Lighting Contactors

[richanton]

I went to buy some lighting contactors today and notice that they come in 3 flavors; mechanically held, magnetically held, and electrically held. Is one type better than the others.

In my application, all I want to do is be able to switch on/off 5 HID fixture ballasts.

 

[jraef]

First off, there are only 2 flavors, Electrically held and mechanically held. Magnetically held means they use electromagnets, which is "electrically held", just using different terminology.

In a nutshell:
Electrically held are safest in that if power fails, they drop out (disconnect), allowing use of a "3-wire" control circuit so that when power is restored, the circuits are not automatically reenergized. The safety comes in the fact that if someone were working on the circuit or changing a lamp while power was out, they will not be shocked if power comes back on unexpectedly. They can also be controlled most simply by just applying power to the coil, 2 wires per contactor. The drawback to Electrically / Magnetically held is that the coils need to be energized continuously, and they make noise and consume power.

Mechanically held also use electromagnets, but only to change state from On to Off or back. Once changed, a mechanical latch holds the contacts in place so the control power can be removed and the contactor is quieter / cooler / more efficient. That fact makes them better for implementing remote control relay schemes which use very low voltage pulses to operate and switch the higher voltage momentarily to the latch and unlatch coils of the contactors. They are also better for installations where there will be nobody available or it is inconvenient to turn the lights back on after a power failure. The downside of Mechanically held contactors is the more complex control circuit because you can't just remove power from the coil to turn them off, you must send one signal to pulse an On condition, and another to pulse the Off condition, making it at least 3 wires and 3 switches.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[ScottyUK]

There are a couple of other types: - mechanically latched, with a mechanism which is electrically actuated, and magnetically latched which requires a coil voltage polarity reversal to de-latch. The former is largely confined to very large types, and the latter is by definition a DC-only type.



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Sometimes I only open my mouth to swap feet...

 

[LionelHutz]

Electrically held means you can use your typical home on-off toggle switch to control the lights. You get a 120V coil contactor and just apply 120V to it with the switch to run the lights.

The other types require another switch type - the mechanically held could use a return to centre double throw switch (a return to centre rocker switch that moves in both directions) and I would expect the magnetically held would need to have DC and the same type of switch but with 2 contacts per position.

 

[richanton]

Thanks for the help.

In this forum, we should have an "ITAQ" for "Infrequently Thought About Questions".