Starting and braking of three phase induction motor

Starting and braking of a three-phase induction motor involve specific methods to ensure safe, efficient, and smooth operation.

Starting Methods:

1. Direct-On-Line (DOL) Starter:

• Used for: Small motors (<5 HP).
• Pros: Simple, cheap.
• Cons: High inrush current.

1. Star-Delta Starter:

• Used for: Motors >5 HP.
• Working: Starts in star (low voltage), then switches to delta (full voltage).
• Pros: Reduces starting current to about 1/3.

1. Auto-Transformer Starter:

• Used for: Large motors.
• Working: Reduces voltage during starting.
• Pros: Better current control.

1. Soft Starter:

• Electronic control to gradually ramp up voltage.
• Smooth acceleration and reduced mechanical stress.

1. VFD (Variable Frequency Drive):

• Controls both voltage and frequency.
• Ideal for precise speed control and soft starting.

🛑 Braking of Three-Phase Induction Motor

Goal: Bring the motor to stop quickly or control its deceleration.

Braking Methods:

1. Plugging (Reverse Current Braking):

• Working: Reverses phase sequence to develop torque opposite to rotation.
• Pros: Fast stopping.
• Cons: High current, more mechanical stress.

1. Dynamic Braking:

• Applies to squirrel-cage motors.
• Working: Disconnect supply, connect DC to stator → generates braking torque.
• Pros: Smooth braking.
• Cons: Needs external circuitry.

1. Regenerative Braking:

• Only in variable-speed drives (like VFDs).
• Working: Motor acts as a generator; power fed back to the grid.
• Pros: Energy-saving.
• Cons: Requires inverter/VFD.