Vidyut Sahayak (Pareshan) Contractual flashcards

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

Thevenin's theorem

Any linear two-terminal network can be replaced by a single voltage source Vth in series with Rth; Vth is open-circuit voltage and Rth is resistance with independent sources deactivated.

Superposition theorem

In a linear network with multiple independent sources, the response is the algebraic sum of responses caused by each source acting alone, with other sources deactivated.

Series resonance

Occurs when inductive and capacitive reactances are equal; impedance is purely resistive and minimum, current is maximum, and power factor is unity at f = 1/(2π√LC).

RMS value

The effective value of an alternating quantity that produces the same heating effect as an equal DC value; for a sine wave it equals peak divided by √2.

Transformer EMF equation

E = 4.44 f N Φm, where f is frequency, N is turns, and Φm is maximum flux; 4.44 equals 2π divided by √2.

Slip of induction motor

s = (Ns − Nr)/Ns, where Ns = 120f/P; rotor frequency equals slip times supply frequency, and rotor EMF is slip times standstill value.

DC motor speed equation

N is proportional to (V − IaRa)/Φ; speed falls with load due to armature drop, and field weakening increases speed but reduces torque per ampere.

Synchronous motor excitation

Overexcited synchronous motor draws leading current and supplies reactive power; underexcited draws lagging current; it runs only at synchronous speed determined by frequency and poles.

Ferranti effect

On a long lightly loaded transmission line, receiving-end voltage exceeds sending-end voltage because line charging capacitance produces a voltage rise along the line.

Per unit system

Actual quantity divided by base quantity; Z_pu = Z_actual/Z_base and I_base = S_base/(√3 V_base); per-unit values are independent of transformer voltage levels.

Buchholz relay

Gas-operated protective relay installed in an oil-immersed transformer between tank and conservator; it detects incipient faults, oil leakage, and severe faults.

Load flow bus types

Slack bus specifies voltage magnitude and angle; PQ bus specifies active and reactive power; PV bus specifies active power and voltage magnitude.

SCR holding current

Minimum anode current below which a conducting SCR turns off; gate loses control after conduction, so commutation is required to stop it.

Buck converter

DC-DC chopper whose average output voltage V_o = D V_s, where D is duty cycle; it steps down voltage and current is continuous through the inductor.

V/f control

Induction motor speed control method that keeps voltage-to-frequency ratio constant to maintain air-gap flux and avoid saturation; used in variable-frequency drives.

PMMC instrument

Permanent-magnet moving-coil meter measures average DC; it has a uniform scale, high sensitivity, and is used with rectifiers or shunts for AC and current measurement.

Current transformer safety

A CT secondary must never be open-circuited while the primary is energized; an open circuit produces dangerously high voltage and core saturation.

Megger

Instrument used to measure insulation resistance of cables and windings; a high DC voltage is applied and resistance is read in megohms.

Routh-Hurwitz criterion

Construct the Routh array from the characteristic polynomial; the number of sign changes in the first column equals the number of poles in the right half of the s-plane.

PID controller

Proportional term responds to present error, integral term removes steady-state error, and derivative term anticipates future error and reduces overshoot.

Gain margin

Amount by which open-loop gain can be increased before the closed-loop system becomes unstable; measured at the phase crossover frequency where phase is −180°.

Universal gates

NAND and NOR gates are called universal because any Boolean function can be implemented using only NAND gates or only NOR gates.

JK flip-flop

Edge-triggered flip-flop with J and K inputs; J=K=1 toggles output, J=1 K=0 sets, J=0 K=1 resets, and J=K=0 holds.

Op-amp virtual short

Ideal op-amp with negative feedback has equal voltages at inverting and non-inverting inputs and zero input current; used to analyze closed-loop circuits.

RCCB

Residual current circuit breaker detects leakage current to earth; 30 mA sensitivity protects people from electric shock; it does not protect against overload.

Earthing

Connecting exposed metal parts to an earth electrode to keep touch voltage safe; earth resistance should be low and is measured by the fall-of-potential method.

MCB Type C

Miniature circuit breaker whose magnetic trip operates at 5 to 10 times rated current; suitable for inductive loads with moderate inrush current.

Betz limit

Maximum theoretical efficiency of a wind turbine is 59.3% of the kinetic energy in the wind; practical turbines achieve about 35 to 45%.

Fill factor

Ratio of maximum power to the product of open-circuit voltage and short-circuit current of a solar cell; it indicates the quality of the cell.

Load factor

Average load divided by maximum demand over a period; higher load factor means better utilization of plant capacity and lower cost per unit.

Gujarati script

Gujarati is an Indo-Aryan language written left to right in an abugida script derived from Brahmi through Nagari; it is the official language of Gujarat.

Syllogism

Logical reasoning where a conclusion is drawn from two or more premises; validity depends on the form of the argument, not on the truth of the premises.