Why does the induction motor Draw a heavy current at starting?

Why does the induction motor Draw a heavy current at starting?

Reason 1: Due to the inductive characteristics

The induction motor may be regarded as a Transformer with a primary coil and a secondary coil loaded with a load that changes its impedance.

The secondary coil is normally a one-turn coil.

The impedance of the load at the rotor is made up of low resistance R and low inductance L hence,

Z=(R+jwL)
where,
w - Frequency
R - Resistance
L - inductance
Z - Impedance

When the rotor of an induction motor is stationary the current in the shorted conductive loops of the rotor is very high as the resistance and the inductance is low while the effective frequency is that of the supply frequency.

This high current in the rotor will create its own magnetic field which opposes the main stator magnetic field, this weakens the stator magnetic field so the back EMF in the stator will drop and the supply voltage will be much higher than the stator back EMF and so the supply current increases to a high value. Those are the starting conditions.

As far as impedance goes the rotor impedance is defined by Z=(R+jwL) were w, the frequency and this is high at starting and as the rotor starts turning the frequency in the rotor will decrease till it is zero at synchronous speed. So, in fact, the impedance in the rotor is decreasing as the rotor speed up.

Reason 2 : Due to starting slip of the motor

The voltage induced in the rotor depends on the relative speed of the synchronous speed of the rotating magnetic field and the speed of the rotor.

At the start, the rotor is at a standstill so its speed is equal to zero. At starting, the difference between the speed of the synchronous speed of the rotating magnetic field and the speed of the rotor is maximum.

The difference between the synchronous speed and the rotor speed is called slip of the motor.

The slip of the motor can be expressed as;
S = (Ns- Nr)/Ns *100   ---------(1)
Where,
S = Slip
Ns= Synchronous speed of the motor= 120 f/P
Nr = Rotor speed

As the speed of the rotor is zero at the start, the rotor conductor will cut the maximum flux and the maximum voltage will be induced in the rotor.

As the motor starts accelerating the speed of the rotor will align in the direction of synchronous speed of the motor and the slip will get reduced.

The voltage induced in the rotor conductor can be expressed as;

Er = S* Es ------------(2)

Where,
Er = Rotor Voltages  
S = Slip 
Es = Stator Voltage

Er = Es    
When Nr =0 & slip=1It is clear that the induced rotor is maximum at the starting of the motor.

Disadvantages of High Starting Currents in Induction Motor

1. High inrush currents

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

2. Increase in the temperature of the machine

For large motors life of the machine depends on the number of startings. High inrush currents can cause an increase in the temperature of the machine, damages the insulation, and can reduce the life of the machine

FAQ

Why a motor takes a large current at starting?

Since there is no voltage drop of induced e.m.f at the start, the current at starting would be greater than normal current. This current is nearly 1.5 to 7 times the rated current or full load current of the motor

Why does a motor consume more current at starting?

So at the starting of motor when there is no speed, there is no emf induced, so there is no oposition to main current so motor draw high current, but as it catch up speed more and more back emf induced, current decreases. At full speed it come to normal current

How do you reduce high starting current in an induction motor?

These illustrate that to some degree the starting current can be reduced with a tuned capacitor bank; however, for better reduction then an energy storage unit is used, such as a battery or a storage capacitor, and this is accessed through a PWM inverter for charging and discharging

What will excessive current drawn during start of motor cause?

Motor overload occurs when a motor is under excessive load. The primary symptoms that accompany a motor overload are excessive current draw, insufficient torque and overheating. Excessive motor heat is a major cause of motor failure

Why induction motor draws high current during starting?

What causes high start-up or inrush current? An induction motor with a stationary rotor behaves like a transformer with shorted secondary windings; there is no back-emf to counter the emf in the stator windings, so a very high current flows

Why does a motor take high current?

Probable causes of high current with load include mechanical overload, excessively high magnetic flux densities and, less frequently, an open rotor. An error in winding data that results in lower-than-design-level magnetic flux also can cause high current with load

Why the starter motor requires a very high electric current during its operation?

Starter motors need the extra current to create a bigger magnetic field, so that rotor poles can pull harder towards the stator poles. More current equals more magnetic field, both in the stator and rotor. Thus the starter motor (a series wound motor) can produce huge starting torque for turning the cold engine over

What causes excessive current draw?

Possible causes for overcurrent include short circuits, excessive load, incorrect design, an arc fault, or a ground fault. Fuses, circuit breakers, and current limiters are commonly used overcurrent protection (OCP) mechanisms to control the risks