A/C Locked Rotor situation

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pierre

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Westchester County, New York
I got a call earlier tonight and here is the situation.

208V, 3 phase supply.
60 amp, 3 phase CH circuit breaker supplying an A/C compressor. (A/C is running for special equipment, not to cool air temp of room)

The unit went into locked rotor, and was humming when the EC got to the site.
He tested for voltage at the supply side of the Circuit Breaker and had 208 volts across all of the phases (normal).

HE tested for voltage at the load side of the Circuit Breaker and had
a-c...208V
c-a...140V
a-b...140V

He changed out the CB and the unit worked fine.

How come the A/C compressor did not burn out?
How come the overloads did not open?

Has anyone seen this type of situation before?
 
Pierre, I think that the motor was humming because of phase loss and not from mechanical failure which would cause the t.o.l's to open.I don't know the locked rotor amperage @ 208 compared to locked rotor @ 140 volt, but i think that there was'nt enough amperage to open them.
Rick
 
How come the A/C compressor did not burn out?
How come the overloads did not open?

Either a locked rotor or single phase condition should have created more than a hum. There might have been a shaded or start winding with a cap, in parallel with a motor winding. In which case, ghost voltages would appear at the two (pitted, open circuit) breaker terminals. This is the best explanation which comes to mind. :)
 
pierre,
I may have missed as to why you had replace the breaker in the first place since the breaker was not nuisance tripping. It is noot uncommon for a breaker that have internal resistance will get warm and trip thermally.
By measuring the voltage at the load end of the breaker it was established that there was a voltage imbalance. But I certainly would have also measured the line side voltages also. If the the line side voltages were balanced then one would conclude that there was a voltage drop within the breaker provided that all cable terminations were verified.
Then to double check your findings it would have also checked the voltage across the line and load of each phase of the breaker which would verify that there was internal resistance in the breaker. But remember that resistance=heat and the breaker should have gotten warm.
You see what I see missing in your description was to assume that it wasn't the lines which feed the breaker, the problem which may have been attributed to an upstream problem which may have been restored after replacing a breaker that wasn't defective in the first place.
 
The contractor did test the line side of the breaker, and all voltages were 208.

I was not there, but he did say the breaker was not hot/warm :?

He gave me the breaker today, and I am going to send it to Cutler Hammer to see what help they may be.
I am leaning towards Peteo's answer, even though I am not sure at all.
Maybe the breaker company can shed some light on this.
 
pierre,
It looks like all bases are covered.
But one must admit that to have a voltage drop across the breaker there must be resistance that will cause heating. If you take the voltage drop X current it should give you heating watts and it isn't going to disappear.
If it were my breaker I would certainly want to have it evaluated also.
Although doing a contact resistance check across the line and load of each phase of the breaker is not a legitimate check for failure or rejection it does provide a reason for a breaker to be evaluated further.
What C-H breaker was it? I know those breakers inside and out.
Larger breakers have actual trip unit assemblies whether they have interchangable trip units or not . They are installed with three screws that must be torqued on the line side and terminals on the load, either of which will cause heating and nuisance tripping when they get hot.
 
:idea: I am guessing it was no more than an open in the one leg of breaker.
Motor is single phasing which will cause hum. you will read 208 V across the one winding and you will read through the other two windings a lower voltage.
 
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