Brief Summary
This lecture discusses DC homopolar machines, their operation as both generators and motors, and introduces the constructional features of formal DC machines with stators and rotors. It covers the generation of DC voltage using a rotating aluminum disc in a magnetic field, the collection of voltage using brushes, and the operation of the same machine as a motor by applying an external voltage. The lecture also introduces the components of a formal DC machine, including the stator with projected poles and field coils, and the rotor (armature) with laminated circular plates, slots, and teeth.
- DC homopolar machines can operate as both generators and motors.
- Formal DC machines consist of a stator with field coils and a rotor (armature) with laminated plates and windings.
- The commutator segments on the rotor help convert AC voltage generated in the coil to DC voltage.
Introduction to DC Homopolar Machines
The lecture begins by revisiting the concept of a simple DC generator using an aluminum disc rotating in a magnetic field. The induced voltage between the center and perimeter of the disc is directly proportional to the magnetic field (B), the square of the radius (R), and the rotations per second (RPS) of the machine. The polarity of the voltage is such that the perimeter is positive and the center is negative. This type of generator is called a homopolar machine, which can be easily constructed unlike single conductor DC machines.
Operation of Homopolar Machines as Generators and Motors
To collect the generated voltage, fixed brushes are used, one at the perimeter and another at the center of the disc. The magnitude of the generated EMF is given by the formula PI * n * B * R^2, where n is the RPS. Homopolar machines can also operate as motors. When operated as a generator, the machine can be modeled as a voltage source with an internal resistance connected to a load resistance. When operated as a motor, an external voltage supply is connected to the brushes, causing current to flow radially through the disc.
Motor Operation and Power Balance
When the machine operates as a motor, the polarity of the applied voltage causes the disc to rotate. As it rotates, each radial line becomes a source of EMF, opposing the inflow of current. The machine's model includes a back EMF (EB) and an internal resistance (r1). The power drawn from the supply (E_supply * I) is equal to the copper loss in r1 (I^2 * r1) plus the mechanical power (EB * I). The lecture emphasizes that this simple DC motor has no complications and its construction is straightforward.
Introduction to Formal DC Machines
The lecture transitions to discussing formal, large-power DC machines, which consist of a rotor and a stator. The rotor has laminated iron circular plates with slots, known as the armature winding. The stator has projected poles. When both rotor and stator coils carry current, they produce force, enabling sustained rotation when the electromagnetic torque balances the opposing torque on the shaft.
Components of a Formal DC Machine: Rotor and Stator
The rotor is made of several laminated circular plates with slots and teeth, where copper conductors or coils are placed. The stator has projected poles. The stator iron is typically solid, unlike induction machine stators, which must be laminated. The stator structure includes coils, and when current is passed through these coils, they create a north and south pole. The terminals of the stator coils are marked as f1 and f2, representing the field coil terminals.
Physical Demonstration and Stator Structure
An actual DC machine armature is shown, highlighting the slots and teeth, and the multi-turn coils placed within the slots. The commutator segments are introduced as a crucial part that helps convert the AC voltage generated in the coil to DC voltage. The rotor rotates on a shaft within the magnetic field created by the stator. The stator has projected poles and is excited with DC current, referred to as the field current.
Field Windings and Armature Structure
The field windings are simple, with projected poles around which coils are wound. These coils are connected in series, and when a DC source is connected, they create a north and south pole, forming the stator poles. The armature, with its slots and teeth, houses coils placed according to a specific logic to achieve DC voltage. The lecture concludes by stating that the constructional features of formal DC machines will be further discussed.
