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Synergy Series Soft Start User Manual
MAN-SGY-017. Version 01. 27/06/2016
174
7.1 Principles
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 magnetising 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 at any load less than it’s
rated full load (at full speed), energy is wasted. 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 the induced rotor currents to increase
also, and so 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, and therefore the energy,
providing the stator field flux remains unchanged at any level of load torque demand. As a consequence,
the efficiency of an induction motor falls as the load falls.
Chapter
7
7. Intelligent Energy Recovery (iERS)
Figure7.1.1: Typical duty cycle for a machine load where the Torque Demand varies.