Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Describe the construction and function of DC motor components.
• Explain the operating principle of a DC motor.
• Distinguish between series, shunt, and compound DC motors.
• Calculate the back EMF in a DC motor.
• Explain how voltage and current affect motor speed and torque.

Lesson summary

This week, we delve deeper into the fascinating world of electric motors. Motors are ubiquitous in modern society, and a solid understanding of their construction, operation, and applications is crucial for any aspiring electrical technician or engineer.

Think about it: from the washing machine at home to the electric fence protecting livestock, and even the electric trains providing transport in our cities, motors are essential components. The South African economy, with its strong mining and manufacturing sectors, relies heavily on electric motors for various processes. This week's focus will provide you with a robust foundation for understanding these vital machines.

Lesson notes

2.1 DC Motor Construction A DC motor is an electromechanical device that converts electrical energy into mechanical energy.

Its main components are: Armature: The rotating part of the motor, consisting of coils of wire wound around a laminated core. This is where the current interacts with the magnetic field to produce torque. Lamination reduces eddy current losses.

Field Windings: Stationary coils of wire that produce a magnetic field. These can be either wound coils (electromagnets) or permanent magnets. In wound field motors, the field winding is supplied with DC current.

Commutator: A segmented cylindrical device mounted on the armature shaft. It reverses the direction of current flow in the armature coils as the armature rotates. This ensures that the torque is always in the same direction, producing continuous rotation. Each segment connects to a different armature coil.

Brushes: Stationary carbon blocks that make electrical contact with the commutator. They provide a path for current to flow from the external power supply to the armature coils. Carbon is used because it's a good conductor and is soft enough to wear down rather than damage the commutator.

Frame (Yoke): The outer casing of the motor, typically made of steel. It provides mechanical support for the motor components and also serves as a return path for the magnetic flux. 2.2 Principle of Operation The operation of a DC motor is based on the principle that a current-carrying conductor placed in a magnetic field experiences a force. This force is given by Fleming's Left-Hand Rule: Thumb: Direction of Force (Motion)

Forefinger: Direction of Magnetic Field (North to South)

Middle Finger: Direction of Current (Positive to Negative) When current flows through the armature coils, each coil side experiences a force. These forces create a turning moment, or torque, which rotates the armature. The commutator and brushes ensure that the current direction in each coil reverses as it passes through the magnetic neutral plane. This reversal maintains the torque in the same direction, ensuring continuous rotation. 2.3 Types of DC Motors DC motors are classified based on how the field winding is connected to the armature winding: Series Motor: The field winding is connected in series with the armature winding. This means the same current flows through both.

Characteristics: High starting torque, speed varies significantly with load (high speed at light load, low speed at heavy load).

Applications: Starting heavy loads that require high initial torque like cranes, hoists, and electric trains. Be careful! Series motors should NEVER be run without a load, as they can overspeed and damage themselves.

Shunt Motor: The field winding is connected in parallel (shunt) with the armature winding. This means the field current is independent of the armature current.

Characteristics: Relatively constant speed under varying load conditions, moderate starting torque.

Applications: Applications requiring relatively constant speed like lathes, fans, blowers, and centrifugal pumps.

Compound Motor: Has both series and shunt field windings.

Characteristics: Combines the characteristics of series and shunt motors, offering a compromise between high starting torque and speed stability. Can be cumulatively compounded (series field aids shunt field) or differentially compounded (series field opposes shunt field - rare due to instability).

Applications: Elevators, rolling mills, heavy-duty machine tools. Provides good starting torque and reasonable speed regulation. 2.4 Back EMF (E b ) As the armature rotates in the magnetic field, it cuts the magnetic flux, inducing an electromotive force (EMF) in the armature conductors according to Faraday's Law of Electromagnetic Induction. This induced EMF opposes the applied voltage (V) and is called back EMF (E b ). E b = K Φ N Where: E b = Back EMF (Volts) K = Constant depending on motor construction (number of poles, number of conductors, type of winding) Φ = Magnetic flux per pole (Webers) N = Speed of the armature (revolutions per minute, RPM) The armature current (I a ) is determined by the difference between the applied voltage and the back EMF, divided by the armature resistance (R a ): I a = (V - E b ) / R a Example 1: A 220V DC shunt motor has an armature resistance of 0.5 ohms. At a certain load, the back EMF is 210

V. Calculate the armature current.

Solution: I a = (V - E b ) / R a I a = (220V - 210V) / 0.5 ohms I a = 10V / 0.5 ohms I a = 20 A 2.5 Speed and Torque Control Armature Voltage Control: By varying the voltage applied to the armature, the speed of the motor can be controlled. Increasing the armature voltage increases the speed, and vice versa. This method is commonly used for shunt and separately excited motors.

Field Current Control: By varying the field current, the magnetic flux can be controlled, which in turn affects the speed and torque. Decreasing the field current increases the speed, but reduces the torque.