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Motor Efficiency


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#21 yuri

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Posted 23 July 2009 - 07:47 PM

Hi Kana.

Units of output power are not not volts and ampers but newtons and meters.
And even the way you have defined output power one cannot calculate (easily) power losses so one cannot calculate efficiency (only on the bases of a nameplate's datas).

Best regards.

#22 kana

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Posted 24 July 2009 - 01:41 AM

Hi yuri,

Yes, output power is P = T.w where T is torque and w is speed in rad/s. Practically its difficult to calculate the losses without knowing the parameters of the Induction Motor but to understand the theory of efficiency we can consider the output power as (input power - losses in motor).

Regards,
Kana

#23 ronnieal

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Posted 27 October 2009 - 07:16 AM

QUOTE (kens @ Jun 14 2006, 11:13 PM) <{POST_SNAPBACK}>
Hi CSTEOH, often the motor will not have the efficiency on the name plate but you can still get a pretty good idea from the information that is there. Our 15 kW motor name plate may have the following data
15 kW 400V 50Hz 1460 rpm 29A p.f. .82. These are all rated at full load. We first need to calculate our kW input from this data so kW =√3 x V x I x cosΦ/ 1000 or 1.732 x 400 x 29 x .82 / 1000 = 16.47 kW. Efficiency = Input Power / Output Power so 16.47 / 15 = .91 or 91%. If you don't have the efficiencies listed you can use the above to get a pretty good idea about different motors performance.
Ken



can we find out motor efficiency by using no load current ? if this can be done, what formula can we use ?

we are trying to measure efficiency of a new eff1 motor vs an old pagemaker motor) both are rated 75hp 220V

thanks

#24 marke

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Posted 27 October 2009 - 05:51 PM

Hello ronnieal

No not directly. You can use the no load current as an indicator of iron loss, but not actual numbers. If the no load current is high, then the core flux will be high and the iron loss will be high. Typically, large two pole machines will have a no load current of around 20 - 25%. If the no load current is higher than this, then the iron loss will be higher, but this does not directly relate to overall efficiency.
The full load speed is another indicator. A motor with a high full load slip has a lower efficiency than a motor with a lower full load slip.

Best regards,
Mark.

#25 ronnieal

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Posted 28 October 2009 - 12:45 AM

QUOTE (marke @ Oct 27 2009, 06:51 PM) <{POST_SNAPBACK}>
Hello ronnieal

No not directly. You can use the no load current as an indicator of iron loss, but not actual numbers. If the no load current is high, then the core flux will be high and the iron loss will be high. Typically, large two pole machines will have a no load current of around 20 - 25%. If the no load current is higher than this, then the iron loss will be higher, but this does not directly relate to overall efficiency.
The full load speed is another indicator. A motor with a high full load slip has a lower efficiency than a motor with a lower full load slip.

Best regards,
Mark.



thanks Mark. we are trying to jsutify replacing an old 1980's motor with a new EFF1 motor but the problem is the factory is not running at the moment due to rehabilitation and we want to replace the old motor because we believe that it wastes energy ...

any ideas on how we can justify ? thanks

#26 marke

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Posted 28 October 2009 - 06:14 AM

The best indicator is the full load speed.
The rated speed should be on the name plate. As the full load speed increases, the efficiency also increases due to a reduction in slip losses.

Best regards,
Mark.

#27 yuri

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Posted 28 October 2009 - 08:31 AM

Hello.
But you can easily measure with a wattmeter what actual no load power consumes the motor of the pagemaker (decoupled from its load) and what consumes the new motor connected to the same voltage supply (also with no load). I am certain the comparison of the results of this simple no load test of the both motors will reflect their power consumption at full loads (so, their efficiencies).
This of course is against of what says Mark in the post 24, but what prevent you from the measuring of the power consumptions and reporting the results here?

#28 marke

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Posted 28 October 2009 - 09:35 AM

Hello Yuri
Measuring the open shaft power will give you essentially the stator iron loss only. It will not indicate the copper loss at full load and this comprises around 50% of the losses and can range from 40% to 70% of the full load losses, so I do not think that this gives any better indication of the efficiency than the no load current.
I believe that the rated full load speed is a better indicator.

Best regards,
Mark.

#29 yuri

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Posted 28 October 2009 - 03:59 PM

Hello Mark.
With that loading up the new motor (and measuring speed/slip) it seems Ronnireal stalls at uninstalling the old motor and installing the new one - if the new one turned no better, then the time and money were wasted. But I cannot understand why he does not wish to use the datas from the motors' nameplates.
Best regards

#30 Broshi

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Posted 07 December 2009 - 09:54 AM

I am new to this forum and likes it very much. Few issues I would like to comment:
1. If you have only one value for motor efficiency, it is the maximum, which is notmally at 75% load (not 100%). Standard motors are less efficiennt under 100% load than 75% load.
2. Normally, when replacing low efficiency motor with high efficiency one, while the motor is loaded, the energy consumption will increase due to the reduced slip for high efficiency motors that causes the motor to do more work. This doesn't say I don't recommend high efficiency motors - they are very important - but we must understand the results.
3. The power factor and efficiency of motors are chnaged depending on the motor load. Some explanations can be found at an article at Energy Central.

Regards, Broshi




#31 ronnieal

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Posted 08 December 2009 - 12:51 PM

QUOTE (Broshi @ Dec 7 2009, 09:54 AM) <{POST_SNAPBACK}>
I am new to this forum and likes it very much. Few issues I would like to comment:
1. If you have only one value for motor efficiency, it is the maximum, which is notmally at 75% load (not 100%). Standard motors are less efficiennt under 100% load than 75% load.
2. Normally, when replacing low efficiency motor with high efficiency one, while the motor is loaded, the energy consumption will increase due to the reduced slip for high efficiency motors that causes the motor to do more work. This doesn't say I don't recommend high efficiency motors - they are very important - but we must understand the results.
3. The power factor and efficiency of motors are chnaged depending on the motor load. Some explanations can be found at an article at Energy Central.

Regards, Broshi

thanks for the replies guys ... i have a few more questions if you don't mind ...

1. are the efficiency of motors made in the 70's and 80's only maximum 70% because of the technology then ? (construction, material, etc)

2. can the efficiency of old motors be lower than 50% ?

thanks in advance

ronnieal




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