ESE Prelims Electrical flashcards

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

Thevenin's theorem

Any two-terminal linear network can be replaced by one voltage source V_th equal to the open-circuit voltage in series with R_th, the resistance seen looking back with sources killed.

Maximum power transfer

A resistive load receives maximum power when its resistance equals the Thevenin resistance of the source; the transfer efficiency at that point is only 50 percent.

Series RLC resonance

At resonance the inductive and capacitive reactances cancel, impedance is purely resistive and minimum, current is maximum, and the resonant frequency is one over two pi root LC.

Norton equivalent

A linear two-terminal network represented by a current source I_N equal to the short-circuit current, in parallel with R_N equal to the Thevenin resistance of the network.

DC machine EMF equation

Generated EMF equals P times flux times speed times total conductors divided by sixty times the number of parallel paths, where P is the number of poles.

Transformer EMF per turn

The RMS induced EMF in a winding is 4.44 times frequency times turns times peak flux, so the volts per turn are the same for primary and secondary.

Induction motor slip

Slip is the synchronous speed minus rotor speed divided by synchronous speed; rotor frequency equals slip times supply frequency, and synchronous speed is 120f divided by poles.

Synchronous impedance method

A method to find synchronous machine voltage regulation in which the open-circuit EMF is computed from terminal voltage plus the drops across synchronous impedance at the load current.

Per-unit impedance base change

When changing base, new per-unit impedance equals old per-unit impedance times the ratio of new to old MVA base times the square of the ratio of old to new voltage base.

Surge impedance loading

SIL equals the square of line voltage divided by surge impedance, the load at which the line's reactive generation exactly balances its reactive absorption.

Ferranti effect

On a lightly loaded or open-circuited long transmission line, the receiving-end voltage exceeds the sending-end voltage because the line charging current raises voltage through series reactance.

Equal-area criterion

A power-system transient stability test in which the accelerating area above the mechanical input must not exceed the available decelerating area below it.

Peak overshoot formula

For a standard second-order system, peak overshoot equals e raised to minus pi zeta divided by root of one minus zeta squared, expressed as a fraction of final value.

Routh-Hurwitz criterion

A test that builds the Routh array from the characteristic polynomial; the number of sign changes in the first column equals the number of roots in the right half-plane.

Steady-state error type

A type-1 open-loop system has zero steady-state error for a step input and a finite error for a ramp; type 0 has finite step error.

Nyquist stability criterion

The number of clockwise encirclements of the minus one point equals closed-loop right-half-plane poles minus open-loop right-half-plane poles.

PMMC instrument

A permanent-magnet moving-coil meter deflects proportional to average current, has a uniform scale, and is used for DC measurements because alternating current gives zero average torque.

Two-wattmeter power factor

In a balanced three-phase load the total power is the sum of both wattmeter readings, and the power factor angle satisfies tan phi equal to root three times their difference over their sum.

Wheatstone bridge balance

The bridge is balanced when the ratio of the two arms on one side equals the ratio on the other, giving zero detector current and an unknown resistance from known values.

Op-amp inverting amplifier

With the non-inverting terminal grounded, the closed-loop voltage gain equals negative feedback resistance divided by input resistance, and the input node is a virtual ground.

De Morgan's theorems

The complement of a sum equals the product of the complements, and the complement of a product equals the sum of the complements of the variables.

Successive approximation ADC

An analog-to-digital converter that tests bits from most significant to least significant, taking one clock cycle per bit and needing n cycles for n-bit resolution.

JK flip-flop toggle

When both J and K inputs are high, the JK flip-flop changes state on each active clock edge, which makes it suitable for binary counter stages.

SCR latching and holding

An SCR conducts after a gate pulse once anode current exceeds the latching current, and it turns off only when anode current falls below the holding current.

Buck chopper voltage

In continuous conduction a step-down chopper gives average output voltage equal to duty ratio times supply voltage, with duty ratio being on-time over period.

V/f induction motor control

Keeping voltage proportional to frequency maintains air-gap flux constant, giving constant maximum torque per ampere below base speed in variable-frequency drives.

Full-wave controlled rectifier

A single-phase fully controlled bridge with continuous current gives average DC output equal to two times peak voltage divided by pi, times cosine of firing angle.

Nyquist sampling theorem

A band-limited signal must be sampled at more than twice its highest frequency to be reconstructed without aliasing; otherwise high frequencies fold to lower apparent frequencies.

Convolution with impulse

Convolving any signal with an impulse located at a given time shifts the original signal to that time, leaving its shape unchanged.

Poynting vector

The cross product of electric and magnetic field intensities gives instantaneous power flow density; its time average for sinusoids includes a factor of one half.

Skin depth

The depth at which current density in a good conductor falls to one over e of its surface value, equal to the square root of two over omega mu sigma.

Voltage standing wave ratio

VSWR equals one plus the magnitude of the reflection coefficient divided by one minus that magnitude; it is one for a matched load and infinite for an open or short.