Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Identify components of a DC motor and their functions.
• Explain the operating principle of a DC motor using Fleming's Left-Hand Rule.
• Describe different types of DC motors and their applications.
• Understand and calculate back EMF.
• List practical applications of DC motors in South Africa.

Lesson summary

Electric motors are the workhorses of modern society. From powering essential appliances in our homes to driving machinery in industry and operating vehicles, their applications are ubiquitous. In South Africa, understanding electric motors is crucial for addressing challenges in various sectors. For instance, efficient motor usage in agriculture can optimize irrigation systems and reduce energy costs for farmers.

Furthermore, the growing electric vehicle (EV) market presents significant opportunities for skilled technicians and engineers who understand motor technology.

Lesson notes

2. 1. DC Motor Construction and Components A DC motor converts electrical energy into mechanical energy.

Its main components are: Armature: This is the rotating part of the motor. It consists of a laminated iron core with slots that hold the armature winding. The winding is a series of coils of insulated copper wire. The armature carries the current that interacts with the magnetic field to produce torque. The laminations reduce eddy current losses.

Field Windings: These are coils of wire that create the magnetic field in which the armature rotates. They can be wound on the stator (stationary part) of the motor. Field windings can be connected in series, shunt (parallel), or a combination of both with the armature.

Commutator: This is a segmented copper cylinder mounted on the armature shaft. It reverses the direction of current in the armature coils as they rotate. This ensures that the torque produced is always in the same direction, resulting in continuous rotation.

Brushes: These are stationary carbon or graphite blocks that make electrical contact with the commutator. They conduct current from the external DC power supply to the armature winding. The brushes are spring-loaded to maintain good contact with the commutator. The carbon material helps to reduce friction and wear.

Yoke: This is the outer frame or housing of the motor. It provides mechanical support for the stator and protects the internal components. It is usually made of cast iron or steel and provides a 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: Fleming's Left-Hand Rule: Extend your left hand with your thumb, forefinger, and middle finger mutually perpendicular.

Forefinger: Points in the direction of the magnetic field (North to South).

Middle Finger: Points in the direction of the current flowing through the conductor.

Thumb: Points in the direction of the force on the conductor (and therefore the direction of motion). When current flows through the armature winding, each conductor within the magnetic field experiences a force. These forces combine to produce a torque, which causes the armature to rotate. The commutator reverses the current direction in the armature coils as they pass under the brushes. This switching action maintains the torque in a constant direction, resulting in continuous rotation of the armature. 2.

3. Types of DC Motors Different types of 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. Series motors have high starting torque but their speed decreases significantly with increasing load. They are used in applications requiring high starting torque, such as electric trains and cranes.

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. Shunt motors have relatively constant speed under varying loads and are used in applications such as lathes and fans.

Compound Motor: This motor has both series and shunt field windings. It combines the characteristics of both series and shunt motors. Compound motors are used in applications requiring both high starting torque and relatively constant speed, such as elevators and printing presses. 2.

4. Back EMF (Electromotive Force) As the armature rotates in the magnetic field, it cuts magnetic flux lines, inducing a voltage in the armature winding. This induced voltage opposes the applied voltage and is called back EMF (Eb).

Equation for Back EMF: Eb = k Φ * N Eb = Back EMF (Volts) k = Motor constant (determined by the motor's construction) Φ = Magnetic flux per pole (Webers) N = Speed of the armature (revolutions per minute or RPM) The back EMF acts as a self-regulating mechanism. When the motor starts, the back EMF is low, so the armature current is high, producing high starting torque. As the motor speeds up, the back EMF increases, reducing the armature current and limiting the speed. The net voltage across the armature is (Va - Eb), where Va is the applied voltage.

Therefore, the armature current Ia is given by Ia = (Va - Eb)/Ra, where Ra is the armature resistance.