Coils in contactors

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Fordean

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New Jersey
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Electrical Contractor
Power to building is getting interrupted. Coils in 2 contractors are melting cause of this replaced them both. Power went off again at PICO side. And coils melted second time. They aren’t fused
 
Fordean said:
Power to building is getting interrupted. Coils in 2 contractors are melting cause of this replaced them both. Power went off again at PICO side. And coils melted second time. They aren’t fused
Click to expand...
How can the coils melt in the absence of power?
 
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Fordean said:
Power to building is getting interrupted. Coils in 2 contractors are melting cause of this replaced them both. Power went off again at PICO side. And coils melted second time. They aren’t fused
Click to expand...

I have an idea what you mean but there are more details needed as has been noted. There is a former thread where I related some incidents of single phase loads seeing roughly 1/2 of the rated voltage during phase loss events due to certain types of feedbacks inherent in the way those systems are wired. Kwired described some of the scenarios how this is possible.
This is not a ghost voltage event. The compressor or load will see in the range 50% voltage.
The easiets way I have been able to solve this, and I have done it on multiple properties is to put phase protection ahead of said control coils. I have 2 single phase 208-240V systems that require this at this facility and all my 3 PH HVACR loads are protected as thus. Formerly I had a field irrigation system where the main contactor in the VFD would experience what you are saying. The Phase monitor stopped these coil failures cold. I do not recall exactly the fault that we suspected we were dealing with at that time.
 
Fordean said:
Power to building is getting interrupted. Coils in 2 contractors are melting cause of this replaced them both. Power went off again at PICO side. And coils melted second time. They aren’t fused
Click to expand...

I had this exact scenario a few years ago. I'm quoting from a thread I commented in here, years ago:


"Where I used to live and work, the city-owned power company is terrible. They have several phase-loss events per year, on average. Last December, a squirrel met its demise on the HV side of the substation transformer, and half the city lost one leg. I guess two-outta-three phases was good enough.

That phase loss event took out our freight elevator; the motor itself was protected as it is run from a VFD, but the 480v-to-120v control transformer was connected to the dropped leg on one side. This lowered the control power's voltage to somewhere (I'm guessing) around 70vac or so. This in turn cooked the coil on the 100A e-stop contactor ahead of the VFD. The switchmode power supplies didn't care about the low voltage, but that contactor sure did.

After replacing the contactor coil, I added phase loss protection to the control power circuit. If the PLM sees anything wonky with the incoming 480v power, it opens the 120v side of the control power circuit; no control power, no worry about damaging the contactor or other 120v loads. :)

We could have saved four days of downtime - and the $150 cost of a new contactor coil - if the original panel builder had included a $60 PLM relay. Needless to say, I'm a fan of PLM's."


What's probably happening is that you're loosing one leg of the supply to the contactors' coils; I'm assuming these coils are powered phase-to-phase. That means the coils aren't seeing enough voltage to pull in; they're seeing phase-to-something voltage, depending on what other loads are connected to the dead leg.

If an AC-powered contactor can't pull in completely, the armature of the contactor (moving part, made of steel that gets pulled on my the electromagnet) can't fully pull in and "close" the magnetic circuit, the coil will draw greater wattage than it was designed to, steady state. It will essentially be pulling its inrush value all the time, rather than lower the steady-state value it's supposed to draw.

This will burn up the coil, and let the magic smoke out. Switch your control voltage to one that's referenced to neutral (120V or 277V), or install a phase-monitoring relay that will drop out the control circuit if you have power quality problems. Fusing won't save your coils. This sounds like a utility problem; work with them to find out why building power keeps getting interrupted. Count yourself lucky - two burned up contactor coils is a small price to pay. I've seen much worse damage from phase-loss events - crashed CNC machines, etc...


SceneryDriver
 
Perception often differs from reality. I got a feeling the coils are burning up either immediately upon loss of power, or immediately after returning from a power loss.

I would do two things.

1. As suggested, see if the controller can accommodate a phase monitor and use that to completely remove power from the control circuit when triggered.
2. Install RC network (snubber) across each of the coils. Size according to mfr recommendations.
 
Besoeker said:
That melt when you lose power?
Makes no sense.
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Something probably happening when power comes back on, mechanical problem and armature isn't fully engaging?

Or power isn't completely out, voltage dropped low enough at some point the contactor dropped out, but never rose again to high enough level to pull back in and cooked the coil at a level somewhere below pull in power needed.
 
80913-0909 EDT

An interesting aspect of AC contactors, relays, and solenoids is:
1. The coil and core combination is an electromagnetic circuit. Much like an electrical circuit.
2. A magnetic circuit has an electrical impedance to AC that is function of flux coupling to the coil of the electromagnetic circuit.
3. A simple solenoid consists of a coil and ferromagnetic movable plunger and fixed parts.
4. A closed magnetic circuit (no air gap) produces the highest AC impedance. As the air gap is increased the AC impedance goes down. Because as the air gap increases there is less flux coupling to the coil.

When a solenoid or relay is de-energized a spring, gravity, or some mechanical force is used to open the magnetic path. Thus, a lower AC impedance and higher current when voltage is applied to the coil.

Energizing the coil produces a current that creates magnetic force that closes the air gap (moving the plunger or armature), now the AC impedance increases, and current drops to a lower level.

The relay or solenoid designed for continuous energization will have a steady state current that won't overheat the coil. If the plunger or armature of this same device does not move from its open or near open position, then because of much higher current the coil will burn out.

If applied voltage and thus current is too low (this current can still be much higher than the current for a fully sealed in device), then the produced magnetic force is not high enough to close the gap, and coil burn out can occur. Very common in AC solenoid valves where the valve gets mechanically stuck. DC solenoids don't have this problem because there is no change in impedance at DC.

If supply voltage was way low, a nominal 120 being less than possibly 90, then I might expect relays or contactors to not close and thus burn out.

.
 
kwired said:
Something probably happening when power comes back on, mechanical problem and armature isn't fully engaging?

Or power isn't completely out, voltage dropped low enough at some point the contactor dropped out, but never rose again to high enough level to pull back in and cooked the coil at a level somewhere below pull in power needed.
Click to expand...
You could well be right. Under voltage rather than loss of power would explain it.
 
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. The typical values are 70% drop-out, 80% pull-in. So on a 120V coil that is already pulled in, it will not drop out until the voltage drops below 84V, but will not pull in again until the voltage rises again to 96V. This is done specifically to avoid this very situation and those values are baked into NEMA specifications.

If the LOAD on the coil powered device (ie a contactor) is large enough to AFFECT the voltage drop because the service is under sized, that’s where you can get into trouble. Let’s say you have a 200HP Motor fed by a 200kVA transformer. It might be enough to start and run it, but if the utility side drops too much, the coil drops out. That sheds the 200HP motor and the transformer recovers, raising the voltage to where the coil picks up again and pulls in the starter. That of course causes the voltage drop again and the coil let’s go again, repeat to destruction. I just used that one large motor to illustrate the point, it could also be 10 x 20 HP motors or 20 x 10 HP motors or 100 x 2HP motors; any total load that is too large for the service.

In order for this to take place, you MUST have a “2 wire control circuit”, meaning whatever is turning the load on and off is maintaining that command. In this case you mentioned a “Pico”, which was a small Allen Bradley “smart relay” or micro-PLC. It’s likely powered by a 24VDC power supply, and that power supply likely has a very wide range of input voltage at which it maintains the 24VDC, meaning at less than 84V, it is staying active, so its output controlling the coil stays closed, allowing this to happen. The simple solution would be the aforementioned line monitoring relay that is fed into an input of the Pico so that if the voltage does drop, it drops out all of the outputs and goes through a restart sequence wherein they don’t all try to turn back on at the same time and start the drop out cycle again. If it’s just one or two loads you can also program in a “starts-per-hour” limit to ensure that the contactors don’t cycle on and off again fire than a couple of times without a significant rest period between them. This by the way is ALSO going to be harming your motors, so it’s a wise plan for more than just the contactor coils.
 
Without phase loss protection you can burn up a relay or other components when a phase is lost.
Many don't want to protect because of the extra cost.
 
AC vs DC Contactor Coil Differences

AC vs DC Contactor Coil Differences

gar said:
80913-0909 EDT

An interesting aspect of AC contactors, relays, and solenoids is:
1. The coil and core combination is an electromagnetic circuit. Much like an electrical circuit.
2. A magnetic circuit has an electrical impedance to AC that is function of flux coupling to the coil of the electromagnetic circuit.
3. A simple solenoid consists of a coil and ferromagnetic movable plunger and fixed parts.
4. A closed magnetic circuit (no air gap) produces the highest AC impedance. As the air gap is increased the AC impedance goes down. Because as the air gap increases there is less flux coupling to the coil.

When a solenoid or relay is de-energized a spring, gravity, or some mechanical force is used to open the magnetic path. Thus, a lower AC impedance and higher current when voltage is applied to the coil.

Energizing the coil produces a current that creates magnetic force that closes the air gap (moving the plunger or armature), now the AC impedance increases, and current drops to a lower level.

The relay or solenoid designed for continuous energization will have a steady state current that won't overheat the coil. If the plunger or armature of this same device does not move from its open or near open position, then because of much higher current the coil will burn out.

If applied voltage and thus current is too low (this current can still be much higher than the current for a fully sealed in device), then the produced magnetic force is not high enough to close the gap, and coil burn out can occur. Very common in AC solenoid valves where the valve gets mechanically stuck. DC solenoids don't have this problem because there is no change in impedance at DC.

If supply voltage was way low, a nominal 120 being less than possibly 90, then I might expect relays or contactors to not close and thus burn out.

.
Click to expand...

Gar, thanks for your explanation. I would like to add my 2 cents which may further support your theory.
Being a "wet-behind-the-ears" FSE for Westinghouse back in the '70's I was sent on a job where the AC contactor coils were burning up at these irrigation pump stations scattered all over Winnemucca, NV. It was reported that brown-outs and voltage fluctuations from the POCO were very common in the area. As I was measuring coil voltage on a contactor at one MCC with a Simpson 260 meter (state of the art analog meter at that time) I saw the needle (voltage) drop momentarily then come back.
The factory fix was to send out a rectifier/coil kit to convert the 120VAC coil voltage to 120VDC. Their explanation was that the DC force (flux) holding the plunger in was less susceptible to voltage fluctuations because of the lower dropout limit. This reduced the high currents and chances of coil failure. After installing the kits on all the stations, I left the site.
After a week or so the report back was "Problem Solved." :D
In hindsight, it probably would have been a good idea to also install a SPD or MOV on the incoming AC line to each MCC.
 
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SPD's won't do anything for a voltage sag.

If a contactor is rapidly cycling (at least if multiple cycles are occurring in just a few seconds) for some reason my experience has usually been contact burnout goes along with possible coil overheating. Doesn't always do the controlled equipment any good either, motors would be about the worst thing to have that happen to because of the high starting curent that will be seen, but I have seen three pole contactor running resistance heating melt down from rapid cycling issues also - that probably mostly from heating in the contactor coil though.
 
kwired said:
SPD's won't do anything for a voltage sag.

If a contactor is rapidly cycling (at least if multiple cycles are occurring in just a few seconds) for some reason my experience has usually been contact burnout goes along with possible coil overheating. Doesn't always do the controlled equipment any good either, motors would be about the worst thing to have that happen to because of the high starting current that will be seen, but I have seen three pole contactor running resistance heating melt down from rapid cycling issues also - that probably mostly from heating in the contactor coil though.
Click to expand...

Yeah, I agree about the v sag but mentioned the SPDs as a precaution for the high as well as the low voltage fluctuations since this is a problem area.
I am curious why you did not comment on the jist of what I was trying to communicate about AC vs DC coil operation which was addressing the OP.
 
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.
 
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.

.
 
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