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Ac Motor Starter Failure


ReberDesign

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Hello Reberdesign

 

Welcome to the forum.

 

If you have a look at a contactor design, you will see that the magnetic circuit is made up of two sets of "E" laminations, one is fixed and the other set moves with the contact mechanism. When there is no power applied to the coil, the magnetic pole faces are separated by a large air gap. When the contactor is closed, there is no air gap.

This means that when power is first applied, there is a lot of current through the coil due to it having a large air gap and low resulting inductance. The high coil current results in a high flux and high magnetic attraction between the poles and forces them together. As the poles move together, the air gap is reduced, the inductance is increased and the current reduces.

When you have two contactors that are mechanically interlocked, the pole faces can not move together, so the excess current continues through the coil and it fails.

It is usual, in addition to the mechanical interlock, to use an electrical interlock to prevent power from being applied to one coil if the other contactor is closed. This is done by looping the feed to the coil on one contactor, through a NC contact on the other contactor. This will interrupt the path to the coil if the other contactor is closed.

 

Best regards,

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I have an OEM customer who uses a lot of reversing contactors and does not use the aux. electrical interlocks, insisting that the mechanical is sufficient. He loses about 20 coils per year. I have explained more than once exactly why he is having this issue, he maintains that the cost of the extra aux. contacts on every starter is higher than the cost of 20 replacement coils per year. From his standpoint as the OEM it is a valid economic argument, but the real cost is borne by his customers, which I say will eventually affect his bottom line by making his customers vulnerable to being taken away by a competitor who has fewer problems.

 

In the mean time, my company (manufacturer of the contactors) is getting a bad reputation for having "defective" coils. We just decided to give him the aux. contacts for free if he agrees to use them. My feeling is that it will cost us less in the long run.

"He's not dead, he's just pinin' for the fjords!"
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I have an OEM customer who uses a lot of reversing contactors and does not use the aux. electrical interlocks, insisting that the mechanical is sufficient. He loses about 20 coils per year. I have explained more than once exactly why he is having this issue, he maintains that the cost of the extra aux. contacts on every starter is higher than the cost of 20 replacement coils per year. From his standpoint as the OEM it is a valid economic argument, but the real cost is borne by his customers, which I say will eventually affect his bottom line by making his customers vulnerable to being taken away by a competitor who has fewer problems.

 

In the mean time, my company (manufacturer of the contactors) is getting a bad reputation for having "defective" coils. We just decided to give him the aux. contacts for free if he agrees to use them. My feeling is that it will cost us less in the long run.

 

I agree with this. The cost of the failure is not just the cost of replacement. The call out cost of maintenance personnel, loss of production, the time to order, the availbility of components etc all needs to be factored in.

I suspect that in combination these issues easily justify the inclusion of an electrical interlock. My view (for what it's worth) is the electrical interlock is just good design practice.

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