close transition wye-delta starter

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boyaxski

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can somebody help me out on how a close-transition wye-delta starter works (reference sites)?

we have a new customer which employs this kind of starter on his 150KW, 440V, 249 FLA compressor motor...he was not able to use the motor because the utility's 225 KVA xf bank fuses all blew a few secs after the motor was started...we monitored the current in another attempt and the sequence were: start-on at wye the load was 530A...10 secs later the delta contactors closed and the load dropped to 130A...another second later the load shot up to 1,270A (almost a locked rotor condition)..this condiion was sustained for 1 sec before the xf bank fuses blew again...the starter supplier is still at a loss of what transpired....what could have caused the problem? i believe it is more likely to be a fault on the starter...

thanks for sharing your thoughts....
 
Re: close transition wye-delta starter

Generally there are three resistors, one connected in series with each motor phase to prevent a short circuit condition, during the changeover from wye to delta.

Ed
 
Re: close transition wye-delta starter

In a normal open-transition starter the motor is temporarily disconnect from the power grid (so no current is flowing) during the transition from wye start to delta run. When the motor is reconnected to power there is a momentary inrush (just like in across the line starting) although because the motor is already turning the load the current spike should be relatively smaller than LRA. Too short of a start time will result in an excessive current spike.

In a normal closed-transition starter a set of power resistors is connected to the motor during the transition.
According to a Square D publication "In order to maintain current flow to the motor during the transition from wye to delta, closed transition starters are available which have an additional contactor (2S) and 3-phase resistor bank. The resistors are sized to match the motor winding impedance so that current flow may be maintained during the brief period after the 1S contactor drops out until the 2M contactor picks up."

So,you appear to have a starter that is behaving like an open-transition device. Things to look for are: properly sized resistors (often people forget to match the motor and the starter), properly functioning resistors (an open resistor will cause an open-transition start), too short of an acceleration time.
 
Re: close transition wye-delta starter

Sometimes we have a tendency to overlook the basic BASES....Make sure there're no loose connections....
 
Re: close transition wye-delta starter

thanks for all the help sirs...

there were indeed resistors in the starter, so it is a closed-circuit transition type...i will meet again with the customer and will present your suggestions...i want the problem resolved soon as the longer it gets for the customer to be energized, opportunity lost piles up....ty again....

jim, where can i find the complete Square D discussion on the matter? can it be dwnloaded in the net? thanks
 
Re: close transition wye-delta starter

for a transformer bank of 225kVA, the full load current at the secondary would be 225000/(1.73*440) = 295 amps. The motor has an FLA=249 A. This is ok IF there are no OTHER LOADS that the transformer is supplying. A website www.franklin-electric.com specifies a 225 kVA transformer for their 150 kW motor.
The values between the transformer and motor ratings are too close for comfort for me. if other loads are already operating when the motor starts, a voltage drop would result. and this would probably INCREASE the current draw of the motor, to compensate.

hope this helps
 
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