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 Glossary of Terms
Glossary of Terms (N-Z)

Houston Transformer Company, Ltd.
Glossary of Terms (N-Z)


Natural Convection: Method of temperature regulation in which the normal convection of ambient air surrounding the transformer will provide its only cooling.

NEMA Enclosure: (National Electrical Manufacturers Association) an enclosure conforming to NEMA specifications, usually constructed of metal that provides some measure of protection against weather and other elements. Different NEMA ratings determine the degree of protection, ranging from "some measure of protection" to "fully weatherproof." Suitable for outdoor use or where indoor location may constitute a shock hazard if connections are left exposed.

Nominal Voltage: The normal or designed voltage level. For three phase wye systems, nominal voltages are 480/277 (600/346 Canada) and 208/120 where the first number expresses phase to phase (or line to line) voltages and the second number is the phase to neutral voltage. The nominal voltage for most single-phase systems is 240/120.

Nonlinear load: A load in which the relationship between current and voltage is not directly proportional.

Network Transformer: Transformer which is electrically and mechanically connected to and coordinated in design with switch-gear or motor control assemblies for use on a utility network power system.

NEMA Standard: Any standard published or sponsored by the National Electrical manufacturers Association (NEMA).

Non-Ventilated Construction: The core and coil assembly is mounted inside an enclosure which has no ventilation openings.

No-load current: The current drawn by a transformer at nominal input voltage in its unloaded (open-circuit) condition. Known also as exciting current or magnetizing current.


Ohm's Law: The relationship between voltage (pressure), current (electron flow), and resistance. The current in an electrical circuit is directly proportional to the voltage and inversely proportional to the resistance. E=IR, or I=E/R, or R=E/I. Where E=voltage, I=current, and R=resistance.

Open Frame: A method of transformer construction utilized when the transformer will be combined with other components inside an enclosure.

Oscillation: The variation, usually with time, of magnitude with respect to a specified reference when the magnitude is alternately greater and smaller than the reference.

Overload: When a transformer’s connected load exceeds the rated output. When overloaded, excessive heat is developed, and insulation system begins to break down prematurely. Life expectancy decreases due to the over heating.


Power Factor: Watts divided by volt amps, kW divided by kVA. Power factor: leading and lagging of voltage versus current caused by inductive or capacitive loads, and harmonic power factor: from nonlinear current.

Primary winding: The coil winding that is directly connected to the input supply.

Peak voltage: Highest voltage measured during an event. Or the maximum voltage obtained from an oscillating voltage wave. With an AC source, this occurs twice and lasts for only a fraction of the cycle. Direct current voltage is considered peak voltage at all times.

Phase: Electrical circuits can be single-phase, three-phase or poly-phase depending on the number of source conductors usually lighting and residential circuits are rated single-phase. Single-phase transformers can be used on a three-phase source when two wires of the three-phase system are connected to the primary of the single-phase transformer. Conversely, three single phase sources can be combined to provide three phase power. Typical power distribution networks combine single phase generators to provide three phase distribution voltages.


Rated Power: The total output power available from all secondary windings, expressed in Volt-amperes (VA) or Kilovolt amperes (kVA).

Reactance: Opposition to changes in flow of alternating current. Capacitive reactance is opposition in change from a capacitor, and inductive reactance is the opposition in change from a coil or other inductor.

Rectifier: An electrical device used to change AC power into DC power.

Regulation: The percentage difference between a secondary winding's output voltage when operating under no-load and open-circuit and full load conditions.

Resin Filled Construction: The core and coil assembly is completely encapsulated with a resin-sand compound and contained in a metal enclosure.


Secondary Winding(s): The coil winding(s) supplying the output voltage to the load(s).

SCR: (Semiconductor, or silicon controlled rectifier) An electronic DC switch which can be triggered into conduction by a pulse to a gate electrode, but can only be cut off by reducing the main current below a predetermined level (usually zero).

Shielding: Imposing a metallic barrier to reduce the coupling of undesirable electromagnetic signals.

Single Phase: (With a three phase source) one or two phase conductors. (Single phase source) A single output which may be center tapped for dual voltage levels.

Sinusoidal Waveform: A waveform that can be expressed mathematically by using the sine function.

Short circuit: A low resistance connection, usually accidental, across part of a circuit, resulting in excessive current flow.

Step up/step-down transformers: A step-up transformer is one in which the output voltage is greater than input voltage. In a step-down transformer, the input voltage is greater than the output voltage.


Transformer Bank / Bank of transformers: Two or more single-phase transformers connected together to supply a three-phase load.

Taps or Voltage Taps: Additional connections to winding allowing different voltages to be obtained from the same winding. Often used on the primary winding to allow the transformer to be used in different countries having different line voltages available.

Temperature Rise: The additional heat, above ambient temperature, that the transformer itself will generate in the normal course of operation.

Test Potential: A voltage applied to a winding to ensure adequate insulation performance.

Three Phase Power: Three separate outputs from a single source with a phase differential of 120 electrical degrees between any two adjacent voltages or currents.

Transformer: An electrical device, which, by electromagnetic induction, regenerates AC power from one circuit into another. Transformers are also used to change voltage from one level to another. This is accomplished by the ratio of turns on the primary to turns on the secondary (turns ratio).

T-connection: A Scott connected three-phase transformer utilizing two primary and two secondary coils called the main and teaser coils.

Transformer Regulation: The percentage difference between voltages at the secondary terminals under no-load condition versus voltage under full-load. This value depends on the load power factor and is usually reported at 1.0 PF and 0.8 PF.

Transient: A high amplitude, short duration pulse superimposed on the normal voltage wave form or ground line.


Voltage Regulation: Maintaining stability of output voltage under conditions of fluctuating input voltage.

Ventilated: Providing circulation of external air.

Ventilated Enclosure: Ventilated Enclosure allows air to flow directly over the core and coil assembly for cooling.

Volt-Amperes: Transformer capacity is rated in volt-amperes (the product of volts and amperes in the input winding). Capacities of very large transformers are rated in thousands of volt-amperes (kVA) and in millions of volt-amperes (MVA). Input VA is equal to output VA.


Working Voltage: The voltage that a winding will operate at, but not necessarily the output voltage of the winding.

Wye: A wye connection refers to a three-phase electrical supply where the source transformer has the conductors connected to the terminals in a physical arrangement resembling a Y. Each point of the Y represents the connection of a hot conductor. The angular displacement between each point of the Y is 120 degrees. The center point is the common return point for the neutral conductor.

Watt: Unit of electrical power when the current in the circuit is one ampere and the voltage is one volt (for DC) and for AC, even the p.f. should be unity.

Weather shield: When added to ventilated enclosures, allow indoor-rated units to be situated outdoors, changing the enclosure rating to NEMA 3R.


Zigzag Transformer: A transformer where the windings are physically inter-connected to achieve specified voltage and current phase relationships. A zigzag winding arrangement is commonly found in harmonic mitigating, phase shifting, or grounding transformers.

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