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Chapter 3: Applications
3.4.2 – Advantages of Energy Optimizing
A soft starter with an energy-optimizing feature alters the motor operation. The energy-optimizing
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 3.4.2.1: ‘Full Speed’ end of the conventional Torque/Current curves.
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
optimizing 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 energy-optimizing feature of a soft starter seeks to do
automatically and continuously.
3.4.3 – 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.
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/Torque 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.
SR44 Series Soft Starter User Manual
1st Ed, Rev B 07/31/2019
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