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Why does induction motor draw heavy current at starting ?

The induction motor draws a heavy current at starting primarily due to the characteristics of its stator and rotor windings, which create certain electrical and mechanical conditions during the initial moments of operation. Several factors contribute to this phenomenon:

1. Rotor Inertia:

  • Stationary Rotor: When the motor is at rest, the rotor is stationary, and its inertia resists the initiation of motion.
  • Heavy Initial Torque Requirement: Overcoming the initial resistance of the stationary rotor requires a heavy current to produce the necessary torque for rotor acceleration.

2. Counter Electromotive Force (EMF):

  • Back EMF Generation: As the rotor begins to rotate, it cuts through the magnetic field, generating a back electromotive force (EMF) in opposition to the applied voltage.
  • Low Back EMF Initially: At the moment of starting, the rotor speed is low, resulting in a low back EMF. This leads to a higher current draw to overcome the lower counteracting force.

3. Low Impedance at Starting:

  • Low Inductive Reactance: The impedance of the motor is mainly inductive, and at the moment of starting, the inductive reactance is relatively low.
  • Ohm’s Law Influence: According to Ohm’s Law (�=�/�I=V/Z), where �I is current, �V is voltage, and �Z is impedance, a lower impedance results in a higher current draw.

4. Stator and Rotor Resistance:

  • Low Speed, Low Counteracting Forces: Initially, the rotor speed is low, and the counteracting forces such as back EMF and electromagnetic torque are low.
  • Low Opposition: The stator and rotor resistance contribute to a lower opposition to current flow, resulting in a higher current draw.

5. High Torque Requirement:

  • Load Inertia and Friction: The motor may be connected to a load with high inertia or mechanical resistance, requiring a higher torque to overcome.
  • Heavy Current for High Torque: To generate the required torque for overcoming inertia and friction, the motor draws a heavy current during starting.

6. Voltage Drop:

  • Voltage Drop in Stator Windings: The heavy initial current draw can lead to a voltage drop in the stator windings.
  • Impact on Operation: The voltage drop may affect the motor’s performance and efficiency during the starting phase.

7. Starting Methods:

  • Direct-On-Line (DOL) Starting: In direct-on-line starting, the motor is connected directly to the power supply, resulting in a higher inrush current.
  • Soft Starters or Variable Frequency Drives (VFDs): Soft starters or VFDs can be used to gradually ramp up the voltage, reducing the initial current spike.

8. Protective Devices:

  • Inrush Current Limitation: To prevent damage to the motor and the electrical system, protective devices such as fuses or circuit breakers may be used to limit the inrush current.
  • Controlled Starting: Controlled starting methods help manage the heavy current draw and provide a smoother starting process.

9. Conclusion:

In conclusion, the induction motor draws a heavy current at starting due to the need to overcome the inertia of the stationary rotor, generate the initial torque required for acceleration, and cope with low counteracting forces such as back EMF. Various factors, including impedance, resistance, load conditions, and starting methods, influence the magnitude of the starting current. Engineers employ methods like soft starters or VFDs to mitigate the impact of the heavy starting current and optimize the motor’s performance.

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