Why does a contactor short cause the HPCO to trip?

[ectofix]

You didn’t specify what type of equipment.  Nonetheless…

 

A reason for a device such as a HPCO is shut down a system in the event that of a malfunction should occur in the main controls which can cause a dangerously high pressure situation. The HPCO abates that situation.

 

A contactor is generally part of a the main control circuit. A contactor can fail and therefore be the contributing factor to a runaway condition.  However, usually it’s the contacts within a contactor that short closed by becoming welded together.

 

In my experience, a contactor coil that has short-circuited will simply stop working altogether. However, that can be contingent upon where within the coil that the short has occurred and also based on the circuit configuration.

With that in mind, I suppose a short within the coil that’s causing a contactor to remain energized IS POSSIBLE, but such an event would be an EXTREMELY rare occurrence.  I’ve never seen it happen.

 

Otherwise, a short-circuit of a control device (or wiring) which operates the contactor coil will usually be the reason why a contactor remains energized when it shouldn’t be.

[fixbear]

His source has to be out of context.  A contactor coil is just a magnet to pull in the armature of the contactor.  If it shorts, it will usually take down the control power by either blowing a fuse or overloading a transformer.  Any short to the coil reduces the magnetic Gauss to pull in the armature.  Pull in magnetism has to be higher than hold level, so it may hold for a cycle,  but not re-energize.  I have seen some old model contractors (30’s to 40’s)  that the coil short caused a swelling to the inside that squeezed on the armature, but it usually relaxed enough to release some degree.  This was only in cloth wrapped coils that were hand wound.  Today’s coils are all machine made with a plastic form on the inside that prevents this. They can be designed with more clearance due to the availability of finer and better wire .  Wire insulation has come a long way.  After all, look how much copper weight has dropped per HP in motors.  I had a 36 inch Bullard vertical turret lathe built in 1929 with a 10 HP motor.  That motor was 30 inches in diameter and 32 inches long without the shaft.

 

Like ectofix said, the problem will be in the control circuit.  Besides, there had to be a over-voltage to short the coil..  Back when wire winding insulation was just enamel,  they failed from cracks.  But today’s polymers have stopped that problem.

 

As for contactor points welding,  The old contractors with a single contact per pole in power control have been relegated to the past.  They are all regulated to have double poles today just for that reason.  The one that I have seen with a problem are the Furnas with the 30 degree angled wiping contacts. They do tend to suffer arc damage more often.

[ectofix]

Thanks for the lesson on contactor construction – both old and new.  Now that you mention it, I have seen a contactor plunger get stuck in due to coil swelling. 

Far more so though, I’ve seen SOLENOID valve coils swell up enough to put a squeeze on the valve’s stem & plunger assembly to a point that I couldn’t remove the coil from it.  I work with steam-heated Hobart dish machines allot and that solenoid valve scenario frequently occurs with the steam valve.  I guess by succumbing to the extreme temperatures of the steam that it controls.

 

Reminds me of a contactor-related story:

I was called by customer because their outdoor W/I cooler was freezing product.  I initially tested its commonly used single-pole thermostat inside of the cooler (a A19 series JohnsonControls tstat).  Per my meter, the thermostat was opening at a suitable temperature. No issues with the thermostat setting or its cut-in/out span of operation.

 

Then I needed to access to the condenser.  Funny that I almost didn’t need a ladder to get up on top the cooler to get at the condenser. This cooler was a dinky one.

 

Its contactor coil was rated at 208v.  I removed power for everything (at the disconnect) and the contactor opened.  I re-applied power and it DIDN’T pull in.  Back inside the box, my meter showed that the tstat wasn’t calling because it was satisfied.  Okay, well…

So then I decided I should force it to run it through all of its paces.  Thusly, I left the cooler door open to heat things up.  With my meter connected to the thermostat, it  eventually showed that it was calling to cool.  This box so happened to be so small that I guess the installer didn’t see a need for a LLSV.  However…when the thermostat closed, I could hear refrigerant start flowing through the TXV.  So, everything was working on cue so far.

 

Okay. Back on top of the box.

Up there at the condenser, I monitored the contactor coil voltage with my meter.  It was reading 208v for awhile.  It  was pulled in and (of course) the compressor was running.  As the cooler temperature dropped, eventually the voltage  reading at the contactor coil dropped off from 208v to 120v.

 

HUH?

Additionally, the contactor was still pulled in…

 

WHAT!?

OH…and the compressor was still running (of course).

 

W-E-L-L…in this particular setup – I knew that when the thermostat is satisfied, the contactor coil voltage should drop to 0v, the contactor should open and the compressor should stop. 

But…it didn’t…

– and what’s with this 120v in a 208v circuit?

 

I shut it down at the disconnect again to study the situation. 

The leg off the contactor coil leading to the thermostat down into the box was READILY evident…since the OTHER leg was simply a jumper off the hot side of one contactor pole.

SO, I’ve got a short to ground somewhere in that thermostat leg. A neutral where there shouldn’t be one (if you will).

 

To verify, I set my meter to ohms, pulled the thermostatically-controlled leg off the contactor coil (in order to isolate), then took a resistance reading to ground.  I don’t remember what the reading was, but it was a r-e-a-l-l-y low resistance…yet still nowhere near continuity.

 

Well, if it HAD read continuity (0Ω), then it should have blown a fuse or tripped a breaker.  RIGHT?

But…it wasn’t, and it didn’t.  Interesting…

 

From there I chased the thermostat wire run from the condenser down into where it penetrated the cooler section.  That led me to a J-box down there – mounted on the exterior wall.  Inside it was the to/from wires for the thermostat leg and two legs to supply the the evaporator fans.

I removed its cover and…it was wet. It was REALLY wet…and…one wire nut in there showed evidence of some shorting to ground via all the wetness. Not a blowout and trip a breaker type of a short.  BUT…it was still a short.

 

That moisture had become part of an impromptu and unintentional source to ground that had enough resistance to NOT trip a breaker or blow a fuse, but enough of a electrical connection to serve as a neutral to keep the contactor coil pulled in.

Mind you, that 120v wasn’t enough voltage to PULL IN the contactor, but it was JUST enough voltage to KEEP the contactor pulled in, keep the compressor running…and freeze their product.

[fixbear]

That is exactly what I found years ago on a walk in.  I have also seen bleed from a defrost heater circuit and evaporator wiring. 

How about airborne salt brine on a circuit board that was’t fully sealed. fooling a IC.  Or someone forgetting to place a seal cap on the bottom of a 6X6X4 junction box so that the mice built a nest in it.  And we all know how they like to chew on THHN.

[alnelson]

This question isn’t in reguards to an ice machine or anything? Is it?

 

if you get a shorted contactor coil on a Manitowoc Indigo quiet cube machine they will do some goofy things and maybe give an HPCO fault.