Basic Electricity and A.C. Motor

Electrical Terms & Definitions

Electrical Safety Precautions

• ✔ Turn off the power before working on electrical components.
  ✔ Use insulated tools to prevent electric shock.
  ✔ Avoid water while handling electrical equipment.
  ✔ Wear rubber gloves & safety shoes when working on live circuits.
  ✔ Use proper earthing to avoid leakage currents.
  ✔ Check wires for damage before connecting them.
  ✔ Use correct fuses & circuit breakers to prevent overload.
  ✔ Follow wiring diagrams to ensure correct connections.

Conductors & Insulators

• 🔹 Copper is the most commonly used conductor in electrical wiring due to its high conductivity and durability.

Measuring Instruments

• 💡 Multimeter is a common tool that combines voltmeter, ammeter, and ohmmeter in one device.

• 🔹 Earthing is the process of connecting electrical systems to the earth to prevent electric shocks and protect appliances.
• Types of Earthing:
  • ✔ Plate Earthing – A metal plate (copper or GI) is buried deep in the ground and connected to electrical equipment.
  • ✔ Rod Earthing – A long metal rod (copper or GI) is inserted into the ground.
  • ✔ Pipe Earthing – A perforated pipe is used for better conductivity.
• Importance of Earthing:
  • ✔ Prevents electric shocks by safely directing leakage current to the ground.
    ✔ Protects equipment from voltage fluctuations.
    ✔ Reduces the risk of fire hazards due to electrical faults.
    ✔ Ensures stable voltage levels for electrical appliances.

• 🔹 Earth Resistance is the resistance between the earth and the grounding system. It should be as low as possible for effective earthing.
  🔹 Measured using an Earth Resistance Tester (Megger).
  🔹 Recommended earth resistance:
  ✔ Less than 1Ω for power stations & substations.
  ✔ Less than 5Ω for residential & commercial buildings.

Insulation & Continuity Testing

• ✔ Insulation Test:
  • Done using a Megger (high-voltage insulation tester).
  • Ensures no leakage current in wires & appliances.
  • Higher insulation resistance = better safety.
• ✔ Continuity Test:
  • Done using a multimeter or ohmmeter.
  • Ensures a continuous electrical path in a circuit.
  • Zero resistance means the connection is good.

Effects of Inductor & Capacitor in an AC Circuit

Inductive & Capacitive Reactance

• 🔹 Inductive Reactance (XL):
  Inductors oppose AC current due to self-induced EMF.
• Formula:

$$X_L = 2\pi f L$$

• • where:
    X_L​ = Inductive Reactance (Ohms, Ω)
    $f$ = Frequency (Hz)
    $L$ = Inductance (Henry, H)
• Higher frequency → Higher X_L → More opposition to AC current.
• Capacitive Reactance (X_C):Capacitors oppose AC voltage by charging and discharging.
  Formula:
  $$X_C = \frac{1}{2\pi f C}$$ 
• where:
  XC​ = Capacitive Reactance (Ohms, Ω)
  $C$ = Capacitance (Farads, F)
• ✔ Higher frequency → Lower XC → Less opposition to AC current.

Impedance (Z) in AC Circuits

• 🔹 Total opposition to current flow in an AC circuit is called Impedance (Z).
  Formula for Series RLC Circuit:
  $$Z = \sqrt{R^2 + (X_L – X_C)^2}$$ 

  ​

  • where:$R$ = Resistance (Ω)
    X_L​ = Inductive Reactance (Ω)
    X_C​ = Capacitive Reactance (Ω)
• ✔ If X_L​>X_C​ → Circuit behaves like an inductor (lagging power factor).
• ✔ IfX_C​>X_L​ → Circuit behaves like a capacitor (leading power factor).

Power Factor (PF) & Phase Angle

• 🔹 Power Factor (PF): $$PF=\cos \theta$$
  • where $\theta$ is the phase angle between voltage and current.
• ✔ Pure Resistor → PF = 1 (No phase difference, Ideal case).
• ✔ Inductive Load → Current lags voltage → Lagging PF (Less than 1).
• ✔ Capacitive Load → Current leads voltage → Leading PF (Less than 1).

Single-Phase vs. Three-Phase Supply

• ✔ Single-Phase: Requires a neutral wire for return current.
• ✔ Three-Phase: More efficient, requires less conductor material for the same power.

Star (Y) Connection

• ✔ Phase Voltage V_P​ = Line Voltage V_L​ ÷ √3
  ✔ Phase Current I_P​ = Line Current I_L​
• 🔹 Used in: ✔ Long-distance power transmission
  ✔ Distribution systems

Delta (Δ) Connection

• ✔ Phase Voltage V_P​ = Line Voltage V_L​
  ✔ Phase Current I_P​ = I_L ÷ √3
  • 🔹 Used in:
    ✔ Motors & Transformers
    ✔ Industrial Loads

• Methods to Improve Power Factor
  • 🔹 Why improve Power Factor?
    ✔ Higher PF → Lower electricity bills
    ✔ Reduces power losses in cables & transformers
    ✔ Increases efficiency of electrical equipment

Power Factor Improvement Methods:

Advantages of AC Motor over DC Motor

• ✔ AC motors are widely used in industries due to their low maintenance & high efficiency.

2. Revolving Field Theory

• AC motors create a rotating magnetic field (RMF) in the stator.
• This RMF interacts with the rotor and induces current via electromagnetic induction.
• The rotor chases the RMF, causing rotation.
• 🔹 Key Principle: RMF speed depends on supply frequency $f$ and number of polesP

$$N_s=\frac{120\times f}{P}$$

• • where:
    ✔ Ns​ = Synchronous Speed (RPM)
    ✔$f$ = Frequency (Hz)
    ✔ $P$ = Number of poles

• 3. Phase Splitting Theory
  • Single-phase motors cannot start by themselves as they lack a rotating field.
  • Solution: Create an artificial phase shift using capacitors or inductors.
• Methods of Phase Splitting
  • 1️⃣ Capacitor Method:
    ✔ Uses a capacitor to introduce a 90° phase shift in the auxiliary winding.
    ✔ Common in capacitor-start and capacitor-run motors.
  • 2️⃣ Inductive Method:
    ✔ Uses a high resistance-start winding to create phase difference.
    ✔ Common in split-phase induction motors.

• 4. Torque in AC Motors
• ✔ Starting Torque: Initial torque required to start the motor.
  ✔ Running Torque: Torque required to maintain motion.

5. Single-Phase Induction Motors

• A. Split Phase Induction Motor
• ✔ Construction: Two windings – Starting & Running
  ✔ Working:
• Start winding has high resistance and low inductance.
• Running winding has low resistance and high inductance.
• When AC supply is given, a phase shift creates starting torque.
• ✔ Application: Fans, Blowers, Small machines
• 🔹 Method of Changing Direction of Rotation (DOR)
  • Swap start winding connections to reverse the magnetic field.

• B. Capacitor Start Induction Run Motor
• ✔ Working Principle:
  • Uses a capacitor in series with the start winding to improve phase shift.
  • Once the motor reaches 75% of rated speed, the centrifugal switch disconnects the capacitor.
• ✔ Centrifugal Switch Function:
  • Disconnects the starting capacitor when motor reaches running speed.
  • Prevents overheating & ensures smooth operation.
• ✔ Application: Compressors, Water Pumps, Drills

• C. Capacitor Start Capacitor Run Motor
• ✔ Working Principle:
• Uses two capacitors:
  • Starting Capacitor (High value) – Increases starting torque.
  • Running Capacitor (Low value) – Improves efficiency & power factor.
• ✔ Application: Air Conditioners, Refrigerators

• D. Shaded Pole Motor
• ✔ Construction:
  • One part of the stator has a shading coil (short-circuited loop).
• ✔ Working:
  • The shading coil delays magnetization, creating a weak rotating field.
• ✔ Torque: Very Low
• ✔ Application: Table Fans, Exhaust Fans, Toys

Torque Comparison of Single-Phase Motors

• ✔ Capacitor Start-Capacitor Run Motor has the best performance.

Starter and Its Necessity

• 🔹 Why Use a Starter?
  ✔ AC motors draw high inrush current during startup.
  ✔ Starters limit the starting current to protect the motor.
• Types of Starters
  • 1️⃣ DOL (Direct On-Line) Starter
    ✔ Used for small motors (<5HP)
    ✔ Connects motor directly to supply.
    ✔ Includes thermal overload relay & contactor for protection.
  • 2️⃣ Star-Delta Starter
    ✔ Used for large motors (>5HP)
    ✔ Starts in star mode (low voltage), then shifts to delta mode (full voltage).

Common Faults, Causes & Remedies in Motors

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