Why does an induction 17 stepper motor need a starter?


Why does an induction 17 stepper motor need a starter?

For induction motor(IM) to be a self starting one, rotating magnetic field(RMF) has to be produced and rotor tries to reach the speed of RMF.Three phase IM has RMF inherently .

So,three phase IM is a self starting electric machine. It does not need any equipment to start.

So called “starter” is for limiting starting current(current drawn by the motor when it is starting) which is around 6–8 times of rated current.This is due to fact that at starting, no back EMF is available to oppose inrush current. As motor picks speed gradually, EMF gets developed and starts opposing inrush current.

So an equipment is needed to limit the inrush current at the time of starting and can be disconnected during running condition.

Whereas single phase induction motor is not a self starting one. According to double field revolving theory( Blondel's theory),the starting torque of single phase induction motor is comprised of two equal and opposite components and hence due to inertia of the rotor,there will be no rotation.RMF is not inherent feature of single phase IM.

In order to start the single phase IM, we have to boost one of the torque components such that the component dominates the opposite component and rotor starts rotating. To boost, we can use another motor called pony motor or starter motor.

The other simple way is to create RMF in single phase IM as well by creating phase shift of 90 degrees between currents flowing through the two windings.Using a capacitor in one of the windings can serve the purpose.Such a motor is called capacitor start single phase IM.


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