3 point starter | 3 point starter dc motor | three point starter of dc motor | three point starter

Summary

This lecture delves into the workings of a three-point starter, a device used to protect DC motors from high starting current. It explains the construction, working principle, and important components like the series-connected resistance, which reduces the current to prevent machine damage. The lecture also covers the starter's terminals, the starting position, the under-voltage protection, and overload protection mechanisms. It highlights the advantages of using a three-point starter for smooth motor starting and speed control in various industrial applications, while also discussing its drawbacks and the concept of back EMF in DC motors.

Takeaways

• 😀 A three-point starter is a device used to protect DC motors from high starting current.
• 🔧 It includes a series resistance that reduces the high starting current, thus protecting the machine from damage.
• 🔌 There are three main terminals in a three-point starter: Line Terminal, Armature Terminal, and Field Terminal.
• 🔄 The Line Terminal is connected to the positive side of the battery, the Armature Terminal to the motor's armature winding, and the Field Terminal to the motor's field winding.
• 🔑 The construction, working principle, and important components of the three-point starter are discussed in the lecture.
• 🌀 The starter uses a series of components to limit the current, including a variable resistor known as the 'r' series, which includes R1, R2, R3, and R4.
• 🛠 The starter provides under-voltage protection by ensuring that the electromagnetic field's strength is reduced if the supply voltage drops or is off.
• 🔄 Overload protection is provided by an overload trip coil, which trips the circuit if the motor is overloaded, preventing motor burnout.
• 🔄 The speed of the DC motor can be controlled using the three-point starter, which is beneficial in applications like factories and spinning mills.
• 🔄 The three-point starter reduces high starting current and provides protection against short circuits and overload issues in the motor.
• 🔄 The term 'back EMF' is introduced, which is the voltage induced in the armature conductors when the stationary flux is cut by the rotating armature, affecting the speed and performance of the DC motor.

Q & A

What is a three-point starter and why is it used?

A three-point starter is a type of device used to protect a DC motor from high starting currents. It includes a series resistance that reduces the high starting current, thereby preventing damage to the motor.

What are the main terminals of a three-point starter?

The main terminals of a three-point starter are the Line Terminal, Armature Terminal, and Field Terminal. The Line Terminal is connected to the positive line, the Armature Terminal to the motor's armature winding, and the Field Terminal to the motor's field winding.

How does the handle of the three-point starter function during the motor start-up?

The handle, initially in the 'Off' position, is manually moved to the start position. As the handle moves over resistance studs, the motor begins to start, with resistances reducing the armature current to a safe value until the motor reaches full speed.

What is the role of the NVC coil in the three-point starter?

The NVC (No Voltage Coil) provides under-voltage protection. It holds the starter handle in the 'Run' position when the motor is running at full speed. If the supply voltage drops, the magnetic field weakens, causing the handle to return to the 'Off' position, cutting off the motor's supply.

How does the OLC coil protect the motor from overload?

The OLC (Overload Coil) provides overload protection by tripping the circuit if the motor draws excessive current. This prevents the motor from burning out by cutting off the supply when an overload condition is detected.

What are the drawbacks of using a three-point starter?

The main drawback of a three-point starter is the large speed variation it can cause in the motor. The NVC coil's magnetic strength can decrease, leading to the handle moving to the 'Off' position unintentionally. This type of starter is also not suitable for variable speed motors.

Why was the four-point starter introduced?

The four-point starter was introduced to overcome the drawbacks of the three-point starter, such as the large speed variation and unintentional handle movement to the 'Off' position.

What is back EMF and why is it important in DC motors?

Back EMF (Electromotive Force) is the voltage generated in the armature conductors of a DC motor due to the generator action when the armature cuts through the magnetic flux. It opposes the supply voltage and is crucial for regulating the current in the motor.

How does the resistance in the three-point starter help during motor startup?

The resistance in the three-point starter helps limit the armature current to a safe value during motor startup by creating voltage drops across the resistances, which reduces the overall current flowing through the motor.

Where are three-point starters commonly used?

Three-point starters are commonly used in applications like lathe machines, factories, and spinning mills where controlling the speed and protecting the motor from high starting currents is essential.