The starting current of an induction motor is quite high for a number of reasons, including the following:
1). Initial Magnetic Field
When an induction motor is first started, the rotor is in a stationary position, and there is no sign of a rotating magnetic field being produced.
The stator windings provide a magnetic field that generates a current in the rotor windings, which in turn helps to build the spinning magnetic field.
The process requires the use of a significant current in order to overcome the initial resistance & inertia of the rotor, as well as to generate an adequate amount of torque for the purpose of beginning the motor’s rotation.
2). Rotor Resistance
The rotor circuit of an induction motor normally has a resistance that is rather high in comparison to the stator circuit.
According to Ohm’s law, which stipulates that
V=IR
it can be accomplished to represent it as I = V/R,
which stands for “I equals voltage divided by resistance.”
a larger starting current will flow as a result of the higher resistance during the starting phase.
The high starting current can be beneficial in generating the required torque to overcome the rotor’s inertia & accelerate it to synchronous speed in order to get it to work.
3). Back electromotive force (EMF)
When the rotor starts to spin and the motor begins to speed up, a back electromotive force (EMF) is created in the rotor windings as a result of the relative motion that occurs between the magnetic fields of the stator and the rotor.
This back EMF works against the voltage that is being provided to the motor, which in turn lowers the current that is being drawn through it.
On the other end, since the rotor remains static throughout the initial phase of the process, there is no back EMF to serve as a protection against the voltage that is being applied, which results in a larger starting current.
Summary
It is important to keep in consideration that the high starting current is the short-term phenomena that only continues for a short period of time up until the motor achieves its working speed.
As soon as the motor achieves its working speed, the current starts dropping until it reaches the usual running value that is established by the load and the motor design.
During the process of starting a motor, limiting the starting current & reducing the stress on both the motor and the power supply may be accomplished by the use of a number of different techniques and devices, such as lower voltage starters.