Basic Electrical and Electronic Fundamentals
Basic Electrical and Electronic Fundamentals
This domain tests circuit laws, AC quantities, power and power factor, three-phase relationships, and the semiconductor and logic basics used on machine panels. Numeric questions are common, so be ready to apply Ohm's law and the power equations quickly and in the right units.
Core rules
- Ohm's law V = IR; power P = VI = I²R = V²/R. Series resistances add; parallel follows 1/R = 1/R1 + 1/R2.
- AC: RMS = peak/√2, so a 325 V peak is about 230 V RMS. Single-phase power P = VI cos φ; three-phase P = √3 V(line) I(line) cos φ.
- Star connection: line voltage = √3 × phase voltage, line current = phase current. Delta: line current = √3 × phase current, line voltage = phase voltage.
- Diode conducts anode-positive; a bridge rectifier gives full-wave DC; a transistor works as a switch when driven into saturation or cut-off; a relay isolates the coil circuit from the contact circuit.
- Resistor colour code: black 0 to white 9; gold ±5 %, silver ±10 %.
Worked example
A three-phase 415 V induction motor draws 8 A at power factor 0.85 with efficiency 0.88. Input power = √3 × 415 × 8 × 0.85 = 4886 W. Shaft output = 4886 × 0.88 = 4300 W, about 5.8 hp. Note that ignoring the power factor would overstate the input by about 18 %.
If the same motor were star-connected at 415 V line, phase voltage would be 415/√3 = 240 V, which is why a 415 V delta motor is started in star to reduce starting current to one third.
Traps
Using peak voltage where RMS is required; forgetting cos φ in AC power; mixing phase and line values in star or delta; assuming a diode drops no voltage (about 0.7 V silicon).