Contator coil failure 1

[lukin1977]

What is the cause of the failure of a conatactor´s coil?

I suspect undervoltage. but, what is the mechanism of the fail?
Could it be that the contactor did not close so CEMF was missing and current on coil got too high?

Here is a picture of damaged coil. V=380 V - 50 Hz

 

[oldfieldguy]

One common failure of AC coils is failure of the contactor to fully close to complete the magnetic path. With the path open,the coil continues to draw too much current, resulting in overheating and failure.

A second common failure of AC coils is caused by mechanical vibration, wherein one or more turns short internally as insulation is mechanically damaged. The shorted turns act to greatly increase the current, again resulting in overheating.

A third failure, common to DC coils, is the failure of the economizer circuit to function, resulting in high current flowing in the coil for much longer than it is rated to accept.

old field guy

 

[lukin1977]

undervoltage on the coil can produce failure on the contactor to fully close?

 

[jraef]

Good quality contactor coils are designed to be "go/no-go" with regard to under voltage conditions, in that they will not pull in if the voltage is below 80%, but will not drop out until it is below 70%, so that makes them hold in longer, then not re-energize while the voltage is still low. So for this type of damage to occur, the voltage would have to be low, but not TOO low, for a long time, as in long enough for you to have detected it (assuming the system was designed to protect against voltage sags).

A more common cause of problems that end up looking like that is rapid opening/closing of the control command. For example a tank pump-up float switch acting directly on the coil without some sort of designed hysteresis in the system. Outflow from the tank will result in the float dropping, turning on the pump, it pumps, raises the float, which immediately stops the pump, but liquid is still flowing out, so the float drops, starts, stops, starts, stops ad nauseum, all taking place in a matter of seconds. Eventually the OL relay should trip because this is damaging the motor too, but often the contactor coil will fail first.

So if you have no other evidence of long term voltage sag, I would be looking at your control circuit very carefully. Sometimes even if it was designed correctly, a failure like this is secondary to another failure further up stream in the control system.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[squeeky]

Sometimes dirt on the pole faces or even rust can cause a chatter but my money is on the fact that it did not fully close. If the former, wipe a small amount of oil across the face with your finger. Also the use of VPCI in the panel would assist to prevent this.

There is a tendancy to move to an AC activated coil but it has a DC converter circuit in it. The coil is actually DC and your contactor will stay energized down to about 50%. If you require details I will dig them up. Please let me know.

 

[lukin1977]

jraef: the contactor is used to start/stop a submersible water pump that feeds a diaphragm rubber tank. The contactor´s control is done by a pressure swicht.
Pressure switch is set for max and min. I dont know what are the values. I will check and see if conctactor is opening and closing repetitivly

Squeeky. I dont think that dirt is the problem in this case. Contactor was new and only worked for one week. The electric cabinet is hermetically seal (rain proof). I personally took it out and every thing was clean. By the way; What is VPCI?

regards,

lukin1977

 

[jraef]

I often find that users do not fully understand the "cut-in / cut-out pressure" concept on pressure switches and attempt to set them too close to each other, resulting in the contactor coming in and out too often, which becomes a race to see what component fails first.

VPCI = Vapor Pressure Corrosion Inhibitors. They are little sponge like stick-on pads that you put in the enclosure which emit a vapor (gas) that coats exposed metal surfaces in such a way as to prevent oxidation and corrosion. Most enclosure mfrs sell them, or they can be found on-line. But be aware they cannot be shipped via air freight, so if ordering them on-line, don't pay the extra shipping costs because they will collect it from you, then ship it ground anyway because it is a hazmat.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Compositepro]

One of the most common failures of such a system is bladder failure or loss of air pressure in the pressure tank. The result is short cycling where the pump will cycle on-off every few seconds. This will also soon lead to pump failure.

 

[lukin1977]

yes. I am suspecting loss of air pressure too. I will be checking the equipments next wednesday

thsnks!

 

[proelect8717]

dear lukin 1977,

from the picture it believes to me that this occurs due to the insulation failure between turns of the coil. As most area of the coil is burnt, but some area at the top of coil is still good, So it more seems inter turn short in the coil rather than any other reason.

Can you kindly share more pictures of coil so that we can understand more better?

THanks.

 

[lukin1977]

proelect: I dont have anymore pictures and the coil has already been thrown to the garbage

Like I said before. The contactor is brand new

 

[lukin1977]

the problem was bladder failure

 

[jraef]

Thanks for the closure. We rarely get that, so it's appreciated when we do. It helps people to learn from the experiences of others.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington