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3 Phase Motor Blowing Fuses
2

3 Phase Motor Blowing Fuses

3 Phase Motor Blowing Fuses

(OP)
I went to troubleshoot a motor that is blowing fuses on one leg of a 3 phase motor,  its fed from a 1 phase to 3 phase converter(no nameplate on the converter) the line side breaker is 100A, Im assuming its a 3 phase delta with a high leg, L1  119v(the problem one), L2 119v, L3 221v, L1 to L2 234v, L2 to L3 249v, L1 to L3 251v. The voltages are the same at start up, running at no load and load.  the motor is a 20hp  51 FLA its on a sander. when I got there the fuse on L1 was blown(60A Time-Delay in a fused disconnect).  I replaced the fuse and everything started fine. The  amps are: L1 28A, L2 18A, L3 18A.(bad motor maybe?) I started and stopped the motor several times with no problem.  All connections from the converter to the motor are tight and OK.     One hour after I left they called me again, same problem.  The operator said the Motor Load Percent Guage(which happens to be on the leg that is the problem one) went from 75% to 0% and was still running.  he decided something was wrong and shut it down, when he hit the stop button(not the E stop) everything stopped and all the control lights went off, and will not restart.  Hes conviced it blew the fuse when he hit the stop button(I dont think so, I think that leg blew the fuse, and the motor continued to run 1 phase until he shut it down)  
Tomorrow i will meg out all the motor leads, anything over 100,000 is OK right?
What esle should I look for/test. after megging the motor im not sure where to go?
This has been running fine for the past 10 years, no changes at all.

Thanks in advance for all the help!

RE: 3 Phase Motor Blowing Fuses

Please check/measure your sander motor's phase winding resistances. My calculations indicate a very low phase load on L1-L2 (based on your readings). Motor could have busted one phase.

RE: 3 Phase Motor Blowing Fuses

(OP)
I will meg it out tomorrow,  do you mean a broken winding when you say busted one phase?

RE: 3 Phase Motor Blowing Fuses

Insulation resistance testing (megger) will tell you whether your motor windings are grounded. What I meant with "busted" is the motor winding burnt open (open-circuit)or partially-open.

RE: 3 Phase Motor Blowing Fuses

2
Check the fuse holder to make sure that it is clean and tight. A loose connection may generate enough heat to blow a fuse.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 3 Phase Motor Blowing Fuses

(OP)
thanks for the help, it was a bad fuse holder, it was still tight but got hot afer only 20 minuites of run time(just old I guess) new disconnect and problem solved.

RE: 3 Phase Motor Blowing Fuses

A lps to Bill for nailing it right out of the box.

Muthu
www.edison.co.in

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