Principle, Working, and Operation of Propulsion System Using DC Motor Drives

Hindi

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🚀 Lesson: Principle, Working, and Operation of Propulsion System Using DC Motor Drives

🎯 Learning Objectives:

By the end of this lesson, students will be able to:

• Understand the principle and working of a DC motor in propulsion systems.

• Explain the role of armature voltage control in DC drives.

• Describe chopper circuits (step-up and step-down) used for controlling motor speed.

• Analyze the control strategies and role of chopper amplifiers in propulsion.

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1. 🧭 Introduction to Propulsion Systems

What is a Propulsion System?

A propulsion system is responsible for moving a vehicle (e.g., electric trains, electric cars, or industrial conveyors). It converts electrical energy into mechanical motion.

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2. ⚙️ DC Motor Drives in Propulsion

Why DC Motors?

DC motors are preferred in propulsion systems due to:

• High starting torque

• Simple speed control

• Quick dynamic response

Types of DC Motors Used:

• Separately Excited DC Motors

• Shunt DC Motors

• Series DC Motors (commonly used in traction due to high torque)

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3. 🔌 Principle of DC Motor Operation

• Working Principle: Based on Lorentz force—when a current-carrying conductor is placed in a magnetic field, it experiences a force.

• Key Equations:

  • Back EMF: $E_b = k_e \cdot \phi \cdot N$

  • Torque: $T = k_t \cdot \phi \cdot I_a$

  • Armature Voltage: $V = E_b + I_a R_a$

Where:

• $V$: Applied voltage

• $I_a$: Armature current

• $R_a$: Armature resistance

• $\phi$: Magnetic flux

• $N$: Speed (RPM)

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4. 🔁 Armature Voltage Control (Speed Control)

• Speed ∝ Armature Voltage

• Used for controlling the speed of the motor.

• Achieved by varying the DC voltage applied to the armature while keeping the field constant.

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5. ⚡ Chopper Circuits

What is a Chopper?

A chopper is a high-speed DC-DC converter that rapidly switches the input voltage ON and OFF to control the average output voltage.

Purpose in Drives:

• Controls the voltage applied to the motor

• Enables efficient speed control

• Converts constant DC into variable DC

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5.1 Step-Down Chopper (Buck Converter)

• Function: Reduces DC voltage.

• Used When: Motor requires a lower voltage than the supply.

• Operation:

  • Switch ON → Voltage applied

  • Switch OFF → Voltage = 0

  • Output Voltage $V_o = \alpha \cdot V_{in}$ (where $\alpha$ is the duty cycle)

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5.2 Step-Up Chopper (Boost Converter)

• Function: Increases the output voltage above the input.

• Used When: Regenerative braking or when load demands higher voltage.

• Operation:

  • Stores energy in inductor when switch ON

  • Releases energy to load when switch OFF

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6. 🧠 Control Strategy

Control Goals:

• Maintain desired speed

• Regulate torque

• Ensure smooth operation during acceleration, braking

Methods:

1. Closed-Loop Control:

   • Uses speed and current feedback

   • Adjusts chopper duty cycle based on error

2. PWM (Pulse Width Modulation):

   • Varies average voltage by changing pulse width

   • Efficient and widely used

3. Current Limit Control:

   • Prevents overcurrent

   • Protects motor and converter

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7. 🔊 Chopper Amplifier

What is a Chopper Amplifier?

• An amplifier that modulates power to the motor using chopper techniques.

• Converts low-power control signal into a high-power signal for motor drive.

Functions:

• Amplifies command input (voltage/current)

• Converts it into high-power variable DC using chopper

• Provides isolation and fast switching

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8. 🛠️ Practical Applications

Application	DC Motor Role	Chopper Role
Electric Trains	High torque for traction	Smooth speed regulation
Electric Vehicles	Acceleration and regenerative braking	Efficient power control
Industrial Conveyors	Load handling and speed control	Adaptive voltage regulation

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9. 🧪 Summary

• DC motors are effective for propulsion due to their torque and speed control.

• Armature voltage control allows precise speed regulation.

• Chopper circuits (step-up/down) are key in DC voltage control.

• Control strategies like PWM and feedback improve performance.

• Chopper amplifiers act as power converters between control systems and the motor.

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📝 Review Questions

1. Explain how a step-down chopper controls the speed of a DC motor.

2. What is the difference between step-up and step-down chopper?

3. Why is armature voltage control preferred over field control in traction systems?

4. What is the role of a chopper amplifier in propulsion?

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