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3.3.6 – Running Induction Motors
Once a start has been completed, the motor operating efficiency becomes of interest. When
working at or near full load, the typical three-phase induction motor is relatively efficient, and
readily achieves efficiencies of 85% to 95%. However, as shown below, motor efficiency falls
dramatically when the load falls to less than 50% of rated output.
Figure 3.3.6.1: Motor Efficiency/Load Characteristic
In fact, very few motors actually experience consistent fully rated operation, and the vast majority
operate at much lower loads due to either over-sizing (a very frequent situation), or natural load
variations. For Fan and Pumping applications, the affinity laws will allow the inverter drive to
show very considerable energy savings over virtually all other methods of control through varying
the speed of the motor in response to changes in load. Where motor speeds cannot be varied, an
optimizing version of semiconductor motor controller, such as the SR44, will also produce energy
savings in lightly loaded motors. Less sophisticated systems of soft starters remain at full
conduction, and the motor then behaves as if it were connected directly to the main supply.
However, at light loads and mains voltages, induction motors always have excess magnetic flux,
and efficiency loss and power factor degradation result. By detecting the load at any instant and
adjusting the motor terminal voltage accordingly, it is possible to save some of the excitation
energy and load loss, and therefore improve motor power factor when the motor is running
inefficiently at light loads.
Figure 3.3.6.2: Motor Efficiency/Loss Characteristic
Figure 3.3.6.2
Motor Efficiency/Loss Characteristic
LOAD
STRAY
EXCITATION
MECHANICAL
LOAD
0 1/2 1/1
LOSSES
Figure 3.3.6.1
Motor Efficiency/Load Characteristic
EFFICIENCY
LOAD
0 1/2 1/1
100%
Chapter 3: Applications
SR44 Series Soft Starter User Manual
1st Ed, Rev B 07/31/2019