Nearly all conventional split system air conditioner condensing units employ a contactor to energize the compressor and condenser fan motor.  A contactor is essentially a heavy duty relay.  When the indoor thermostat calls for cooling, it sends 24 VAC to the contactor coil.  When energized, the contactor coil creates a magnetic field which attracts the contact bar and draws the high voltage contacts together.

These contacts when closed, will complete the circuit for the compressor and condenser fan motor.  Contactor contact points carry the full amperage drawn by the motors.  As such, they must be designed to handle the load.  The larger the compressor and fan, the higher the amp rating of the contacts.

We don’t normally associate electrical components with mechanical wear.  Contactor contact points are a big exception to this.  Contact points are in fact wear items similar to the brake pads on your van.  They are meant to wear out and get used up.

Every time the contacts close or open, a small arc is created between them for a brief instant.  This arc is similar to a welding arc, and the results are similar as well.  A small amount of the metal contact material vaporizes and burns away.  Over time, more and more of the material is worn away, and the contact point begins to change shape.

Both mating surfaces of the contact points are convex, or rounded outwards.  Essentially, they are both shallow domes that come together on the round parts.  As they wear, the surfaces become flatter and flatter.  As the contact points flatten out, the arcing is augmented and the wear accelerates.

As contact points experience this normal wear and tear, a number of bad things become more and more likely to happen.

As the wear becomes pronounced, the contact point surfaces become pitted and rough.  Due to the flattening, it becomes more likely that the contact points may not draw together evenly.  Both of these conditions will create a high resistance path for current flow.  This can cause three potentially damaging conditions.

First, as the current flows through a high resistance path through the contact points, heat is generated.  This heat can build in the contact points and lead to a total meltdown of the contactor and a no cooling call.

Second, the additional resistance of the contact points can create a voltage drop across them.  In some cases, the compressor will receive voltage that is too low as a result.  This leads to additional heat in the compressor motor windings and to compressor failure.

Third, the contacts can become so worn that even though the contactor bar is down, there will still be  no current flow across the contacts, and the unit won’t run at all.

A fourth problem can also develop.  As the contact points flatten out and the arcing becomes more pronounced, the likelihood that they can weld together increases.  In a single phase unit, this can lead to the condensing unit running wild and either overcooling or freezing the coil.  If not detected soon enough, this can also lead to compressor failure.

In three phase units controlled with a two pole contactor, if one set of contacts welds shut but the other breaks at the end of the cycle, the compressor will experience a single phase condition and may be damaged.

In short, contactors should be replaced proactively whenever they become pitted, flattened, and worn.  This practice will prevent a no cooling call and a compressor failure.

I once had a chance encounter with a retired engineer from the Cutler Hammer company.  He had begun his career there in the mid ‘50s and stayed there until he retired, designing contactors and motor starters.  He gave me this bit of advice which I pass along to you :  He said that if you replace a contactor with one that is the next amp rating higher (As in replace a 30A contactor with a 40A contactor), the new contactor will have lifespan that is five times longer than the original.

I have used this advice to great success on water source heat pumps that seem to wear out contactors very quickly.  And now it is yours.

-EricShidell

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