WBCHSE HS Vocational Power flashcards

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

Ohm's law

The current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance, written V = IR for a linear resistive element.

Power factor

The ratio of active power in watts to apparent power in volt-amperes, equal to cos φ, where φ is the phase angle between voltage and current.

Kirchhoff's voltage law

Around any closed loop of a circuit, the algebraic sum of all EMFs and all voltage drops is zero, expressing conservation of energy in the loop.

Series resonance

Condition in an RLC series circuit where inductive and capacitive reactances cancel, giving minimum impedance and a resonant frequency of 1/(2π√(LC)).

Cable voltage drop

The fall in voltage along a cable, approximately (2 × length × current × resistivity)/area for a two-wire run, usually limited to a few percent.

Earth conductor colour

In Indian wiring practice the protective earth conductor is identified by green or green-with-yellow stripes and must never be used as a live or neutral conductor.

Conduit fill factor

The ratio of total conductor cross-sectional area to the internal area of the conduit; it is kept around 40 percent to allow heat dissipation and easy drawing.

Transformer EMF equation

The RMS induced EMF in a winding is E = 4.44 f N Φm, where f is frequency, N the turns, and Φm the peak core flux.

Slip of induction motor

The fractional difference between synchronous speed and rotor speed, s = (Ns − Nr)/Ns; rotor frequency equals slip times supply frequency.

Star-delta starter

A starting method that connects the induction motor stator in star during run-up, reducing phase voltage to 1/√3 and starting torque to one third.

DC series motor caution

A DC series motor develops very high speed at light load because flux falls with current, so it must never be started without mechanical load.

Load factor

The ratio of average load over a period to the maximum demand in that period; a high load factor indicates efficient use of installed generating capacity.

Ferranti effect

On a long, lightly loaded transmission line the receiving-end voltage rises above the sending-end voltage because line capacitance supplies charging current.

Skin effect

At alternating current, current tends to concentrate near the conductor surface, raising effective resistance; it grows with frequency and conductor cross-section.

Per-unit impedance

Impedance expressed as a fraction of a base impedance, Zbase = Vbase²/Sbase; the per-unit value is the same on both sides of an ideal transformer.

Megger test

An insulation resistance test applying a high DC voltage between conductors and earth; a healthy low-voltage installation reads above one megohm.

Energy meter constant

The number of revolutions the disc makes per kilowatt-hour consumed; energy in kWh equals revolutions divided by this constant.

Two-wattmeter method

A method of measuring three-phase power with two wattmeters; total power is W1 + W2 and the power factor follows from their difference and sum.

Current transformer safety

A CT secondary must never be open-circuited while primary current flows, because a dangerous high voltage would be induced across the open terminals.

SCR holding current

The minimum anode current below which a thyristor, once triggered, stops conducting and returns to its blocking state even with gate signal removed.

Buck converter

A DC-to-DC chopper that steps voltage down; its output equals input voltage multiplied by the duty cycle D of the switching device.

IGBT

An insulated-gate bipolar transistor combining MOSFET-like voltage-controlled gate drive with low ON-state conduction loss, widely used in PWM inverters and drives.

V/f control

A variable-frequency drive method that keeps the voltage-to-frequency ratio constant so that motor air-gap flux and maximum torque remain nearly unchanged.

Residual current device

A protective device that trips when the vector sum of live and neutral currents exceeds a set residual value, typically 30 mA for personnel protection.

Class II equipment

Electrical equipment with double or reinforced insulation that does not rely on a protective earth connection for basic safety against electric shock.

Lockout tagout

A safety procedure in which the supply is isolated, locked, and tagged, then the circuit is proved dead before work begins.

HRC fuse

A high rupturing capacity fuse with a silver element in a filled ceramic body, able to interrupt very large short-circuit currents without explosion.

Inverse-square illumination law

Illuminance at a surface from a point source equals luminous intensity divided by the square of the distance, for normal incidence.

Affinity laws for fans

For a centrifugal fan or pump, flow varies with speed, head with speed squared, and shaft power with speed cubed, so small speed reductions save much energy.

Crimping terminal lug

A mechanical joint made by compressing a correctly sized lug onto a stripped conductor with a crimping tool, giving a low-resistance, gas-tight connection.

Break-even quantity

The number of units that must be sold so total revenue equals total cost, calculated as fixed cost divided by contribution per unit.

Udyam registration

The Indian online registration for micro, small, and medium enterprises, which gives access to credit schemes, tender preferences, and delayed-payment protection.