A 4-point starter limits the starting current of a DC motor and ensures smooth acceleration. The key components of a 4-point starter include a starting resistance, a no-volt coil, an overload coil, and a starting handle. Here's how it works to limit the starting current:
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Starting Resistance: When the motor is initially started, a high starting current can occur due to the low resistance of the armature winding. The 4-point starter includes a series of resistors connected in series with the motor armature. These resistors limit the initial current by providing higher resistance at the start.
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Starting Handle: The operator moves the starting handle to progressively short-circuit these resistors, gradually reducing the resistance in the circuit. This step-by-step reduction in resistance allows the motor to accelerate smoothly without drawing excessive current.
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No-Volt Coil: This coil protects the motor from sudden power failures. If the supply voltage drops or is interrupted, the no-volt coil de-energizes, causing the starting handle to return to the 'off' position. This action prevents the motor from restarting automatically with high current when the power is restored.
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Overload Coil: The overload coil provides protection against excessive current due to overload conditions. If the current exceeds a predetermined safe level, the overload coil becomes energized and trips the starter, disconnecting the motor from the supply and preventing damage.
In summary, the 4-point starter limits the starting current of a DC motor by using a series of resistors to provide initial high resistance, which is then gradually reduced as the motor speeds up. The no-volt and overload coils provide additional safety features to protect the motor from power failures and overload conditions.