Power Sources
This domain covers workshop electrical supply, protective devices, motor types, starters, earthing and safe practice.
Supply and protection
Indian practice is 230 V single phase, 415 V three phase at 50 Hz. A fuse or MCB protects the wiring against overload and short circuit, while an ELCB or RCCB tripping at 30 mA protects people against earth leakage. An MCB alone will not save a person from shock. Earthing provides a low-resistance fault path, and the earth conductor (green or green-yellow) must never carry normal current. Under IS 732 the phase colours are brown, black and grey, neutral is blue, and earth is green-yellow.
Motors and starters
Three-phase squirrel-cage induction motors drive most workshop machines; single-phase motors need a capacitor for starting; hand drills and angle grinders use a universal series motor that runs on AC or DC. A direct-on-line starter suits small motors; a star-delta starter reduces starting current and starting torque to one third for larger motors; soft starters and variable frequency drives give controlled acceleration.
Star and delta
In star, line voltage is root three times the phase voltage, so a 415 V supply puts about 240 V across each winding. In delta, line voltage equals phase voltage. Reversing any two of the three supply phases reverses a three-phase motor.
Worked example
A 3.7 kW, 415 V three-phase motor with efficiency 0.85 and power factor 0.8 draws I = P / (root three x V x cos phi x efficiency) = 3700 / (1.732 x 415 x 0.8 x 0.85) = about 7.6 A. Choose cable, MCB and overload relay rated above this, with the relay set near 7.6 A. Reasoning: efficiency divides because input power is larger than the rated output.
Traps that cost marks
- Believing an MCB protects a person.
- Using the earth wire as a neutral.
- Forgetting efficiency and power factor when calculating current.
- Reversing all three phases instead of any two to reverse rotation.