Today I encountered a faulty 400A/200kW contactor (datasheet attached).

The system and contactor work as follows: 230VAC control circuit comes on after power-on and goes to many devices, including this contactor (on terminals A1 and A2, typical nomenclature for the coil) but the contactor does NOT energize (by design) when 230V is applied to A1 and A2. The contactor does not energize until a separate 24VDC signal is applied to another pair of terminals on the coil labeled "PLC". My initial assumption about the reason for this, was that some designers prefer to use 24VDC I/O for everything, so a contactor with 24V coil would be desired, but a 24V coil of this size would draw far too much current for a PLC output. So this is a hybrid that's actually using a 230V coil, which is controlled one level higher by 24VDC. Essentially an embedded interposing relay.

The issue was as follows: when 230V control power is energized the breaker immediately trips. There was a <0.2ohm short across the control circuit which I traced to this contactor on the A1/A2 terminals. When the contactor is disconnected, issue is gone. I placed an order for a new contactor but with several days lead time and production pressure from on high, I was encouraged to "find another (temporary) way."

I dissected the contactor and found a PCB inside with way more components than I expected. I only expected a small relay as I said. But something else was going on here. I could not make out what was going on because the board was thickly potted and only some ferrites and snubbers were distinguishable. Only slightly daunted, and with nothing to lose really, I decided to test my initial assumption.

• I broke loose the coil ends from the epoxy and soldered on extension wires, ran to the exterior of the contactor. Reassembled the contactor.
• Outside the contactor I installed a 24VDC relay. PLC energizes the relay, relay sends 230V inside to the coil, completely bypassing the PCB.
• That didn't work. Coil just hummed. So I assumed there must be a rectifier in the PCB.
• I installed a diode between the relay and the coil, half-wave rectifier to give it low-stress DC test. That didn't work. Again coil just hummed.
• I reversed the diode.That didn't work. Again coil just hummed.
• I made a full bridge rectifier out of 1N4004 diodes and put it between the relay and coil. THIS WORKED. The contactor pulled in immediately and stayed in, no humming.
• It worked for under a minute and then the coil started billowing smoke.

Considering that half-wave 230 isn't enough but full wave 230 is too much, I deduced the PCB is responsible for some advanced control scheme where initially a high energy pulse or longer timed event closes the armature but thereafter the power is reduced to a lower "holding" power. I have found references to this sort of technique in earlier research on advanced control of solenoids. Further supporting this theory (IMO) is the fact that the coil seemed very small/insufficient for a contactor of this size; I have seen much larger/thicker coils on much smaller contactors. This kind of scheme should save a considerable amount of energy. Separate "closing power" and "holding power" ratings in the datasheet seem to hint this way, but I'm unsure because an AC coil contactor naturally changes impedance when it closes, without any PCB.

I am just wondering if anyone here knows that my theory about the role of this PCB is correct or knows it is not. Or if anyone has any more likely speculation as to what this PCB does. But if speculation is offered please identify it as such.

 

I'm assuming this is a normal solenoid contactor, not a motorised one.
As it is a DC coil, then it would need circuitry to reduce the operating power once the contacts have closed. Is there an auxiliary contact which connects to the circuitry to tell it when the main contacts have closed?

 

I'm assuming this is a normal solenoid contactor, not a motorised one.

Correct.

As it is a DC coil, then it would need circuitry to reduce the operating power once the contacts have closed.

I think most DC contactors don't have this. I've never seen one anyway. I think they just normally design the coil to dissipate excess heat.

Is there an auxiliary contact which connects to the circuitry to tell it when the main contacts have closed?

There are auxiliary contacts but they are on the outside and provided for feedback to machine controls. No physical sensor, switch, or contact provides feedback to the PCB. I think it should be possible for the PCB to sense closure by a change in inductance of the coil though.

 

Edging further in to the realms of speculation . . .
Because it's a high-current contactor, there's an advantage if the contacts can switch really fast, and that could be achieved more easily with a DC coil, the higher power the faster. At some point the power is so large that it cannot be kept at full power continuously.
Albright have several power options for the same coil voltage on their contactors, presumably for better switching speed.

On the other hand, the cynic in me thinks that companies add electronics to things that don't need electronics, just because they can, or because the marketing department thinks it's a good idea.

 

Because it's a high-current contactor, there's an advantage if the contacts can switch really fast, and that could be achieved more easily with a DC coil, the higher power the faster.

That would make sense. I have read some app notes about electromagnetic brakes and brake modules made by SEW. I can't remember all the details and I'll probably be wrong if I try to get too specific, but as I remember, their brake module doesn't just apply/remove nominal voltage. It does some kind of clever trick I think by applying a small voltage of wrong polarity while at rest, so that when the brake is ordered to release and proper polarity is applied, it is met with residual magnetism of opposing polarity, resulting in more force and faster opening. Maybe this contactor could be doing the same or similar. (Or just applying really high current at start).

the cynic in me thinks that companies add electronics to things that don't need electronics, just because they can, or because the marketing department thinks it's a good idea.

I don't think anyone who has ever replaced a modern printer cartridge could disagree with a straight face, but I hope that isn't the case here. I'd like for this to be something cool to add to the mental notes.