Page 4–4 Stellar
®
SR55 Series Soft Starter User Manual – 1st Ed, Rev F – 09/18/2019
Chapter 4: Principles of iERS (intelligent Energy Recovery System)
advanTages of iers
A soft starter with an iERS energy-optimizing feature alters the motor operation. The iERS
function reduces the terminal voltage applied to the motor so that the energy needed to
supply the field is more closely proportioned to the torque demand. The effect is shown in the
Figure below.
figure 4.2.1: ‘full speed’ end of The ConvenTional Torque/CurrenT Curves.
A
Full load Torque
at rated voltage
Reduced
Torque
demand
Speed Synch
Torque at
rated voltage
Torque at
reduced voltage
Torque
100% M
80% M
B
Voltage/To rque balance
The present considerations do not affect
soft-starting options or strategies.
Point A on the current curve is the
operating point of the motor when the
motor terminal voltage is at its ‘nominal’
or rated value, and when the load is the
maximum for which the motor is rated.
If the load decreases, a motor supplied
at a fixed voltage will speed up slightly,
the current demand will reduce, and the
operating point moves along the curve to
point “B”. Because the torque developed
by a motor is proportional to the square of
the applied voltage, lowering the terminal
voltage reduces the torque. If the reduced voltage is correctly chosen, the working point at
the reduced torque demand becomes the point “A”. By reducing the terminal voltage, the
motor has in effect been ‘exchanged’ for one which has a lower rated power output. A reduced
terminal voltage also means a reduced field energy requirement, and this simple relationship
enables the iERS function to maintain the efficiency of the motor over nearly the entire load
range from ‘no load’ upwards. In practical terms, ‘no load’ means no external load. There are
still internal mechanical and electrical losses to overcome – friction and windage of the rotor
at speed, and the electrical heating and hysteresis losses. The ideal response to the ‘no load’
condition would be to supply precisely the amount of magnetizing current needed to provide
the armature reaction to balance the losses. This is what the iERS seeks to do automatically
and continuously.
addiTional benefiTs in praCTiCe
It is normal to select a standard motor with a rating somewhat higher than the maximum
demand of the driven load. The motor selected for any given application will almost certainly
be over-rated for this reason alone, and therefore energy could be saved even at full load
when supplied at rated voltage. Furthermore, there are those applications where the size of
motor has to be chosen to provide for high loadings which occur only intermittently, or for an
arduous start, even though the load demand at most times is much lower.
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