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motor star delta starter problem

motor star delta starter problem

motor star delta starter problem

(OP)
Motor started normally on star connection during delta transition supply protection circuit breaker is tripping.
75 KW Induction motor directly coupled to screw air compressor
Moulded case circuit breaker is rated for 160 Amps thermal & 1600 Amps magnetic protection
ganesan

RE: motor star delta starter problem

Star-delta starters..  Worst thing going.

This is very common when using those foul things.  During the transition the motor is disconnected from the power source.  It immediately starts to go out of phase with the source but since it still has its magnetic fields its actually a generator.  So what you have in your case is a 75kW generator out of sync with the supply that is now reconnected to the supply at some completely random phase angle.  You can get 2x short circuit currents.  Every start is a crap-shoot of when the the transition contactor closes.

Wye-Delta starters cause a lot of head scratching - "why did the breaker blow?" - questions.

Keith Cress
kcress - http://www.flaminsystems.com

RE: motor star delta starter problem

To continue, since every start has this gambling aspect to it the more starts the more the odds of a breaker trip.

A higher efficiency motor which typically has lower impedance will likely have a greater propensity for trips.   A compressor that starts, perhaps, dozens of times a day has bad odds.

The delay between shifts can make a large difference in the odds too.  Perhaps your contactors have gotten slower in transitioning?  Perhaps faster? If your setup has a transition delay timer it could be changing in the direction that causes bigger or more common surges.  Your breaker could be going out of calibration.  The adjustment screw mechanism changing.  It could be getting warmer where your breaker is so now it's tripping a little easier?

I'd switch out to a soft starter.  You'll stop shocking your power network and everything else sharing it.  All the other stuff is also seeing those occasional voltage blasts and current surges.

Keith Cress
kcress - http://www.flaminsystems.com

RE: motor star delta starter problem

Have you checked whether the starter is properly wired, especially in Delta ?

Muthu
www.edison.co.in

RE: motor star delta starter problem

My opinion is to connect the motor directly on delta.

RE: motor star delta starter problem

Not withstanding the above, is the compressor itself behaving correctly, for example check the unloader valves if fitted?
  

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If we learn from our mistakes I'm getting a great education!
 

RE: motor star delta starter problem

If the mains are capable to take the start inrush current then this is the cheapest solution. Of course the best way to deal with this is to install a soft starter as itsmoked suggested.  

RE: motor star delta starter problem

Missed the 'screw compressor' in the OP - at 75kW this could have been a recip, which would have been awkward for a soft start. A screw compressor is a good application for a soft start, although you might want to check with the compressor manufacturer to see if a VFD is compatible with the machine, just to get the energy savings.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: motor star delta starter problem

If this is a new installation, I would check the transition timer setting and check the connections at the starter to make sure that no wires are crossed between phases and that the motor rotation is the same for start and run.

If it is an existing installation, I would check the timer, the contactors, and the compressor for proper function. The breaker itself may also be an issue.

The motor is probably not the problem.  

RE: motor star delta starter problem

Nice post rhatcher. Covers all possibilities.

Muthu
www.edison.co.in

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