Motor Overload Problems

[westmc]

Hi,

A motor i was working on today started, then after about 0.1 seconds trips on "start-up current" the motor FLC is 100Amps and its start up in rush is 600Amps but the overload unit doesn't give the motor a chance to get upto running speed to allow the inrush current to drop. The motor spins free.

Sounds like the overload unit is buggered to me any other input from people much appreciated.

 

[cokeguy]

A bit more info is required, but assuming you have ruled out short circuits, checked the wiring and megged the motor, and are using an MCP for short circuit portection with adjustable trip point, what´s its current setting? Remember some motors can take up to 1200% FLA on startup. Are you starting direct from the line or use wye-delta starter, or something else?

 

[DickDV]

US overloads have a current-time curve described as Class 10, Class 20, or Class 30. You may have a Class 10 or 20 overload block when you need a Class 20 or 30.

Be careful with making this change however. The motor is ultimately the determining factor on which Class to use. Using a longer time curve overload block than the thermal rise time of the motor will permit simply fails to properly protect the motor.

 

[waross]

Is this a new installation or an previously good system which has failed?
respectfully

 

[westmc]

It is a DOL starter on a previously good system. The system has been shutdown for 2 months and now requires starting which just is not happening. The motor has been meggered and spins freely

 

[54494060]

westmc,
You said that the motor tripped on start (1/10 second?) after it ran. I'm not sure it ran, other than the inrush moved the rotor. I'm assuming it tripped the supply breaker on instantaneous. If that is correct, it tripped at the supply breaker, so that rules out overloads.
If I am correct in my interpretation of your issue, you may need to test or replace your supply breaker or, you could have moisture in your motor which you could try and bake out.
If I am incorrect in my interpretation, I appologize.

12fish
As Iron Sharpens Iron, so does a man sharpen the coutenance of his friends.

 

[jraef]

A motor i was working on today started, then after about 0.1 seconds trips on "start-up current" ...

Still need more info on your system. From the above, I interpret that you have something in the Overload Relay that is telling you this, i.e. some sort of intelligent OLR. Is this the case? If not, how are you determining "start-up current" is the cause of your trip?

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[GGOSS]

If you could advise us of the type of overload being used, it would be a great help.

Regards,
GGOSS