DC relays in AC circuits

[kal.a]

I've seen this wired in a couple of panels and though I had access to the wiring diagram it did not show anything other than the DC relay wired to AC lines without showing any rectification. So I took a couple of pictures showing the relays with capacitors connected to their input terminals and concluded that one of the wires must have had a rectifier diode as it looked bulky in one section of it and had heat shrink on it.
So I got to work and wired a couple of them at home and they worked. The reason for the DC relays was to create delay/interlock between the motor starters, ; about 1.5 seconds and .1 seconds. I liked the idea and recently had a need for cheap, small way to modify a control circuit to have a delay between reversing contactors. I wired a small board as shown in the **broken link removed**which was loosely based on what I saw on that old panel and added a resistor the drop the voltage to within relay specs. I hooked it up to a PLC to test it out by energizing on and off continuously for 10 hours. It worked well.

So what I missing? I'm going to wired it up on live panel next week and wanted to make sure I'm not missing something specifically if life cycle is going to be greatly affected.


Cheers
Kal

 

[MaxHeadRoom78]

Although the hysteresis on most relays is fairly large, I would not consider a good idea to power a relay this way, if there is any delay or slowness to the pull in, you may get contact arcing.
Do you need any more delay than the normal delay necessary to extend the electrical and mechanical interlock on any motor reversing contactor?
Max.

 

[kal.a]

Thanks Max for your reply.
Yes I need to stop the machine before reversing direction to reduce mechanical wear. This is done routinely with timers and AC relays if it's hard wired or "logically" if PLC or solid state controller of some sort. But there are still many of these old panels around and I got curious if I could do that and what it encourages me is that the panel where I got the idea from is still being manufactured to this day and is used in automated car washes.

I appreciate your input. I think I should do more testing on it and see how long I lasts.

 

[cowboybob]

You're going to get arcing regardless. Best you can do is reduce it as much as possible, which you seem to have done.

Could it be improved? Yes, of course, but there's a trade off between effort and result. Best to follow your instincts (continued testing and inspection) to assess what's needed (or not needed).

 

[kal.a]

Just to be clear, you guys mean contact arcing as a result of the coil not immediately energizing or de-energizing? Or both?
I'm thinking de-energizing.

 

[MaxHeadRoom78]

Could be both.
If these relays are subsequently switching AC contactor coils, you want the contact to close without any hesitation, as AC coils have Very high initial current due to inrush plus low initial inductance.
If anything you would be better off with DC contactor coils if you have a choice.
M.

 

[kal.a]

Thanks a lot. Excellent feedback.
Much appreciated.

Kal

 

[cowboybob]

Just to be clear, you guys mean contact arcing as a result of the coil not immediately energizing or de-energizing? Or both?
I'm thinking de-energizing.

Yes, excluding energizing bounce, de-energizing is more prone to arcing.

But, it is very difficult to control the de-energizing arc as that is a function of the mechanical design and construction of the relay (how quickly and to what extent the contacts separate). From your description I'm assuming your relays are this type (metal to metal "dimples").

High voltage/current relays, however, often have flat brass to flat graphite (lots of surface area and minimal slide) construction. They still arc, but with considerably more surface area such that the erosion is spread out over a much longer period of time.

 

[Diver300]

Many car manufacturers will not allow freewheel diodes in parallel with relay coils, because the current decays too slowly, increasing the arcing. The preferred solution is to have a resistor in parallel with the coil, that takes around 10% of the coil current. When the supply to the coil is opened, the coil current flows through the resistor, resulting in around 10 times the supply voltage on the coil, and the coil current decays quickly. The surge voltage is well defined, so whatever supplies the relay coil, normally a semiconductor, can be specified for that.

If you have a DC relay supplied from AC with a rectifier, you will get the current decaying slowly. I recently repaired a refrigerator where that had been done, and the relay had failed as the NO contact was welded closed. That is the contact that would have suffered the worst due to the coil current decaying slowly.