TSSPDCL Sub-Engineer flashcards

36 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 V_th in series with R_th, where V_th is the open-circuit terminal voltage and R_th is the resistance seen with independent sources killed.

Superposition theorem

In a linear network with multiple independent sources, the response in any branch equals the algebraic sum of responses caused by each source acting alone, with the others killed.

Series resonance condition

A series RLC circuit resonates when X_L equals X_C, so impedance is purely resistive and minimum; the resonant frequency is f_r = 1/(2π√(LC)) and current is maximum.

Two-wattmeter method

In a balanced three-phase load, total power equals W1 + W2 and the power factor angle is tan⁻¹[√3(W1 − W2)/(W1 + W2)]; a negative reading means power factor below 0.5.

Transformer EMF equation

The RMS induced EMF per winding is E = 4.44 f N Φ_max, where f is frequency, N is turns and Φ_max is peak core flux; volts per turn is equal on both windings.

Maximum efficiency of transformer

A transformer reaches maximum efficiency at the load fraction x = √(P_iron/P_copper at full load), because at that point variable copper loss equals constant iron loss.

DC motor speed relation

For a DC motor, speed N is proportional to (V − I_a R_a)/Φ, so speed falls with load in a shunt motor and can be controlled by armature voltage or field flux.

Induction motor slip

Slip s = (N_s − N_r)/N_s, where N_s is synchronous speed and N_r is rotor speed; rotor frequency equals s times supply frequency, and slip is small at full load.

Star-delta starter torque

Star-delta starting applies 1/√3 of the phase voltage during starting, so starting current and starting torque are each reduced to one-third of their direct-on-line values.

Load factor

Load factor is the ratio of average load to maximum demand over a period; a higher load factor means better utilization of installed plant capacity.

Surge impedance loading

Surge impedance loading is V_L²/Z_c, where Z_c = √(L/C) is the line surge impedance; at this loading the line delivers power at unity power factor with flat voltage profile.

Ferranti effect

On a lightly loaded or open-circuit transmission line, the receiving-end voltage rises above the sending-end voltage because charging current through line inductance adds voltage; shunt reactors compensate.

Per-unit impedance

Per-unit impedance is actual impedance divided by base impedance V_base²/S_base; with bases related by transformer turns ratio, per-unit values are the same on both sides.

Differential protection principle

A differential relay compares currents entering and leaving a protected zone; any difference indicates an internal fault, and the Merz-Price circulating current scheme applies this to generators and transformers.

SCR latching and holding current

Latching current is the minimum anode current needed to keep an SCR on immediately after gate removal; holding current is the lower value below which a conducting SCR turns off.

Three-phase converter output

A fully controlled three-phase bridge gives average DC output V_dc = 1.35 V_L cos α, where V_L is line voltage and α is the firing angle; it can invert when α exceeds 90°.

Buck chopper voltage

A buck (step-down) chopper gives output V_o = D·V_s, where D is the duty cycle and V_s the source voltage; a boost chopper gives V_o = V_s/(1 − D).

V/f induction motor control

Keeping the voltage-to-frequency ratio constant maintains air-gap flux in an induction motor, giving constant torque below base speed; above base speed field weakening gives constant power.

PMMC instrument

A permanent-magnet moving-coil instrument deflects proportional to current, reads average value, works only on DC, and has a uniform scale with high accuracy and sensitivity.

Current transformer safety rule

A current transformer secondary must never be open-circuited while primary current flows, because the loss of secondary mmf causes dangerously high core flux and voltage.

Routh-Hurwitz stability criterion

A linear system is stable if all elements of the first column of the Routh array are positive; the number of sign changes equals the number of right-half-plane poles.

Second-order peak overshoot

Peak overshoot of a second-order system depends only on damping ratio ζ, given by exp(−πζ/√(1 − ζ²)); settling time for a 2% band is about 4/(ζω_n).

Inverse square law of illumination

Illuminance on a surface normal to the light is E = I/d², where I is luminous intensity in candela and d is distance in metres; for an inclined surface use E = I cos θ/d².

Luminous efficacy

Luminous efficacy is the luminous flux produced per unit electrical power, in lumens per watt; LED and high-pressure sodium lamps exceed fluorescent and incandescent lamps.

Tonne of refrigeration

One tonne of refrigeration equals the heat removal rate of 3.5 kW (210 kJ per minute), the latent heat absorbed by melting one tonne of ice in 24 hours.

Drooping arc welding characteristic

Arc welding supplies need a steeply drooping volt-ampere characteristic so that arc length changes cause little current change, keeping the arc stable and the weld uniform.

De Morgan's theorems

De Morgan's laws state that the complement of a sum equals the product of complements, (A+B)' = A'·B', and the complement of a product equals the sum of complements, (AB)' = A' + B'.

Full-wave rectifier ripple factor

A full-wave or bridge rectifier has average output 2V_m/π and ripple factor 0.48, while a half-wave rectifier gives V_m/π and ripple factor 1.21.

JK flip-flop toggle mode

When both J and K inputs of a JK flip-flop are high, the output toggles on every active clock edge, which makes the JK flip-flop useful in counters and frequency dividers.

RCCB function

A residual current circuit breaker trips when the difference between live and neutral currents exceeds its setting, typically 30 mA, protecting people against earth leakage shock.

Power factor correction kVAR

Capacitor rating for power factor improvement is kVAR = kW(tan φ1 − tan φ2), where φ1 and φ2 are the initial and desired power factor angles.

Two-part electricity tariff

A two-part tariff charges a fixed amount based on maximum demand plus a running charge per kWh consumed, so consumers pay for capacity reserved as well as energy used.

Syllogism validity rule

In syllogisms, a conclusion is valid only if it holds in every possible Venn diagram of the given premises; conclusions that hold in only some diagrams are rejected.

Compound interest formula

Compound amount is A = P(1 + R/100)^T where P is principal, R the annual rate and T the number of years; for half-yearly compounding use R/2 and 2T periods.

Cache memory

Cache memory is a small, fast, volatile memory between the CPU and main memory that stores frequently used data and instructions to reduce average access time.

IP address function

An IP address is a unique numeric label assigned to a device on a network, used to identify the host and route packets between networks using the Internet Protocol.