Semiconductor contactors for motor duty

[ScottyUK]

A potential supplier whose bid I'm reviewing has proposed use of semiconductor motor contactors for some drives of roughly ~11kW rating. I instinctively don't like it. I've used semiconductor switches for resistive loads and for small motors but never in an industrial MCC. I have the suspicion that a motor fault which a contactor might more-or-less survive - e.g. a winding failed s/c to earth - will probably destroy a semiconductor type.

Has anyone used semiconductor contactors for motor duty? The fast-acting fuses which serve to protect heater type loads won't be suitable for protecting a motor starter unless they are outrageously oversized. Heat losses must be a problem, unless they are internally bypassed. Any other thoughts on pros / cons?


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

 

[matthewm1965]

We were considering them for use in distributed motor starters but knocked them back when we discovered that only two of the three poles were switched.

Matt

 

[cswilson]

We have used both semiconductor and hard-contact relays for the soft-start bypass in our servo amps over the years (I think close enough to what you are looking at, even though on a DC circuit). Bottom line: no problems ever with the mechanical contactors, lots of problems with the solid-state, even at lower powers (<10kW) and voltages where we thought they would be robust enough.

Curt Wilson
Delta Tau Data Systems

 

[jraef]

Think about t a bit Scotty. A soft starter is essentially a "semi-conductor contactor" in the same light as your concern. Sure they can be damaged by shorts, but then again, so can contacts.

But why are they proposing them? The only real legitimate reason is if your duty cycle is so high that it will consume contacts too quickly. Otherwise, they are usually more trouble than they are worth if you don't get the added benefits of soft starting.

Heat losses are roughly 4-1/2 Watts per running load amp on a 3 phase starter. You must deal with that heat or it builds up. Soft starters do it with ventilation or bypass contactors. If you are using a semiconductor contactor just for DOL stating, then adding a bypass contactor is just redundant. So that leaves only ventilation as a cooling method, and if that can introduce contaminants, then that is another problem.

At 11kW you might be able to deal with the heat via radiant convection through the enclosure walls, but that is a complex issue that involves investigation. Many enclosure manufacturers provide software to calculate heat build-up (for the purpose of selling air conditioners), but they can be used backwards to determine minimum enclosure size as well.

As an unofficial rule-of-thumb, most SSC manufacturers stop at about 4kW for motor contactor applications. Above that, it makes better sense to use a soft starter and have a bypass. Most of the small inexpensive soft starters now have integral bypass contactors.


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[ScottyUK]

Hi jraef,

Does the semiconductor contactor ramp up the current? Because if it doesn't then it and the fuses are exposed to LRC for however long it takes to accelerate, and unless they have been very generous with the silicon the I²t of the silicon is going to be smaller than the fuses. Kaboom.

Most decent contactors survive a winding fault cleared by the MCC cubicle breaker. I'm not at all convinced that a semiconductor equivalent would do the same. The MCC has a fault level of just over 50kA although obviously it would be much lower for a fault at the motor - maybe 10kA or so.

Oh, and it is in a hot sandy country - bad news for temperature, bad news for dust!

I'm intending to reject this proposal, but thought I'd see if I was just being old fashioned.


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