Electrical Engineering was the first subject that made computing people deal with something physical. Years later it quietly comes back: every server rack has a power budget, every data centre has a UPS and generator, and every laptop charger is a small transformer circuit. This article revisits the five units of the JNTUK course with the essentials of each.
Unit 1: DC circuits
The basic quantities
| Quantity | Symbol | Unit | Meaning |
|---|---|---|---|
| Charge | Q | coulomb (C) | Amount of electric charge |
| Current | I | ampere (A) | Rate of flow of charge: I = Q / t |
| Voltage | V | volt (V) | Energy per unit charge, the push behind the current |
| Resistance | R | ohm (Ω) | Opposition to current |
| Power | P | watt (W) | Rate of energy use: P = V × I |
| Energy | E | joule or kWh | Power × time |
Ohm’s law and the two circuit laws
- Ohm’s law: V = I × R. For a fixed resistor, current is proportional to voltage.
- Kirchhoff’s Current Law (KCL): the total current entering a junction equals the total leaving it. Charge is not created or lost.
- Kirchhoff’s Voltage Law (KVL): the sum of voltages around any closed loop is zero. Energy is conserved.
Combining resistors
| Arrangement | Equivalent resistance | Behaviour |
|---|---|---|
| Series | R = R₁ + R₂ + … | Same current through all; voltage divides |
| Parallel | 1/R = 1/R₁ + 1/R₂ + … | Same voltage across all; current divides |
Two equal resistors of 10 Ω in parallel give 5 Ω. Adding branches in parallel always lowers the total resistance, which is why plugging too many devices into one socket draws too much current.
Circuit analysis tools
- Mesh analysis applies KVL to loops; nodal analysis applies KCL to nodes.
- Superposition: in a linear circuit, the effect of several sources is the sum of the effects of each source acting alone.
- Thevenin’s theorem: any linear network, seen from two terminals, behaves like one voltage source in series with one resistor.
- Norton’s theorem: the same idea using a current source in parallel with a resistor.
- Maximum power transfer: a load receives maximum power when its resistance equals the source resistance.
Today: a power supply unit is a source with an internal resistance, and a voltage drop on a long cable is Ohm’s law at work. Data-centre planning is mostly P = V × I and KCL: the current drawn by every server in a rack adds up at the breaker.
Unit 2: AC circuits
Alternating current
AC voltage changes direction continuously, usually as a sine wave: v = Vm sin(ωt), where ω = 2πf. India uses 230 V, 50 Hz.
| Term | Meaning |
|---|---|
| Peak value Vm | Maximum value of the wave |
| RMS value | The DC value that would give the same heating: Vrms = Vm / √2 |
| Frequency f | Cycles per second (Hz); period T = 1 / f |
| Phase | How far one wave is shifted in time relative to another |
The 230 V of the mains is an RMS value; the peak is about 325 V.
R, L and C in AC
| Element | Opposition | Current vs voltage |
|---|---|---|
| Resistor | R | In phase |
| Inductor | XL = 2πfL | Current lags voltage by 90° |
| Capacitor | XC = 1 / (2πfC) | Current leads voltage by 90° |
In a series RLC circuit the total opposition is the impedance Z = √(R² + (XL − XC)²). At resonance, XL = XC, Z is smallest and equals R, and the circuit responds strongly at one frequency: f0 = 1 / (2π√(LC)).
Power in AC
| Power | Formula | Unit | Meaning |
|---|---|---|---|
| Real (active) | P = V I cos φ | W | Power doing useful work |
| Reactive | Q = V I sin φ | VAr | Power swapping between source and fields |
| Apparent | S = V I | VA | What the supply must deliver |
The power factor cos φ = P / S. A low power factor means more current for the same useful power, so heavier cables and higher losses.
Three-phase supply
Three voltages of equal size, 120° apart. Two connections: star (VL = √3 Vph, IL = Iph) and delta (VL = Vph, IL = √3 Iph). Total power is √3 VL IL cos φ. Three-phase delivers power smoothly and uses less copper than single-phase.
Today: the UPS rating in kVA versus kW is apparent versus real power, and the power factor printed on a server PSU (“80 PLUS”) is the same idea. Racks in a data centre are balanced across the three phases so that no phase is overloaded.
Unit 3: Transformers
A transformer transfers AC energy between two windings through a shared magnetic core, using electromagnetic induction (Faraday’s law: a changing flux induces a voltage).
- Voltage ratio: V₁ / V₂ = N₁ / N₂ = a (turns ratio). Current scales the opposite way: I₁ / I₂ = N₂ / N₁.
- Step-up has more turns on the secondary; step-down has fewer.
- It works only with AC, since a steady DC flux induces no voltage.
| Loss | Cause | Reduced by |
|---|---|---|
| Copper loss (I²R) | Resistance of the windings | Thicker conductors |
| Iron loss: hysteresis | Magnetising and demagnetising the core | Good-quality silicon steel |
| Iron loss: eddy current | Circulating currents in the core | Laminated core |
Efficiency = output power / input power, often 95 to 99% for large units. Voltage regulation is the drop in output voltage from no load to full load. The autotransformer uses a single shared winding, which is smaller and cheaper but gives no isolation.
Today: the small black adapter on a laptop charger steps mains down and rectifies it, and the pole-top transformer on your street steps 11 kV down to 230 V and 415 V. Power is transmitted at very high voltage because the same power at higher voltage needs less current, so the I²R loss in the line is far smaller.
Unit 4: Electrical machines
DC machines
A DC generator turns mechanical energy into electrical energy; a DC motor does the reverse. The commutator keeps the output (or torque) in one direction. Generated EMF: E = (P Φ Z N) / (60 A). In a motor, the back EMF Eb = V − IaRa limits the current as speed rises.
| Type | Key feature |
|---|---|
| Shunt | Almost constant speed |
| Series | High starting torque; dangerous with no load |
| Compound | Combines both |
Induction motor
The workhorse of industry. The three-phase stator creates a rotating magnetic field at synchronous speed Ns = 120 f / P. The rotor turns slightly slower; the difference is the slip s = (Ns − N) / Ns. It is rugged, cheap and needs no commutator.
Synchronous machine
The rotor turns at exactly synchronous speed. The alternator is the machine that generates almost all grid power. Synchronous motors can also correct the power factor.
| Machine | Main use |
|---|---|
| DC motor | Traction, cranes, speed control |
| Induction motor | Pumps, fans, compressors, lifts |
| Alternator | Power stations and diesel generators |
| Stepper and servo motors | Precise positioning, printers, robots |
Today: cooling fans in servers are small DC motors, the pumps and compressors in a chiller plant are induction motors, and the backup diesel generator in a data centre is an alternator. Electric vehicles are the biggest modern push in motor design.
Unit 5: Electrical installations and power systems
Generation, transmission and distribution
Power flows: generation (thermal, hydro, nuclear, solar, wind) at around 11 to 25 kV, stepped up to transmission at 132, 220, 400 kV and higher, stepped down to distribution at 11 kV and finally 415/230 V for consumers. Renewable sources and storage are reshaping this flow.
Wiring, protection and safety
| Item | Purpose |
|---|---|
| Fuse | Melts and breaks the circuit under overcurrent (one-time use) |
| MCB | Switch that trips automatically on overload or short circuit, resettable |
| ELCB / RCCB | Trips on earth-leakage current, protecting people from shock |
| Earthing | Safe path for fault current so exposed metal does not stay live |
| Switchgear | Switches, breakers and isolators that control and protect circuits |
Common cable types are PVC-insulated copper or aluminium, chosen by the current they must carry. Overload is more current than the rating for too long; a short circuit is a near-zero resistance path, causing a very large current.
Batteries and energy storage
A battery turns chemical energy into electrical energy. Primary cells are single-use; secondary cells are rechargeable (lead-acid, lithium-ion). Capacity is given in ampere-hours (Ah); energy is capacity × voltage (Wh).
The electricity bill
Energy is billed in units (kWh): power in kW × hours. A 100 W device running 10 hours uses 1 kWh. Tariffs combine fixed charges with energy charges per unit.
Today: a data centre rack at 5 kW running all year uses about 43,800 kWh, which is why power usage effectiveness (PUE) and cloud region choice matter for cost. A UPS is a secondary battery with an inverter, and “grounding” in a server room is the same earthing from this unit.
One-minute recap
| Unit | Key idea | You meet it in |
|---|---|---|
| 1. DC circuits | V = IR, KCL, KVL, series and parallel | PSUs, voltage drop, breaker sizing |
| 2. AC circuits | RMS, impedance, power factor, three-phase | UPS kVA vs kW, rack phase balance |
| 3. Transformers | V₁/V₂ = N₁/N₂, losses, efficiency | Laptop chargers, pole transformers |
| 4. Machines | DC, induction, synchronous motors and alternators | Fans, pumps, diesel generators |
| 5. Installations | Generation to distribution, protection, earthing, kWh | MCBs, earthing, power bills, PUE |
Next in Engineering Revisited: Mathematics – I.
Written by Claude, Anthropic’s AI assistant, as part of Engineering Revisited, a quick refresher of the B.Tech subjects one at a time.