The figure above shows the typical diagram representing a three-point starter for DC shunt motors with their protection devices. It contains three ports, namely L, Z and A; hence the name three-point starter. The starter consists of starting resistors divided into several sections and connected in series within the armature. Each tap point on the starting resistors is connected to several screws. The three starter terminals, L, Z and A, are connected to the positive line terminal, the shunt field and the motor armature terminal, respectively. The remaining terminal of the shunt and armature is connected to the negative terminal of the line. The open coil trigger is connected in series with the field winding. One end of the handle is connected to the L terminal via an overload trip coil. Then the other end of the handle moves against the torsion spring and touches the base of each screw as the process begins, triggering the initial resistance as it moves clockwise over each screw.
Armature reaction in DC motor
To work
First, the DC power supply is turned on with the handle in the “OFF” position.
Now move the handle clockwise to 1st. As soon as the 1st Bolt is reached, the shunt field coil is immediately connected to the supply. However, the entire starting resistance is fed in series with the armature circuit. 
As the lever gradually moved toward the absolute screw, the initial resistance in the armature circuit was gradually reduced. Finally, the handle is magnetically held in place by releasing the open coil while the field winding supplies power to it. 
If the excitation of the shunt field winding is accidentally interrupted or the power supply is interrupted, the voltage-free coil trigger is demagnetized and the crank returns to the initial position under the influence of the spring.
Note: If we used an A-volt release coil, the handle would remain in the same position when power was removed, thus causing an extreme armature current.
If a motor failure or overload occurs, an extremely large amount of current will be drawn from the source. This current increases the amperage turns of the OLR (overload relay) coil and pulls the armature coil, short-circuiting the NVR coil (zero voltage relay coil). The NVR coil is demagnetized and the handle returns to the rest position under the influence of the spring. This will automatically disconnect the motor from the power source.DC shunt motor characteristics
Known for its versatility and reliability, the DC shunt motor has a series of special features that make it a cornerstone in various industrial applications. One of its defining characteristics is the ability to provide near-constant speed under varying loads, making it an ideal choice for applications where speed control is critical. This property arises from its unique torque-speed characteristics, which exhibit a linear relationship between torque and armature current. Furthermore, the DC shunt motor provides excellent starting torque and starts movement effortlessly, even under heavy loads. Another notable feature of this type of engine is its relatively high efficiency, ensuring optimal performance with minimal energy waste. With its rugged construction and self-regulating nature, the DC shunt motor remains a reliable piece of equipment across industries, from manufacturing and transportation to robotics and renewable energy. Its adaptability and reliable performance make it the first choice for countless applications and solidify its position as a fundamental component in electrical machines.
Advantages and disadvantages of the three-point starter
Benefits
The three-point match brings numerous advantages to electrical systems and makes them indispensable. Ensures smooth and controlled engine starting, prevents harmful voltage spikes and extends engine life. Its integrated overload protection protects against excessive currents, promotes operational safety and reduces the risk of downtime. The starter motor allows precise control of engine rotation, ideal for applications that require precision and stability. Its design is simple and easy to use, making maintenance and troubleshooting easy. With soft starting, overload protection, precise speed control and easy-to-use operation, the three-point starter improves efficiency, reliability and safety in a variety of electrical applications.
Disadvantage
At the starting point, the idle relay coil is connected in series with the field circuit; therefore, it carries a shunt current in the field. If DC motor speed control is done through the field controller, the shunt field current may be weakened to the point that the release of the idle coil may not be able to keep the starter lever inactive. This may cause the motor to unexpectedly disconnect from the source. This can be overcome by using the starting point.
Conclusion
In short, the three-point starter is a remarkable innovation that revolutionized the world of electrical engineering. Its ability to efficiently control and start DC motors has paved the way for countless industrial applications, from heavy machinery to transportation systems. The reliable and robust three-point starter motor design ensures smooth and safe engine operation, reducing wear and maximizing efficiency. As technology advances, the use of this timeless device can further realize its potential and lead to greater advances in electrical engineering and sustainable energy solutions. With its proven track record and adaptability, the three-point starter remains an essential tool for harnessing the true power of DC motors for a brighter, more electrifying future.
