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mrmuneb
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Join Date: Oct 2012
Posts: 44

Induction Motor Output Efficiency

12/12/2014 12:54 AM

dear fellow members,

i am seeking a simple method to find out the output efficiency of an induction motor, i.e. on how much effeiciency a motor is runnin on, i have found out the following formula but having difficuilty in understanding/solving ,

efficiency = (0.7457 X Hp X Load)/Pi

where,

Load = output power as % of rated power

Pi= three phase power in Kw

what exactly is the difference between "load" and "Pi" ????

pls help.

or is there any other suitable way to do this .
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Crabtree
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#1

Re: induction motor output efficiency

12/12/2014 1:56 AM

The simplest way is to look at the plate attached to it. The next simplest way is to look it up on the data sheet that came with it. The third simplest way is to pick up the telephone and talk to its manufacturer.

Who needs equations?

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IdeaSmith
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#2

Re: induction motor output efficiency

12/12/2014 3:34 AM

As you will know not all the power that you have to put into the motor is available to drive the load. So the efficiency is the relation of power inserted into the system and power available on the other end.

Clear now?

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mrmuneb
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#3
In reply to #2

Re: induction motor output efficiency

12/12/2014 5:43 AM

"power available at other end" ???

please guide me to find out that, that would be very simple, effiency = output/input X 100

i know the input but how to determine the outpur??

please help........
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SHOCKHISCAN
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#5
In reply to #3

Re: induction motor output efficiency

12/12/2014 8:17 AM

The only way to accurately test the output of the motor is to attach it to a dynamometer that can apply known load values.

Other than the above method it can be calculated by using a power factor meter to take readings when the motor is fully loaded by the application then compare the gathered DATA to the motor ratings declared on the motor nameplate by the manufacturer.

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Crabtree
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#7
In reply to #3

Re: induction motor output efficiency

12/12/2014 12:04 PM

Shaft power is torque multiplied by angular velocity.

What do they teach in science classes these days? Sack the lot of them!

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Rixter
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#4

Re: Induction Motor Output Efficiency

12/12/2014 6:02 AM

0.745 x HP converts the rated horsepower of the motor to KW. Multiply by Load to get the actual power output from the motor. For example, if Load is .5, the motor is delivering half its rated output. Divide by the power input, Pi, to determine its efficiency.
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Aghvel Niazi
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#6

Re: Induction Motor Output Efficiency

12/12/2014 8:49 AM

Taking example of 11 kW induction motor.

Taking following information from nameplate of the Induction motor

11 KW, 400 V, 50 Hz,D, 21 A, P.F 0.85, 1450 RPM

Here

Mechanical Power/ Output Power: 11 kW

Electrical Power/Input Power: Mechanical Power/Efficiency

Mechanical Power = Electrical Power x Efficiency

or

Mechanical Power = (1.732 x V x I x P.F) x Efficiency

Therefore

Efficiency = Mechanical Power / (1.732 x V x I x P.F)

Putting values of V, I and P.F

Efficiency = 11,000 /1.732 x 400 x 21 x 0.85

Efficiency = 88.95 %

Note: 88.95 is the efficiency at 100 % load. Efficiency is the function of load. As load goes below 60 %, it decreases very rapidly.
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#8

Re: Induction Motor Output Efficiency

12/12/2014 12:33 PM

SHOCKHISCAN has it, there is no way to "calculate" the load expressed on a motor, you can only MEASURE it. That is why others have said, read the efficiency rating of the motor, because the motor mfr is supposed to have done that, at least at rated load.

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Crabtree
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#9

Re: Induction Motor Output Efficiency

12/12/2014 12:41 PM

If the answer exceeds 100%, then take it from me, there's something wrong.

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IdeaSmith
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#10
In reply to #9

Re: Induction Motor Output Efficiency

12/13/2014 12:26 AM

Or its a generator seen backwards!

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Ed Watts
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#11
In reply to #9

Re: Induction Motor Output Efficiency

12/13/2014 12:27 AM

Heh.
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7anoter4
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#12

Re: Induction Motor Output Efficiency

12/24/2014 4:30 PM

If Pi represents the input power in kW for each load it is a correct definition of the

efficiency for any load. However it is not a way to calculate the efficiency.

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