How long can a relay module be energised?

Other Hardware Motors, Mechanics, Power and CNC
1

I want to turn on/off a cistern pump using Arduino. The pump is 240VAC 1/2 HP and will consume about 375 W and hence draining less than 2A, if I am not wrong. Two of these regular relay modules, ratet up to 10A should do. I intend also using an extra DC source to energise the relays coils saving the Arduino pins from that burden. My question is: would be a problem if the relay stays energised for as long as 30 minutes or are them too fragile for that?

2

Typically the relay ratings are for continuous duty. They also contain an inductive and resistive ratings. As long as you do not exceed them they can be left on indefinitely or as long as you have the correct voltage on the coil.

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3

Not what you asked however, are you planning on using one of the Arduino "relay boards" with the blue relay module?

Leaving them on continuously is not the issue, however these relays when pushed to operate on mains have been less than reliable.

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4

Unless you really enjoy getting up in the middle of the night to replace relay boards, get a real relay rated for at least a 1/2 horse AC motor. Your motor with a pump connected will draw a WHOLE lot of current while starting the pump and adding enough energy to get the water column moving!
Paul

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5

Relays rated for 10A resistive will not handle 10A inductive load like a motor - if the relay doesn't have an inductive current rating you probably shouldn't use it.

Does your pump motor have a snubber circuit?

And yes, as mentioned the start current may be very much higher than the steady state current (10 x larger is not unknown).

However there are different AC motors, for instance induction or universal, and universal are much less demanding to switch - do you know what type of motor?

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6

Thanks for the hints. I will check if the pump motor has a protection circuit. I understood that the issue with the blue relay modules typical in Arduino's projects is not the time they remain energised. The danger is that it is not supposed to work with high inductive load. I will go after a proper relay and circuit. Thanks again.

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