Why a factory needs electricity
Motors, lamps, heaters and computers in a factory all use electrical energy. A circuit is a closed path: a source pushes charges through wires to the machine and back. If the path is broken, nothing flows. The push of the source is the voltage (V). The flow of charge is the current (I), measured in amperes (A).
DC circuits
Direct current (DC) flows in one direction all the time. Batteries and solar panels give DC. Electronic parts, sensors and small controllers need DC.
Ohm's law links the three basics: V = I x R. R is the resistance in ohms. For parts joined one after another (series) the same current flows through all, and resistances add up. For parts joined side by side (parallel) the voltage is the same across each, and the current splits between them.
AC circuits
Alternating current (AC) keeps changing its direction. One full back-and-forth is one cycle. The number of cycles in one second is the frequency, in hertz (Hz). India uses 50 Hz, so one cycle takes 1/50 = 0.02 s. The mains voltage is about 230 V.
Why AC? A transformer can raise or lower AC voltage easily. Power stations send electricity at very high voltage so that little energy is lost in the long wires, then transformers bring it down for factories and homes. Factories also use three-phase AC: three supply wires, each swinging a little later than the other. It gives smooth power for big motors.
Power: P = V x I (watts). Energy: E = P x time. One kilowatt-hour (kWh) is 1000 W used for 1 hour, and it is one "unit" on your electricity bill.
Electrical installations and safety
An electrical installation is all the wiring and safety parts that bring power to machines.
- Wires: thicker wire for bigger current, or it gets hot.
- Fuse or breaker: cuts the circuit when the current is too high (overload) or when wires touch (short circuit). A breaker can be switched on again; a fuse wire melts and must be replaced.
- Earth wire: joins the metal body of the machine to the ground. If a live wire touches the body, the current goes to the ground and the breaker trips, so a person is not shocked.
- Switchboard: one box with the main switch and a breaker for each line.
Safe working: switch off and lock the supply before touching any machine, never use wet hands, and only trained people work on high voltage.
Key formulas and definitions
- V = I x R
- P = V x I
- Energy (kWh) = power (kW) x time (h)
- Frequency f = 1 / T (T = time of one cycle in seconds)
- Series: R total = R1 + R2 + ...
Worked examples
1. A 12 V battery drives a DC motor of resistance 4 ohm. Find the current.
I = V / R = 12 / 4 = 3 A.
2. One cycle of an AC supply takes 0.02 s. What is the frequency?
f = 1 / T = 1 / 0.02 = 50 Hz.
3. A factory heater takes 10 A from a 230 V supply. What is its power?
P = V x I = 230 x 10 = 2300 W = 2.3 kW.
4. The heater runs for 5 hours. How much energy does it use?
E = 2.3 kW x 5 h = 11.5 kWh = 11.5 units.
5. A line has a 16 A breaker on a 230 V supply. What is the largest power it can safely carry?
P = 230 x 16 = 3680 W, about 3.7 kW. More than this makes the breaker trip.
6. Three machines draw 3 A, 4 A and 5 A from the same 230 V line, which has a 10 A breaker. What happens?
Total current = 3 + 4 + 5 = 12 A. This is more than 10 A, so the breaker trips. Fix: move one machine to another line.
Common mistakes
- Thinking AC and DC are the same. DC keeps one direction; AC reverses many times a second.
- Mixing power and energy. Watt is power (a rate); kWh is energy (power x time).
- Thinking a breaker is only for people. It protects the wires from heat; the earth wire protects people.
- Putting too many machines on one line because "the plug fits".