800 Hp Intermittent Starting Problem

[nmark]

Hello,

 

I have seen a problem with the starting of this motor the last 1 out of 3 starts. And have been told its happened at least once before. The problem is after starting motor (4160 800hp DOL) starter closes momentarily drops out then recloses (approx. 1 second between opening and closing). After starter engages for the second time it runs fine.

The starter coil is 120 volt supplied via a 4160/120 transformer. I did get a fluke 87 with a min max feature on the control voltage (the start control is from a remote panel)to the motor starter before started last time. The motor started fine(contactor closed one time) the min voltage recorded was 101, avg 123 and max 124.

Does anyone have any suggestions as to what the problem might be?

 

Thanks

[AB2005]

Hello,

 

The problem is after starting motor (4160 800hp DOL) starter closes momentarily drops out then recloses (approx. 1 second between opening and closing).

 

The starter coil is 120 volt supplied via a 4160/120 transformer. I did get a fluke 87 with a min max feature on the control voltage (the start control is from a remote panel)to the motor starter before started last time. The motor started fine(contactor closed one time) the min voltage recorded was 101, avg 123 and max 124.

 

 

Thanks

 

Hello nmark;

 

"800HP DOL" is it real? I am sure that the problem is "Voltage Drop".

What is the starting time of motor?

How did you get the 101 min voltage reading? Digital meters are not so quick. Did you use analog meter to measure the voltage during problem?

 

[yuri]

Disconnect the motor's leads and try out the starter working - maybe for hundred or more times, if needed - untill the fault is found. Dont risk the motor's life.

[marke]

Hello nmark

I have seen this happen before where there was an auto - OFF - manual switch that dropped the contactor briefly as switched from manual to auto mode.

 

Best regards,

Mark.

[jraef]

Hello,

 

I have seen a problem with the starting of this motor the last 1 out of 3 starts. And have been told its happened at least once before. The problem is after starting motor (4160 800hp DOL) starter closes momentarily drops out then recloses (approx. 1 second between opening and closing). After starter engages for the second time it runs fine.

The starter coil is 120 volt supplied via a 4160/120 transformer. I did get a fluke 87 with a min max feature on the control voltage (the start control is from a remote panel)to the motor starter before started last time. The motor started fine(contactor closed one time) the min voltage recorded was 101, avg 123 and max 124.

Does anyone have any suggestions as to what the problem might be?

 

Thanks

It's actually quite a common problem on large MV motors when the system is relatively weak. The coltage drop caused by the starting current causes a colapse of the supplying transformer and the voltage drops below the hold-in threshold of the contactor, typically about 80% of rated voltage. For this reason, some people use latching contactors for MV motors. But this is frought with many other potential risks and should not be undertaken unless you know and understand all of them.

 

Probably the reason it is intermittent is that sometimes there are other loads on the line when you start this which contribute to the effect of adding this 800HP to the grid. There are a number of ways to address this if that is the problem. But it is very important that you spend the time and money to investigate exactly why the contactor is dropping out. This is not a trivial event or just a nuisance; that immediate re-close could result in a torque transient that can shear off your motor shaft! Get a good recording meter and look at the control voltage signal over a period of time that includes a drop-out event if possible. Don't delay!

 

By the way, DO NOT decouple the motor and cycle that contactor hundreds of times. MV contactors are not the same as LV contactors, they have a much lower mechanical lifespan.

[yuri]

"Hundred or more times if needed" was an overstatement underscoring the gravity of the situation referred to , but the cost of the motor's repair/reinstallation may without any overstatements hundred times exceed the cost of a new contactor.