L19+L20: Electrical Machines Flashcards
What is an Electrical Machine (EM)
An EM can be defined as an electromechanical energy converter. Their design implements the common idea - interaction of the magnetic fields B generated by permanent magnets and/or electromagnets to produce the force F and torque T.
What are the advantages/positives of an EM
- High efficiency
- Less maintenance
- Low inertia J (quick response)
- High reliability
- Simple construction
- Robust construction
- Less expensive
- Long expensive
- 4 Quadrant Operation
- Quiet
Applications of EM’s
Recent advanced in power electronics and high-speed microprocessors have led to considerable attention in EMs with regard to their applications in industrial drives
- Electric Generation
- Machines in Manufactory
- Electric & Hybrid Vehicles
- Home Appliances
How can an EM be made to operate as a motor or generator
EM is a general term for electric motors and electric generators. In otherwise the same electrical machines can operate as either a generator depending on the direction of power flow. EM involves the interchange of energy between electrical system and mechanical systems.
What direction of power flow makes an EM operate as Motor
What direction of power flow makes an EM operate as Motor
How can an EM be operated as an AC machine
The EMs operating on AC supply are called AC machines.
How can an DC be operated as a DC machine
The EMs operating on DC supply are called DC machines.
Why are DC machines preferred to AC machine
- Existence of DC power sources (e.g cars, trucks, aircraft)
- When wide speed range is required
What are some applications from DC Machines
- Traction systems
- Cranes
- Air compressors
- Vacuum cleaner
- Fans
- Blowers
- Conveyors
- Toys
What parts make up the DC Machine’s Structure
- Stator
- Rotor
- Windings
- Enclosure
- Fan
- Brushes
- Commutator
- Bearing
What is the Stator in a DC Machine’s Structure
The stator is a the stationary part of a machine
- It is the electrical circuit that sets up the stator magnetic field
- The stator is either composed of two or more permanent magnet pole pieces to set up stator magnetic field B
- Or the magnetic field B can alternatively be created by an electromagnet. In this case, a DC coil is wound around a magnetic material that forms the stator. And the stator core is made up of several hundred thin laminations.
What makes up the rotor/what is the rotors function in a DC Machine
- The rotor is the inner part of the motor
- It is the rotating part of the electromagnetic circuit
- Rotor is made of ferromagnetic material
- The rotor is composed of windings (called armature windings)
- Armature windings are connected to the external circuit through a mechanical commutator.
How do the Stator and Rotor fit together in a DC Machine
- The stator is mounted inside the frame
- The rotor fits inside the stator with a slight air gap separating it from the stator
- Simply the rotor and the stator are separated by air-gap
What makes the windings/what do the windings do in a DC Machine
Windings are made up of series/parallel connection of coils
* They are usually made of copper
* There are two windings:
- Armature winding: Is the winding on the rotor
- Field winding: Is the winding on the stator
What does the Enclosure do in a DC Machine
The enclosure consists of a frame and two end brackets
- It holds the parts together
- It helps heat dissipation
- Protects internal components from the environment
What does the Fan do in a DC machine
The heat generated due to the copper losses Pcu in the windings is transferred to the surroundings by air circulation within the casing together with cooling air blown by an integral fan along the casing’s exterior.
What do the Brushes do in a DC Machine
The brushes are just two pieces of springy metal or carbon that make contact with the contacts of the commutator.
- The rotor is supplied by DC current through the brushes, commutator and coils
What does the Commutator do in a DC machine
The commutator is connected to the slotted iron core
- It is constructed from a copper ring split into segments with insulating material between the segments
- The commutator mechanically reverses the armature loop connections to the external circuit.
What does the Bearing do in a DC Machine
To maintain the separation between the bearing races to lower μ if two flat surfaces were sliding against each other
Structure of a Permanent Magnet DC Motor (image diagram)
How can DC machines be classified
DC machines can be classified according to the electrical connections for the armature windings and the field windings.
The magnetic field B of the field winding can be formed either by:
* An Electromagnet - temporary magnets which have a magnetic field only when current passes through them
* Permanent Magnet - Is a material/object that produces its own persistent magnetic field.
Summary of what is in a DC Machine’s Structure
