Message Board Thread - "Motor wiring and net efficiency"

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Motor wiring and net efficiency Scott Willits 6/8/2004
I know how to use the ITC provided indirect power loss tool for specific shapes, but I'm wondering if there is any kind of rule of thumb approach for conservatively estimating the hit to efficiency in situations where you find a motor running hot throughout its entire circuit.

The example I'm looking at right now is a 100HP booster pump in a municipal water supply system. The motor is on a VFD even though it is generally running under a fairly consistent condition near full load. The operator indicates that most of his VFD equipped motors run hotter than he'd like them to. This particular motor runs warm but not alarmingly so, but the primary conductors are running >45°F over ambient on a warm day.

I'm suggesting this is a probable deficiency that warrants further investigation, but my problem extends a little deeper.

I'm doing this work for a local energy authority, and my specific task here is to identify not so much hazards or maintenance issues as to identify and, to the extent that it's possible, quantify energy savings opportunities.

So what I need to figure out is, given the information above, how can I come up with a ballpark figure representing the energy being wasted by the poor condition of this motor, beyond just measuring the heat coming off its constituent parts and converting from watts to dollars? If a motor is supposed to be premium efficiency rated at 92% but it's running like this, can we assume a 5% or 10% or worse hit in efficiency? What's the best way to quantify this without overstating the case?

Thanks much,
Scott Willits
 
Re:Motor wiring and net efficiency Scott Willits 6/8/2004
Here's a pic of the conductors.
 
Re:Motor wiring and net efficiency Scott Willits 6/8/2004
Here's a pic of the conductors.
 
Re:Motor wiring and net efficiency Carl M 6/8/2004
Premium efficiency motors do run at higher temperatures (seems contradictory, but that's the facts.)
The conductor heat loss is unrelated. Could be harmonic current losses or undersized conductors (I²R). Used to be a rule of thumb, increasing one size conductor over max curretn per NEC 310.16 allowable ampacities, would pay for the upsize itself in seven years. I haven't done any investigating on this lately. What is your motor FLA, and what size and type conductor are they using? They do look kinda warm. The motor temps look about right.
 
Re:Motor wiring and net efficiency Gary Orlove 6/8/2004
Have you checked the power factor? If it is less than one, energy is being wasted and power factor correction is indicated. Check out these links:

Variable-Frequency Drives - Heat Loss and Necessary Ventilation
See http://www.engineeringtoolbox.com/33_656.html

Power Factor for a Three-Phase Electrical Motor
See http://www.engineeringtoolbox.com/33_654.html

Motor Power Factor and Energy Conservation
See http://www.usmotors.com/Products/ProFacts/1-115.htm

Motor Application Guide
See http://www.usmotors.com/PB100/MotorAppGuide.pdf

Gary Orlove
Infrared Training Center
 
Re:Motor wiring and net efficiency electricpete 1/5/2005
If cables are running hot, it is likely associated with the mechanical load placed on the motor (which also explains motor running hot). There are very few deficiencies in the motor which will cause it to draw significantly more current to heat the cables.

Reducing losses in the cable would usually involve installing larger cables... not cheap. I don't think 50F rise on cable is necessarily abnormal. Cables carrying current create heat proportional to current squared.

I agree with the comment that high efficiency motors typically run hotter. They cut down on the fan action which saves energy.
 


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