Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Define the basic components of DC and AC motors and their functions.
• Explain the operating principle of a DC motor using Fleming's Left-Hand Rule.
• Explain the difference between series, shunt, and compound wound DC motors.
• Identify common applications of DC motors in South Africa.

Lesson summary

Electrical motors are absolutely fundamental to modern life in South Africa, powering everything from the pumps that bring us clean water and the fans that keep us cool in summer, to the machinery in factories that produce the goods we use every day and the vehicles that transport us. Understanding how motors work, how they're constructed, and where they're used is crucial for anyone interested in a career in electrical technology. South Africa's industrial sector and infrastructure development rely heavily on skilled technicians who can install, maintain, and repair these vital machines.

Lesson notes

2. 1. Fundamental Principles of Electric Motors All electric motors work on the principle of electromagnetic induction. A current-carrying conductor placed within a magnetic field experiences a force. This force is what causes the motor to rotate. 2.1.

1. Fleming's Left-Hand Rule: This rule helps determine the direction of the force on a current-carrying conductor within a magnetic field. Hold your left hand with your thumb, forefinger, and middle finger mutually perpendicular.

Forefinger: Represents the direction of the Field (North to South).

Middle finger: Represents the direction of the Current (conventional current, positive to negative).

Thumb: Represents the direction of the Motion (or Force) acting on the conductor. 2.1.

2. Torque: Torque is the turning force that causes rotation. It is the product of the force acting on the conductor and the perpendicular distance from the axis of rotation. Torque (T) = Force (F) x Radius (r). 2.

2. DC Motor Construction A DC motor consists of the following key components: Stator: The stationary part of the motor, which provides the magnetic field. This can be created using permanent magnets or electromagnets (field windings).

Rotor (Armature): The rotating part of the motor, consisting of coils of wire wound around an iron core. These coils are connected to the commutator.

Commutator: A segmented cylindrical device that reverses the current direction in the armature windings as the rotor rotates. This ensures that the force acting on the armature conductors always produces torque in the same direction, enabling continuous rotation.

Brushes: Stationary conductors (typically made of carbon) that make electrical contact with the rotating commutator, allowing current to flow into and out of the armature windings.

Field Windings: Coils of wire wrapped around the field poles to create a magnetic field when current flows through them. 2.

3. DC Motor Types DC motors are classified based on how the field windings are connected to the armature windings: Series Wound DC Motor: The field winding is connected in series with the armature winding. This means the same current flows through both. Series motors have very high starting torque but poor speed regulation. As the load increases, the speed decreases significantly, and the current increases substantially.

Application Example in South Africa: Starter motors in vehicles (high starting torque to crank the engine).

Shunt Wound DC 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 regardless of the load, but lower starting torque compared to series motors.

Application Example in South Africa: Pumps in water treatment plants (constant speed is important for consistent flow rates).

Compound Wound DC Motor: A combination of both series and shunt windings. This provides a balance between high starting torque and good speed regulation.

There are two types: cumulative compound (series field aids the shunt field) and differential compound (series field opposes the shunt field). Cumulative compound motors are more common.

Application Example in South Africa: Elevators in buildings (requires both high starting torque for initial movement and good speed regulation for smooth operation). 2.

4. AC Motor Construction (Brief Intro - More Detail Later) While this week focuses on DC motors, it's important to touch briefly on AC motors for context.

Stator: Contains the windings that create a rotating magnetic field when AC current is applied.

Rotor: There are two main types: Squirrel Cage Rotor: Consists of conducting bars embedded in slots in a laminated iron core, short-circuited at both ends by end rings. Simple and robust.

Wound Rotor: Has windings similar to the stator, with slip rings connected to external resistors. Allows for adjusting starting torque and speed. 2.

5. Worked Examples Example 1: Applying Fleming's Left-Hand Rule Imagine a wire carrying current upwards (conventional current) placed in a magnetic field directed from left to right. Use Fleming's Left-Hand Rule to determine the direction of the force on the wire.

Forefinger (Field): Points from left to right.

Middle Finger (Current): Points upwards.

Thumb (Motion/Force): Points outwards, away from you.

Therefore, the force on the wire is directed outwards.

Example 2: Calculating Torque A motor exerts a force of 50 N at a radius of 0.1 meters. Calculate the torque produced by the motor. Torque (T) = Force (F) x Radius (r) T = 50 N x 0.1 m T = 5 Nm (Newton-meters)

Example 3: Identifying Motor Type based on Characteristics A motor has very high starting torque, but its speed drops significantly when a load is applied. Which type of DC motor is most likely being used?

Answer: A series wound DC motor. The high starting torque is a key characteristic of series motors, and their speed is highly dependent on the load.