# Thread: Coils in contactors

1. Originally Posted by gar
180917-2141 EDT

On the subject of switching the coil in a contactor and using rectified AC to power the coil;

My experiment on an AB #2 starter. 709 #2, 709COD 120 V coil.

On AC this solidly pulls in at 70.9 V 0.11 A steady state after pull in.

At 120.7 V current is 0.24 A. DC resistance is 40.5 ohms. Approx power dissipation at 120 is 2.3 W.

Dropout between 65 and 70 V and very noisy.

Powering the same coil with a bridge rectifier.

Pull in at 48.3 V and 1.08 A. Calculated R = 44.7 for a check. Power dissipation 48.3*1.08 = 52 W. With greater voltage, which is necessary for a good operating point, this won't go down.

Dropout is about 6.5 V. and power dissipation at this point about 6.5^2/40.5 = 1.04 W.

With AC you get high current when the armature is open. This provides the force to close the armature, and after closure the impedance goes way up and current drops a great deal. Once the armature is closed it requires much less current to keep it closed.

My above results seem almost unbelievable. But I don't think I made a mistake. Smaller AC relays with a much smaller initial air gap I don't believe show as much disparity between AC and DC. I haven't checked one recently. My recollection was that I could run a 120 V P&B AC relay on 24 V DC.

Possibly someone else will run some experiments.

.
Tks GAR for taking the time to do the leg work and explain the difference between the 2 systems.
Based on your results, looks like those Westinghouse engineers back then knew what they were talking about!
I love George W and Nicola T. Thomas E not so much

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One way to handle the high pull-in power requirement on DC is to use a large series resistor (calculated to deliver the hold-in current only) to charge a capacitor and then connect the capacitor to the relay coil to energize it. Of course it may take a pretty large capacitor, probably determined experimentally rather than calculated with the minimal data you have on the relay armature movement.

Not good for all applications, as the cap needs to be given time to recharge before the next activation.

3. Originally Posted by GotShocked
Very good discussion going on with this post, I would also suggest that you check your neutral conductor and connections. When I worked at the utility, we would see massive voltage swings depending upon the loading on each 120V leg. With a poor neutral connection, the voltage on each 120V leg will fluctuate between 0 - 240V. Essentially the circuit becomes a voltage divider.
voltage swing of 5-10% is understandable and maybe a result of simple voltage drop, overloading conductors, poor balancing etc. but is very extreme condition and more likely to be a failed neutral conductor if you are dropping to near zero and/or seeing near 240 volts.

4. Originally Posted by Jraef
As a general rule however, good quality coils are designed with a “hysteresis” of flux that will either pull in OR drop out a coil, it is very very difficult for it to flutter in a zone in which it does both.
Apologies for truncating your post.
The pulled in VA is far less than VA before the contactor pulls in. If there is insufficient voltage to pull the contactor into the closed condition it will sit at a current much above the design steady state rating and will burn out. That's what I imagine happened here.

5. Originally Posted by GoldDigger
One way to handle the high pull-in power requirement on DC is to use a large series resistor (calculated to deliver the hold-in current only) to charge a capacitor and then connect the capacitor to the relay coil to energize it. Of course it may take a pretty large capacitor, probably determined experimentally rather than calculated with the minimal data you have on the relay armature movement.

Not good for all applications, as the cap needs to be given time to recharge before the next activation.
A long time since we used DC contactors. Last time was on variable speed drives on a Russian submarine bought by the Indian navy. A convoluted tale I won't go into here. The power supply was a battery bank.
The resistors for the contactors were called economy resistors.

6. gar
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180917-1217 EDT

I suggest that some of you run some actual experiments to actually see what happens. You will need a Variac, some different AC and DC relays, a bridge rectifier, and a smoothed adjustable DC supply. Then you will be able to speak from experience.

You can use the bridge rectifier, and a large filter capacitor, with Variac as a smooth variable DC supply.

.

7. Originally Posted by gar
180917-1217 EDT

I suggest that some of you run some actual experiments to actually see what happens. You will need a Variac, some different AC and DC relays, a bridge rectifier, and a smoothed adjustable DC supply. Then you will be able to speak from experience.

You can use the bridge rectifier, and a large filter capacitor, with Variac as a smooth variable DC supply.

.
With much respect Mr gar, I am speaking from decades of experience.

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