How to Find AC/Induction Motor Load and Efficiency

Determining Motor full load and its efficiency has always been a quite hard task in industry . But after reading this we will definitely be expert in finding out any motor full load and its efficiency . Most of the motors are designed to run at 50% to 100% of load , the most efficiency of the motor is at 75% load.Thus a 20 HP motor will have its peak efficiency at 15 hp load.
How to find motor full Load ?

1)Calculate the three phase input power to motor using below formula -

Pi = V x I x PF x √3/1000

Where:
Pi = Three-phase power
V = RMS voltage, mean line-to-line of 3 phases
I = RMS current, mean of 3 phases
PF = Power factor as a decimal

2) Pir = hp * 0.7457 / ηfl

Where:
Pir = Input power at full-rated load in kW
hp = Nameplate rated horsepower
ηfl = Efficiency at full-rated load

3) Now to find load apply the below formula

Load = Pi/Pir * 100 %

Where:
Load = Output power as a % of rated power
Pi = Measured three-phase power in kW
Pir = Input power at full-rated load in kW

Now from the above method you can easily calculate motor load , if u can see the motor nameplate data then there is another usefull equation below .
How to get Load from motor nameplate

Load = I/Ir * V/Vr * 100 %

Where:
Load = Output power as a % of rated power
I = RMS current, mean of 3 phases
Ir = Nameplate rated current
V = RMS voltage, mean line-to-line of 3 phases
Vr = Nameplate rated voltage

To calculate motor load using slip method is use full when only operating speed measurements are available.The synchronous speed of an induction motor depends on the frequency of the power supply and on the number of poles for which the motor is wound. The higher the frequency, the faster a motor runs.Subscribe to Automation-Talk by Email. The more poles the motor has, the slower it runs.By using a tachometer to measure actual motor speed, it is possible to calculate motor loads.

Load = Slip/Ss - Sr * 100%

Where:
Load = Output power as a % of rated power
Slip = Synchronous speed - Measured speed in rpm
Ss = Synchronous speed in rpm
Sr = Nameplate full-load speed

Below is an example as how to find motor load using slip method .

Example: Slip Load Calculation
Given: Synchronous speed in rpm = 1800
Nameplate full load speed = 1750
Measured speed in rpm = 1770
Nameplate rated horsepower = 25 hp
Determine actual output horsepower.

Now applying the above equation we get 60 % load. So Actual output horsepower would be 60% x 25 hp = 15 hp.As most of the motors shaft are designed in such a way that tachometer can be connected to measure the speed so slip method is preferred widely to find load due to its simplicity .The accuracy of the slip method, however, is limited. The largest uncertainty relates to the 20% tolerance that NEMA allows manufacturers in their reporting of nameplate full-load speed.Thus a voltage correction factor can be inserted in to slip load method to get better results .

Load = Slip / (Ss- Sr )*(Vr / V)2 * 100 %

Where:
Load = Output power as a % of rated power
Slip = Synchronous speed - Measured speed in rpm
Ss = Synchronous speed in rpm
Sr = Nameplate full-load speed
V = RMS voltage, mean line to line of 3 phases
Vr = Nameplate rated voltage

Now after studying all above formulas to calculate motor load , i think calculating motor load now will not be a big task . Finding Motor efficiency is also required several times for many applications .Subscribe to Automation-Talk by Email.To find the motor efficiency , we will discuss here some use full formulas . How to calculate Motor efficiency .definition of energy efficiency is the ratio of its useful power output to its total power input and is usually expressed in percentage as shown in below equation.

η = 0.7457 x hp x Load/Pi

Where:
η = Efficiency as operated in %
Por = Nameplate rated horsepower
Load = Output power as a % of rated power
Pi = Three-phase power in kW

Below is a graphical representation of motor conversion of electrical energy in to mechanical energy .Motor losses are the difference between the input and output power. Once the motor efficiency has been determined and the input power is known, you can calculate output power.


Note :- Click on the picture for a clear view.

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