Contact Cycling
- Thread starter fifty60
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mgookin
Senior Member
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- Fort Myers, FL
What is your objective?
If it is to stress test the system, the load would be preferred. Even a load in excess of the normal anticipated load may be preferred.
If it is to cause a working system to last the longest possible service life, no load or when the sine wave is at zero works best.
From the electrical perspective, contact surfaces last longer if opened under no load. AFAIK, operations or cycle ratings are supposed to be representative of being closed and opened under load. That's the purpose of contacts.
From the mechanical perspective, makes little difference.
I have a PID controller with 2 outputs. Under certain circumstances I only want one of my outputs to cycle, and the other one not to cycle. I cannot prevent this on the controller, it is both outputs or no outputs.
I can break the load at the output, so that the controller cycles but the load does not.
The load is a relay driving another load. I can break the load directly at the output of the controller so that the controller contacts see ideally Zero voltage. Or I can break the load at the load relay contacts. This way, the controller would still be turning on and off the relay, but the relays contacts will then be seeing Zero volts. I would rather replace a relay than a controller.
I can break the load at the output, so that the controller cycles but the load does not.
The load is a relay driving another load. I can break the load directly at the output of the controller so that the controller contacts see ideally Zero voltage. Or I can break the load at the load relay contacts. This way, the controller would still be turning on and off the relay, but the relays contacts will then be seeing Zero volts. I would rather replace a relay than a controller.
The conventional method is to control loads via the control circuit of the load relays.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Trusting that when you say "I cannot prevent this on the controller..." you are correct (because this seems odd to me, but I can't see what you have), then cycling the contactor on and off with zero load is only going to affect the mechanical life of the contactor. Having a load on it will shorten the electrical life of the contacts no matter how low the load is, compared to having zero load on them.
It's an all or nothing PID. I have 2 sets of PID's stored on the controller. the 2nd set is activated by an event input. For local mode I have PID set one which has heat and cool, while the remote only has heat. The local has a system that needs to be controlled by both outputs of the PID. For the remote I only need one output. Both systems use the same controller.
I've heard that having no load is not desirable because some load can slow down the oxidation that occurs on the contacts.
I've heard that having no load is not desirable because some load can slow down the oxidation that occurs on the contacts.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Meh, somewhat true if what you are switching is ALWAYS going to be low energy, but pretty much irrelevant if, when you ARE going to use the contactor to switch a load, there is going to be a serious load involved.
The "oxidation" issue happens when, for example, you are trying to use a cheap little relay with silver based contacts to try to switch something like a 5VDC TTL logic level circuit. Over time, the very low energy of that circuit will allow a buildup of silver oxide. Ozone, a byproduct of interrupting current flow with an air gap, reacts with the silver in the contact material to form an insulating layer of Ag2O on the contacts. When the energy is really low like that, there is insufficient heat to "punch through" that thin layer of Ag2O and it acts as an insulator, so it eventually interferes with the contact conduction. But when you have sufficient energy in the circuit, that layer gets "zapped" by the arc itself and the AG2O reforms back into the molten contact material at the surface. That's why you see people use gold flashed contacts on low energy relay circuits, the gold doesn't react with ozone.
So bottom line, that issue is not applicable to your situation. When you are banging that contactor open and closed with NO load, you get NO ozone formation, so the problem is not created. Ozone in the area formed by OTHER contactors may cause that film to build up anyway, but the first time you run a large load through, it will be as if it was never there. It's no different than a contactor sitting in a box with other contactors near by, but not being used. Yes, for a fraction of a microsecond the first time it is used there is slightly more resistance, but the net effect is totally insignificant.
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Thanks for the explanation. The controller load is actually a 120VAC relay. The relay is then controlling a refrigeration solenoid. So even for the 120VAC load, i probably do not have to worry about the Ag2O forming on the contacts, even the the current is minimal for the relay it is controlling? Is it mainly a problem with 5VDC circuits?
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