RRB JE CBT 2 Electrical key terms

At any node of a circuit, the algebraic sum of currents entering equals the sum leaving, because charge cannot accumulate at a point. It underpins nodal analysis and current distribution calculations.

Around any closed loop, the algebraic sum of all voltage rises and drops is zero. It expresses energy conservation and is the basis of mesh analysis and loop equations.

Any linear two-terminal network can be replaced by one voltage source in series with one resistance. The source equals the open-circuit voltage and the resistance the input resistance with sources dead.

A linear two-terminal network is replaced by a current source in parallel with a resistance. The current is the short-circuit current and the resistance the same as the Thevenin value.

In a linear bilateral network with several sources, the response in any branch equals the sum of responses caused by each source acting alone, with the others replaced by their internal impedances.

A source delivers maximum power to a load when load resistance equals the source's internal resistance. The efficiency at that point is only fifty percent, so it suits signal rather than power circuits.

The cosine of the phase angle between voltage and current in an AC circuit, equal to real power divided by apparent power. A low value means larger current for the same useful power.

The total opposition a circuit offers to alternating current, combining resistance and reactance as a complex quantity measured in ohms. Its angle gives the phase difference between voltage and current.

The condition in an RLC circuit where inductive and capacitive reactances cancel, leaving impedance purely resistive. Series resonance gives minimum impedance; parallel resonance gives maximum impedance at the resonant frequency.

Apparent power is volts times amperes, active power is the real watts consumed, and reactive power is the wattless component exchanged with reactances. They form a right triangle with power factor as cosine.

The induced electromotive force in a circuit equals the rate of change of magnetic flux linkage. A faster change or more turns gives a larger induced voltage.

The direction of an induced current opposes the change in flux that produced it. It explains back EMF in motors, damping, and the negative sign in Faraday's law.

The voltage generated in a rotating machine's armature that opposes the applied voltage. In a DC motor it limits armature current, so a stalled motor draws dangerously high current.

The distortion of the main field by the magnetic field of armature current. It shifts the neutral axis, weakens the field under load, and is corrected by interpoles or compensating windings.

The reversal of current in a coil as it passes the brush axis in a DC machine. Sparking at brushes indicates poor commutation, often cured by interpoles or brush shifting.

The RMS induced voltage per winding equals 4.44 times frequency, turns and peak flux. It shows voltage ratio follows turns ratio and that a transformer cannot work on DC.

The ratio of primary to secondary turns of a transformer. It fixes the voltage ratio and the inverse current ratio, so a step-up transformer raises voltage while lowering current.

The rise in secondary terminal voltage from full load to no load, expressed as a percentage of rated voltage. Smaller regulation means the output stays steadier as load changes.

The difference between synchronous speed and actual rotor speed of an induction motor, expressed as a fraction of synchronous speed. Slip is zero at no load and rises with load.

The speed of the rotating magnetic field, equal to 120 times frequency divided by number of poles. An induction motor always runs slightly below it, while a synchronous machine runs exactly at it.

The curve relating an induction motor's torque to slip. Torque peaks at the breakdown slip; the motor is stable only on the rising portion below that point.

The tendency of alternating current to concentrate near the surface of a conductor, raising effective resistance. It increases with frequency and conductor size, which is why bundled and hollow conductors are used.

Ionization of air around a high-voltage conductor when surface voltage gradient exceeds breakdown strength. It causes power loss, radio interference and ozone, and is reduced by larger conductor diameter.

The rise of receiving-end voltage above sending-end voltage on a long lightly loaded line, caused by capacitive charging current. It is countered by shunt reactors.

An instrument transformer that steps high current down to a standard low value for metering and relays. Its secondary must never be open-circuited while primary current flows, as dangerous voltage develops.

An instrument transformer that steps high voltage down to a safe standard value for meters and relays. It is connected in parallel with the circuit and works nearly on open circuit.

The total apparent power in volt-amperes that the secondary of a CT or PT must supply to connected instruments. Exceeding rated burden causes ratio and phase-angle errors.

A four-arm resistance network used to measure an unknown resistance by balancing two ratios so that no current flows through the galvanometer. It suits medium resistance values.

A modified bridge with two sets of ratio arms that measures very low resistances while cancelling contact and lead resistance errors. It is used for cable and winding resistance measurement.

A portable instrument that applies a high DC test voltage to measure insulation resistance in megohms. It verifies insulation health of cables, motors and transformers before commissioning.

A device that converts one form of physical quantity, such as temperature or pressure, into an electrical signal for measurement or control. Examples include thermocouples, strain gauges and LVDTs.

A transducer using the voltage generated across a current-carrying semiconductor in a magnetic field. It senses magnetic flux, position, speed and current without contact.

A four-layer semiconductor switch that conducts only when forward biased and gate-pulsed, then latches until current falls below holding level. It is widely used for controlled rectification and power control.

A power electronic circuit that converts fixed DC voltage into variable DC voltage by rapidly switching a semiconductor device on and off. Duty cycle controls the average output.

A converter that changes direct current into alternating current using controlled switching devices. Output frequency and voltage can be adjusted, making it central to variable-speed drives and UPS systems.

The fraction of each switching period during which a device conducts, expressed as a ratio or percentage. In choppers and PWM drives it sets the average output voltage.

Feeding a portion of the output back to the input in opposition to the input signal. It reduces gain but improves stability, bandwidth, linearity and reduces distortion in control systems and amplifiers.

The ratio of the Laplace transform of output to that of input with all initial conditions zero. It characterises a linear system's dynamics, poles and zeros.

The residual difference between desired and actual output as time approaches infinity for a given input type. It depends on system type and error constants, and is reduced by higher loop gain.

A high-gain differential amplifier with very high input impedance and low output impedance. With feedback it performs addition, integration, differentiation, comparison and filtering in analog circuits.

A basic digital circuit performing a Boolean operation such as AND, OR, NOT, NAND or NOR on binary inputs. Combinations of gates form adders, flip-flops and processors.

A programmable integrated circuit that fetches, decodes and executes instructions from memory to perform arithmetic, logic and control operations. It forms the central processing unit of a computer or embedded system.

A switching device that makes and breaks circuits under normal and fault conditions, extinguishing the arc automatically. Unlike a fuse, it can be reset and reused after clearing a fault.

A device that detects abnormal current, voltage or other quantities and commands a circuit breaker to trip. Its sensitivity and speed determine how much equipment damage a fault causes.

Electric railway drive arrangement using DC series or three-phase induction motors fed through converters. Key concerns are tractive effort, speed control, regenerative braking and current collection from overhead equipment.

Production of heat by resistance, induction, dielectric or arc methods for industrial and domestic use. Resistance ovens and induction furnaces are common examples in utilization of electrical energy.

Joining metals by melting them with electric arc or resistance heating, sometimes with filler material. Arc welding uses a low-voltage high-current supply with drooping characteristics.

Design and provision of artificial light, described by luminous flux, intensity, luminance and illumination level in lux. Good design balances glare, colour rendering and energy efficiency.