lighting contactor on motor?

"Although an electromagnetic motor starter contactor looks similar to a lighting contactor, it shouldn't be used for lighting applications. Certain lighting loads can have very high inrush currents, with characteristics much different than a lighting load.

Contact bounce is another phenomenon to consider when applying an electromagnetic contactor. When a contactor coil is energized, the movable contacts strike the stationary contacts with considerable force. Even though the distance involved is relatively small, the movable contacts actually bounce off the stationary contacts at least once before firmly seating themselves.

The inrush current of tungsten lamps can reach as high as 18 times normal operating current for a very short time period. This inrush period is coincident with the contact bounce. During this period an arc hot enough to melt silver strikes across the contacts. Other types of lamps and ballasts have different characteristics that could also damage a non-lighting-load contactor. Thus, a lighting designer should confirm a contactor's rating and the lighting loads being served."

So a lighting contactor is at least in some respects superior to a motor starter contactor. I can't say for certain that there are no other factors to consider. I believe the second sentence quoted has a typo and should have read "much different than a motor load."

The biggest difference when switching inductive loads is that the open gap has to be wider. Ideally, the contacts should open very fast also. You want to lengthen and cool the gap as fast as possible to minimize contact errosion.

Closing the circuit is usually the least part of the problem of switching currents. For the size motor you are describing, I wouldn't anticipate a problem with that contactor. That is a pretty small load. Use what you have and don't worry about it.

Earle Rich Mont Vernon, NH

The last thing I saw that was labeled "lighting contactor" was a latching relay - i.e. separate coils to open and close the contacts. What makes it special for lighting is that it mechanically remembers its state so a power glitch won't cause all the lights to go off. Now obviously on the other hand you want all the power tools to go off hence the more commmon contactor style.

I have seen many "definite purpose" contactors that were not explicitly horsepower rated but did have tables for LRA (locked-rotor i.e. starting amps) and FLA (normal full-load amps). For example one might show 20 FLA and 120 LRA.

I think what in part makes them "definite purpose" is that one might be designed for e.g. a specific 5 hp motor with a known LRA, as opposed to any arbitrary 5 hp motor.

The LRA on a 1/2 hp 3 phase motor ought to be around 10 amps, wild guess.

As for contact bounce, opening distance, arcing differences I am ignorant.

Bob