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ronnieal

Member Since 27 Oct 2009
Offline Last Active Dec 10 2009 12:26 AM
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Posts I've Made

In Topic: Motor Efficiency

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

In Topic: Motor Efficiency

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

In Topic: Motor Efficiency

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