Induction on Starter Coil

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mull982

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I am having a problem with a starter in my plant, where after the starter has pulled in, we cannot get it to drop back out even after dropping out the necessary control contacts that feed the starter coil. In other words when we go to start the mtor, the starter pulls in fine but when we hit the stop button on the control circuit the starter stays pulled in and does not drop back out when control power is removed from the starter coil.

When we break the control circuit thus dropping the control voltage to the coil we are noticing that we are still reading 65V on the wire going to the coil. The coil is an 120V and 160VA rated coil. My question is weather or not the 65V which we believe is being caused by inductance would be enough to hold this coil in? I always thought that below 70% rated voltage a coil would not be able to stay latched.

With induction, only a voltage is induced on the wire, not current, and the current is determined by the load on the wire?? Is this correct?? What could have caused this induction to just appear out of the blue, since we have not had a problem for years until now with this circuit?
 
I had a somewhat similar situation several years ago. One of the local casinos around here moved the bulk fuel tank for their generators about 2000 feet away. They ran 1-1" pvc from the day tank all the way to the bulk tank. (I'm not kidding, before they called us in to connect it, they even paved the parking lot!).

The pumps at the bulk tank were 2 HP, 480 3PH. The starters were at the bulk tank. The day tank controller sent 120AC to the bulk tank to start a pump. The starters were 480. I installed a small interposing relay at the bulk tank, 120 coil.

While testing, I discovered (messily!) that the inductance from the 480 motor current would hold the 120 relay in. Actually, about 150 volts across the coil with the other end open. In fact, as soon as one pump started, the other one would also start.

My solution was to install a 100 ohm 150 watt resistor across each relay coil. These resistors were mounted in a separate can, that's alot of heat.

This 'loaded' the relay coils enough that they dropped out as expected. With one pump running, I still read about 20 volts across the open relay.

This was about 6 or 8 years ago, I've been back many times for other stuff, and have never heard of a problem with this set-up. I left a note in the can that stated 'Leave resistors connected or day tank will overflow!!'

I realize the current of a relay coil is much less than a starter, but inductance can be quite powerful.

Linework.... If it ain't grounded, it ain't dead. Every lineman knows this, and it's mainly because of inductance. If you have one phase of an overhead line isolated (not grounded, no load), and the other phases are carrying current, and you grab the 'dead' phase, YOU'LL get dead. Even if the span is only a couple of poles. Inductance is very serious at these voltages.
 
micromind said:
My solution was to install a 100 ohm 150 watt resistor across each relay coil. These resistors were mounted in a separate can, that's alot of heat.
Click to expand...

I am assuming that what is holding the coil in is the current that is created as a result of the resistance on the voltage induced circuit to the coil. So by adding a resistor across the coil, are you simply creating a parallel path for the current thus lessening the current that flows through the coil to energize it? I' guess I'm trying to picture in my mind how this would work, and am thinking that adding this resistance would lessen the current that passes through the coil.

By adding this resistor I am guessing that based upon ohms law, at the 65V I mentioned i had on the circuit this increased total impedance on the circuit would decrease the current and thus decrease the current across the coil to not completely energize it. Then based upon the fact that I=V/R, that when the normal 120V was supplied to this circuit the increasing voltage would lead to an increase in current thus allowing enough current to flow through the coil to energize it. Am I on the right track here?

Should 65V be enough to pull the coil in, or is it really the amount of current rather then voltage that determines weather the coil will energize??
 
On long runs (i.e. the previously mentioned 2000') of control circuits where the stop contacts are located far away from the starter coil, it is usually capacitive coupling that causes the starter to not drop out.

I have never seen any standard that requires a general purpose coil to drop out at any specific voltage level. While most coils do drop out at 70% you can never be sure which is one reason why dedicated under-voltage relays exist.

Instead of putting a resistor across the coil, I would use a coil "surge suppressor" which usually contains an RC circuit so it does not affect the voltage seen by the coil when it is supposed to be on.
 
Jim; I didn't think about the RC network, that would have been a better solution.

Mull; the resistor is wired in parallel with the relay coil. The idea here was to place additional load on the control circuit, to cut back on the effects of the other conductors in the conduit.
 
The "70% drop-out" rating of an AC coil means that it will NOT drop out at 70% voltage or greater , but that has nothing to do with what voltage it WILL drop out at. In some cases it could be as little as 50% voltage. With newer style electronic coils, it could be as low as 10%!

Best practice; don't run your control wires in the same raceway as power. Second best, use the RC network suppressor mentioned above.
 
This is an interesting situation. At a facility where I used to work we had several circuits some hundreds of feet long with the 120V control wiring inthe same conduit with the 480V motor circuit and never experienced this problem. The OP doesnt say what supply voltage was. In the case I mentioned it was a 480V three phase ungrounded system.
 
Instead of putting a resistor across the coil said:
Can someone please explain to me electrically what happens by putting this RC circuit across the coil??

Does it have to do with decreasing the circuit impedance thus increasing the current on the circuit in turn leading to less of a voltage across the coil??
Click to expand...
 
mull982 said:
Can someone please explain to me electrically what happens by putting this RC circuit across the coil??

Does it have to do with decreasing the circuit impedance thus increasing the current on the circuit in turn leading to less of a voltage across the coil??
Click to expand...

No.
The purpose of these RC devices is to suppress the surge voltage created by the decaying magnetic field of the starter coil when it is turned off.

Except in very large coils we rarely reduce the voltage to an energized coil.
 
jim dungar said:
No.
The purpose of these RC devices is to suppress the surge voltage created by the decaying magnetic field of the starter coil when it is turned off.
Click to expand...

I am familiar with RC snubbers and as you stated they are to suppress transient voltages. These transient voltages are typically a higher frequency event. This is why the RC works.
The capacitor "C" appears as a low resistance to the higher frequency and thus the low resistance "R" loads the circuit during that event.
The RC snubber is designed such that it appears as a high impedance (~ 26k ohms for a 0.1 uf at 60hz) under normal energized conditions.

I am missing how the RC snubber would work effectively in place of (Microminds) 100 ohm resistor if the induced voltage is from a 60 hz line?
 
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