Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Explain the principle of electromagnetic induction in motors.
• Describe the construction and operation of DC motors (shunt, series, compound).
• Identify the key components of a DC motor.
• Discuss applications of DC motors in South Africa.
• Calculate back EMF and torque of a DC motor.

Lesson summary

Motors are ubiquitous in modern life, especially here in South Africa. From the borehole pumps providing water to our communities to the conveyor belts in our factories and mines, and even the washing machines in our homes, electric motors are critical. A solid understanding of their construction, operation, and applications is essential for anyone pursuing a career in electrical technology. This week, we will focus on the fundamental principles of motor operation, different types of motors, and their common applications within the South African context. Understanding these concepts will enable you to troubleshoot, maintain, and even design electrical systems that rely on motor technology.

Lesson notes

2.1 Electromagnetic Induction and Motor Principle: The operation of an electric motor is based on the principle of electromagnetic induction. Michael Faraday discovered that a conductor carrying a current within a magnetic field experiences a force.

The magnitude of this force is given by: F = B I L Where: F = Force (in Newtons) B = Magnetic flux density (in Tesla) I = Current (in Amperes) L = Length of the conductor within the magnetic field (in meters) This force causes the conductor to move, and by strategically arranging multiple conductors in a coil (the armature) and placing them within a magnetic field created by field windings or permanent magnets, we can create continuous rotational motion. This is the fundamental principle behind all electric motors. The direction of the force can be determined using Fleming's Left-Hand Rule. 2.2 Construction of a DC Motor: A DC motor consists of the following key components: Armature: The rotating part of the motor, containing the conductors that carry the current and experience the force. It is usually made of coils of wire wound on a laminated iron core.

Field Windings (or Permanent Magnets): These create the magnetic field that interacts with the armature current. Field windings are coils of wire wrapped around salient poles. DC motors can have shunt, series, or compound field windings. Permanent magnet DC motors are smaller and simpler but typically have lower torque capabilities.

Commutator: A segmented ring that reverses the direction of the current in the armature coils as they rotate. This is crucial for maintaining continuous rotation in one direction. Each segment of the commutator is connected to a different armature coil.

Brushes: Conductors (usually made of carbon) that make sliding contact with the commutator, providing the electrical connection between the external circuit and the rotating armature.

Yoke: The outer frame of the motor, providing mechanical support and completing the magnetic circuit.

End Shields: Protect the internal components and house the bearings that support the armature shaft. 2.3 Types of DC Motors: DC motors are classified based on how the field winding is connected to the armature winding: Shunt Motor: The field winding is connected in parallel (shunt) with the armature winding. Shunt motors have relatively constant speed under varying loads and are used in applications like lathes, fans, and centrifugal pumps. They offer good speed regulation.

Series Motor: The field winding is connected in series with the armature winding. Series motors have very high starting torque but their speed varies significantly with load. They are used in applications requiring high starting torque, such as cranes, hoists, and electric vehicles. Running a series motor without a load can cause it to run at dangerously high speeds (runaway).

Compound Motor: Has both shunt and series field windings. This combines the characteristics of both shunt and series motors. Compound motors offer a balance of high starting torque and good speed regulation. They are used in applications like elevators, rolling mills, and compressors. There are two main types of compound motors: cumulative compound and differential compound. Cumulative compound motors have series and shunt fields that aid each other, producing high starting torque. Differential compound motors have series and shunt fields that oppose each other, providing excellent speed regulation but lower starting torque. 2.4 Operation of a DC Motor: When a DC voltage is applied to the motor, current flows through the armature and field windings. The interaction between the magnetic field produced by the field windings and the current-carrying armature conductors creates a force, causing the armature to rotate. As the armature rotates, the commutator segments switch the current direction in the armature coils, maintaining the torque in the same direction. As the armature rotates, it also generates a back EMF (electromotive force) which opposes the applied voltage. The back EMF is proportional to the speed of the motor. Back EMF (Eb) = k Φ N Where: Eb = Back EMF (in Volts) k = Motor constant (determined by motor construction) Φ = Magnetic flux per pole (in Webers) N = Speed of the motor (in RPM) The armature current (Ia) is determined by: Ia = (Vt - Eb) / Ra Where: Vt = Terminal voltage (applied voltage) Ra = Armature resistance The torque (T) developed by the motor is proportional to the armature current and the magnetic flux: T = k Φ Ia Where: T = Torque (Nm) k = Motor Constant Φ = Magnetic Flux per pole Ia = Armature current 2.5 Applications of DC Motors in South Africa: Mining Industry: DC motors are used in various mining applications, including driving conveyor belts for transporting ore, powering winches and hoists for lifting materials, and controlling the speed of crushers and grinders.