First Flashcards
What is an Electromechanical Device
This is a device that converts electrical to mechanical and vice versa
2 ElectroMechanical devices Classifications
Static And Dynamic
Give atleast two examples of static machines and two examples of dynamic
Static:
-Transformers
- Rectifier
Dynamic:
- Motors
- Generators
What is a Transformer
A static electrical device that transfers AC electrical power from one circuit to the other at the same frequency but with voltage usually changed.
How are machines classified
-DC Machines
- AC Machines
Why is an AC system used as opposed to a DC one in the transmission and distribution of electric power?
Alternating current (AC) is used in the transmission and distribution of electric power instead of direct current (DC) because it is easier, cheaper, and more efficient. AC voltage can be more easily increased and decreased than DC voltage, which is more complex and expensive to do with. AC can also transmit power over long distances without much energy loss, while DC power can’t
Formula for Resistance in a wire
R = pl/A
Formula for Power
P = IV = (I^2) * R
Types of transformer
- Step Up
- Step Down
- Isolation
- Auto
Characteristics of coolants
- Low Viscosity
- High Flash Point
- Very Low Moisture/ water content
- High specific resistance
Give 4 Applications of Transformers
- To Change the level of voltage and current in electrical power systems
- As impedance matching device for maximum power transfer in low -power electronic and control circuits
- As a coupling device in electronic circuits
- To isolate one circuit from another
- To measure voltage and currents (instrumental transformers)
What is the Basic Principle of Transformers
Electromagnetic Induction
Classes of Concentric winding
- Spiral
-Helical - Crossover
-Continuous disc winding
What are the classes of insulation
- Major Insulation
- The insulating cylinders between the low voltage winding and the core and those between the high voltage and the low voltage windings, the insulating barriers which are inserted between adjacent limbs when necessary and the insulation between the coils and the core yokes etc., fall into the category of major insulation. * These insulating cylinders and barriers are usually made of pressboard or synthetic resin-bonded paper. * These cylinders are made in such a way that they are not only excellent insulators but also mechanically strong. * The permittivity of insulating cylinders is about 4. The major insulation is transformer oil which has a permittivity of 2.2.
- Minor Insulation
The passage explains how transformers insulate individual wire turns and layers:
- Turn insulation: Materials like paper, cotton, or fiberglass tape are wrapped around each wire turn. Fiberglass is only used for air-cooled transformers.
- Impregnation: To improve insulation, windings are soaked in a substance. Traditionally, varnish was used, but now transformer oil is preferred.
-
Oil impregnation process: Here’s a summary of the 3-stage process:
- Pre-dry and shrink the coils.
- Further dry them in a hot air oven until they reach a good insulation resistance.
- Remove any vapor or gas from the core and windings using a vacuum at high temperature (around 100°C).
- Importance of proper insulation: Don’t use too much insulation, as it increases cost and makes heat transfer less efficient.
Explain Bushings
Here’s the summary of transformer bushing construction:
- Bushings: These are insulators that allow electrical conductors to pass through the grounded transformer tank safely.
-
Types based on voltage:
- Up to 66kV: Simpler bushings like porcelain or oil-filled are used.
- Above 66kV: Condenser bushings are used for cost-effectiveness.
-
Oil-filled bushings:
- Have a hollow conductor in a porcelain cylinder filled with oil for insulation.
- Include a glass chamber to show oil level and allow for expansion.
- May use bakelite tubes to prevent oil breakdown from particles.
-
Condenser bushings:
- Use layers of aluminum foil within paper insulation for a more compact design.
