Capacitive Load through Relay Issue

[StudentSA]

Good Day,

I am hoping someone can point me in the right direction.

I needed to run four AC "high" power electronic ballasts for fluorescent tube lights. Each electronic ballast is rated 75W and globe is matched at 75W. The four globes total 300W. All ballasts are wired in parallel. (i.e. all lives tied together likewise for neutral and earth).

I calculated a normal running current of under 2A and included a 10A relay (in N/O configuration) to drive this setup. The relay was added to be able to remotely activate the setup.

The issue I am facing is that after a couple cycles of operation (+-100) I am finding the Relay is sticking i.e. it is not opening when power is released from its coil.

I've read up a bit about this and understand the key reason is linked to exceeding the relay specs. I also learned that each electronic ballast is most likely a "capacitive" load which means there will be a high turn on current draw.

Is there a way to protect the relay in this setup to prevent the sticking from occurring?

Thanks

 

[Diver300]

Fit a relay rated at a higher current or put an NTC inrush limiter in series. You could put a limiter in series with each ballast, or just one for all four.

Something like this:- https://uk.farnell.com/amphenol-advanced-sensors/cl-140/ntc-thermistor-50r-disc-11-43mm/dp/2847974 would be about the rating you would need for two balasts.

The NTCs will be hot when running.

 

[StudentSA]

Thank you Diver300, I've been searching the web and only came across recommendations for snubbers and chokes. Now that you've pointed me in the right direction I see that an NTC is exactly what I need.

I managed to source locally the following NTC (Datasheet) which is a 30 Ohm 5 Amp Inrush NTC. I have placed this in series with all four ballasts (i.e. the load).

Out of interest, I was looking at a 50 Ohm version as well, would it be possible to use two of these in parallel? Objective being:
1. It would reduce the running heat across the two.
2. Increase the recovery time, I.e. if less heat it would recover its 25deg resistance faster.
3. In the worst case it also adds a layer of redundancy as a single one should suffice on its own as well.
4. Increased lifespan?

Regards,

 

[Diver300]

I wouldn't put those in parallel. If you want to use two 50 Ohm ones, suggest that you connect two ballasts to each.

With two in parallel, there is no guarantee that the current is shared equally. If one took more current, it would get hotter, which would make it lower resistance, so it would take still more current, and so on.

Those NTC thermistors are only for loads up to about 3 kW. I think that larger ones would heat unevenly, and parts would get hot while others stay cold.