Why Starter is Needed in DC Motor? Working, Need and Simple Explanation
Why is a Starter Needed in a DC Motor? Simple Explanation for Beginners
Search Description: Learn why a starter is required in a DC motor, how it reduces high starting current, protects the armature, and helps the motor run safely.
A DC motor starter is used to protect a DC motor from drawing very high current at the time of starting. When a DC motor is switched ON, its armature is initially at rest, so there is no back EMF to oppose the applied voltage. Because the armature resistance is very low, a dangerously high current can flow through the motor.
Table of Contents
- What is a DC Motor Starter?
- Why DC Motor Draws High Starting Current
- Role of Back EMF in DC Motor
- Simple Starting Current Calculation
- How a Starter Protects the Motor
- Types of DC Motor Starters
- Modern Applications
- FAQs
What is a DC Motor Starter?
A starter is an electrical device connected in series with the armature circuit of a DC motor during starting. Its main job is to limit the starting current to a safe value. As the motor gains speed, the starter resistance is gradually reduced and finally removed from the circuit.
In simple words, a starter works like a temporary current limiter. It protects the motor during the first few seconds after switching ON.
Why Does a DC Motor Need a Starter?
In a DC motor, the armature winding receives electrical supply and the field winding produces the magnetic field. When the motor starts rotating, it generates a voltage called back EMF. This back EMF opposes the applied voltage and naturally reduces the armature current.
But at the starting instant, the motor speed is zero. Therefore, back EMF is also zero. Due to this, the full supply voltage appears across the armature resistance.
Ia = (V - Eb) / Ra
Where V = supply voltage, Eb = back EMF, and Ra = armature resistance.
At starting, Eb = 0, so:
Simple Example: Starting Current of a DC Motor
Suppose a DC motor is connected to a 230 V supply and its armature resistance is only 2 Ω.
I = 230 / 2
I = 115 A
This means the motor may draw about 115 A at starting. Such a high current can damage the armature winding, commutator, brushes, and other motor parts. It can also cause voltage dips in the supply system.
How Does a Starter Reduce Starting Current?
A starter adds external resistance in series with the armature at the time of starting. This extra resistance limits the current to a safe value. As the motor speed increases, back EMF also increases. Then the starter resistance is gradually cut out step by step.
Finally, when the motor reaches normal speed, the starter resistance is completely removed and the motor runs normally.
What Happens If a DC Motor is Started Without Starter?
- Very high starting current flows through the armature.
- Armature winding may overheat or burn.
- Commutator and brushes may get damaged.
- Supply voltage may drop suddenly.
- Motor life may reduce.
- Protective devices may trip frequently.
Role of Back EMF in DC Motor Starting
Back EMF is the voltage generated in the armature due to rotation in the magnetic field. It always opposes the applied voltage. When the motor speed is low, back EMF is low and current is high. When speed increases, back EMF increases and current decreases.
Types of DC Motor Starters
1. Two-Point Starter
A two-point starter is mainly used for DC series motors. It has two terminals: one connected to the supply and the other connected to the motor.
2. Three-Point Starter
A three-point starter is commonly used for DC shunt and compound motors. It has three terminals: line, armature, and field. It provides starting resistance, no-voltage protection, and overload protection.
3. Four-Point Starter
A four-point starter is an improved version of the three-point starter. It is used where field current control is needed without affecting the no-voltage coil operation.
Comparison: Starting With and Without Starter
| Condition | Without Starter | With Starter |
|---|---|---|
| Starting Current | Very high | Limited to safe value |
| Motor Safety | Risk of damage | Motor remains protected |
| Armature Winding | May overheat | Safe operation |
| Commutator | Sparking may occur | Sparking is reduced |
| Supply System | Voltage dip may occur | Stable starting |
Modern Use of DC Motor Starters
Today, traditional manual starters are still used in laboratories and small DC motor applications. In modern industries, electronic DC drives are often used instead of old-style resistance starters. These drives control the starting current smoothly and also provide speed control, overload protection, and better efficiency.
DC motors and their starting methods are still important in electric traction, cranes, rolling mills, elevators, battery-operated systems, and industrial drives.
Interview Questions on DC Motor Starter
- Why is a starter required in a DC motor?
- What is back EMF?
- Why is starting current high in a DC motor?
- What happens if a DC motor is started without starter?
- What is the difference between three-point and four-point starter?
Frequently Asked Questions
Why is starter necessary in DC motor?
A starter is necessary because at starting, back EMF is zero and the armature resistance is very low. This causes a very high starting current, which can damage the motor.
What is the main function of a DC motor starter?
The main function of a DC motor starter is to limit the starting current by adding external resistance in series with the armature circuit.
Can a small DC motor run without starter?
Very small DC motors may run without a starter because their current rating is low. But medium and large DC motors require starters for safe operation.
What is back EMF in DC motor?
Back EMF is the voltage generated in the rotating armature of a DC motor. It opposes the applied voltage and helps control the armature current during normal running.
Which starter is used for DC shunt motor?
A three-point starter or four-point starter is commonly used for DC shunt motors.
Conclusion
A starter is very important for a DC motor because it protects the motor from excessive starting current. At the starting moment, the back EMF is zero and the armature resistance is very small, so the motor can draw a dangerously high current. By inserting external resistance during starting, the starter allows the motor to accelerate safely. As the speed increases and back EMF develops, the starter resistance is gradually removed.
Therefore, for safe operation, longer motor life, and reliable performance, a starter is essential for medium and large DC motors.
Posts
No comments