Max safe coil temp for 200A 12V contactor/relay?

[Psi]

So I have a 12V DC 200A relay that has been working well, and has not given me any issues so far.

However, after taking the case off and using both the thermal camera and DMM probe I measured the coil temp at 97deg C after the relay had been on for 20min.  That is the temp at the outer windings so obviously inner core windings must be hotter.
(This reading is with minimal current flowing through the relay contacts themselves. The heat is all from the coil)

This seems excessive to me. Does anyone have experience with high current relays and know what's normal?

I know from experience that it is normal for big solenoids/relays to have a warm outer case when active. So I imagine their coil does get pretty toasty in normally and acceptable operation.   I just don't know what is considered abnormal?

[Siwastaja]

That sounds marginally normal to me.

Many high quality relays/contactors run really hot; they cope by using high quality wiring insulation material (bare minimum Class F, 155 degC, but class H, 180 degC is not colossally expensive either, maybe a few cents extra for the manufacturer), and plastics that can take the heat without becoming brittle.

Now the big questions are, how close to the extremes are you operating. I.e., what if your ambient temp rises by 20 degC? You possibly should derate current in such case. Another question is, is this really high quality relay, i.e., known brand?

[Psi]

Na, it's a no name generic china relay brand.

I was not super worried about the relay quality when I spec'ed it because there are mosfets in the system which do all the high current connection/disconnection. The relay contacts never energize or deenergize when high current is flowing. The relay is mainly their to provide reverse polarity protection and act as a global power switch for the entire system.
I never really thought about relay coil temp as a consideration.

However, I am currently controlling the relay from 12V SLA with a mosfet to turn it on/off as needed. The relay will get full battery volt (less 1 diode drop) which can be up to 14V so it's getting more voltage than 12V.
In any case, I can update the code to use the mosfet for some PWM, start at 100% then drop back to 60% or something after 1 second to hold the relay in.  That should give a good reduction in heat.

[srb1954]

However, after taking the case off and using both the thermal camera and DMM probe I measured the coil temp at 97deg C after the relay had been on for 20min.  That is the temp at the outer windings so obviously inner core windings must be hotter.

You could get an estimate of the average temperature rise of the winding by measuring the coil resistance change between cold and hot - about +0.39% change per degree C for copper. Comparing this  average temperature to the external temperature you have measured you would be able to calculate an estimate of the peak coil temperature.

Depending on the size of the relay you probably need to leave it on for more than 20 min for the temperature to stabilise.

[Siwastaja]

The PWM thing works well, it's internally implemented in some higher-end DC relays/contactors. You need a lot of current to pull the contacts together because they start up far away, but much less is actually needed to maintain good contact. Contactors with AC coil can automatically reduce holding current by mechanically changing the magnetic path to increase the inductive reactance thus limiting current, but this isn't possible on DC contactors, hence active PWM trickery is needed to achieve the same.

Freewheeling diode, its correct placement at the driver, not at the relay, proper layout, and input voltage bypassing becomes extra necessary with such PWM, and de-energization delay is then a possible problem but if you otherwise guarantee you are not breaking current with this contactor, you can ignore this issue.

[Psi]

Is there a general rule of thumb for how low % PWM you can go before getting issues with current flow due to not enough force holding the contacts together?

Obviously there are also risks that some vibration or impact might cause the relay to unlatch if the PWM gets too low.

I tested the relay in question, min holding voltage was 3.5V and latch voltage was 8.2V with a 12VDC automotive rating on the relay.

[Psi]

Freewheeling diode, its correct placement at the driver, not at the relay

Are you able to explain the reasoning behind putting the diode across the mosfet and not the relay?

(This relay draws 580mA at 12V and around 260mA at 8V)

[Siwastaja]

I'm quite positive your proposed 60% duty won't cause problems with holding force, but obviously all bets are off when using products outside their intended specification.

Not "across" the FET, but next to it. Parallel to the relay coil, but not physically where it sits. Freewheeling diode provides continuous path for the current flowing in the inductance, preventing voltage peaks which would happen if the loop is interrupted. The inductance is the coil inductance + wiring inductance. If you place it at the coil, you are not bypassing the wiring inductance. This is obvious no-brainer when people design motor controls, everybody knows to minimize loop area and tightly couple the two semiconductor switches, but somehow some people get lost when instead of a motor there is a relay coil, despite it being electrically equivalent situation. This has been discussed on these forums a few times and it always causes issues, hopefully not this time.

[Psi]

ah, i see.   

Yeah for this application the diode across the relay is currently ~8mm from the Mosfet on same PCB.

Relay wires travel at most 200mm away from PCB.

[Zero999]

Is there a general rule of thumb for how low % PWM you can go before getting issues with current flow due to not enough force holding the contacts together?

Obviously there are also risks that some vibration or impact might cause the relay to unlatch if the PWM gets too low.

I tested the relay in question, min holding voltage was 3.5V and latch voltage was 8.2V with a 12VDC automotive rating on the relay.

Check the data sheet for the holding voltage. The minimum duty cycle will be the percentage of the holding vs nominal voltage.