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kwired

Electron manager
Location
NE Nebraska
I wouldn't trust the coils to be similar enough to divide the voltage equally.
I don't see them being that much different that they won't still be within a reasonable tolerance. HVAC guys parallel control transformers when "twinning" units sometimes, I think there is more risk of unequal division issues with something like that.
 

MidwestJest141

New User
Location
Iowa
Occupation
Electrical Engineer - Industrial Control Panel Design
In this scenario what stops the current from choosing one relay over the other? I see a potential imbalance of load on one of the relays. In the case of this imbalance you might burn out one of the relays... This issue comes about because of the possible difference in coil resistance...this can not be guaranteed to match perfectly from relay to relay even of the same p#. Just my 2 cents, Thanks!
 

gar

Senior Member
210730-1145 EDT

If these were relays with a DC coil I doubt there would be any problem. Coil resistances, and currents at a voltage are likely quite close.

Generally relays have a pull in voltage considerably below their nominal operating voltage, possibly 70%, and dropout is lower. Probably a greater ratio for a DC than an AC relay.

In an AC relay you have both coil resistance and inductance to look at. Coil resistance will be quite close and it is not affected by armature position. Armature position has a great effect on inductance, and therefore impedance dependent upon no excitation, and fully powered. I might expect to see much greater variation in the steady state impedance when energized than what I see in resistance.

Dynamically when the relays are closing the impedance vs time comparison between the two relays may be the greatest variable. An AC relay coil impedance greatly increases as the relay is energized.

One would certainly need to run tests on series AC relays to see how they perform on both the transient turn on and steady state conditions, and over a range of lower voltages.

The problem, if one exists, could be corrected by putting an appropriate capacitor, or resistor in parallel with each relay coil. The other way is to parallel the two coils, if two relays are really needed, and use a step-down transformer for power.

.
 

kwired

Electron manager
Location
NE Nebraska
In this scenario what stops the current from choosing one relay over the other? I see a potential imbalance of load on one of the relays. In the case of this imbalance you might burn out one of the relays... This issue comes about because of the possible difference in coil resistance...this can not be guaranteed to match perfectly from relay to relay even of the same p#. Just my 2 cents, Thanks!
If they are in series, current is always going to be the same through both coils. Voltage drop across each individual coil can vary if impedances are not the same though.

Input voltage of 492, with variances you could have 239 across one but that means the remaining 253 has to drop across the other, but if they are the same model I wouldn't expect them to even be that far off from one another.
 
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