Induction motors draw heavy current at starting due to the high initial load placed on them as they accelerate from standstill to their operating speed. This heavy starting current is primarily caused by two factors:

1. Initial Stator Current: When power is applied to the motor, the stator winding initially draws a large current to create a rotating magnetic field. This current, known as the starting or locked rotor current, is significantly higher than the normal operating current because the rotor is stationary, presenting a high impedance to the rotating magnetic field. As a result, the motor draws a heavy current to overcome this initial resistance and start rotating.

2. Rotor Induction: In an induction motor, the rotor is not directly connected to an external power source but rather induced to rotate by the rotating magnetic field produced by the stator. During starting, the rotor initially experiences a high slip (the difference between the synchronous speed and the actual rotor speed) since it is not yet rotating at synchronous speed. This high slip results in increased rotor current, contributing to the overall heavy starting current drawn by the motor.

The heavy starting current drawn by induction motors can cause several issues, including voltage drops, overheating of motor windings, and excessive mechanical stress on the motor and connected equipment. To mitigate these issues, various starting methods and devices are employed, including:

1. Direct Online (DOL) Starting: This is the simplest method where the motor is directly connected to the power supply without any additional control. However, it results in the highest starting current and can cause voltage dips and mechanical shocks.

2. Star-Delta Starting: This method involves starting the motor in a low-voltage star configuration to reduce starting current and then switching to a higher-voltage delta configuration once the motor reaches a certain speed. This reduces the starting current but may result in reduced torque during starting.

3. Soft Starters: Soft starters gradually ramp up the voltage and frequency applied to the motor during starting, reducing the starting current and mechanical stresses. They provide smooth acceleration and are suitable for applications where controlled starting is required.

4. Variable Frequency Drives (VFDs): VFDs control the motor speed by varying the frequency and voltage supplied to the motor, allowing for soft starting and precise control of acceleration. They are effective in reducing starting current and energy consumption and are commonly used in applications requiring variable speed control.

By employing appropriate starting methods and devices, the heavy starting current drawn by induction motors can be mitigated, ensuring reliable operation, reduced energy consumption, and prolonged motor life.