WBBSE Madhyamik Power (Vocational) flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

Ohm's law

For a linear conductor at constant temperature, voltage equals current times resistance (V = IR). It fails for non-linear devices such as diodes and filament lamps.

Electrical power formula

Power in a DC circuit is P = VI, also written P = I²R or P = V²/R. The unit is the watt, and 1 kW equals 1000 W.

RCCB function

A residual current circuit breaker compares current in phase and neutral and trips when the difference, usually 30 mA, indicates leakage to earth. It protects people.

Electrical fire extinguisher

Electrical fires are Class C. Carbon dioxide or dry powder extinguishers are used because they do not conduct electricity; water must never be used on live equipment.

Parallel circuit rule

In a parallel circuit the voltage is the same across every branch, and the total current is the sum of branch currents. Total resistance is less than the smallest branch resistance.

Kirchhoff's voltage law

Around any closed loop of a circuit, the algebraic sum of all voltage rises and drops is zero. It follows from conservation of energy.

Thevenin's theorem

Any linear two-terminal network can be replaced by one voltage source V_th in series with one resistance R_th, where V_th is the open-circuit voltage.

Power factor

Power factor is cos φ, the ratio of true power in watts to apparent power in volt-amperes. A low value means more reactive current and larger line losses.

Phase and neutral colours

In Indian wiring the phase conductor is red or brown, neutral is black or blue, and the protective earth is green or green-yellow. Fuses go only in phase.

Two-way switch wiring

Two two-way switches control one lamp from two places. Their common terminals go to the lamp and supply, and two traveller wires link the remaining terminals.

Insulation resistance test

A megohmmeter applies a high DC voltage, typically 500 V, to measure insulation resistance in megohms. A healthy low-voltage installation reads at least 1 MΩ.

Pipe earthing

A galvanised iron pipe is buried vertically in the ground and surrounded by charcoal and salt to lower resistance. The earth wire connects to it.

Faraday's law of induction

The emf induced in a coil equals the negative rate of change of magnetic flux linkage, e = −N dΦ/dt. This principle underlies generators and transformers.

Alternator frequency formula

The frequency of an alternator is f = PN/120, where P is the number of poles and N is speed in revolutions per minute. Indian grid frequency is 50 Hz.

Hydroelectric generation

Stored water at height has potential energy. Falling water turns a turbine coupled to an alternator, converting mechanical energy into electrical energy without fuel cost.

Wind power cube law

Available wind power is proportional to the cube of wind speed: P = ½ρAv³Cp. Doubling wind speed increases power about eight times.

Star and delta relations

In star, line voltage is √3 times phase voltage and line current equals phase current. In delta, line current is √3 times phase current and voltages are equal.

Why transmit at high voltage

For a given power, raising transmission voltage lowers current. Since line loss is I²R, higher voltage greatly reduces losses and conductor size.

Distribution transformer connection

A distribution transformer steps 11 kV down to 400/230 V and is usually delta-star connected so that a neutral is available for single-phase consumers.

ACSR conductor

Aluminium conductor steel reinforced has aluminium strands for conductivity around a steel core for tensile strength. It is widely used on overhead transmission lines.

Transformer emf equation

The rms emf of a transformer winding is E = 4.44 f N Φmax. Voltage ratio equals turns ratio, and current ratio is its inverse for an ideal transformer.

Transformer losses

Copper loss is I²R in the windings and varies with load squared. Iron loss is hysteresis plus eddy current and is nearly constant at normal voltage and frequency.

Induction motor slip

Slip s = (Ns − N)/Ns, where Ns is synchronous speed 120f/P and N is rotor speed. The rotor must run slower than synchronous speed to induce current.

DC series motor use

A DC series motor has high starting torque because field and armature currents are the same. It is used in cranes and traction, never run unloaded.

Thyristor turn-off

An SCR starts conducting with a gate pulse when forward biased. It turns off only when anode current falls below the holding current, as in natural AC commutation.

Full-wave bridge rectifier

Four diodes in a bridge rectify both half-cycles, giving twice the ripple frequency of the supply. Average output without a filter is about 0.637 of peak voltage.

TRIAC application

A TRIAC conducts in both directions when triggered, so it controls AC power. It is used in fan regulators and lamp dimmers instead of a rheostat.

Inverter and UPS

An inverter converts DC from a battery into AC. A UPS adds a charger and changeover so the load continues without interruption when mains fails.

Solar panel Wp rating

Peak watt rating is the power output at standard test conditions of 1000 W/m² irradiance and 25 °C cell temperature. Real output is lower in heat and shade.

Betz limit

No wind turbine can extract more than about 59.3% of the kinetic energy in the wind passing through its swept area. This maximum is called the Betz limit.

Energy meter unit

One unit of electrical energy is one kilowatt-hour, the energy used by a 1000 W load running for one hour. It equals 3.6 million joules.

Clamp meter use

A clamp meter measures current by surrounding one conductor and sensing its magnetic field, so the circuit need not be opened or broken for the reading.

Break-even point

Break-even units equal fixed cost divided by selling price minus variable cost per unit. At that volume total revenue exactly covers total cost.