For competitive exams like DRDO (CEPTAM), ISRO, and NCVT/ITI RAC Mechanic, electricity and motors are high-weightage topics. Here is a condensed, factual guide optimized for quick revision.
Part 1: Basic Electricity Essentials
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Current ($I$): Flow of electrons. Unit: Ampere (A). Measured by Ammeter (connected in Series).
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Voltage ($V$): Electromotive force (EMF). Unit: Volt (V). Measured by Voltmeter (connected in Parallel).
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Resistance ($R$): Opposition to current. Unit: Ohm ($\Omega$). Measured by Ohmmeter.
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Ohm’s Law: $V = I \times R$.
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Power ($P$): Rate of doing work. Unit: Watt (W). $P = V \times I \times \text{Power Factor (for AC)}$.
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Frequency: Number of cycles per second. India Standard: 50 Hz.
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Capacitance: Ability to store charge. Unit: Farad (F) (RAC uses Microfarads $\mu F$).
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Circuit Protection:
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Fuse: Sacrificial wire; melts on overcurrent.
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MCB (Miniature Circuit Breaker): Electromagnetic/thermal switch; resettable.
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OLP (Overload Protector): Specifically protects compressor motors from overheating.
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Part 2: Single-Phase AC Motors in RAC
Since single-phase power cannot create a Rotating Magnetic Field (RMF) on its own, RAC motors use an Auxiliary (Starting) Winding and a Capacitor to create a phase shift.
| Motor Type | Key Characteristics | RAC Application |
| Split Phase | Low starting torque; uses a centrifugal switch to cut out starting winding. | Small fans, old domestic fridges. |
| Capacitor Start Induction Run (CSIR) | High starting torque. Uses a Start Capacitor and a Potential/Current Relay. | Domestic and small commercial compressors. |
| Permanent Split Capacitor (PSC) | No centrifugal switch; Capacitor is always in circuit. High efficiency, low starting torque. | Window/Split AC fan motors, some compressors. |
| Capacitor Start Capacitor Run (CSCR) | Two capacitors (Start + Run). Highest torque and efficiency. | Heavy-duty AC compressors. |
| Shaded Pole | Very low efficiency and torque. No capacitors or switches. | Small freezer fans, evaporator fans. |
Part 3: Motor Components & Testing
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Stator: Stationary part; contains Main Winding (thick wire, low resistance) and Starting Winding (thin wire, high resistance).
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Rotor: The rotating part; usually Squirrel Cage type in RAC.
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Capacitors:
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Start Capacitor: High $\mu F$ (e.g., 80–100), Electrolytic type, short duty cycle.
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Run Capacitor: Low $\mu F$ (e.g., 2–45), Oil-filled type, continuous duty.
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Relays:
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Current Relay: Used in small fridges; works on high starting current.
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Potential (Voltage) Relay: Used in ACs; works on Back-EMF.
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PTC (Positive Temperature Coefficient): Solid-state device; resistance increases with heat to cut off starting winding.
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Pro-Tip for Exams: To identify terminals on a compressor ($C, S, R$):
Measure resistance between all three pairs.
The pair with the highest resistance is $S$ to $R$. The remaining terminal is Common ($C$).
From $C$, the terminal with higher resistance is Start ($S$), and the lower is Run ($R$).
Part 4: Three-Phase Motors (Industrial RAC)
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Usage: Used in large chillers and industrial plants (e.g., Cold Storage).
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Self-Starting: 3-phase motors produce a natural RMF; no capacitors or starting windings needed.
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Star-Delta Starter: Used to reduce high starting current in large motors.
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Phase Prevention: If one phase fails (Single Phasing), the motor will overheat and burn; a Single Phase Preventer is used for protection.
Part 5: Troubleshooting Summary
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Motor Hums but won’t start: Faulty capacitor, seized compressor, or low voltage.
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Motor starts but trips on OLP: Shorted windings, refrigerant overcharge, or restricted capillary.
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Ground Test: Use a Megger or Multimeter to check for continuity between any terminal and the motor body (should be “Open” or Infinite resistance).
Would you like a specific deep dive into the Potential Relay wiring diagram or Capacitor testing procedures?