Starting Methods

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

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.