Basic Electrical and Electronics Applications in Mechanical Systems
Basic Electrical and Electronics Applications in Mechanical Systems tests the electrical knowledge a mechanical technician uses around motors, drives, sensors and control panels: Ohm's law, motor starting, protection and transducers.
Core rules
- Ohm's law V = IR; power P = VI = I²R = V²/R. Series: same current, voltages add. Parallel: same voltage, currents add.
- Single-phase power P = VI cos φ; three-phase power P = √3 V_L I_L cos φ. kVA is apparent power; kW is real power.
- Three-phase induction motors are the workshop workhorse. They draw heavy starting current, hence DOL starters for small motors and star-delta starters for larger ones; in star the phase voltage is line voltage ÷ √3.
- Protection: fuse (one-shot), MCB (resettable), overload relay, earthing for safety. The earth conductor is green/yellow.
- Sensors: thermocouple (mV from a temperature difference), RTD/PT100 (resistance rises with temperature), thermistor (large non-linear change), LVDT (displacement), strain gauge (force via resistance change), proximity switch (presence).
- Electronics: a diode rectifies, a transistor switches or amplifies, a relay lets a small signal control a large load.
Worked example
A 415 V, three-phase induction motor draws 5 A at a power factor of 0.8. P = √3 × 415 × 5 × 0.8 = 2875 W ≈ 2.9 kW. Apparent power = √3 × 415 × 5 = 3.59 kVA, which is why cables and switchgear are rated in kVA while energy charges depend on kW.
Traps that cost marks
Multiplying line voltage by current without √3 in a three-phase circuit; treating kVA as kW; confusing the star and delta voltage relations; assuming an MCB protects a person from shock (it protects the circuit - use an RCD/ELCB); wiring a thermocouple as if it were a resistance sensor.