Minimum recommended motor power rating

[roteng518]

When sizing an appropriate motor, although API-610 10Ed 6.1.3 using table 11 indicates that motor power rating as a percentage of the rated pump power, looks like the vendors don't follow API instead of using the pump max power at max impeller diameter as the number to determine minimum motor power rating. I think this selection is conservative and is for some future considerations such as capacity upgrade by replacing with a larger impeller.

Any insight on this? Thanks.

 

[BigInch]

That's why you have to carefully check the proposals. Theoretically according to API they should provide a pump motor and a pump casing that would allow changing the impeller and reaching a 10% higher head over what is required at the specified operating point, without changing the motor, but sometimes .. they don't.

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"The problem isn't finding the solution, its trying to get to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[pumpking]

Most pump manufacturers, and i believe API touch on this from memory, will select a motor that is next up from +25% absorbed power for motors up to 7.5kw, +20% for 7.5kw to 15kw and then +10% for 15kw+.

Having a motor which is suitable for end of curve and or with maximum impeller duty only, is good for site to be able to change duties in a later life, however, motors are probably running nowhere near their best efficiency and so actually energy costs far outweigh costs for later modification IF increase is required.

I hope that makes sense, its late and my head knows what i want to say but not so sure my fat fingers are writing it in a 'make sense' manner ?????

Ash Fenn

http://www.cdrpumps.co.uk

 

[Artisi]

In many instances the available motor sizes dictate what you will fit for a particular flow / head/ pump combination.
For instance if the end of curve power is 10Kw then the next available motor is from memory,11Kw giving no margin for any error let alone any future upgrade whereas the next is 15Kw so this is what you will probably fit.

However, in some cases it becomes an engineering decision as the next availabe motor size may be marginal in terms of the codes requirement whereas the next motor size is way over-the-top.

 

[TenPenny]

Table 11, as referenced, shows that the motor selection should be based on the power required at RATED conditions. It makes no mention of max diameter power, etc.

Also, BigInch, I see that the requirement is for an increase of 5% head at rated flow by changing the impeller, not 10%.

 

[BigInch]

Made you look!

Thanks for the correction.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[rshayesteh]

Electric motors have their best efficiency point in the range of 60-80% of rated load and smaller ones near 75% of rated power. so selecting a motor which at 75% power fitts (or approaches) the pump rated power is a good suggestion. an electric motor shall not be operated bellow 50% load for longterm.
you can find some good information in the bellow mentioned link:

http://www.p2pays.org/ref/40/39569.pdf

Reza

 

[BigInch]

1/0.75 = 1.33 x Pump_power.
While that might be OK for small motors, I think that would be far too conservative for the very large, higher cost, motors, especially given such flat curves in the higher range. I wouldn't feel right recommending a motor that might cost 25% more than the one I really needed. Normally 10-15% is well more than enough. Besides, if you had to move lower flows where power consumption is 50-60% of pump rated power and selected your motor at 1.33 instead of 1.10, that would put your lowest efficient motor operating range at 67% of pump power, instead of 55%

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[electricpete]

The rule about highest efficiency point between 50 - 75% power was more relevant in the old days where the efficiency curve had a pronounced peak and efficiency sloped steeply away from the peak. New higher efficiency motors tend to have a much flatter (constant efficiency) curve near full power and typically don't vary more than a few tenths of a percent between 75% and full power (for larger motors).
One example: no difference in efficiency between 75% and 100% power:

http://www.reliance.com/pdf/pdf/aced/W06127-C-J001.pdf

Actually it wasn't my intent to create any new generalizations but to warn against generalizations. The curve for the motor you buy is the most relevant.

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[electricpete]

There is a logical reason for the change as well. In the mid-90's efficiency standards were introduced. NEMA MG1 specifies minimum efficiencies for various classes of motor (energy efficient and premium efficiency) AT FULL LOAD. It doesn't say anthing about other loads. So the manufacturer has incentive to move the peak efficiency nearer to full load.

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[electricpete]

Actually, higher efficiency standards were introduced in the 90's, but finally implemented in the mid 00's

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[rshayesteh]

BigInch, that's correct and as I mentioned also, 75% was recommended for small (obsolete!)motors.
electricpete, thanks for the good info.


Reza