Determining electric motor load and efficiency

As we known,the electric motor is the one of important equipment to drive the operation smoothly from beginning to end of product.

Electricity cost normally around 60% came from electric motor running.It become more worth if installed motors are oversized,low efficiency,under design,poor maintenance or mismatched.

So to know the actual motor performance, we need to determine electric motor load and their efficiency to eliminated all problems that can cause the high electricity cost and breakdown.

We need to collected several important data from our electric motor to made comparison and analysis about load and efficiency.

Why need to determine electric motor load and efficiency?

To compare the operating costs of an existing standard motor with an appropriately-sized energy efficient replacement, you need to determine operating hours, efficiency improvement values, and load.

Commonly electric motors are designed to operate at 50% to 100% of rated load and maximum efficiency value is
around 75% of rated load. Example for the 5 hp motor has an acceptable load range of 2.5 to 5 hp and the motor efficiency is running at 3.5 hp.

A motor’s efficiency value will be decrease dramatically below 50% from load. However, the range of good efficiency varies with individual motors and tends to extend over a broader range for larger motors.A motor is considered under loaded when it is in the range where efficiency drops significantly with decreasing load.

Other aspect that we need to concern is about overloaded.This issue can caused a motor overheat and lose efficiency.Normally manufacture designed with several value of service factor.

What is a service factor?

For easy understanding,it is a value that allows occasional overloading and is a multiplier that indicates how much a motor can be overloaded under ideal ambient temperature.

Example :-

We have a 5 hp of electric motor with 1.15 service factor.That mean a motor can run until 5.75 hp load for short periods of time without incurring significant damage.



Measuring motor loads is relatively quick and easy when you use the techniques discussed in my next post.I will explain detail how to collect a data’s and formula to calculate for input power measurement,line current measurement,motor slip load and motor efficiency.

I recommended to all of you to perform a motor load and efficiency analysis on all of your major working motors as part of your preventative maintenance and energy saving program. After that we can segregate to 3 categories of electric motor condition into the following :-

1)  Motors that are significantly oversized and underloaded

– Replace with more efficient, properly sized models at the next opportunity, such as scheduled plant downtime.

2) Motors that are moderately oversized and underloaded

– Replace with more efficient, properly sized model when they fail.

3) Motors that are properly sized but standard efficiency

– Replace most of these with energy-efficient models when they fail. The cost effectiveness of an energy-efficient motor purchase depends on the number of hours the motor is used, the price of electricity, and the price premium of buying an energy-efficient motor.


  1. […] loads calculation Electric Motor On my last post,i discussed about why we need to determine motor loads and efficiency.So for this post i want to share how to get the motor load value using simple calculation […]

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