Star Delta changeover fault 4

[dave57]

We have a 75kw motor and recently replaced the all the contactors, following this when the motor ran up, it would trip the overload as soon as it switched to delta, If the motor had no load it switched over fine, we found by increasing the magnetic overload level the breaker will stay in under loaded conditions. I have since put a current clamp meter on all three legs and found two spike from 90A to 160A as the delta contactor comes in, the current level then settles and the motor runs fine, any suggestions as to what is happening here??

 

[EEbody]

Marke

Thanks for your information. It has been very helpful as I along with many others are having these spikes occuring with star to delta transition. It is disapointing that more people do not show their appreciation by giving you more stars.

Looking from a logical perspective, If you must have a fast transition to reduce deceleration, so the rotor flux has not decayed completely, would swapping your delta phases help, as your phase angle upon closing would be completely different? I would think in practice, the result would be different.

 

[Marke]

Hello EEBody

From my experience, this only holds true if the motor is operating at virtually synchronous speed. The problem is that at the instant the first contactor opens, the voltage becomes that of the "generator" being the rotor with it's residual magnetic field. The frequency is not related to the line frequency and so the phase angle is also not related. At the very instant that the contactor opens, it may be true that the voltages would be close to equal, but I do not think so. Perhaps Electricpete can shed some light on that. In reality, you must have some delay in order to enable the arcs to extinguish on the first contactor to open.In this half cyle or so, the phase will slip by an amount proportional to the difference in rotor speed and line frequency.
Provided that you had an accurate control of the timing of the reclose, the extinguishing of the arc and the rotor slip, it is theoretically possible to reduce/eliminate the transient. In practice, I do not believe that this is achieved.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[aolalde]

I agree with Marke; a momentary (average of 10 cycles or 10/60seconds for 60 HZ) generator condition appears while in Wye connected, the power supply to the motor is open..

The frequency of that voltage will decay following the rotor speed ( for high inertia and reduced mechanical load torque in the shaft the speed change is minimum but for light inertia and high load torque the speed could drop a lot).

The connection is now changed to delta and the voltage is reapplied.

We have now two contributors to build up the current.

The first one is the motor current- speed performance (below the breakdown torque speed the current curve has high values), so if the speed dropped too much the current will be high.

The second contributor to build up the current is a random phenomenon. It depends of the phase angle and intensity of the voltage generated by the decaying rotor field when the contactors are closed..
If we are lucky the power is closed near or at a zero crossing with minimum transient, but it could be a maximum offset and peak voltage with results similar to an out of step synchronization.

In my opinion the best benefit of a closed transition is that it keeps the phase reference of the line voltage while the change from Wye to Star is being done.