Message Board Thread - "Hot spots and motor efficiency - Still more Q's"

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Hot spots and motor efficiency - Still more Q's Scott Willits 11/8/2004
I've asked before about the possibility of correlating motor frame temperature to net motor electrical operating efficiency and it led me to some pretty good resources on motors generally.

My question now is more specific to hot spots in the circuitry. I'm wondering if we can draw any correlation, even to within some reasonable range, of motor efficiency degredation and, say, a 150°F hot spot on a single leg of its power supply on a lug at the breaker or mag. Say the motor is only a couple of years old and rated as a premium efficiency unit.

Just a couple days ago I got my hands on a borrowed circuit power data logger and have seen at least one instance 125 hp motors with a known hot spot on the C-phase at the breaker. As you would expect, the power profile indicates a major load imbalance. I'm still sorting out what this means in terms of net operating efficiency, and am not even sure I can get there without understanding more about the magnitude of the load actually placed on the motor.

But given the dramatic difference in power factor from phase to phase in the diagrams below, would we expect to see very similar patterns emerge if we looked at a lot of these in the same manner?

If so, is there any way we can correlate these conditions directly to energy efficiency?

What I'm getting at here is trying to develop a marketing tool. I run into more than a few potential clients who are perfectly happy to let their equipment run in poor condition and not deal with it until they have a failure. I'm making headway in selling predictive maintenance as a philosophy, but it's a continual challenge. My goal is to find new ways to make the savings from thermography more tangible, to offer them something they'll see as an immediate improvement in their bottom line. If I can convince them that fixing problems like this will immediately cause their energy bills to go down, and more importantly, if I can give them an idea by how much they will go down, that job becomes much easier.
 
True power plot Scott Willits 11/8/2004
True power plot
 
Power factor plot Scott Willits 11/8/2004
power factor plot
 
Re:Hot spots and motor efficiency - Still more Q's electricpete 11/8/2004
I am pretty sure you may find info at pdma.com
They have long been hyping - I mean marketing - the negative effects of voltage imbalance upon the motor.

Also here:
http://www.reliance.com/mtr/b7087_5/b7087_5_7.htm

Figure 25 if motor losses vs unbalance.

If you have a 100KW motor initially 90% effieicnt, that is 10KW losses (balanced condition). Add a 6% voltage unbalance and according to this figure your losses go up by 40%. i.e. from 10KW losses to 14KW losses.

Your plot shows one phase has watts near zero compared to the other phase. I would be interested to hear the three current magnitude measurements and three phase-to-phase voltage measuremements if you have them recorded.
 
Thanks for the response, Pete Scott Willits 11/8/2004
Below is a table showing the I, V, P, VA, and PF data for all three legs of the circuit in question. I'm beginning to question the the PF data, and please forgive me for not understanding this particular component as well as I should.

Note that when I polled the data, total PF was measured at about what the chart shows. I'd think if the C-phase was as anomalous as shown here it would have jumped out at me at the time, but it didn't. It troubles me that the True Power is so low while the current, voltage, and thus VA appear to fall within relatively normal bounds. The owner of the analyzer tells me there must have been something up with the voltage probe, but those values in the table look OK. I'm beginning to think there must be some kind of bug in the data though, or else there's something fundamental about power factor I just don't understand.

************************************************
File C:\WINDOWS\Desktop\Oldmycom.log
Test began at 11/05/04 17:54:39
Test ended at 11/05/04 18:12:02



Measurement Value Units
Voltage, A Phase, Ave: 258.1 volts
Voltage, B Phase, Ave: 257.3 volts
Voltage, C Phase, Ave: 259.8 volts
Current, A Phase, Ave: 104.6 amps
Current, B Phase, Ave: 107.3 amps
Current, C Phase, Ave: 109.5 amps
Current, Neutral, Ave: 0.0 amps
True Power, A Phase, Ave: 23484.1 Watts
True Power, B Phase, Ave: 26116.5 Watts
True Power, C Phase, Ave: 1260.4 Watts
Total True Power: 50860.9 Watts
VA Power, A Phase, Ave: 28031.0 VA
VA Power, B Phase, Ave: 28774.9 VA
VA Power, C Phase, Ave: 29758.0 VA
Total VA Power: 86563.9 VA
Power Factor, A Phase, Ave: 0.84
Power Factor, B Phase, Ave: 0.91
Power Factor, C Phase, Ave: 0.09
Total Power Factor: 0.59
Cost, Total Elapsed: $1.92 $
Cost, estimated per month: $4830.00 $
(at $0.13000/KWH)
Peak Demand: 56160.8 Watts @ 11/05/04 17:59:18
Peak Ave VA: 94777.6 VA @ 11/05/04 17:59:33
 
Update Scott Willits 11/8/2004
Turns out I very likely have an issue with the voltage values here, but it wasn't apparent because it stems from the outdated circuit architecture. It's a 3-phase, 3-wire delta, with no 4th wire carrying the neutral. The tool lending library suggested I try it again and place the 4th voltage probe on a neutral ground within the box and that should correct the problem.

Although this doesn't explain why I didn't see the same issue with another motor I checked at the same time as a control, and got a total TPF of .85. Either way, I'll get more data tomorrow and hopefully be able to draw some more concrete conclusions. In the meantime, I welcome guidance or commentary from more experienced thermographers, particularly those with expertise in motor circuit analysis.
 
unbalanced voltage effect on motor efficiency electricpete 1/5/2005
another data point - the below link shows one particular motor decreased efficiency 1.5 - 2% when 2.5% voltage imbalance was introduced.

http://motors.copperwire.org/Files/doe-inbalance.pdf
 
Re:Hot spots and motor efficiency - Still more Q's ringet 8/8/2005
Check out these sources. Lots of good info on motors and more.
Good references for motor issues
http://reliabilityweb.com/
and
www.motordoc.net
 


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