Replacing motors with high efficiency motors 2

[c2883]

Has the state of the art in large motor design and manufacturing reached a point where it makes sense to consider changing out motors for the electrical savings that you might obtain from the use of a modern high efficiency design? In particular, I'm interested in motors above 1000 Hp. Although this represents a traditional kind of engineering economics problem, I am interested in knowing if it even makes sense to consider these evaluations for aging power plants.

 

[electricpete]

I have heard a lot of discussion of the options... fueled by DOE and the vendors. But I haven't heard of many people investing in new motors SOLEY for the improved efficiency.

A quick rough calc on annual benefit of 1% efficiency improvement on 1000hp (750KW)motor operating 7000hr per year with cost of power = 2 cents per kw_hr (20 Mils/Mwhr):

1% * 750kw* 7000hr/yr * $0.02/(kwhr) = $1,000 per yr.

If you invest approx $150,000 in a new 1000hp motor, you might hope to gain at most 5% efficiency or $5,000/yr payback... not a very good return. Also you can scale up the savings if cost of power is higher etc. It doesn't look like a good payback to me.

You MIGHT expect higher reliability out of the new motors.... so that needs to be factored into your decision.

One trap is that higher-efficiency motors typically have lower rotor resistance hence lower slip and operate at a sligthly higher speed. Since power is proportional to speed cubed, a small increase in speed can result in larger increase in power consumption. If the extra pumping power is unneeded, it will likely be gobbled up by system throttel valves etc... amounting to a waste which eats of some of the savings (unless you trim impellers or adjust fan blade pitch).

More savings can be gained by adding variable speed drive. Doesn't help in case of constant-load motor (like nuke plants)... there all you need to do is trim the impeller. Once notable exception is circ water motors. The system demand does vary seasonally and overall plant efficiency is affected by delivering precise right amount of cooling (too much cooling flow not only wastes electric power... it decreases net electrical output of the plant).

Lots more info in IEEE739 and NEMA MG-10.

 

[BJC]

Large motors have been highly efficient for some time. The current interest in the EPA Act Standard and the CEE premium motors is about low voltage motors. Nation wide converting to high efficeincy motors is supposed to save a tanker of oil every day or something llke that.
If you know the efficiency of your present motor run the numbers. It's basicall the I^2 R loss difference in the winding that you save. Higher efficiency motors have less r and that is where you savethe $. You can sell that power for about $250 a megawatt hour.
If you go for a new motor go for a gaurenteed efficiency.
Electric pete is correct about the variable speed. 5 KV units are requlary installed on ID and FD fans in power plants.
In a nuke plant You could install them on Service water, cirwater, hot well pumps and of course lots of 480 volt loads.

 

[electricpete]

Good comments BJC.

I had not previously heard of VSD being used on the nuc plant systems you mentioned but it makes sense. Our "well water" pumps (I assume that's similar to service water) have a fairly unpredictable demand due to various auto-cycling loads. Operators can only choose from 1,2 or 3 pumps... and usually there are more pumps running than needed. In years past we had serious reliability problems with pumps operating at low flow and high vibration. We installed an auto-relief valve (to reservoir) at a certain discharge pressure to provide some regulation. But a huge waste of energy all around. And still among our highest-maintenance pumps, presumably due in part due to prolonged operation at low loads (along with outdoor environment, limited plant attention because they are not "critical" etc).

I believe NYPA put in VSD's for circ water in one of their nuc's. I saw an EPRI writup on it. I've been tossing the idea around with our plant thermal engineer, but we're still not ready to jump out on a limb and try to justify the huge capital outlay.... with potential concerns about reduced reliabiliy.

$250 per MW-hr? At a power plant? That is a factor of 10 high from my frame of reference. But I guess conditions do vary depending on local rate structure and market etc.

Our power plants produces at incremental cost of approx $20/MW-hr. At my home I pay retail price of approx 9 cents/kw-hr which is $90/MW-hr and I still complain about that being high. Maybe $250/MW-HR in California where the unusual is commonplace ;-)

 

[jbartos]

Suggestions:
1. If the existing motor is aging and has the poor efficiency and power factor, then the modern high efficiency and high power factor motor could be considered.
2. Additional modern aspects could also be considered, e.g. if the AC motor drive could provide savings, etc. (This would need an inverter duty rated motor.)

 

[BJC]

Electricpete.
I was wrong- crainial rectosis struck again.
Recently did a job where the owner though he could sell all his power for that on the spot market. The number stuck in the wrong place. The cost should be the wholesale price the utility can sell the power for, which as you say is 5 -10 cents a KWH.
Still lots of motors that could be veriable speed. Lots of motors on non-saftey systems that run full speed all the time.

 

[electricpete]

BJC
I'm not sure I fully understood your last comment. I don't see a benefit of VSD for the vast majority of nuke plant non-safety motors which operate most of the time at a steady state value and never require significant capability beyond that steady state level. In that case the impeller can be trimmed (or fan pitch adjusted) to reduce the flow to desired level without throttling losses. (Possibly operates at slightly less efficient point for pump and motor, but it's much more efficient than throttling.) I see the big benefit of VSD comes where the pump is required to operate for extended periods at many different flow levels (not common in my nuke world).