Potential energy savings in motors operating under partial loads

[mrspcs]

I am not EE and need to understand this issue a bit better (from the process energy use optimization perspective).
When an AC Induction motor operates under partial load conditions, the Power Factor and the motor Efficiency go down. There are a number of ways to resolve this issue but any of them will have to be justified on the basis of cost/benefit issues.
The QUESTION is: what is the potential energy savings on the motor alone (other opportunities related to the load, the process etc. are considerate separately) ?? Is it just that gained by improving its efficiency up to rated values again (e.g. to full load condition) ??
As an example, take the case of a motor driving a pump.
Peak demand requires that the motor be of 100 hp.
However, the pump operates a significant amount of time at 50 or 60 % of the peak condition.
The Current (amps reading) for the motor go down from its full load condition. So the Power Factor goes down.
From tables (provided by the manufacturer) the Efficiency also goes down (e.g. the portion of Electric Energy fed to the Motor that gets converted to shaft power is smaller and the rest is wasted).
The potential savings in Electric Energy from resolving this issue are only those corresponding to the lost Efficiency ??

(again, I am looking at the motor only. Other possible savings like pressure drops, flow re-circulations etc are evaluated separately)

Thank you all for your time and attention.

 

[dpc]

First off, do not confuse efficiency as a percentage, with energy loss. Even if a motor at 50% is slightly less efficient than the same motor at 100% load, the total kW of loss will be much less at 50% load.

Secondly, the motor is likely the most efficient part of the entire system, so improvements in motor efficiency will no have a major impact on overall efficiency.

Lastly, the efficiency drop-off at lower loads is not really significant until the motor is very lightly loaded and may actually non-existent.

Data I have for a typical 1800 rpm 100 hp motor:

Nominal FL efficiency: 93%
75% Load efficiency: 94.8%
50% Load efficiency: 94.8%



David Castor

http://www.cvoes.com

 

[mrspcs]

Thanks for the comments dpc.
About your first comment: Sorry if anything in my question hints at the fact that I may be equating % Load with % Energy loss. I believe I made only qualitative assertions, not quantitative. That is, I assumed that a partial load implies some loss of efficiency (not totally true judging from your answer .. ). A loss of efficiency obviously implies some energy wasted (e.g. conversion of Electric Power into Shaft Power). Far from saying that % load = % energy lost. So, that's clear.

I understand your answer to be that the energy saved (at the motor only) by resolving partial load and Power Factor issues is basically negligible ?

From the values you provide it seems that Efficiency at 75 % load is actually higher than efficiency at 100 % load and below 75 % it does not drop at all even if the load is only 50 % of nominal. Good point, by the way. This would be in fact the "best efficiency point" for the motor which is usually claimed by manufacturers to be at about 75 % load (not at full load).

My objective in asking this question is to get support for dropping entirely the calculation of potential savings in the motor by investing on improving its PF or on running it at 75 % of full load all the time.
(savings, if any, will come from other aspects of the system like reducing pressure drops, flow re-circulations, etc).

Your answer seems to support that.

By the way, it is also clear to me that in a mass transport system where the work is done by rotating equipment driven by electric motors, the motor is probably the element with the highest efficiency.

Thanks again for your help.

MS

http://www.msuarez.com

 

[waross]

PF savings are easy. Take a look at your power bill. If there is a penalty for poor power factor, the power factor may be improved to the point that there is no more penalty.
Part of the penalty may be hidden in demand KVA charges or KVAHr charges. Power factor correction will most often reduce these charges.
You have to be familiar with the local tariffs and the affect of PF on these tariffs.
It may be possible to save by reducing your peak demand. If you are in a position to reduce your demand by curtailing the use of a large piece of equipment when the demand from the rest of the plant is peaking you may see savings.
Beware that you do not curtail too much. This can lead to a situation where you may be forced to use the normally curtailed machine when the rest of the plant is at peak load. You will pay back all your savings and then some if a few hours of use of the wrong machine raises your demand for the month. This may come about if the target demand set point is set too low.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Power factor has very little effect on efficiency. There are only 2 valid reasons to correct power factor. You do it to save on your power bill or you do it to free up system capacity.

Motor current is not directly related to efficiency.

As already pointed out, most general use motors will have very similar efficiency numbers from 50% load to 100% load. It has been proven here before from actual motor data that using a 10hp motor can be more efficient than using a 5hp motor.

I wouldn't just drop the consideration without any investigation. It becomes a simple matter of looking at the motor data sheet and comparing the 50%, 75% and 100% efficiency numbers. You can then drop that consideration if they look like the numbers dpc posted.

You have to resolve pumping system efficiency into a number such as gallons pumped per kWh or gallons pumped per dollar. I've seen to many people see the input power to the pump motor go down and they automatically believed this meant they were saving energy. The problem, for example, is that the power may go down by say 1/2 but the gallons pumped drops to 1/4.

Of course, you may be flow controlling the pump for process control reasons. In this case, efficiency takes a back seat to the process.

 

[mrspcs]

Yes, the issue of charges and fines for low PF will be taken into consideration. Thanks Waross.

About "efficiency" (LionelHutz comment), indeed there has to be a consistent definition as to which variable we are trying to optimize. I would put it as "effective mass transferred per kWh" (which is basically the same as to say 'per monetary unit').
That would be the 'efficiency' of the entire system. The notion being that in basically all instances where electric motors are used, the purpose is to move mass from one place to another (pumps, blowers, compressors, conveyors etc etc... moving solids, liquids or gases).
In that respect, I am trying to look at each element of the drive train and try to optimize its own efficiency and this is not necessarily the same as that of the entire system.
In the case of the Motor, it would be to optimize the conversion of Electric Power to Shaft (mechanical) Power. In the case of the Pump (or any rotating equipment) it would be the conversion of Mechanical Power to Hydraulic Power (e.g. torque to head).
I believe optimizing each element's efficiency is just the first step. I'll then have to look at the overall efficiency of the system (here is where the notion of mass transport vs. kWh or money comes into play) and that implies taking the concept into account. Recirculations, restricted flows etc. are all great contributors to lower the system's efficiency as described.
For instance, a recirculation motor and pump may each be working at very high individual efficiencies (high conversion of electric-to-mechanical and mechanical-to-hydraulic) and yet, the entire recirculation system is lowering the total system efficiency by moving mass back to its point of origin and not its destination.
I trust this is the point you were trying to make ??
Thanks all for the help, really appreciated.


MS

http://www.msuarez.com

 

[wcarter]

mrspcs,

The down and dirty, getting away from the technical side, from operating specs like that, you would benefit from a VFD. From what I've seen, the drives can be a bit pricey (for some people's tastes), but one example, at a carwash, they installed drives, and in three years, the drives paid for themselves in energy costs. I know salesmen for a number of company's have the exact numbers and calculations factoring everything from the electrical side. One of those might be able to give you an exact idea of what you're looking at.

 

[jraef]

Let's look at another aspect of something you said. As load goes down and efficiency drops, more energy is "wasted". Waste is a relative term here. It takes a certain amount of electrical energy to magnetize the motor, in other words make it into a motor. You can't really call that "waste" if you cannot eliminate it. It is, instead, "overhead". That stays basically constant regardless of load, so as load decreases, this begins to represent a larger percentage of power used. But again if you removed it, the motor doesn't work!

Ill just step in here to head off another looming issue.

A small amount of that magnetizing energy is lost in relation to the applied voltage, so reducing the voltage under light load conditions will slightly reduce that small loss. But don't be misled by the purveyors of devices that claim to "optimize" the motor by exploiting that. It technically works, but you save a fraction of a small amount of a low percentage (I'm being facitious to make a point). The other side of that is that the technology to do this has specific limitations that often don't fit with industrial installations, so it rarely has a reasonable ROI, if at all. It is seriously over hyped though, so be careful. Many of us in here have ranted about this issue repeatedly, yetvthese companies continue to find [strikethrough]victims[/strikethrough] er, " customers".

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