Poor Power Factor
- Thread starter rpkuhl
- Start date
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Yes, we had it monitored the last 72 hours. The breaker was tripping every couple of hours in the excessive az heat.It did not tripp while we were monitoring it. From the report we got, there was 5500 watts across all three phases but c phase was only using about 300. The power factor was all over the place. 1.0 to .05 to .00 to .06 to 1.0
gedwards
Member
- Location
- Birmingham, AL
The first thing that jumps out is that you stated that this is a 3-phase heat pump, if so you should see near identical currrent readings for all three phases, my first suggestion is to look at the actual phase currents. Thermal-magnetic breakers are current devices, they do not care about power factor. The breaker either will open under a short circuit (magnetic element) or under an overload (more current = more heat, thermal element).
The second thing is that you stated that the breaker did not trip during a monsoon, by looking at the last forecast it appears that Phoenix (assuming you are in this area) was @ 82 deg F today. Your problem may possibly be linked to ambient temperature. Standard circuit breakers are designed for use in a 40 deg. C (104 deg F) ambient environment. I would recommend first looking back on the days that the breaker tripped to see if the ambient temperature was 104 deg F or above. If this appears to be the case I would then recommend that you contact the mfg. of the heat pump and have them advise on a recommended breaker for a higher ambient temperatures.
The second thing is that you stated that the breaker did not trip during a monsoon, by looking at the last forecast it appears that Phoenix (assuming you are in this area) was @ 82 deg F today. Your problem may possibly be linked to ambient temperature. Standard circuit breakers are designed for use in a 40 deg. C (104 deg F) ambient environment. I would recommend first looking back on the days that the breaker tripped to see if the ambient temperature was 104 deg F or above. If this appears to be the case I would then recommend that you contact the mfg. of the heat pump and have them advise on a recommended breaker for a higher ambient temperatures.
The current readings are all the same! We dont know why the breaker keeps tripping. It draws 55 to 56 amps across all 3 phases. What raises a flag with us is the power factor. The True Power across all phases is 5500 to 5600. Apparent Power across A & B was the same. C Phases was at 300. Would this have anything to do with the a/c unit and inturn cause something else to good wrong and tripp the breaker? The power room is indoors. The unit is on the roof. Temp was well over 105 F last week when it was tripping. There is 3 identical units. 3 - 70 amp breakers the all are doing this. One is more then others. One stopped.
What is the Minimum Circuit Ampacity (MCA) of each unit? The 70A breaker might be too small.
You didn't say what the apparent power is on phase A and B? But if the current is about the same on each leg, and the true power is about the same, then the apparent power would have to be about the same. So I am guessing the phase A and B apparent power would be in the 100 to 500 VA range - which shouldn't be a problem.
Two other things to check: what's the voltage at the units? Get a measurement on the starting voltage at the unit if possible. Any chance the controls are causing the unit to turn on and off several times fairly quick? That would trip a breaker.
Steve
You didn't say what the apparent power is on phase A and B? But if the current is about the same on each leg, and the true power is about the same, then the apparent power would have to be about the same. So I am guessing the phase A and B apparent power would be in the 100 to 500 VA range - which shouldn't be a problem.
Two other things to check: what's the voltage at the units? Get a measurement on the starting voltage at the unit if possible. Any chance the controls are causing the unit to turn on and off several times fairly quick? That would trip a breaker.
Steve
Min circuit ampacity is 65 amps. Max ocp is 80 amps. We did exactly what the engn. spec. For us to change them it would cost around $600 per breaker. Someone will need to pay for them. Not us! We are trying to find out whats wrong to make sure its not in the unit or charge the engn. for the mistake if there is one. A and B phase were at around 5500VA and C was at 300VA. There is a energy managment system on the unit. It only trips when it extremly hot outside.??? In a three phase system shouldnt all three phases VA to be close to the same? Would something be wrong with the unit if it isnt?
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
RP, if I were a betting man, I'd say you have an intermittent open on C phase. The clues are (1) more than one unit affected and (2) the low 300va on that phase. The resultant single-phasing is causing an overcurrent condition on the other phases.
Let me know if this makes sense.
Let me know if this makes sense.
5500 VA on 2 phases and 300 VA on one phase doesn't agree with the current being the same on all three phases. I thought you meant one value was true power, and the other was apparent power. But on second thought, the two shouldn't be that different anyway.
My WAG: I think the 300 VA might be a loose or disconnected voltage lead on the meter. So the meter is seeing single phasing, but the AC unit is still getting all three phases. If that were the case, i think you are back to the breaker being undersized.
Steve
My WAG: I think the 300 VA might be a loose or disconnected voltage lead on the meter. So the meter is seeing single phasing, but the AC unit is still getting all three phases. If that were the case, i think you are back to the breaker being undersized.
Steve
wirenut1980
Senior Member
- Location
- Plainfield, IN
rpkuhl said:
If the currents on each leg are the same, but you are measuring different VA on phase C than on A and B, then I think you are getting a bad reading. How are you getting these measurements? If the currents are balanced on each leg, that would lead me to believe that the voltages are also balanced. Something is not right here. I would think that the ambient temperature might be affecting the breaker operation and I wonder if conductor ampacity has been adjusted for higher ambient temperature? Need to figure out why you are getting measurements that contradict each other.
rcwilson
Senior Member
- Location
- Redmond, WA
What voltage? What is Apparent Power (VA?) on C.
56A at 208V 3 phase would be 20 kVA total or 6700 VA per phase. 5600 watts per phase indicates an average power factor of 0.83 which is about right.
What type of meter are you using to measure the loads? If the meter is connected as a 2-1/2 element meter two phases could read positive watts and one will read negative but the total power will be correct. Your 3-phase power meter may be connected up so it cannot accurately give the per phase KW & KVA readings. Like Steve said it may be a loose voltage wire or a clamp on CT not quite closed.
Your problem may just be a low voltage during a heat wave causing enough current to trip the beaker.
56A at 208V 3 phase would be 20 kVA total or 6700 VA per phase. 5600 watts per phase indicates an average power factor of 0.83 which is about right.
What type of meter are you using to measure the loads? If the meter is connected as a 2-1/2 element meter two phases could read positive watts and one will read negative but the total power will be correct. Your 3-phase power meter may be connected up so it cannot accurately give the per phase KW & KVA readings. Like Steve said it may be a loose voltage wire or a clamp on CT not quite closed.
Your problem may just be a low voltage during a heat wave causing enough current to trip the beaker.
rcwilson
Senior Member
- Location
- Redmond, WA
Something is wrong with the meter connections for C phase. The numbers look like the meter is reading voltage a-b for C phase-neutral voltage. Based on your graph, the C current is about 86 degrees out of phase with the voltage on C phase as seen by the meter. This could happen if the VT's are connected open-delta. But if that were the case, I would also expect to see C phasse with 173% more current. The magnitude of the currents and voltages appearOK otherwise you would not get the balanced VA readings. Can your meter read phase angle of votlages and currents?
I don't think an open phase problem could get theses readings because the open phase would read zero amps.
But we still have not explained why it's tripping??
Good luck
I don't think an open phase problem could get theses readings because the open phase would read zero amps.
But we still have not explained why it's tripping??
Good luck
Last edited:
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Okay, next guess: C phase is grounded. 
The meter read the VA's correctly, but I don't believe the real power on C is only about 300W.
I think they may have had the voltage or current probes for C phase reversed in polarity.
The thing I notice is that these ran for 2 days with barely a break.
I still think the breaker might be under sized.
I think they may have had the voltage or current probes for C phase reversed in polarity.
The thing I notice is that these ran for 2 days with barely a break.
I still think the breaker might be under sized.
wirenut1980
Senior Member
- Location
- Plainfield, IN
The difference in watts and VA could be a bug in the software. Because if volts and amps, and phase angles are measured and the rest is calculated in the software, then that could explain the descrepancy.
I think the breaker is tripping due to the high ambient temperature, and also that the load is shown to be almost continuous for at least 3 days straight!
I think the breaker is tripping due to the high ambient temperature, and also that the load is shown to be almost continuous for at least 3 days straight!
qucherbichn
New member
Not possible
Not possible
The Powersight PST 50 meter that was used is self correcting for a reversed polarity.
Not possible
steve66 said:
The Powersight PST 50 meter that was used is self correcting for a reversed polarity.
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