Electrical Distribution, Transformers, and Rotating Machines

Vocabulary practice flashcards covering electrical distribution voltage classes, protection equipment, transformer principles and losses, and DC/AC motor and generator operations.

Extra Low Voltage

An AC supply voltage class below $30\,\text{V}$ that does not present a serious threat of electrocution under normal conditions, but is susceptible to voltage drop and unsuitable for large power loads.

Low Voltage

An AC electrical supply voltage class ranging from $30\,\text{V}$ to $750\,\text{V}$ that poses a serious risk to life upon physical contact and is sufficient to supply common residential and commercial power loads.

High Voltage

An AC supply voltage class exceeding $750\,\text{V}$ where voltage is sufficient to arc through the air over small distances to a grounded body, requiring installations to be locked and accessible only to authorized personnel.

Identified Conductor

The grounded circuit conductor that is connected to the earth (via a grounding rod or buried metal pipe); it is the only conductor permitted to be white in colour and measures zero volts to ground.

Grounding Conductor

A bare or green-insulated conductor that connects non-current-carrying metal parts of equipment to earth to provide a low-resistance path that safely carries fault current to trip protective devices; it connects to the identified conductor only at the service entrance.

Feeder

A set of electrical conductors that forms the distribution network extending from the first protective devices to the final overcurrent devices protecting branch circuits.

Branch Circuit

The portion of a wiring system extending beyond the final overcurrent protective devices that delivers electrical energy directly to individual utilization loads.

Motor Control Centre (MCC)

A centralized assembly of enclosures that provides a convenient means to power, control, protect, and lock out electric motors in a facility.

Plug Fuse

A screw-in overcurrent protective device featuring an alloy strip with a low melting point that melts to open the circuit when rated current is exceeded.

Cartridge Fuse

An enclosed cylindrical overcurrent protective device used for current ratings higher than $30\,\text{A}$ and voltages between $125\,\text{V}$ and $600\,\text{V}$, available in ferrule-type and knife-blade designs.

Continuous Current Rating

The maximum current that a protective fuse is designed to carry indefinitely under normal operating conditions without melting.

Interrupting Rating

The maximum short-circuit current that a fuse or circuit breaker can safely interrupt without rupturing or sustaining damage.

Safety Switch

An externally operable disconnect switch designed to manually isolate or disconnect parts of an electric circuit during maintenance or shutdown.

Tie Circuit Breaker

A normally open breaker connecting two independent distribution buses in a double-ended power center to maintain power continuity from one transformer during an emergency without paralleling the sources.

Uninterruptible Power Supply (UPS)

An auxiliary power system consisting of a rectifier, battery bank, inverter, and static transfer switch designed to provide instantaneous, uninterrupted AC power to critical facility loads.

Schematic Diagram

A drawing that uses standardized graphical symbols to illustrate the electrical functions, sequences, and relationships of a circuit without showing physical component sizes or actual locations.

Riser Diagram

A simplified diagram that uses single lines and standard symbols to illustrate the vertical distribution of electrical feeders and panels across multiple floors of a building.

Lenz's Law

A fundamental law stating that an induced electromotive force always produces a current whose magnetic field opposes the change in magnetic flux that induced it.

Self-Inductance

The property of an electrical conductor or coil whereby a counter-electromotive force is induced in itself in opposition to any change in current flow.

Mutual Inductance

The generation of an electromotive force in one coil caused by the changing magnetic flux produced by a neighboring coil; the foundational operating principle of a transformer.

Step-Up Transformer

A transformer in which the secondary winding has more turns than the primary winding ($N_s > N_p$), resulting in a secondary voltage that is higher than the primary voltage ($E_s > E_p$).

Step-Down Transformer

A transformer in which the secondary winding has fewer turns than the primary winding ($N_s < N_p$), producing a secondary voltage lower than the primary voltage ($E_s < E_p$).

Eddy Currents

Circulating currents induced in the iron core of a transformer or machine by alternating magnetic flux, which cause electrical power loss as heat; minimized by assembling the core from thin, insulated laminations.

Magnetic Hysteresis

The lagging of magnetic domain alignment behind an alternating magnetizing field, resulting in core power loss that is minimized by using high-grade silicon steel.

Single-Phase Transformer Configurations

The two primary core configurations for single-phase transformers: core type (where windings surround two iron legs) and shell type (where the iron core surrounds the central winding leg).

Current Transformer (CT)

An instrument transformer designed to step down large primary currents to a safe standard secondary current of $5\,\text{A}$ for metering and protection; its secondary terminals must never be opened while energized.

Potential Transformer (PT)

A specialized step-down instrument transformer used to reduce high system voltages to a standard secondary rating of $120\,\text{V}$ for safe instrumentation and relay operation.

Auto-Transformer

A transformer that utilizes a single continuous tapped winding common to both primary and secondary circuits, offering material savings and high efficiency but lacking electrical isolation.

K-Type Transformer

A transformer engineered with additional primary impedance and enhanced shielding to withstand the severe overheating effects of harmonic currents generated by non-linear loads.

Dielectric Strength

The maximum electric field that transformer insulating oil or solid insulation can withstand without electrical breakdown.

Stator

The stationary outer portion of an electrical machine that houses the field poles and frame in a DC machine, or the laminated iron core and AC windings in an AC machine.

Rotor

The rotating inner component of an electrical machine that carries the armature and commutator in a DC machine, or the rotating field or shorted bars in an AC machine.

Yoke

The heavy outer frame of a DC machine, fabricated from cast steel or iron to provide mechanical support and a low-reluctance magnetic path for field flux.

Commutator

A mechanical cylinder composed of insulated copper segments mounted on the shaft of a DC machine that rectifies alternating currents in the armature coils into direct current for the external circuit.

Armature Reaction

The distortion and shift of the main stator magnetic field caused by the magnetic field produced by current flowing through the armature conductors.

Interpoles

Narrow auxiliary magnetic poles situated between the main field poles of a DC machine connected in series with the armature to neutralize armature reaction and prevent brush sparking.

Shunt DC Motor

A direct-current motor whose field windings are connected in parallel with the armature, providing excellent speed regulation across varying mechanical loads.

Series DC Motor

A direct-current motor whose field windings are connected in series with the armature, producing very high starting torque with a speed that increases dangerously if load is removed.

Compound DC Motor

A direct-current motor combining both series and shunt field windings to achieve high starting torque while maintaining safe, stable speed control under light loads.

Back Electromotive Force (Back EMF)

The counter-voltage generated in the rotating armature conductors of a motor by generator action that opposes the applied voltage and regulates armature current proportional to speed and load.

Alternator

An AC generator typically constructed with a rotating DC-excited electromagnet field and a stationary laminated stator output winding.

Synchronous Speed

The rotational speed of an alternator or the rotating magnetic field of an AC motor, calculated by $N = \frac{60f}{p}$, where $f$ is line frequency and $p$ is pole pairs.

Synchroscope

An instrument that indicates whether an incoming alternator is in phase, running at the correct frequency, and ready to be paralleled with an energized electrical bus.

Squirrel Cage Rotor

An induction motor rotor composed of conducting aluminum or copper bars fitted into slots of a laminated steel core and short-circuited at both ends by heavy end rings.

Wound-Rotor Induction Motor

An AC induction motor featuring insulated wire coils on the rotor connected to slip rings, allowing external variable resistance to be inserted for controlling starting current and torque.

Synchronous Motor

A constant-speed AC motor that operates at exact synchronous speed with its supply frequency; when overloaded beyond its pull-out torque, it stalls and trips rather than slowing down.

Variable Frequency Drive (VFD)

An electronic controller that varies the speed of an AC induction motor by modulating the frequency and voltage of the electrical supply.

Totally Enclosed Fan Cooled (TEFC)

A motor enclosure classification that prevents free exchange of outside air into the motor interior while utilizing an external shaft-driven fan to blow air across the ribbed housing for cooling.

Service Factor (SF)

A multiplier printed on a motor nameplate indicating the permissible percentage of continuous temporary overload the motor can handle under specified conditions without thermal breakdown.

Active Power

The real power measured in watts ($\text{W}$) or kilowatts ($\text{kW}$) that is converted into useful work such as heat, light, or mechanical output, calculated as $P = EI \times \text{Power Factor}$.

Apparent Power

The total electrical power delivered to an AC circuit by the utility, calculated as the direct product of RMS voltage and current ($S = EI$) and expressed in volt-amperes ($\text{VA}$) or kilovolt-amperes ($\text{kVA}$).

Power Factor

The ratio of active power ($\text{kW}$) to apparent power ($\text{kVA}$) in an AC circuit, reflecting the phase displacement caused by inductive or capacitive reactance.