Electromechanical latching relay pull-in voltage

[vinod]

We are using MIL standard Latching Electro-mechanical relays having dual ON/OFF coils. During screening process the relays are being subjected to pull-in voltage

measurements for both ON & OFF coils in SRC (25 degree Celsius),COLD (8 degree Celsius) & HOT (80 degree Celsius) conditions. For pull-in measurement, the coil voltage

starts at 0V and gradually increasing to 28V in continuous manner. Pull-in will occur in 6-16V range. Status monitoring (Open/Close) is done at relay poles.

The anomaly is that the pull-in voltage measured at Hot is showing values lesser than Cold and Cold value is higher than SRC. This observation is against the normal

intuition of pull-in voltage HOT>SRC>COLD.
For example, The pull-in of RL-1 ON coil shows 11.496V (ISRC), 15.042V (Cold), 17.719V (Hot). That is SRC reading is 3.5V lesser than Cold.Also the difference voltage

(3.5V) shows inconsistency under repeated tests of the same part.The curreny limit of 30V source is set at maximum coil current of relay as per data sheet.
The relay specifications are 450 Ohm coil, 15A/28V DC contact, 30V DC coil/500mA.

The deviation is observed only for 0.2% of total relays screened per batch.All others are operating well.

I hope for feedback.

[Externet]

The anomaly is that the pull-in voltage measured at Hot is showing values lesser than Cold and Cold value is higher than SRC. This observation is against the normal
...
I hope for feedback.

And the question is... ?

I would say 0.2% of anomaly is a very good small percentage of rejection.

I guess you are focusing in the pull-in needed voltage versus a temperature-to-copper-coil resistance. That is not the only thing to consider. All the parts of a relay are warmed up and those not-coil parts also change behavior with temperature.
Either in reed or spring type, the springy action to engage is also affected by temperature and would make contact earlier when warm.

[sghioto]

Found this on the web:

In DC relays, after continuous passage of
current in the coil, if the current is turned
OFF, then immediately turned ON again,
due to the temperature rise in the coil, the
pick-up voltage will become somewhat
higher. Also, it will be the same as using it
in a higher temperature atmosphere. The
resistance/temperature relationship for
copper wire is about 0.4% for 1°C, and
with this ratio the coil resistance
increases. That is, in order to operate of
the relay, it is necessary that the voltage
be higher than the pick-up voltage and the
pick-up voltage rises in accordance with
the increase in the resistance value.
However, for some polarized relays, this
rate of change is considerably smaller.

Steve G

[haklesup]

"then immediately turned ON again,due to the temperature rise in the coil, the pick-up voltage will become somewhat higher."

I think he was saying it was opposite this expectation. I suspect the heat may have changed other mechanical aspects of the armature making it move more easily (it should not be lubricated but that's what one would expect from a lubricated system) perhaps there is a plastic bearing or a metal part that expands. Less intuitive is if your oven were vibrating from a fan or something, the external mechanical input may be influencing the relay mechanical system.

look at the current instead of the voltage (or both, use a curve tracer as the source). The magnetic field is more of a function of current and current is a function of voltage and resistance. by looking at both V and I, you will be able to deduce the change in coil resistance directly, the rest of mechanical.

[Lenp]

I used to see this problem with ac coils on gas valves if they were in a high heat area. The resistance of the coil would increase and the valve would not operate. Replacing the coil fixed the problem.