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Basic principles

Devices, symbols, and circuits

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- 1. PART 1 Prepared by Nabeel Hassan Al-Zamror
- 2. Outline 1. Basic principles 2. Devices, symbols, and circuits
- 3. Basic principles
- 4. Objectives 1. To refresh basic electrical concepts 2. To define basic concepts of transformer 3. To refresh single-phase power concepts 4. To refresh three-phase power concepts.
- 5. Basic electrical concepts Electricity is found in two common forms: ◦ AC (alternating current) ◦ DC (direct current). An electrical circuit has the following three basic components irrespective of its electrical energy form: ◦ Voltage (volts) ◦ Ampere (amps) ◦ Resistance (ohms).
- 6. All three are bound together with Ohm’s law, which gives the following relation between the three: V = I × R
- 7. Power In DC circuits, power (watts) is simply a product of voltage and current. P =V × I
- 8. Power For AC circuits, the formula holds true for purely resistive circuits, power is not just a product of voltage and current. Real power or true power is the power that can be converted into work and is measured in watts.
- 9. Power Reactive power If the circuit is of an inductive or capacitive type, then the reactive component consumes power and cannot be converted into work. This is known as reactive power and is denoted by the unit VAR.
- 10. Relationship between powers Apparent power (VA) = V × A True power (Watts) = VA × cosf Reactive power (VAR) = VA × sinf
- 11. Power factor Power factor is defined as the ratio of real power to apparent power. The maximum value it can carry is either 1 or 100(%), which would be obtained in a purely resistive circuit.
- 12. Percentage voltage regulation
- 13. Electrical energy This is calculated as the amount of electrical energy used in an hour and is expressed as follows: Kilowatthour = kW× h
- 14. Types of circuits There are only two types of electrical circuits – series and parallel.
- 15. series circuit A series circuit is defined as a circuit in which the elements in a series carry the same current, while voltage drop across each may be different.
- 16. parallel circuit A parallel circuit is defined as a circuit in which the elements in parallel have the same voltage, but the currents may be different.
- 17. Transformer A transformer is a device that transforms voltage from one level to another. They are widely used in power systems. With the help of transformers, it is possible to transmit power at an economical transmission voltage and to utilize power at an economic effective voltage.
- 18. Schematic diagram of a single- phase transformer
- 19. Potential-induced The ratio of the primary potential to the secondary potential is the ratio of the number of turns in each and is represented as follows:
- 20. step-up and step-down The concepts of step-up and step-down transformers function on similar relation. A step-up transformer increases the output voltage by taking N1 > N2 and a step-down transformer decreases the output voltage by taking N2 > N1 .
- 21. Current-induced When the transformer is loaded, then the current is inversely proportional to the voltages and is represented as follows:
- 22. EMF equation of a transformer rms value of the induced emf in the primary winding is: E1 = 4.44 × f × N1 ×ɸm rms value of the induced emf in the secondary winding is: E2 = 4.44 × f × N2 × ɸm
- 23. Types of transformers
- 24. As per the type of construction a) Core type: Windings surround a considerable part of the core. b) Shell type: Core surrounds a considerable portion of the windings.
- 25. As per cooling type a) Oil-filled self-cooled: Small- and medium-sized distribution transformers. b) Oil-filled water-cooled: High-voltage transmission line outdoor transformers. c) Air Cooled type: Used for low ratings and can be either of natural air circulation (AN) or forced circulation (AF) type.
- 26. As per application a) Power transformer : These are large transformers used to change voltage levels and current levels as per requirement. Power transformers are usually used in either a distribution or a transmission line. b) Potential transformer (PT): These are precision voltage step-down transformers used along with low-range voltmeters to measure high voltages. c) Current transformer (CT): These transformers are used for the measurement of current where the current- carrying conductor is treated as a primary transformer. This transformer isolates the instrument from high- voltage line, as well as steps down the current in a known ratio. d) Isolation transformer : These are used to isolate two different circuits without changing the voltage level or current level.
- 27. Connections of single-phase transformer
- 28. Series connection of two single- phase transformers
- 29. Parallel connection of two single- phase transformers
- 30. Three-phase transformers
- 31. Commonly used three-phases are Three-phase three-wire (delta) Three-phase four-wire (star).
- 32. Delta connection
- 33. Three-phase four-wire star connections
- 34. Output power of a transformer in kW
- 35. Testing transformers
- 36. Testing transformers The following tests are carried out on transformers: Measurement of winding resistance Measurement of Voltage ratio Test phasor voltage relationship Measurement of impedance voltage, short-circuit impedance and load loss Measurement of no load loss and no load current Measurement of insulation resistance Dielectric test Temperature rise.
- 37. Why is transformer rating defined in kVA usual to express the rating of the transformer as a product of the rated voltage and the rated current (VA or kVA). This however does not mean that you can apply a lower voltage and pass a higher current through the transformer. The VA value is bounded individually by the rated voltage and rated current.
- 38. Types of electrical machines DC machines Shunt motor Self excited Separately excited Series motor Compound motor AC machines Squirrel-cage induction motor Wound rotor motor Synchronous motors
- 39. Meters used in troubleshooting
- 40. Multi-range voltmeters
- 41. Clip-around or clamp-on ammeters
- 42. • Electrostatic voltmeters (high- voltage measurements)
- 43. Multimeter or volt-ohm-milli- ammeter (voltage, resistance, current, etc.)
- 44. Thermocouple meters (indirect current measurement)
- 45. True wattmeters (directly measurement of power in watts)
- 46. Digital voltmeters
- 47. Digital frequency meters
- 48. Continuity testers
- 49. Insulation testers
- 50. Logic level probe testers
- 51. Logic analyzers (diagnosing logic systems problems)
- 52. Devices, symbols, and circuits
- 53. Objective 1. To understand basic electrical symbols 2. To understand power and control circuits 3. To read electrical drawings
- 54. Devices and symbols See page 25
- 55. Direct-on-line (DOL) starter power circuit
- 56. Control circuit
- 57. A control circuit is for the automatic control of equipment, for safety interlocking, and sequencing the operations of the plant equipment and machines.
- 58. Control circuits hardware consists of relay contacts, wires, hardware timers, and counters, relay coils, etc. These consist of input contacts representing various conditions; the output coils are energized or de-energized depending on the input conditions represented by the control circuit.
- 59. There are two types of contacts NO (normally open) and NC (normally closed). Input contact: These are contacts of relays, contactors, timers, counter, field instrument switches, pressure switches, limit switches, etc. Output coil: These have two states – On or Off. Output coil can be auxiliary contactor or Main contactor coil.
- 60. ‘AND’ operation circuit Figure shows a simple control circuit (AND operation) with two input contacts (NO) representing two conditions that must be true to complete the circuit to switch on the output relay coil and change the state of output from ‘Off’ to ‘On’.
- 61. ‘OR’ operation circuit Figure shows a circuit with three input contacts (NO) representing that at least one of the three conditions should be true to complete the circuit to switch On the relay coil and change output state from ‘Off’ to ‘On’.
- 62. ‘AND with OR’ operation circuit Figure shows a control circuit, consisting of a combination of AND and OR operations. There are two parallel (OR condition) paths with two input contacts (NO) connected in series in each path representing AND conditions. The path for coil K3 will be completed when one of the path conditions comes true. The circuit then will switch ‘On’ the relay coil and change the output state from ‘Off’ to ‘On’.
- 63. Practical Example Workshop practical.
- 64. Reading and understanding electrical drawings
- 65. Practical Workshop page 28
- 66. End of Part 1

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