24vdc, Contactors, PLC and voltage spike

S

SSDriver

Senior Member
Location
California
Occupation
Electrician
I'm only talking about the controls in this post.

I know I get a lot of voltage spikes when I have larger Contactors open going back to the control circuit. Enough that 120v LED pilot lights flash when the contactors de-energize. I have tried surge suppressors, RC, and varistors mounted on the contactors that do help but not eliminate the issue. They help enough that the LED's don't burnout but you will get a smaller flicker.

I have a project that I will have a PLC, HMI, 3 pilot lights, and three 40A contactors. Space is very limited and I was wondering if I could use the same power supply for the contactors, pilot lights, PLC, and HMI. Everything that could run at the same time would be under 40va and I was looking at a 48va Power supply that would work for inrush and space wise as well. Will I run into issues? I was planning on putting some " IronHorse Surge Suppresser, varistor, RC "modules on each contactor. Only 2 would ever open at the same time. I don't use DC contactors that much and wasn't sure if the spike would be worse or better than on an AC circuit.

Thanks.
 
By 'surge suppressors, RC, varisistors mounted on the contactors', do you mean across the contactor coils or across the controlled contacts?

When you open the contactor, the inductance of the coil tries to keep current flowing. This can create a voltage spike on the circuit controlling the coil. With an AC contactor, any device trying to snub that voltage spike has to not conduct when you actually want to power the coil.

With a DC contactor, you can use a simple diode to snub the voltage spike. When you power the coil the diode is reverse biased by the power supply. When you de-energize the coil the current continues to flow through the diode until it decays into the coil resistance.

Note to that this voltage spike is separate from and voltage regulation issue when the load changes on your power supply.
 
My max with
ptonsparky said:
Use what the maker of the contactor recommends.
Usually, I've just seen a diode.
Click to expand...
ptonsparky said:
How much under 40va?


Bench test it in the shop.
Click to expand...
They don't have a recommendation other the RC Surge suppressor I linked up top. I was curious as to how that might help (or not help) for the spike. I'm liking the diode Idea. My total load with maximum inrush would be 38 VA. I have a 48VA power supply already that I would like to use.
winnie said:
By 'surge suppressors, RC, varisistors mounted on the contactors', do you mean across the contactor coils or across the controlled contacts?

When you open the contactor, the inductance of the coil tries to keep current flowing. This can create a voltage spike on the circuit controlling the coil. With an AC contactor, any device trying to snub that voltage spike has to not conduct when you actually want to power the coil.

With a DC contactor, you can use a simple diode to snub the voltage spike. When you power the coil the diode is reverse biased by the power supply. When you de-energize the coil the current continues to flow through the diode until it decays into the coil resistance.

Note to that this voltage spike is separate from and voltage regulation issue when the load changes on your power supply.
Click to expand...
Ok I'm going to look for a suitable diode. The contactors only dray 9va inrush, and 6va holding at 24VDC.

hbiss said:
If it's DC put a diode across the coils.

-Hal
Click to expand...
Any recommendations for a diode for a Contactor at 24VDC, 9va inrush, 6va holding? I'm assuming the striped end would go on positive?
 
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