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Questions and Answers
A 3 phase, dual voltage motor has how many leads brought out to its terminal block?
Which of the fire extinguishers is ideal for putting off electric fires?
In a workshop, gangways must be kept clear for which of the following reasons?
In a dc machine, inter-poles are fitted in between the field poles to achieve what?
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Soft solder used for electrical joints is an alloy of which metals?
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What is the synchronous speed of a 4 pole induction motor connected to a 50Hz supply?
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Why are copper and aluminium used in their pure state as electrical conductors?
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Before removing an ammeter from the secondary terminals of a current transformer, what must be done?
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What is armature reaction in a DC machine?
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What is the purpose of connecting a capacitor in parallel with an inductive load?
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Study Notes
Induction Motors
- 4-pole induction motor connected to a 50Hz supply has a synchronous speed of 1500rpm.
Electrical Conductors
- Copper and aluminium are excellent electrical conductors in pure form.
- Alloying materials increase electrical resistance, making them unsuitable.
Current Transformers
- Before removing an ammeter from a current transformer's secondary terminals, short circuit them to prevent dangerous voltage surges.
DC Machines
- Armature reaction is the magnetic field created by armature currents affecting the main field, causing distortion.
- Interpoles are placed between field poles to improve commutation, minimizing sparking at the brushes.
Bearings
- Heat-treat bearings to about 80°C using an oil bath for easier assembly.
Power Factor Correction
- Capacitors connected in parallel with an inductive load improve power factor, reducing losses and increasing efficiency.
Carbon Brushes
- Carbon and copper are commonly used materials in carbon brushes for electrical machines, offering good conductivity and wear resistance.
3-Phase Motors
- A 3-phase, dual voltage motor has 9 leads connected to its terminal block.
Fire Extinguishers
- Class C fire extinguishers are ideal for electrical fires, utilizing non-conductive agents.
Workshop Safety
- Keep gangways clear to allow for free movement and prevent accidents.
Lifting Loads
- Correct lifting technique involves securing a good footing, bending knees, keeping the back straight, and lifting with leg muscles.
Motor Protection
- Motors above 0.37KW require motor starters for safe operation and overload protection.
Meter Movements
- Moving coil meters have a linear scale and are used for measuring DC current.
Series Circuits
- Components connected in series share the same current.
Soft Solder
- Soft solder used for electrical joints is an alloy of tin and lead.
Split Phase Motors
- Capacitor run and start motors are a type of split phase motor.
Temporary Installations
- Electrical installations on construction sites require inspection and testing every 3 months.
Tong Tester
- A tong tester instrument can also be called a clip-on ammeter.
Transformer Core
- Lamination of a transformer core reduces eddy current losses.
Nickel Cadmium Cells
- Advantages of nickel cadmium cells include:
- High discharge rate capability
- Long life cycle
- Tolerance to overcharging
Ohm's Law
- Ohm's Law states that the current (I) flowing through a conductor is directly proportional to the voltage (V) applied across its ends, and inversely proportional to its resistance (R).
- Equation: I = V/R
- To calculate the current flowing through a 50Ω resistor with a 100V voltage:
- I = V/R
- I = 100V / 50Ω
- I = 2A
- To calculate the power (P) dissipated by the resistor:
- P = V * I
- P = 100V * 2A
- P = 200W
Power Factor
- Power factor is the ratio of real power (kW) to apparent power (kVA) in an AC circuit. It represents the efficiency of power utilization, with a value between 0 and 1.
Moisture Absorbent
- Silica gel is used to absorption moisture in a transformer unit.
Inductive Load on Power Factor
- An inductive load of 80% power factor (0.8) leads to:
- Increased current flow
- Higher energy consumption
- Reduced system efficiency
DC Motor Components
- Armature: Rotating part of a DC motor, containing windings that carry current and generate a magnetic field.
- Commutator: Segmented ring attached to the armature, reverses the direction of current in the armature windings to maintain continuous rotation.
Ampere-Hour and Watt-Hour Efficiency
- Ampere-hour efficiency: Ratio of output ampere-hours to input ampere-hours, representing charging and discharging capacity.
- Watt-hour efficiency: Ratio of output watt-hours to input watt-hours, representing overall energy conversion efficiency.
Fuse Purpose
- A fuse is a safety device that protects electrical circuits by interrupting the flow of current when exceeding a predetermined limit.
Lightning Protection - Arcing Horns
- Arcing horns are metal horns mounted above a transformer to protect against direct lightning strikes.
- When struck, the horns create an arc, directing the lightning current to ground, preventing damage to the transformer.
Discrimination
- Discrimination in electrical protection refers to the ability of protective devices to isolate only the faulty section of a circuit without affecting healthy sections. This ensures selective tripping and minimizes disruptions.
Synchronous Speed
- Synchronous speed is the speed at which the magnetic field in an AC motor rotates. It is determined by the frequency of the supply voltage and the number of poles in the motor.
- Equation: Ns = 120f/p (where Ns is synchronous speed in rpm, f is frequency in Hz, and p is the number of poles).
- For a 6-pole machine connected to a 60Hz supply:
- Synchronous speed Ns = (120 * 60Hz) / 6 poles = 1200 rpm.
Slip
- Slip represents the difference between the synchronous speed (Ns) and the actual rotor speed (Nr) of an induction motor.
- Equation: Slip = (Ns - Nr) / Ns
- For a 6-pole machine with a rotor speed of 1000rpm:
- Slip = (1200rpm - 1000rpm) /1200rpm = 0.167 or 16.7%
Lightning Protection - Lightning Arrestors
- Lightning arrestors (surge arrestors) are devices designed to protect electrical equipment from transient overvoltages caused by lightning strikes.
- They divert the surge current to ground, preventing damage to equipment.
Measuring Instruments
- Ammeter: Measures electric current, connected in series with the circuit.
- Voltmeter: Measures voltage, connected in parallel with the circuit.
- Ohmmeter: Measures resistance, connected in parallel with the circuit.
- Megger: Measures insulation resistance, utilizing a high voltage source.
- Tong tester (Clip-on Ammeter): Measures electric current, used on conductors without breaking the circuit.
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Description
Test your knowledge on key concepts in electrical engineering, including induction motors, conductors, transformers, and DC machines. This quiz covers essential principles that every electrical engineer should understand, focusing on both theoretical and practical aspects.
