3 Phase Induction Motors - Starting Methods
Why starting methods? On the one hand, direct start-up can cause mechanical problems, such as mechanical shocks or stress on gearboxes, workpiece, etc. The 3-phase-induction motor has the problem of high starting current, which amounts to approx. 7 ... 10 times the rated current.
The 3-phase-induction motor used today is the so-called squirrel cage motor. Starting aids, such as Star-Delta starting or Soft Starters, follow the principle of reducing the stator voltage during startup. The Variable Frequency Drive is also used as a starting aid, but it can do even more, namely to control the speed while adjusting the voltage to the frequency. That is why this electronic device is clarified here in a separate chapter.
All stator starting methods have one problem: as soon as the voltage is reduced, the torque decreases to a square.
Characteristic of the three-phase induction motor when operating with reduced voltage
Pmech = η × Pelectr => Pmech ~ Pelectr
T × n ~ 3 UCoil × ICoil ,whereby n = constant, because f = 50 Hz constant
with Icoil = Ucoil / Z results in:
T ~ VCoil2 / Z , whereby Z = constant, because f = constant
=> T ~ VCoil2
Therefore, when using the star-delta starting or the soft starter, always check whether the starting torque and the saddle torque are still sufficient.