156 unit 27

a certain type of three-phase power connection that exhibits a higher voltage on one leg when measured to ground than on the other legs.

High leg

It is not uncommon for isolation transformers to be designed with windings that have more than one set of lead wires connected to the primary or secondary. These are called

multiple-tapped windings

The ability of one coil to induce a voltage into another coil is called

mutual induction.

one method of color coding the high leg connections of some three-phase power systems.

orange wire

As current increases from zero to its peak positive point, a magnetic field expands

outward around the coil

Impedance is expressed as a

percent (%Z or %IZ)

In a 60-hertz AC circuit, the voltage changes

polarity 60 times per second

The autotransformer does have one disadvantage. Because the load is connected to one side of the powerline, there is no line isolation between the incoming

power and the load.

The primary winding of a transformer is the

power input winding

The amount of core material needed for a particular transformer is determined by the

power rating of the transformer

This can cause problems with certain types of equipment and must be a consideration when designing a

power system.

The current of the secondary induces a countervoltage in the secondary windings that is in opposition to the countervoltage induced in the

primary

When the primary of a transformer is connected to power but there is no load connected to the secondary, current is limited by the inductive reactance of the

primary

All transformation of voltage and current takes place between the

primary and secondary windings.

An autotransformer has only one winding used for both the

primary and secondary.

As a general rule, connecting the secondary windings of transformers in series does not present a

problem

Loads that are intended to operate on 240 volts, such as water heaters, electric-resistance heating units, and central air conditioners are connected directly across the lines of the

secondary

The magnetic field of the primary induces a voltage into the

secondary

Loads that are intended to operate on 120 volts connect from the center tap, or neutral, to one of the

secondary lines

When a coil induces a voltage into itself, it is known as

self induction

Transformer secondary windings connected in

series

Voltage spikes are generally of very

short duration

a transformer that produces a lower voltage at its secondary than is applied to its primary.

step down transformer

a core made by winding a continuous length of magnetic metal into a spiral.

tape wound core/ toroid core

single-phase transformers connected in such a way as to operate on a three-phase system.

3 phase bank

A blank is a magnetically operated machine that can change values of voltage, current, and impedance without a change of frequency

A transformer

transformers that use only one winding for both primary and secondary.

Auto transformers

a three-phase transformer connection with a delta-connected primary and a wye-connected secondary.

Delta wye

A three-phase transformer connection that can provide a six-phase output.

Diametrical connection

a type of insulating oil often used to cool transformers.

Dielectric oil

When calculating the values of an isolation transformer with multiple secondary windings, each secondary must be treated as a

Different transformer

a method of identifying conductors or components.

Tagging

A MEGGER will reveal problems of insulation breakdown that an ohmmeter will

not

a type of wiring diagram that shows basic power runs and does not show actual connections.

one leg diagram

a type of three-phase transformer connection that employs only two transformers.

open delta

The control transformer is used to reduce the line voltage to the value needed to

operate control circuits.

the winding of a transformer to which the power line is connected.

primary winding

. The most common type of control transformer contains two

primary windings and one secondary

The secondary winding should be disconnected before disconnecting the

primary wing

Street lamps are often connected in

series

a transformer that produces a higher voltage at its secondary than is applied to its primary.

step up transformer

Flux leakage is the amount of magnetic flux lines that do not follow the metal core and are lost to the

surrounding air

Transformer impedance is determined by the physical construction of the

transformer

. Another very important rule is that only phase values of voltage and current can be used when calculating

transformer values

All values of a transformer are proportional to its

turns ratio.

the ratio of the number of turns in the primary winding of a transformer as compared to the turns of wire in the secondary winding.

turns ratio.

Series-connected lamps require a single conductor to be connected from lamp to lamp instead of

two conductors

The neutral carries the sum of the

unbalanced load

If power is applied to the transformer bank before the delta connection is closed, the voltmeter should indicate 0

volts

determined by dividing the voltage applied to the primary of a transformer by the number of turns of wire in the primary.

volts per turn ratio

These dots signify that the polarity is the same at that point in time for each

winding

the winding of a transformer to which a load is connected.

Secondary wing

loads that operate on only one phase of an AC power system.

Single phase load

Blank are the most efficient machines known

Transformer

If the rotary switch shown in Figure 27-41 were to be removed and replaced with a sliding tap that made contact directly to the transformer winding, the turns ratio could be

adjusted continuously.

Magnetic materials contain tiny magnetic structures in their molecular material known as

magnetic domains

Isolation transformer is

magnetically coupled, not electrically coupled

If the phase winding of one transformer is reversed, an extremely high current will flow when power is

applied

The tape-wound core is one of the most efficient core designs

available

The polarity of transformer windings can be determined by connecting them as an autotransformer and testing for additive or subtractive polarity, often referred to as a

boost or buck connection

making the final connection to complete a delta connection.

closing a delta

The primary leads of a control transformer are generally cross

connected

a transformer that supplies a constant amount of current to a varying load.

constant-current transformer

a common type of isolation transformer generally used to change line voltage of 240 or 480 volts into 120 volts to operate motor-control equipment.

control transformer

see constant-current transformer.

current regulator

When lamps are connected in series, some device must be used to continue the circuit in the event that one or

more lamps fail

A wye-delta transformer, for example, has its primary winding connected in a wye and its secondary winding connected in a

delta

Three-phase transformers are connected in

delta or wye configurations.

External gauges are generally provided to indicate the level of

dielectric oil, temperature, and internal pressure

he transformer is shown before it is mounted in an enclosure, which will be filled with a

dielectric oil.

a common transformer used by utility companies to change their line voltage into the 240/120 volts necessary to operate homes and small businesses.

distribution transformer

Constant-current transformers contain primary and secondary windings that are movable with respect to

each other

These dots signify that the polarity is the same at that point in time for

each winding

An isolation transformer has its primary and secondary windings that are

electrically separated from each other.

(1) the DC used to produce electromagnetism in the fields of a DC motor or generator, or in the rotor of an alternator or synchronous motor; (2) the AC that flows in the primary of a transformer with no load connected.

excitation current

DC through an inductor rises at an

exponential rate.

When the current decreases from its peak positive point toward zero, the magnetic field

fields

When closing a delta, connections should be checked for proper polarity before making the

final connection and applying power

The countervoltage of the secondary weakens the countervoltage of the primary and permits more primary current to

flow

When a load is connected to the secondary, current begins to

flow

the number of magnetic flux lines that radiate into space and do not cut through the conductors of an inductor or secondary winding of a transformer.

flux leakage

The H core, however, surrounds the windings on

four sides instead of two

Impedance is expressed as a percent (%Z or %IZ) and is measured by connecting a short circuit across the low-voltage winding of the transformer and then connecting a variable voltage source to the

high-voltage winding

It is the winding that is connected to the

incoming power supply

. Some isolation transformers are built with a turn's ratio of 1:1. A transformer of this type has the same

input and output voltage and is used for the purpose of isolation only.

the amount of current that is drawn when an inductive device is first connected to the power line.

inrush current

something made of thin sheets stacked together.

laminated

A film cutout device shorts and maintains the circuit if the

lamp fails

An inductor maintains the circuit if the

lamp fails

. Constant-current transformers are designed to deliver a constant output current, generally 6.6 amperes, under varying

load conditions.

As a general rule, the low-voltage winding is placed closest to the core and the high-voltage winding is placed over the

low-voltage winding.

The amount of inrush current in the primary of a transformer is limited by three factors:

the amount of applied voltage, the resistance of the wire in the primary winding, and the flux change of the magnetic field in the core. The amount of flux change determines the amount of inductive reactance produced in the primary winding when power is applied.

The open-delta transformer connection can be made with only two transformers instead of

three

If three-phase transformation is needed and a three-phase transformer of the proper size and turns ratio is not available, three single-phase transformers can be connected to form a

three-phase bank.

The isolation transformer greatly reduces the

voltage spike

. All windings should be tested for grounds, and the ohmmeter should indicate infinity for each

winding

The secondary winding is the load

winding, or output winding.

A delta-wye transformer would have its primary winding connected in delta and its secondary connected in

wye

The primary winding is first tied into a

wye connection

a three-phase transformer connection that has its primary connected in wye and a secondary connected in delta.

wye delta