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Synergy Series Soft Start User Manual
MAN-SGY-017. Version 01. 27/06/2016
176
7.2 Advantages of IERS (continued)
If the load falls, 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 ‘changed’ 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 whole of the
load range from ‘no load’ upwards.
In practical terms, ‘no load’ means no external load. There are the internal mechanical and electrical losses
to be 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
magnetising current needed to provide the armature reaction to balance the losses. This is what the iERS
feature of a soft starter seeks to do, continuously and automatically.
7.3 Additional Benefits in Practice
It is usual 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, when supplied at rated voltage, energy could be saved even at full load.
Furthermore, there are those applications where the size of motor has to be chosen to provide for high
loadings which occur only intermittently, although the load demand at other times is much less.
7.4 How Much Energy?
The amount of energy used by a squirrel cage induction motor operating with a soft starter in iERS mode is
shown in Fig. 7.4.1, for the same duty cycle as Fig. 7.1.1. By reducing the voltage when torque demand is
below maximum, the magnetising current is proportioned to the torque current.
Compare Fig. 4, energy-optimising, with Fig. 7.1.2, non - iERS.
(These graphical representations are illustrative only, not to scale.) To arrive at any exact figure for the
energy saved requires each individual case to be examined in detail, taking into account the following
variables;

Motor rating, type, and any special characteristics;
 Load, load characteristics, duty cycle;

Supply voltage; Supply authority tariffs and the user’s particular terms.
7. Intelligent Energy Recovery (continued)
Current demand with Energy Saving in operation
100
Magnetizing current
Torque current
50
FLC
0
40 60 80 100 120 140 160
seconds
Figure 7.4.1: Energy Savings