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Anonymous Poster

Troubleshooting an ac coil Magnetic Contactor,

07/28/2008 8:55 PM

my 24volt ac coil 3phase magnetic contactor (used on my air conditioner)burn frecuently.what can couse a 24 acvoltage coil to burn.

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#1

Re: Troubleshooting an ac coil Magnetic Contactor,

07/28/2008 10:01 PM

1] use multmeter check ac voltage apply on your coil to see if its too high to afford.

2] to see the specification is it 24v or 12v

3] to see the specfication is it ac or dc?

4] check your voltage with a multimeter when air conditioner start and stop, and see if it can effect on your coil.

5] air conditioner is using single phase, why do you use 3 phases contactor? is it a large concentration conditioner?

8] check you contactor's joint carefully. make sure not to have a gap between them!!! as well as ferrite core and plate.

6[ change a new name brand one.

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#2
In reply to #1

Re: Troubleshooting an ac coil Magnetic Contactor,

07/29/2008 6:23 PM

the air conditioner is one of several identical AC units that work perfectly.Coil voltage is 24volt a/c,current is the same as in all other units,the transformer that provide the coil ac voltage had being replaced.the coil burn when it is not under tes(note parts are the same as in other units)

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#3

Re: Troubleshooting an ac coil Magnetic Contactor,

07/30/2008 12:32 AM

If the coil burned, you either had too much voltage or too little impedance. Too much voltage would be caused either by a spike in supply voltage to the transformer, or a failed transformer. Too much current could result from a short circuit between the two leads of the coil, causing a dead short, through damage to the insulating coating. This could have resulted from poor quality control during manufacture, excessive moisture in the environment, or even excessive vibration in the equipment. Or somebody poking around with a screwdriver...

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Anonymous Poster
#4

Re: Troubleshooting an ac coil Magnetic Contactor,

07/30/2008 5:07 AM

The coil is verified it is really 24 V ac and with a multimeter verifies if coil is receiving 24 V ac and removes the terminals of connection of the coil uses a fine sandpaper for cleaning and re-do the connections

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#5

Re: Troubleshooting an ac coil Magnetic Contactor,

07/30/2008 7:16 AM

If the specs on the contactor are correct for the application and it is configured the same as others that are not burning out, the only cause of a contactor coil burning out would be an obstruction preventing the contactor armature from sealing in fully when it is energized. This results in an open circuit in the magnetic field resulting in reduced coil impedance. With a reduced contactor coil impedance, the coil will draw exessive current that will generate more heat that it can dissipate, resulting in failure of the winding insultaion.

Check to see if a nearby wire or other object is getting in between the contactor armature and stator.

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#6
In reply to #5

Re: Troubleshooting an ac coil Magnetic Contactor,

07/30/2008 7:36 AM

This answer makes sence and makes good use of the given information.This is my original feeling about the couse of the problem.

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#7
In reply to #6

Re: Troubleshooting an ac coil Magnetic Contactor,

07/30/2008 9:37 AM

Adding to my previous comment:there is no object getting between the contactor and the stator.Considering the reduced coil impedance produced when the contactor armature is not sealing in fully(reduced magnetic field)/ question:can it be cause by an intermitent,erratic,rapidly changing control signal telling the contactor to close but not permiting it to fully close for a period long enough to produce excessive heat in the coil?

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#8
In reply to #7

Re: Troubleshooting an ac coil Magnetic Contactor,

07/30/2008 10:33 AM

It seems joeconed is on the right track with the magnetic circuit. If you have access to one, I recommend using a recording ammeter (for example, a Fluke clamp-on with data logging capability) to determine whether the coil current is spiking, or is continuously higher than a normal coil.

Other items to check which could lead to the same result:

Armature spring resistance: with power off, manually push in the armature of a known good contactor, and compare the spring resistance with the suspect contactor. If the suspect contactor requires more force, the spring may be preventing the armature from completing the magnetic circuit.

Damaged armature: a cracked or warped armature would also cause an open in the magnetic circuit, with the same result.

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#9
In reply to #8

Re: Troubleshooting an ac coil Magnetic Contactor,

07/30/2008 12:09 PM

No sense in checking contactor because it had being replaced several times. The fluke will point to what was mentioned before: problems with the control signal going to the coil. The problem with the spike theory is that all the other exact model A/C units are connected to the same power line, one after the other. Question: Is it possible for a control signal to be intermittently bad, erratic in such away as to permit the contactor to be "electrically close" but not permitting the stator and the armature to make full physical contact to allow the magnetic field to increase the impedance of the coil and control the current?

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#10
In reply to #9

Re: Troubleshooting an ac coil Magnetic Contactor,

07/30/2008 12:53 PM

The control signal to the contactor coil results from the closure of a contact from a thermostat which connects the control voltage coming from the secondary of the control transformer to the contactor coil. Assuming no one has ever changed the control transformer, the secondary voltage should be correct for the contactor coil. The possible causes of the control transformer secondary voltage being too low are:

  1. The line voltage being too low which would result in other promlems such as the compressor not starting or the compressor overload tripping.
  2. A shorted turn in the secondary of the transformer, will cause a lower output voltage. This would not persist too long before the transformer burns up. Shorted turns in a transformer generate hot spots which always progress to additional adjacent turns shorting until the transformer burns up. This will occur even without any secondary load but will progress faster with a load on the transformer secondary.
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#11
In reply to #9

Re: Troubleshooting an ac coil Magnetic Contactor,

07/30/2008 1:01 PM

Yes, it is possible. The contacts will close slightly before the armature reaches full travel. The extra travel ensures pressure on the electrical contacts to minimize resistance. Any binding or interference near the end of travel could cause the problem you are experiencing.

However, don't assume that any current spikes would be caused by the source to the coil. Any mechanical problem in the contactor which causes the magnetic circuit to open will appear in the electrical circuit as an increase in current. The effect is similar to switching a resistor in or out of a constant voltage circuit. The change in resistance alone will drastically affect the current.

Question: Most electricians would not replace the entire contactor, but only the burnt coil (much cheaper). Was the entire assembly replaced, or just the coil?

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#12
In reply to #5

Re: Troubleshooting an ac coil Magnetic Contactor,

07/30/2008 9:46 PM

your describe is clear

--obstruction preventing the contactor armature from sealing in fully when it is energized. This

and mine is vague at article VI.

I only know the word of gap, can you or top poster correct my this sentence? I shall appreciate.

and I hope to know can you guess wht I said in the article 6?

then I will not mke such mistake.

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#13
In reply to #11

Re: Troubleshooting an ac coil Magnetic Contactor,

08/03/2008 6:32 PM

Sir, as mentioned several times before, there is no possibility of mechanical problem with contactor; it has being replaced several times before. The transformer that provides the 24 volt ac had being replace too. All those obvious attempt had being tried.

The contactor is not the problem, the transformer that provides the 24 volts is not the problem both had being replaced and are of the correct type for that A/C unit.

Whatever is producing this problem is coming through the contactor control signal. It can be at the primary or the secondary of the 24 volt ac. The control signal tells the contactor to activate or deactivate. Let assume something is wrong with the circuitry that tells the contactor to close or to open, example: dirty relay ,burned sensor contacts in series with the 24 ac line going to the coil. All this can produce an intermittent erratic coil control signal, intermittent means that it will happen when less expected, erratic means that the shape or velocity of change can not predicted. The question is: do some body know for a fact or by real experience if a rapidly changing control signal to the coil be in such a way as to permit the coil to close and stay close but not permitting the stator and the armature to remain under full physical contact so that the magnetic field be strong enough to produce the impedance necessary to control the coil current.

This might be my last attempt to clarify or find a solution to this problem.

thank you all for your help

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Anonymous Poster
#14
In reply to #13

Re: Troubleshooting an ac coil Magnetic Contactor,

08/06/2008 9:16 AM

Sir, as mentioned several times before, there is no possibility of mechanical problem with contactor; it has being replaced several times before. The transformer that provides the 24 volt ac had being replace too. All those obvious attempt had being tried.

The contactor is not the problem, the transformer that provides the 24 volts is not the problem both had being replaced and are of the correct type for that A/C unit.

Whatever is producing this problem is coming through the contactor control signal. It can be at the primary or the secondary of the 24 volt ac. The control signal tells the contactor to activate or deactivate. Let assume something is wrong with the circuitry that tells the contactor to close or to open, example: dirty relay ,burned sensor contacts in series with the 24 ac line going to the coil. All this can produce an intermittent erratic coil control signal, intermittent means that it will happen when less expected, erratic means that the shape or velocity of change can not predicted. The question is: do some body know for a fact or by real experience if a rapidly changing control signal to the coil be in such a way as to permit the contactor to close and stay close but not permitting the stator and the armature to remain under full physical contact so that the magnetic field be strong enough to produce the impedance necessary to control the coil current.

This might be my last attempt to clarify or find a solution to this problem. This is the only probable explanation that I can think of.

Thank you all for your help

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Anonymous Poster
#15
In reply to #13

Re: Troubleshooting an ac coil Magnetic Contactor,

08/18/2008 1:19 PM

Theoretically: yes, this condition might exist. But practically... try to find the problem not assuming that coil won't close fully enough because the signal change. Most probably if you have switch in series with coils than undervoltage condition will exist if thermostat is in trouble. And this undervoltage condition might be presented on one particular coil (maybe this is farest coil in circuit). Than you will hear rather chattering noise. Anyway, to eliminate this put another small relay to separate your switch signal from coils power. At least you will prove yourself that there is no problem with circuits.

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Anonymous Poster
#16

Re: Troubleshooting an ac coil Magnetic Contactor,

06/30/2009 11:17 AM

your home a/c i take it?? If so, u don't have 3-phase power. mayyyybe if you own something like the Biltmore, a power company would give you three wires that are three phase, not two wires for 240 v and one ground which is just single phase. my guess is u meant three contact points??

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