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Topic Title: Star Delta Starter - Transition Issue
Topic Summary: Any star Delta experts?
Created On: 22 July 2013 10:03 PM
Status: Read Only
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 22 July 2013 10:03 PM
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Joined: 25 October 2006

Good Evening, I have an issue at work which I would love to get some opinions on relating to the change over of a hydraulic power pack motor from star to delta.

The motor is fitted to the crane of a ship under repair at our shipyard. We have removed the motor, it was despatched to a repair workshop and overhauled. The motor is a 105KW 440v 60 Hz induction motor started by a standard open transition star delta starter. the FLC current of the motor is 155A.

The vessel is currently connected to our grid system via a hook up cable. The first circuit breaker this supply goes through is a 630A MCCB which is set to 0.6 of this to give an overload setting of 378A. The instantaneous magnetic setting is X10 this 378A giving 3780A.

The motor is started and runs in star ok but when transitioning to delta this "shore supply" MMCB trips.

We have also removed an identical motor , overhauled it and replaced it. This starts on the same supply with no issue at all.

The connections have been checked and rechecked. The motor windings balance out ok (checked accurately with a micro-ohmmeter), insulation resistances are fine. When started both motors take 30A per phase during the start period.

There is no sign of excessive arcing at the contactors of the motor that trips the supply, both timers for transition are set identically.

Now I feel that this could be normal in the sense that during the transition there could well be a transient of well in excess of the instantaneous setting of the MCCB and I propose that we try the motor again on the ships generators ( the generator ACB`s are much higher rating and have instantaneous settings of over 10KA) The problem with this is that the vessels superintendent is adamant the motor is at fault and wishes us the remove the motor for testing. We cannot try it on generators for another 24hrs and his schedule is tight. My worry is also that if we remove it find nothing and then replace it and we are powered via the ships generators and starts ok, there is no proof that we were not at fault.

The other thing worth mentioning is that the vessel is a 440v 60 hz supplied system and we are currently supplying it at 410v 50 HZ. We did however briefly connect a package generator to the ship via the same MCCB and tried to run the motor, it tripped the same.

I am told by the mechanical engineers it is not possible for a mechanical issue to be the cause and given the star current per phase is identical on both motors I am inclined to agree.

Am I missing the obvious here or am I right to think the MCCB setting could be the weak link? Any views/ help gratefully received.
 23 July 2013 01:01 PM
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Joined: 23 April 2002

What about star/delta change over timer setting is it electronic or electromechanical.
Are the coils on the star/delta contactors suitable for the frequency. Both electrical and mechanical (see-saw) interlocks need to be confirmed.
As you say one motor works and another does not would tend to suggest the motor. Just been overhauled.....reversed motor windings polarity?
Hope you find a solution.
 23 July 2013 01:15 PM
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Check the star/delta transistion timer, is it electromechanical or electronic, is it frequency dependant? Polarity of motor windinds after overhaul, they may have been swopped over/mixed up. Tightness of terminals on top & underside of connection terminal block. Vibration during start up may cause loose connection/unbalance not noticed during static micro-ohmmeter test.
Frequency of star/delta contactor hold on coils. Electrical and mechanical (see-saw) interlocks ? All shots in the dark, lets know how you get on.
 26 July 2013 12:11 PM
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Thanks for the input irnbru - much appreciated. In the end it was just the capacity of the shore connection MCCB. We held our nerve until ships poer was available and then tried the unit again, it worked like a charm.

Thanks again

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