I've got an old granulator with what I believe to be a 20 horse motor on it, 575V 3 phase, 20 or 30 years old, maybe. The ID plate is no longer legible.
I put a fuse block in for 30A CC time-delay fuses, midget size, and this motor blows them like popcorn.
The motor only draws 11A when running, and the fuses are rated for 150A for at least 10 seconds, so I was amazed when the fuses blew. I put a meter on the motor lines and found that the inrush current is 165A.
I did a megger test and the motor passed without trouble. The amperage isn't perfectly even, ranging from 9.7A to 11.2QA, but this amount of inrush has me baffled.
I can switch to RK5 fuses to I can put 40A fuses on there, but then I have to change the wire gage, etc.
Can anyone suggest why the inrush current is so high for this motor?

You said 20 horsepower at 575volts.

That equates to FLA of about 18.5A (assuming power factor = efficiency = 0.9).
[18.5=20hp*(746 watts/hp)/SQRT(3)/575/0.9/0.9 ]

You measured 165A. That is about 9 times full load amps. Higher than most, but not undheard of.

The allowable range for locked rotor amps can be inferred from the KVA code letter. Or in your case since we don't know the KVA code letter, we can find it from LRA. . The motor drew around 164kva (=SQRT3*575v*165A*1KVA/1000va) in locked rotor conditions. That is 8.9 kva per horsepower. It would correspond to KVA code class K

Most motors are down around F, G, H with less LRC. But motors were made in those higher classes for various reasons. One reason is that you can meet all the performance objectives (starting torque etc) at lower cost if you remove restrictions (such as starting current limit). Another possible reason is to accomodate special-purpose high starting torque requirements.

Another possibility, if your voltage is higher than 575V, your LRA will increase as well.

One other thing is that depending on how your meter processes the transient, you may be seeing the transient decaying DC offset present in the inrush. The "true peak" starting current (for example the highest value you could see on an o-scope) can be up to 2.8 times the locked rotor current during the first half cycle.

This message has been edited. Last edited by: electricpete,

quote:

Originally posted by electricpete:
You measured 165A. That is about 9 times full load amps. Higher than most, but not undheard of.

The 11A _was_ running without load, and if I had a brain (optional equipment on my model year... I went for the heavier chassis instead) then I would have clued in to that.
What was really weirding me out is that I had just changed the fuse block and fuses because I noticed that this machine had been running with 30A 250V fuses for the last five years or so. Running 575V through 250V fuses doesn't seem like a good idea to me, so I changed out the holder and fuses to a proper 600V-rated set, and now they blow. Weird. I guess the 250V RK5 fuses had somehow shorted out internally? Or they just magically could handle 15X their rating in inrush current at a voltage more than twice what they were rated to carry.

The final mental indignity is that the fuses don't blow the first time. Really. You put new fuses in and it starts up, runs all day, then the next time you go to start it up they all blow. You get one free run from each fuse. Bizarre. It's saving my bacon for the moment, but it's still bizarre.

quote:

Originally posted by Metalworker Mike:
so I changed out the holder and fuses to a proper 600V-rated set, and now they blow.

As an update, I installed a holder for 600V RK5 fuses, put in a set of 40A fuses, and the machine starts just fine. I ran heavier wire from the disconnect to the motor starter, too, just for the sake of doing it (it was #10, I ran #8).