Training Manual on Energy Efficiency - APO Asian Productivity ...

<strong>Training</strong> <strong>Manual</strong> on Energy Efficiency for Small and Medium Enterprises
Motor efficiency versus load
Figure 6-3 Load and power
Output Input−Losses 746×HP Output
Efficiency = ×100 = ×100 =
×100
Input
Input
Watts Input
Electric motors are electromagnetic energy converters whose function is based
on the force exerted between electrical currents and magnetic fields, which are
usually electrically excited as well. Due to their principle of functioning, electric
motors have a relatively high efficiency. A typical efficiency value for an 11 kW
standard motor is around 90%, for 100 kW, up to 94%. The efficiency levels of
large motors are higher than those of smaller motors.
It should be noted that peak efficiency occurs at about 75% loading and drops
significantly when loading is below about 30%.
Core losses:
Stator and rotor resistance losses:
6.3 THE TYPES OF MOTOR LOSS
Core loss is around 22% of total loss at full load. Core losses represent the
energy required to magnetize the core material (hysteresis) and are expended
by small electric currents that flow in the core (eddy currents). Core loss of a
motor is constant and is independent of the motor load current, and thus it
accounts for a much higher percentage of the losses at low motor loads.
Loss due to stator and rotor resistance is about 56% of total loss at full load. It
occers due to the current flow (I) through the motor conductors of resistance
(R). Loss is proportional to the square of the stator and rotor current and is
also called I2R loss; it is influenced largely by loading on motor.
- 58 -