phase, cap start 480 motor that keeps blowing fuses, randomly.
Load, amp draw, meggar, PI, all fit spec.

Question: While most common, probable causes passed. I started looking for other causes and noticed the disconnect switch was rated for 60HP max @480v the motor is rated 75HP. Is that a cause to blow fuses?

The insides of Disconnect switches look the same to me, but there must be a reason..difference.

 

jmcquarrie... there is someting unusual about your observations or I have misinterpreted your problem::

1) Why would a 3-ph,480V motor, need a "cap-Start?"

2) Do you have a fused-disconnect-switch? Or are the two elements, the switch and the fuse-blocks, in different enclosures?

3) And, yes it is possible to overheat the fuse-clips of a 60Hp-rated disconnect if it is carrying the current of a 75Hp-rated motor!

4) Does the fuse "operate" only when starting the motor?

5) Does does the same fuse "operate" each time?

5) What is the type and size of the fuse?

6) Can you provide complete Motor nameplate data?

7) Do you have access to an Infrared-reading temperature-measuring tester?

Regards,
Phil Corso ([email protected])

 

When did the problem start?

Does the motor trip during starting alone?

what kind of load is the motor carrying,variable load or fixed load?

What's the rating of the load on the motor?

what is the means of starting?

what is the fuse rating?

Have you meggered the motor supply cables?

Is the motor using contactors and if yes have you checked the contact surfaces of the contactors?

Is the motor getting the voltage it requires or the supply voltage fluctuates at a point in time?

Is the motor bearings lubricated very well?

Is the motor aligned very well to the load?

Are the motor and load mounted very securely?

Lets answer these questions and something useful may come out.

For the disconnect switch,as long as the load on the motor does not exceed the rating of the disconnect,it should be working fine.However please check if the contacts are latching very well and check the contact surfaces.

Does the motor use a thermal overload protection?, if yes then please check it as well.

I hope this helps.