6 Electricals and Electronics

Updated on 01/04/26 by Mananjay MahatoShare on WhatsApp

Electricals and Electronics in RAC (Refrigeration & AC)

These notes cover the essential electrical concepts, components, and troubleshooting facts required for competitive technical exams like DRDO and ISRO for RAC Mechanics.

1. Fundamental Electrical Terms

  • Voltage ($V$): Potential difference measured in Volts.

  • Current ($I$): Flow of electrons measured in Amperes.

  • Resistance ($R$): Opposition to flow, measured in Ohms ($\Omega$).

  • Power ($P$): Rate of work, measured in Watts ($W$).

    • Formula: $P = V \times I$

  • Capacitance ($C$): Ability to store charge, measured in Microfarads ($\mu F$).

  • Frequency: Number of cycles per second, measured in Hertz (Hz). In India, standard is 50 Hz.

2. Essential RAC Electrical Components

Motors and Starters

  • Compressor Motor: Usually Single Phase Induction Motors (Capacitor Start/Run).

  • LRA (Locked Rotor Amps): The high current drawn when the motor starts from a standstill (usually 5-7 times the running current).

  • RLA (Running Load Amps): The normal operating current.

Capacitors

  • Start Capacitor: High capacitance, used briefly to provide starting torque. Disconnected by a relay once the motor reaches 75% speed.

  • Run Capacitor: Lower capacitance, stays in the circuit to improve power factor and motor efficiency.

Relays (Starting Devices)

  • Current Relay: Operates based on high starting current; used in small systems like domestic refrigerators.

  • Potential (Voltage) Relay: Operates based on Back-EMF; used in large commercial AC units.

  • PTC (Positive Temperature Coefficient): A solid-state device that increases resistance as it heats up to cut off the start winding.

Protection Devices

  • OLP (Overload Protector): A bimetallic strip that trips the circuit if the compressor draws excess current or overheats.

  • Thermostat: A temperature-controlled switch that cycles the compressor on/off to maintain the setpoint.

  • LP/HP Cutouts: * Low Pressure (LP): Trips if refrigerant leaks or pressure drops too low.

    • High Pressure (HP): Trips if condenser cooling fails or pressure gets dangerously high.

3. Electronics and Inverter Technology

  • PCB (Printed Circuit Board): The “brain” that processes sensor data (room temp, coil temp) to control the system.

  • Inverter AC Technology: * Uses a VFD (Variable Frequency Drive) to control compressor speed.

    • Converts AC to DC, then back to variable-frequency AC.

    • Benefits: Saves energy, maintains precise temperature, and avoids high LRA spikes.

  • Sensors (Thermistors):

    • NTC (Negative Temperature Coefficient): Resistance decreases as temperature increases. Most common in ACs.

4. Wiring and Troubleshooting Logic

Symptom Possible Electrical Cause
Compressor won’t start (Humming) Failed Start Capacitor or seized bearings.
Compressor trips on OLP Low voltage, overcharge, or failed condenser fan.
Indoor unit works, Outdoor doesn’t Failed contactor or faulty communication wire.
System completely dead Blown fuse, tripped MCB, or faulty Transformer on PCB.

5. Key Knowledge for Exams

  • Phase-Neutral Voltage: Normal range is 220V–240V AC.

  • Star-Delta Starter: Used in very large chillers to reduce starting current.

  • Earthing: Essential for safety; prevents electric shock from “leakage” to the metallic body.

  • Solenoid Valve: An electromagnetic valve used to control refrigerant flow (e.g., in heat pumps or liquid lines).

Do you need a specific deep dive into the star-delta starter or inverter AC circuit diagrams?

6 Electricals and Electronics