AC Motor Efficiency - A Guide to Energy Savings

What we have done, and what we are doing
to help reduce energy consumption


| Efficiency | Watts Loss | Improving Efficiency |


Motor efficiency is a measure of the effectiveness with which a motor converts electrical energy to mechanical energy. It is defined as the ratio of power output to power input or, in terms of electrical power, watts output to watts input and can be restated as the ratio of output to output + losses (Figure 2).

Motor Efficiency = Output   = Output
    Input     Output + Losses
  =   HP x 746    
      HP x 746 + Watts Losses    

Figure 2
Motor Efficiency /Losses

The difference - watts loss - is due to electrical losses plus friction and windage. Even though higher horsepower motors are typically more efficient, their losses are significant and should not be ignored (Figure 3). In fact, higher horsepower motors offer the greatest savings potential for the least analysis effort, since just one motor can save more energy than several smaller motors.

Watts Loss Determines Motor Efficiency

Every AC motor has five components of watts losses which are the reasons for its inefficiency (Figure 4). Watts losses are converted into heat which is dissipated by the motor frame aided by internal or external fans. Stator and rotor l2 r losses are caused by current flowing through the motor winding and are proportional to the current squared times the winding resistance (I2 r). Iron losses are mainly confined to the laminated core of the stator and rotor and can be reduced by utilizing steels with low core loss characteristics found in high grade silicon steel. Friction and windage loss is due to all sources of friction and air movement in the motor and may be appreciable in large high-speed or totally enclosed fan-cooled motors. The stray load loss is due mainly to high frequency flux pulsations caused by design and manufacturing variations.

Figure 3

  Typical Losses (Watts)
Loss Components Std. XE
1) Iron
2) Stator I2 r
3) Rotor I2 r
4) Friction & Windage
5) Stray Loss Load

Figure 4
AC Motor Components of Motor Loss
Typical Design B Motor 10 HP, 1750 RPM, TEFC

Improving Efficiency By Minimizing Watts Losses

Improvements in motor efficiency can be achieved without compromising motor performance - at higher cost - within the limits of existing design and manufacturing technology. See comparison of loss breakdown in Figure 4. The formula for efficiency (Figure 2) shows that any improvement in motor efficiency must be the result of reducing watts losses. In terms of the existing state of electric motor technology, a reduction in watts losses can be achieved in various ways.

All of these changes to reduce motor losses are possible with existing motor design and manufacturing technology. They would, however, require additional materials and/or the use of higher quality materials and improved manufacturing processes resulting in increased motor cost.


Watts Loss Area Efficiency Improvement
1. Iron Use of thinner gauge, lower loss core steel reduces eddy current losses. Longer core adds more steel to the design, which reduces losses due to lower operating flux densities.
2. Stator I 2 R Use of more copper and larger conductors increases cross sectional area of stator windings. This lowers resistance (R) of the windings and reduces losses due to current flow (I).
3. Rotor I 2 R Use of larger rotor conductor bars increases size of cross section, lowering conductor resistance (R) and losses due to current flow (I).
4. Friction & Windage Use of low loss fan design reduces losses due to air movement.
5. Stray Load Loss Use of optimized design and strict quality control procedures minimizes stray load losses.

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