Industrial Power Utilisation and Drives
Industrial Power Utilisation and Drives
This domain covers electric heating, welding, electrolysis, illumination, traction and the selection and control of motors in industrial drives.
Core rules
- Heating: resistance (direct or indirect), induction (eddy currents in the charge), dielectric and arc furnaces; efficiency and heat losses govern energy cost.
- Welding: arc welding needs a drooping characteristic supply; resistance welding uses I²Rt at low voltage and high current.
- Electrolysis follows Faraday's laws; mass deposited is proportional to charge and electrochemical equivalent.
- Illumination: luminous flux in lumen, illuminance in lux (lumen/m²), efficacy in lumen/W. LED lamps far exceed incandescent efficacy.
- Traction: DC series motors and three-phase induction motors with VFDs; regenerative braking returns energy.
- Drives: motor torque must exceed load torque at every speed. V/f control keeps flux constant; above base speed, field weakening reduces torque.
- Power-factor correction capacitors reduce line current and losses but not the real kW consumed.
Worked example
Heat 50 kg of water from 25 °C to 75 °C using a 3 kW heater at 90% efficiency. Specific heat = 4.186 kJ/kg·K. Energy = 50 × 4.186 × 50 = 10 465 kJ. In kWh: 10 465/3600 = 2.91 kWh. Input energy = 2.91/0.9 = 3.23 kWh. Time = 3.23/3 = 1.08 h, about 65 minutes.
Traps that cost marks
- Forgetting efficiency, or mixing kJ with kWh (1 kWh = 3600 kJ).
- Assuming capacitors reduce the energy bill; they reduce current and losses only.
- Mismatching motor and load torque and expecting a large kW rating alone to carry the load.
- Confusing lux with lumen.