Automation Direct Stellar SR55 Series, User Manual

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Page 4–3 Stellar
®
SR55 Series Soft Starter User Manual – 1st Ed, Rev F – 09/18/2019
Chapter 4: Principles of iERS (intelligent Energy Recovery System)
p
rinCiples of i
ers
Every wound-field electric motor must consume some minimum amount of energy to
provide a magnetic field which enables it to work at all. With DC motors the field is under
separate control, so that the amount of magnetizing energy can be adjusted to be sufficient
to overcome losses and provide an armature reaction appropriate to the load. The squirrel
cage AC induction motor has no such provision, with the result that energy is wasted at any
load less than its rated full load (at full speed). When a squirrel-cage motor is supplied at a
constant terminal voltage, as when it is connected directly to the supply without a controller
of any kind, the strength of the field flux is fixed by the supply voltage. At normal running
speed the field will take a fixed quantity of energy regardless of the torque demanded by the
mechanical load. The energy required to support the load torque is determined by the torque
demand. As load torque increases, the rotor slows down a little (i.e. ’slip’ increases), causing
induced rotor currents to also increase in order to increase the torque. These additional
currents in the rotor are balanced by additional current in the stator coils. Conversely, if load
torque demand falls, the slip decreases, the rotor currents decrease, and the current in the
stator decreases accordingly. But at constant terminal voltage, the current providing the
stator field flux remains unchanged at any level of load torque demand. As a consequence,
the efficiency of an induction motor decreases as the load decreases.
f
igure
4.1.1: T
ypiCal duTy CyCle for a maChine load Where The
T
orque
d
emand varies
.
Load Torque Demand at working speed
40 60 80 100 120 140 160
seconds
M
demand
%
100
50
0
f
igure
4.1.2:
T
orque
d
emand ConverTed To an equivalenT CurrenT WiTh The moTor magneTizing CurrenT
added
Current Demand due to torque and flux
40 60 80 100 120 140 160
seconds
Magnetizing current
Torque current
%
FLC
100
50
0