Here's a breakdown of its components and how it works:
Components:
* No-Volt Coil (NVC): This coil is connected in parallel with the armature winding and is used for safety. It's energized when the starter is in the "ON" position.
* Overload Relay (OLR): This device protects the motor from excessive current. It's usually a bimetal strip that bends when it heats up due to high current, opening the circuit.
* Starting Resistance: A series of resistors that are gradually reduced as the motor speeds up. This limits the starting current and protects the armature winding.
* Start Button: Used to initiate the starting sequence.
* Stop Button: Used to interrupt the power supply to the motor.
Working Principle:
1. Starting:
* When the start button is pressed, the NVC is energized, completing the circuit.
* The motor armature circuit is closed through the starting resistance and the NVC.
* Since the armature is initially at rest, the back EMF is zero. This allows a high starting current to flow, but it's limited by the starting resistance.
* As the motor begins to rotate, the back EMF increases, reducing the armature current.
* The operator moves the starter lever to the next position, cutting out some of the starting resistance. This increases the motor speed further.
* This process of increasing motor speed and reducing starting resistance continues until all the resistance is cut out and the motor reaches its full speed.
2. Stopping:
* When the stop button is pressed, the NVC is de-energized, interrupting the current flow to the motor armature.
* The motor then slows down and eventually stops.
Advantages:
* Simple and inexpensive: Compared to other types of starters, the two-point starter is relatively simple and affordable.
* Easy to operate: The operation of a two-point starter is straightforward and easy to understand.
Disadvantages:
* Limited control: The two-point starter only provides basic control over the motor's speed and has no provision for reversing the motor's direction of rotation.
* No field control: It doesn't allow for adjusting the motor's field current, which can be important for some applications.
* Safety: The two-point starter only offers basic protection through the NVC and OLR. More sophisticated protection features may be necessary for critical applications.
Applications:
* Small DC shunt motors with limited speed requirements
* Applications where simplicity and low cost are prioritized
* Systems where speed control is not critical
In summary, the two-point starter is a basic and cost-effective solution for starting and stopping DC shunt motors. While it's simple to operate, it offers limited control and protection. More sophisticated starters are used in applications that require more precise speed control, reverse capability, and enhanced protection.
