Induction Motor data - what is correct

[tomad]

We were involved in an analysis of replacing "normal" efficiency motors by "high" efficiency motors. During the process one issue appeared. Below is a typical example.

One of the motors in question had following data:
(per NEC 2005 - Table 430.250 - typical motors)
Output power: 20 HP ( 14.92 kW)
Voltage: 230V (LL)(Y connection)
Current: 54A
3 phases.
That leads to an input power of 21.5 kVA.
Based on these values, the product of efficiency * power factor = 14.92/21.5 = 0.694
Catalogs indicate for typical "normal" efficiency motors a value of 0.88, which would lead to a power factor of 0.694/0.88 = 0.788.
For typical/average "high" efficiency motors values are 0.92 (effi) and 0.892 (power factor), i.e. a product of 0.82.
If using high efficiency motors, for the same output ( 20 HP = 14.92 kW), the input power would be 18.18 kVA.
That, in turn, would lead to a lower current of 45.6 A, which is basically about 84.5% of the value indicated in NEC. These values have been labeled as "average/typical".
If determining power consumptions, a difference of 15% is significant.
What values would be the most correct one to be used and why ?

 

[edison123]

The product of eff x pf does seem a bit low. What is the speed of the motor ? Typically, both of these nos. drop with speed.

Muthu

http://www.edison.co.in

 

[rhatcher]

Either by luck or by nature, I am responding tonight in a skeptical manner.

I believe the correct terminology for what you are talking about is the difference between 'apparent' power (V*A*1.73) and 'real' power (V*A*1.173*pf). 'Apparent' power is total kVA and 'Real' power is kW.

The premise behind the argument that you are hearing is that high efficiency motors reduce 'power' costs by having lower losses. Real power losses in a motor include friction (bearings) and windage (shaft mounted fans). That is not changed by a high efficiency motor.

What is changed is the power factor. While a higher power factor may seem to be a good thing, this may not actually result in 'power cost savings' to you.

The difference to you economically is based on how your utility charges you for power. If your bill is based on kVA, then the high efficieny motors will benefit you. If your bill is based on kW, then there is no direct benefit that I know of.

Of course, there are other members with more experience that will hopefully speak up...

 

[davidbeach]

If kVA goes down, then I^2R losses through out the system supplying the motor go down.

 

[Skogsgurra]

If you change the words 'That leads to an input power of 21.5 kVA' to 'That leads to an apparent input power of 21.5 kVA'then you will realize that kVA is not at all the same as kW (the real power that you are actually paying for).

Using kW makes the numbers more justice and is the correct way of calculating efficiency.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[LionelHutz]

"(per NEC 2005 - Table 430.250 - typical motors)"

Did you read those power, voltage and current numbers from a table or off the rating plate of the motor?

The currents in the NEC table are to account for motor variations so that most motors are covered by the table.

You will have to pick a manufacturer and look at the actual specific motor data if you want to perform the cost savings analysis like you intend to do.

Using the NEC tables for the old motors won't give you the numbers you want but rather will give overly low low efficiencies as you've already noted.

If the old motors have a steady load you could do some power measurements on them loaded and open shaft and get some decent data that way. Assume the output power goes from 0 - rated when the current goes from no load - full load.

 

[dpc]

The purpose of the NEC table is to provide a baseline that is used for sizing feeders and protection. It is necessarily conservative for that reason.

This has nothing to do with the actual efficiency of any given motor.

kVA is NOT an indicator of motor efficiency. High efficiency motors often have lower power factors than "standard" motors and can actually draw more current. As Gunnar said, efficiency is based on the kW used by the motor, not the kVA.

 

[7anoter4]

First of all, there are some change from 2 poles up to 6poles [3600-1200 rpm] in efficiency and p.f.
NEC states 54 A, may be, for 6 poles [or more] squirrel-cage induction motor.
If we'll take Siemens Catalog as example then:
For General Purpose - TEFC Motors 6 poles the apparent input power [SkVA] will be 20.7 [52 A only], SkVA=sqrt(3)*0.46*26 = 20.7 kVA [recalculated for 230 V=52 A] efficiency 90.2% and PF about 0.8.See:

http://www.sea.siemens.com/us/inter...ndDrives/NEMAMotors/docs/2008 - GP10-RGZP.pdf

For NEMA Premium™ Efficiency TEFC motors:
SkVA=sqrt(3)*0.46*27 = 21.5 kVA [recalculated for 230 V=54 A] efficiency 91.7% and PF about 0.76
See:

http://www.wermac.com/RGZEESD.pdf

So, I agree with LionelHutz and dpc, you have to take the manufacturer catalog data [still better the rating plate data from the motor itself]

 

[electricpete]

General comments that you might role into your evaluation of replacing motors for improved efficiency:

1 – Higher efficient pumps often operate at slightly higher speed (lower slip). This means pump outputs more. If flow requirement is constant, this may result in increased throttling to maintain desired flow. Increased throttling losses which effects overall efficiency.

2 – Watch out for your molded case breaker instantaneous settings. High efficiency motors generally warrant a higher instantaneous setting than the motors they replace, even if everything on the nameplate is the same (except efficiency).


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[electricpete]

Correction in bold:
Higher efficient motors often operate at slightly higher speed (lower slip).

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[tomad]

rhatcher

Real power losses are also losses in windings. A higher power factor lowers the input current, so lower losses in feeders. But at the same time the reactive current (that provides magnetization) is also taken into account, so in the end it influences the motor efficiency.

Skogsgurra

The motor draws apparent power S, the real input power would be P = S * pf. It is true that you pay for the real power consumed (kW) but at the same time you may pay a penalty if the power factor of your load is to low.

LionelHutz,

The numbers used were indicated as typical in a DOE report, they do not refer to a specific manufacturer. For high efficiency motors, where the power factor was given, they refer to a Siemens motor that had average values. In the same list there were motors with higher efficiency & pf listed, as well as some with lower values, but within a certain range.

dpc,

You are right about feeder & protection for NEC. The problem is that when you analyze the load you have to enter a value. If you use the numbers given in NEC you end up with a load that may be 117% of the actual load, and this adds up. You improve efficiency by 4% and calculate with an error of +17%. Normally, for a low number of motors they can be assessed individually, but when it comes to tens or hundreds of motors, a typical value has to be used.

Zanoter4,

The system real voltage level is 480V, your example used 460V (as given in the attached Siemens cutsheet), so there will be a difference: if you keep the apparent power (kVA) as given, the current will be lower, if you consider the current, the rating (kVA) will increase.

 

[dpc]

So, at this point, I have no idea what your question is or if you actually have one.

 

[Skogsgurra]

I think that tomad wants to educate us.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[tomad]

The question, as mentioned in my first post, was what load to use in a power flow calculation and to determine the impact on the total energy comsumption by using more efficient motors.
Values based on National Electric Code (in the US), or similar codes in other countries, are not meant to serve for design, but are largely used by inspectors/reviewers in the process of approving a project.
In the NEC there is a clause that stipulates that, in some cases, values that deviate from Code recommended, can be used, "under engineering supervision".
This term "under engineering supervision" is mentioned very vaguely, its interpretation and subsequent application varies from engineer to engineer, inspector, etc.
As mentioned in several responses before, the numbers in the code are very conservative because the NEC is part of the NFPA (National Fire Protection Association) so their main concern is safety and not efficiency.
The question is to use numbers that are justifiable (as part of the engineering supervision) or use the conservative numbers in the codes.
When designing something, you most likely add a safety margin ( 15-20%, etc) but that margin is added to something that is real, resulting from calculations.
When there is no distinction between what results from engineering judgment/calculation and what represents an added safety factor (per codes or other administrative documents), most likely the results from studies are affected by increased margins of error.

 

[electricpete]

I'm not sure I get your drift. Does that mean you finally figured out that the manufacturer's catalogues are a better place to go for information about motor performance than the NEC?

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[edison123]

Why do I feel like a student here ?

Muthu

http://www.edison.co.in

 

[tomad]

I agree that the manufacturer's catalogs are the best when dealing with a small number of motors, you look them up and use those numbers. The question was to find a "typical/default" value based on the latest requirements/standards (NEMA or similar) to be used in calculations.
In my understanding, there are (minimal) values for efficiency & power factors that have to be met by manufacturers of motors (for general use). (I.e. if the prescribed minimum efficiency for a particular type of motor is 92% +/- 0.2%, than all manufacturers numbers shall be between 91.8% and 92.2%). Thus, an assumption of 92% may be used, irrespective of manufacturer.
As mentioned before, the numbers indicated in various codes have not been updated to keep track of those new adopted values. For instance in the NEC no efficiency/power factor is indicated. Only typical full load currents. These currents should be updated to reflect the changes in efficiency/power factor, but they are not.

 

[LionelHutz]

I don't follow either. If you want to prove your energy savings then you have to look at real data for the old vs new motor. Otherwise, you're just blowing smoke and hoping others suck it in.

You mentioned DOE = Department of Energy? If so and if the DOE is like the Canadian equivalent then their VFD and high efficient motor "propaganda" is a joke compared to what happens in the real world.

The NEC currents are worst case to cover most loads. So, they will be higher than many motors really are. The NEC is concerned with safety, not energy savings.

As I already posted, if you want to really know you have to either find the data for the old motors or measure them. Then, pick new motors and compare how much better they are and how much will be saved.

 

[tomad]

The DOE (Department of Energy)indicated was DOE/CE-0384 and is a document that had, among other info, average efficiencies for "standard" motors and "energy efficient" motors, also the average acquisition price for normal and more efficient motors. They considered between 7 - 20 manufacturers for each statistical estimation.
The estimation of the potential energy savings by using high efficiency motors is in the primary phase meaning it was based on the type & quantity of "old" motors that are recommended to be replaced by more efficient motors and we were trying to estimate how much could be saved (energy and money-wise) by this replacement.
If the estimated amount is attractive enough, probably they will decide to proceed and in this case specific motors will be indicated, accurate calculations performed, etc.

 

[electricpete]

The question was to find a "typical/default" value based on the latest requirements/standards (NEMA or similar) to be used in calculations. In my understanding, there are (minimal) values for efficiency & power factors that have to be met by manufacturers of motors (for general use). (I.e. if the prescribed minimum efficiency for a particular type of motor is 92% +/- 0.2%, than all manufacturers numbers shall be between 91.8% and 92.2%). Thus, an assumption of 92% may be used, irrespective of manufacturer.

Now you’re talking. Download NEMA MG-1 condensed version here:

http://www.nema.org/stds/mg1condensed.cfm

And take a look at Table 54 (derived from Table 12-14 of NEMA MG1 full version)


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