Electrical papers Flashcards
(27 cards)
What is a growler ?
A growler is a device used to test the armature of electric motors for shorted coils. It works by inducing a magnetic field and detecting irregularities using a metal strip or “feeler.”
Internal growler
Definition: An internal growler is designed to be placed inside the armature or stator.
Use Case: Used when the windings or coils are large enough to insert the growler within them.
Test Method: A Magnetic field is induced from the inside of the windings.
Advantage: Often provides a more direct and intense magnetic field on larger equipment.
External Growler
Definition: An external growler is used on the outside of the armature or stator.
Use Case: Suitable for smaller motors or when internal access is not possible.
Test Method: Magnetic field is applied around the windings from the outside.
Advantage: Easier to use without disassembling machinery
Capacitor start single phase induction motor
Working Principle:
Start-Up:
When power is supplied, both the start winding (with capacitor and centrifugal switch) and run winding are energized.
The capacitor creates a phase shift, producing a rotating magnetic field to start the motor.
During Operation:
As the motor reaches around 70–80% of full speed, the centrifugal switch opens.
This disconnects the start winding and capacitor, leaving only the run winding to maintain motion.
Application:
Commonly used in domestic water pump motors due to high starting torque and reliable operation.
Type of starter
Motor starter foward /reverse direct on line
type of diagram
Nema or schematic diagram
Components
1) Stop Push Button
2) Forward)Push Button
3) Reverse Push button
4) Forward latching contact relay
5) Reverse latching contact relay
6) Reverse Interlock contact relay
7) Forward Interlock contact relay
8) Forward coil contact
9 Reverse coil contact
10) Overload contact
Operation of the forward /reverse circuit
The dash lines represents
mechanical Interlocking
What is phase spiltting in single phase motors
Phase spiltting is how single-phase motors get started up since it cannot start for itself it creates a phase difference
Why is phase splitting important?
To start single phase motors without it they would hum and be unable to spin
It allows use everywhere - because of phase splitting we can use single phase motors that are in our fans and washing machines
What is a pilot exciter
It is a small AC generator mounted on the same shaft as the main alternator
Role of the pilot exciter on an alternator
1) It supplies the field current to the main exciter which excites the rotor in the main alternator
2) It eliminates the need for an external Dc supply , making the machine self sufficient and reliable
Two types of excitation systems in alernators
1) Static excitation - Uses a static rectifier and power tools to supply field current directly to the rotor
2) Brushless excitation is more modern and reliable. It uses an exciter alternator and rectifiers mounted on the same shaft to feed DC into the rotor without brushes or slip rings.
reduces maintenance
Five basic steps when servicing a dc generator
1) Disconnect the generator from power
2) Inspect and clean commutator and brushes
3) Check and tighten all electrical connections
4)Lubricate bearing
5)Test insulation resistance and continuity
Types of conductors used in domestic and commercial wiring
PVC-Insulated Copper Cable
Widely used for general wiring in homes and offices. Flexible and flame-resistant.
Armoured Cable (SWA - Steel Wire Armoured)
Common in commercial installations for underground or outdoor use; provides mechanical protection.
Twin and Earth Cable
Common in domestic installations for lighting and socket wiring; includes a live, neutral, and earth conductor.
Flexible Cord (e.g. 3-core flex)
Used for connecting appliances like washing machines, kettles, etc.; has multi-strand conductors for flexibility.
Explain the term mechanically and electrically sound
Electrically sound: The system operates correctly with no faults like shorts or broken circuits.
Mechanically sound: The components are physically intact, secure, and free from damage.
Volt Meter
connection: In parallel with the circuit or component.
Use: Measures voltage (potential difference) between two points used to find voltage drops
Ammeter
Connection: In series with the load.
Use: Measures current flow in amperes (A)
Used to diagnose overloads, check the current draw of motors or appliances.
Insulation Resistance Tester (Megger)
Measures the resistance of insulation materials used in electrical equipment. It applies a high voltage to the insulation and measures the resulting leakage current. Insulation resistance testers are used to assess the quality of insulation and identify potential insulation failures.
Ohm Meter
Connection: Across the component when the power is OFF.
Use: Measures resistance in ohms (Ω).
Used to check for open/short circuits, continuity, or resistor values.
Full-Wave Rectifier (Center-Tap)
Principle: Converts both half-cycles of the AC signal into pulsating DC.
Operation:
The transformer has a centre-tapped secondary winding and two diodes.
During the positive half-cycle, one diode conducts (forward-biased) and the other is off (reverse-biased), and current flows through the load.
During the negative half-cycle, the second diode conducts, and the first is off, but current still flows in the same direction through the load.
Result: Both halves of the AC waveform are used.
Output: A smoother pulsating DC compared to a half-wave.
Semi conductors
Germanium and silicon
What is a slip
This is the difference between the synchronous speed and the actual rotor speed
