Automation Direct Stellar SR55 Series, User Manual

Page: 185 / 202

Appendix B: Soft Starter Application Considerations
Page B–13 Stellar
®
SR55 Series Soft Starter User Manual – 1st Ed, Rev F – 09/18/2019
b.3.6 – r
unning
i
nduCTion
m
oTors
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.
f
igure
b.3.6.1: m
oTor
e
ffiCienCy
/l
oad
C
haraCTerisTiC
Motor Efficiency/Load Characteristic
EFFICIENCY
LOAD
0 1/2 1/1
100%
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
SR55, 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.
f
igure
b.3.6.2: m
oTor
e
ffiCienCy
/l
oss
C
haraCTerisTiC
Motor Efficiency/Loss Characteristic
LOAD
STRAY
EXCITATION
MECHANICAL
LOAD
0 1/2 1/1
LOSSES