American National Standards Institute. A recognized
body which approves standards for transformers. ANSI
C 57.12 series contains the standards most often used
for dry type transformers.
A transformer cooled by the natural circulation of
air over and/or through the core and coils.
Current (or voltage) Current that alternates
regularly in direction, is periodic and has an average
value (over a period of time) of zero.
Noise Level The existing or inherent sound
level of the area surrounding a transformer installation.
Measured in decibels.
Temperature The temperature of the air
surrounding the transformer.
The current-carrying capacity of an electrical conductor
The practical unit of electric current.
Decrease in signal
voltage or power.
A transformer in which part of one winding is common
to both the primary and the secondary circuits associated
with that winding.
Basic Insulation Level.
A measure of the ability of the insulation system
to withstand very high voltage surges. For example,
a 600 volt class transformer has a 10 KV BIL rating.
Two or more single-phase transformers connected together,
or banked, to supply power. Three single-phase transformers
banked together will produce a KVA capacity of three
times the nameplate rating of the individual single-phase
transformers. For example, three 5 KVA single-phase
transformers connected together for a three-phase
load will have a 15 KVA capacity.
Mark to indicate third party approved or self- certification
to European Community requirements.
Canadian Standards Association. The Canadian
equivalent of Underwriter's Laboratories (UL).
Mark to indicate UL certification to CSA standards.
Same as Centigrade. To convert Centigrade to Fahrenheit,
use the following formula: °F = 1.8 x °C +
A number of turns of conductor wound as a coil.
Transformer A transformer with a turns
ratio which provides a higher rated voltage at no-load
and rated voltage at rated load. Normally used on
units rated 2 KVA or smaller.
Duty The service requirement that demands
operation at a constant load for an indefinite period
Rating The load that a transformer can
handle indefinitely without exceeding the specified
Transformer Usually referred to as an Industrial
Control Transformer. Designed for good voltage regulation
characteristics when low power factor and/or large
inrush currents are drawn (5 to 15 times normal).
Losses Losses in the transformer winding
that are incidental to the carrying of the load. These
losses include those due to resistance as well as
to stray and eddy currents.
The steel that carries the magnetic flux in a transformer.
Loss Losses caused by a magnetization of
the core and its resistance to magnetic flux.
One complete sequence of values of an alternating
quantity, including a rise to maximum in one direction,
a return to zero, a rise to a maximum in the opposite
direction, and a return to zero.
(db) A unit used to express the magnitude of a change
in signal or sound level, either an increase or a
Connection A method used for connecting
the three windings of a three-phase transformer (or
three single-phase transformers). The windings are
connected in series, the three-phase supply being
taken from or supplied to the junctions.
Wye The method of connection for both primary
and secondary windings of a three-phase transformer
Tests A series of tests conducted at a
much higher than rated nameplate voltage to assure
the frequency of insulating materials and electrical
Transformer Any transformer rated between
3 and 500 KVA and a primary voltage of 601 volts or
Wound Transformer See Isolating Transformer.
Isolation Transformer A transformer designed
to withstand the additional heat and mechanical stress
caused by DC drives.
Type Transformer A transformer cooled by
a medium other than a liquid, usually through the
circulation of air.
Winding A winding that consists of two
separate windings which can be connected in series
to handle a specific voltage and KVA or in parallel
to handle the same KVA at one-half the series connected
Currents Additional currents
caused by a time varying magnetic field.
Voltage or Current .707 times the peak
value of AC voltage or current. Effective value is
also designated RMS value (Root Mean Square). When
AC voltage is referred to, the effective value is
understood unless otherwise noted. Symbols "E"
and "I" without subscripts indicate effective
The efficiency of a transformer is the ratio of its
power output to its total power input.
Shield A grounded conductor placed between
the primary and secondary winding to greatly reduce
or eliminate line-to-line or line-to-ground noise.
Often referred to as a Faraday shield.
Current The steady rate current that keeps
the transformer energized after the inrush has dissipated,
with all other windings open-circuited. Also called
magnetizing or no-load current.
Wattage The no-load loss of a transformer.
and FCBN Taps Full Capacity
Above Nominal and Full Capacity Below Nominal. The
FCAN designation is used to indicate that a transformer
will deliver rated KVA when connected to a voltage
source which is higher than rated voltage. The FCBN
designation indicates that a transformer will deliver
rated KVA when connected to a voltage source which
is lower than rated voltage.
Cooled A means of accelerating heat dissipation
to lower the temperature rise of the transformer.
This has the effect of increasing the transformer
The number of complete cycles per unit for a periodic
quantity such as alternating current, sound waves
or vibrating objects.
An overcurrent protective device with a circuit-opening
fusible member which is directly heated and severed
by the passage of overcurrent through it, or by a
A conducting path, whether intentional or accidental,
between an electric circuit or equipment and the earth,
or some other conductor.
Connected to the earth or some other conductor.
Horsepower. Energy required to raise 33,000 pounds
one foot in one minute. Equals 746 watts, or .746
A sinusoidal waveform with a frequency that is an
integral multiple of the fundamental 60 Hz frequency.
waveforms from non-linear loads appear distorted because
the non-linear waveform is the result of adding harmonic
components to the fundamental current.
Distortion Non-linear distortion of a system
characterized by the appearance in the output of harmonic
currents when the input is sinusoidal.
Distortion, Total The square root of the
sum of the squares of all harmonic currents present
in the load, excluding the 60 Hz fundamental. Usually
expressed as a percent of the fundamental.
(Hz) Cycles per second.
Voltage Windings In a transformer with
two windings, designates the winding with the greater
voltage. Usually marked with an "H" designation.
Tendency of a magnetic substance to persist in any
state of magnetization.
Total oppostion of a component
or circuit to the flow of an alternating or varying
current (symbol Z).
That property of a circuit or circuit element
opposing a change in current flow (symbol L). Measured
The power or signal fed into an electrical
device, or to the terminals involved.
Current The initial high peak of current
during the first few cycles of energization which
can be 30 to 40 times the rated current.
Transformer Transformer in which input
winding(s) connected to the line are completely isolated
from those connected to the load.
Material with high electrical resistance.
Device used for supporting or separating
conductors of electricity.
Transformer Another term for isolation
A numerical value taking into account both the magnitude
and frequency of the component of a current waveform.
Used to indicate a full-rated transformer specifically
designed to handle non-linear loads.
(KW) 1,000 Watts.
Kilowatt hour, one kilowatt for one hour.
Kilovolt-ampere, or thousand volt-ampere. When multiplied
by the power factor, will give kilowatts, or KW.
Loads Loads where the current
waveform conforms to that of the applied voltage,
or loads where a change in current is directly proportional
to a change in applied voltage. For example: resistance
heating, incandescent lighting, water heater.
Thin sheets of steel making up the core of the transformer.
Voltage The voltage of the power line.
Loads Loads where the current
waveform does not conform to that of the applied voltage,
or where a change in current is not proportional to
change in applied voltage. For example: computer power
supplies, motor drives, fluorescent lighting.
Construction The core and coil assembly
is mounted inside an enclosure which has no ventilation
Factor (PF) A capacitive
or inductive circuit condition that results in the
applied current leading or lagging the applied voltage.
Voltage The voltage or current of an AC
sinusoidal wave when it reaches its peak or maximum
level. This occurs twice and lasts for only a fraction
of the cycle. Direct current voltage is peak voltage
at all times.
Filled Construction The
core and coil assembly is completely encapsulated
with a resin-sand compound and contained in a metal
Circuit A low resistance
connection, usually accidental, across part of a circuit,
resulting in excessive current flow.
Having the form of a sine (or cosine) wave.
Transformers A transformer can either step
up or step down voltage. A step-up transformer is
one in which the output voltage is greater than the
input voltage. With a step-down transformer, the input
voltage is greater than the output voltage.
Incoming plant voltage varies according to the distance
from the substation and other factors. Taps allow
a distribution transformer to provide secondary voltage
as close as possible to the desired operating voltage.
Taps are usually supplied on the primary winding to
allow matching of the supply voltage to the voltage
rating of the transformer connection. A tap position
above the nominal connection will lower the secondary
output and vice-versa.
Regulation The percentage difference between
voltage 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.
Ratio The relationship between the number
of turns on the transformer's two windings. Voltage
is always transformed in exact accordance to this
ratio. The amperes, or amount of current, changes
in an inverse ratio to the turns ratio. When voltage
increases, current decreases in the same proportion,
Underwriter's Laboratories. A non-profit safety testing
Providing circulation of external air.
Enclosure Enclosure with openings which
allow air to flow directly over the core and coil
assembly for cooling.
Transformers are rated in volt-amperes
(the product of volts and amperes in the input winding).
The capacities of very large transformers are rated
in thousands of volt-amperes (kilovolt-amperes or
KVA) and in millions of volt-amperes (megavolt-amperes
or MVA). For all practical purposes, input KVA is
equal to output KVA.
Unit of electrical power when the current in the circuit
is one ampere and the voltage is one volt.
When added to ventilated enclosures, allow
indoor-rated units to be situated outdoors, changing
the enclosure rating to NEMA 3R.