# Contactor: when and how to use it

I hope my question is not too stupid. I'm controlling a brushed 48V 50A motor with a motor controller and I use two gel batteries as a power supply.

I would like to use a low power signal to switch on and off the power circuit, and for this reason I've bought a solenoid contactor (the coil works at 12v, the contacts are rated up to 500A). The contactor connectors open and close the positive main wire and it stays closed for all the working time (it is placed between the positive battery lead and the motor controller).

This is the solenoid: https://www.amazon.it/gp/aw/d/B01N49AYTN?psc=1&ref=ppx_pop_mob_b_asin_image#

The contactor is on all the time, and for this reason I realized that the temperature is very high (max 60 degrees).

Is it normal? Or should the contactor be closed only for short periods of time?

• What does the data sheet tell you. If there is no data sheet because you bought it from a non reputable source, then that’s a good lesson to learn. Commented Jul 24, 2019 at 11:02
• 500A is its peak ampere capacity while continuous current capacity will be somehow LOW than peak current , but i think it will good enough for you and for temperature rise problem it says it can handle upto 85C so its not problem Commented Jul 24, 2019 at 11:04
• At the moment, I didn't connected the contactor to the high power circuit. I only connected the coil to check how the contactor works. I left the coil switched on for 4 hours and then I measured the temperature and I found out it is steady at 60 degrees. Can I suppose it is normal? Moreover, my idea is to keep the coil activated for all the operating time of the system. Is this how it is supposed to work? Or he contactor must be used only for temporary operations (few seconds) ? Commented Jul 24, 2019 at 12:49
• I only speak English, but if the contacts are only rated for 12V, I think using 400% of the rated voltage for a contactor may very well lead to early failure due to arching. It could end up welded (stuck closed). As with any device, you may also run an increased risk of isolation issues.
– Bort
Commented Jul 24, 2019 at 16:06
• I'm sorry, guys, there is a misunderstanding and it's my fault. I'm operating the coil at 12Vdc. The motor runs at 48v and I use the contactor to short circuit the 48v positive lead from the battery with the wire from the motor controller board. Commented Jul 24, 2019 at 19:21

The specifications on the linked website say the temperature range goes up to 80°C (though I'm unsure if that's operating or ambient). Either way, at 60°C you're within this range. In my experience, the main source of heat is from the coil and not the contacts so it is important to supply the proper coil voltage. Overvoltage or undervoltage will cause the coil to warm up more.

It is normal for a contactor to be on for a long duration of time. As an example, I just installed a ~77 A motor starter for a three-phase motor. A motor starter is nothing more than a contactor + overload and this motor will run almost 24/7. This should be no different for a DC motor.

With that said, everything is application dependant. The coil of the motor starter in my example is rated for continuous duty. Some contactors may not be suited for 24/7 operation. Some contactors don't like being opened and closed quickly. The important point is to examine the datasheet accompanied with the contactor and make sure that its specifications meet your requirements.

Edit: I missed the switching voltage of the contacts on the linked page. I had assumed it was at least your 48 VDC and the 12 VDC/14 VDC related to the coil. If you use contacts rated for 14 VDC at 48 VDC you'll run the risk of welding them shut. I'd advise finding a contactor with a higher contact rating.

• I'm switching the coil at 12v when I use the two poles from the contactor to short circuit the 48v wire. Commented Jul 24, 2019 at 19:24
• Thanks for the support, Lange! Commented Jul 24, 2019 at 19:25
• @MarcusBarnet, using 12 V to power the 12 V coil isn't a problem, that's OK. Using contacts that are rated for less than 48 V to switch your 48 V motor might lead to contact welding.
– user199402
Commented Jul 24, 2019 at 19:30
• I'm pretty sure current surges and current overloads have a monopoly on contact welding.
– JACK
Commented Jul 25, 2019 at 12:55
• @Iamengineer I wouldn't call it a monopoly. Current surges/overloads are less a problem with the low resistance of closed contacts. When the contacts are close but not touching (during opening/closing) under load an arc forms. This causes a very large amount of heat at the contact surfaces. This can lead to welding, or even melting/explosion in extreme cases (the arc creates air pressure, which is why some contactors have a vacuum or alternate gas inside.). This is why the rated switching current is always lower than the operating current. Switching too fast is bad even at lower currents.
– Bort
Commented Jul 25, 2019 at 13:57

This contactor is designed for automotive or marine use. It is rated for 12Vdc for the coil and 12 - 24 volts for the contacts. The 60 to 80 degree C. is the max ambient temp the contactor to rated to operate in. 500 amp peak and 200 amp long term. The heat you are feeling from the unit is normal. The coil is heavy duty to make/break high current and it will run hot.

The only problem I see is that you want to switch the contacts at 48 volts and that exceeds the rating of 12 to 24v. Will that matter??? Probably not but you are using a device manufactured for a different purpose. Good luck and have fun.

• I'm switching the coil at 12v. Commented Jul 24, 2019 at 19:26
• @marcus .. The contacts will be switching 48v... and the coil will be operating at 12v
– JACK
Commented Jul 25, 2019 at 1:51