Starting Current in Induction Motor:
Starting current of induction motor is as high as 5 to 7 times the
normal full load current. Therefore different starting of induction motor,
methods such as (star delta starter, auto transformer starter and other
starting methods) are employed in order to reduce the high starting currents of
induction motor.
Why High Starting Currents:
Induction motor can be compared to an electrical transformer with
the secondary short circuited. Primary winding of the transformer can be
compared to the stator winding of the induction motor and the rotor winding is
considered as the short circuited secondary winding of the transformer.
Induction
motor circuit model is shown in the figure. From the model of Induction motor
we can observe that induction motor consists of two branch circuits which are
in parallel
- Magnetizing
component circuit
- Resistance
and reactance circuit.
Magnetizing component of current flowing through induction motor
is proportional to the applied voltage and is independent of load on the motor
similar to transformer
Resistance and leakage reactance circuit consist of resistance and leakage reactance of stator and rotor of induction motor connected in series. A load resistance (variable) is connected in series to the fixed rotor and stator impedance. During starting of the motor, slip will be one. Therefore if we calculate the total impedance offered (stator and rotor impedance) to the inrush currents during starting of induction motor which is minimum resulting in high inrush currents during starting of the motor.
Resistance and leakage reactance circuit consist of resistance and leakage reactance of stator and rotor of induction motor connected in series. A load resistance (variable) is connected in series to the fixed rotor and stator impedance. During starting of the motor, slip will be one. Therefore if we calculate the total impedance offered (stator and rotor impedance) to the inrush currents during starting of induction motor which is minimum resulting in high inrush currents during starting of the motor.
When 3 phase voltage applied across the stator winding for
starting of induction motor, high inrush currents magnetize the air gap between the stator and rotor. An induces emf
is generated in the rotor windings of the induction motor because of the
rotating magnetic field. This induced emf produces electrical current in rotor
windings. Current generated in the rotor windings produces a field which
in turn produces torque to rotate the motor. Once the rotor starts picking up
the speed, current drawn by the machine decreases. The time required for
staring of the motor depends on the time required for the acceleration which
depends on the nature of the connected load.
Disadvantages of High Starting
Currents in Induction Motor:
High inrush currents drawn by induction motor during starting can
result in large dip in connected bus voltages. This dip in bus voltages can
impact the performance of other motors operating on the bus. Voltage dips
during starting of large motors can trip some of the motors operating on the
same bus. Care should be taken to limit the inrush currents during starting of
the motor by employing proper starting methods.
For large motors life of the machine depends on the number of starting.
High inrush currents can cause increase in the temperature of the machine,
damages the insulation and can reduce the life of the machine
