Transformers

The winding to which voltage is supplied is called the _____ winding.

PRIMARY

The X1 lead is directly across not diagonal from the H1 lead the transformer is

Subtractive

What type of information can be gained by performing a dissolved gas analysis?

the cause of the degradation of insulating fluids

If a transformer has a turn ratio of 10 to 2 and the secondary voltage is 24, what is the applied voltage of the primary?

(10/2 = x/24) = (2x = 240) = (x = 120)

What are the 2 functions served by the liquid in a liquid-immersed transformer?

- cooling\n-dielectric material

3 ways guarding can be accomplished.

- limiting access to equipment\n- use of partitions/barriers\n- elevation to restrict access

----- is a force that acts at a distance and is caused by a magnetic field

Magnatism

If the transformer is used as a _____ transformer, the high voltage winding is the primary.

STEP-DOWN

Two coils wound on a closed iron core form a simple _____.

TRANSFORMER

Taps are typically located on the ----- windings of a transformer.

primary (though they can be on either)

6 things to check in a transformer during an internal inspection.

-damage\n- loose laminations\n- loose nuts and bolts\n- coil damage\n- position of coils\n- build up of dust and contaminants

The voltage tolerance on most motors is ----

10 percent

Dry type transformers installed indoors and not over 112 1/2 kVA, shall have a clearance from combustible materials of at least -----

12 inches

What is a fault?

A partial or total local failure in the insulation or continuity of a conductor.

of the two losses in a transformer, which one will increase as the transformer load goes up.

Copper Losses

With no load on the secondary winding, a small current flows in the primary called _____ _____.

EXCITING CURRENT.

In an autotransformer, tap ----- connects to tap -----

H2\nX1

Anything above 600 volts is _____?

HIGH VOLTAGE

Three major parts of a transformer are

High-voltage winding low -voltage winding core

----- is the property of ferromagnetic materials where the magnetic induction of a coil lags the magnetic field that is charging the coil

Hysteresis

----- is the porperty of a device or circuit that causes it to store energy in a magnetic field.

Inductance

The output or capacity of a transformer is rated in _____ - _____.

KILOVOLT - AMPERES (kVA)

The capacity of a transformer is rated in ____ _____.

KILOVOLT - AMPERES (kVA).

Autotransformers are not safe when they supply a _____ voltage from a high voltage source.

LOW

The transformer winding to which voltage is supplied is called the _____ winding.

PRIMARY

Three conditions that must be met before transformers can be paralleled.

Proper polarity must be observed. Similar primary and secondary voltages Similar % impedance.

What is leg(s)?

Reference to a jumper or conductor, to or from a piece of electrical equipment.

The turns of both windings are cut by the same flux; therefore the induced emf in each turn of both windings is the _____.

SAME

To minimize flux leakage in transformer construction, the primary and secondary windings are placed on the _____ leg(s) of the core.

SAME

In a transformer the _____ current will always flow in such a direction that its magnetizing action will oppose the magnetizing action of the primary winding.

SECONDARY

The transformer winding to which the load is connected is called the _____ winding.

SECONDARY

The winding to which the load is connected is called the _____ winding.

SECONDARY

What three things are transformers used for.

Step Up/ Down Voltage Step Up/Down Current To electrically isolate Facilitate safe transmission of electrical energy over long distances with minimal loss Change Magnitude of Resistance/ Inductance/Capacitance

What is Hysteresis?

The loss of power required to reverse the core magnatism each time the current reverses. This loss is caused by friction between the molecules of iron as these particles are aligned first in one direction and the in the opposite direct as current direction reverses.

What does "rated KVA of a transformer" mean?

The output which can be delivered for the time specified at rated secondary voltage and rated frequency without exceeding the specified temperature limitations.

What does transformer efficiency mean?

The ratio of useful power output to its total power input.

What is magnetic flux?

The total numbre of lines or force issuing from a pole. (north or south)

What does 'ratio of a transformer" mean?

The turns ratio between the windings (primary & secondary) of a transformer unless otherwise specified.

What is primary winding?

The winding of a transformer to which electrical energy is supplied.

two 100kVA transformers are paralleled. Transformer 1 has 5% Z and transformer 2 has 3%. which one will carry more then its fair share of the load?

Transformer 2 will carry more due to its lower impedance rating

What are class 400 transformers?

Transformers designed for 4800 volt primary operation, but the secondaries are connected 240/480 volts. Winding ratio is 10:1

What are class 200 transformers?

Transformers with 4800 volt primaries and 120/240 volt secondaries, either pole or subway type, with two secondary windings terminating at internal links which provide for parallel operation of the secondary windings at 120 volts or series operation at 120/240 volts. Winding ration in series is 20:1

The polarity of voltage that causes current to flow from H1 to H2 in the primary of an autotransformer results in current flow from ---- to ----- in the secondary

X1\nX2

In a dry-type transformer, ----- is used to cool the coils

air

Electrical power circuits must be insulated by ----- where they enter the transformer tank.

bushings

A ----- is an extra electrical connection on a transformer coil that allows a varying number of turns of a coil to be part of the circuit.

coil tap

The majority of 3 phase transmission circuits in the US are designed with 3 phase xfmrs ------ to obtain 3 phase power.

connected together

The most efficient way to send electric power over transmission lines as at the lowest ------ possible.

current

Buck boost transformers have ------ primaries and secondaries.

dual voltage

In a 3 phase circuit, power varies during a cycle but never ----.

falls to zero

A ------ is a material that is easily magnetized.

ferromagnetic

A ------ is a machine that converts mechanical energy into electrical energy by means of electromagnetic induction.

generator

A ----- is a test instrument that can be used to evaluate insulation resistance by measuring leakage current

hi pot tester

Dry type transformers 600V or less are ----- to be installed in a readily accessible space

not required

Liquid-immersed transformers are typically immersed in ----

oil

In an electric generating power plant, all generators must be connected in -----

parallel

----- are protective devices that take a transformer off-line in the event of a fault

relays

a wye system is also known as a ----- system

star

Liquid-filled transformers are fitted with a thermometer to measure the temperature at the ----- of the fluid

top

The ------ determines if a device is a step-up or step-down transformer

turns ratio between the coils and the primary winding's physical relationship to the core

Since a transformery typically consists of a primary and secondary coil wound over one another, ----- electromagnets exist in a transformer

two (each of different polarity)

A hipot tester uses a very low ----- for testing.

voltage

What is telescoping in a transformer?

when the windings tend to deform in response to the stress applied by the electromagnets during a short circuit

What does polarity refer to?

It refers to the direction of current flow in a closed circuit.

Instrument transformers reduce high voltages and currents to safe, usable values for the operation of _____ equipment.

MEASURING

Instrument transformers reduce high voltages and currents to safe, usable values for the operation of _____ _____.

MEASURING EQUIPMENT

What is a phase?

A single electrical circuit.

What are constant current transfomers?

A transformer which when supplied from a constant potential source automatically maintans a constant current in its secondary circuit under varying conditions of load impedance. Used on series street light circuits.

A transformer that has its primary and secondary windings electrically connected is called an _____.

AUTOTRANSFORMER

A variable transformer is a continuously adjustable _____.

AUTOTRANSFORMER

The _____ has the primary winding and secondary winding electrically connected.

AUTOTRANSFORMER

Core losses are _____ loss and _____ - _____ loss.

HYSTERESIS; EDDY-CURRENT

The energy is always transferred without a change in frequency, but usually with changes in _____ and _____.

VOLTAGE; CURRENT

Explain how placing a load on the secondary affects current flow in the primary.

When a load is connected across the secondary the induced voltace causes a current to flow in the secondary this current creates an opposite polarity field from the primary which allows more current to flow creating more inductance which induces more voltage in the secondary etc.... until the load gets the current it needs

Transformer mutual inductance is also referred to as a ------

coefficient of coupling

----- is power loss in a transformer due to currents induced in the metal field structure from the changing magnetic field.

eddy current loss

A magnetizing current is also known as a ---- current.

exciting

When a magnetic circuit reaches a point of saturation, further increases in ----- are no longer possible

flux density

Are transformers normally operated at the point of magnetic saturation?

no

A material is magnetic if it has ------ molecular magnets so that the individual fields ----- together

organized \nadd

The closed, laminated core of a transformer gives a pathe of low ----- for the flux to flow, which aligns the flux and allows the maximum number of ----- to be cut

reluctance\nconductors

Explain magnetic saturation

the condition where a magnetic core has substantially all the magnetic domains alligned with the field and any increases in current no longer result in a stronger electromagnet

What is done to minimize telescoping in a transformer?

the primary and secondry windings are electrically centered on the core leg

What it the turns ratio?

the ratio of the primary coil turns to the secondary coil turns (eg. 2:1)

Three phase transformers are seldom connected in a delta to ----- connection

delta

A ------ is a 3 phase connection method that has the wire from the ends of each coil connected end to end to form a closed loop

delta connection

In a balanced 3 phase ----- connected system the line current is equal to ----- times the current in one of the coils.

delta\n1.73

A ------ is a pipe, 4" in diameter or more that extends a few feet above the cover of a transformer and is curved toward the ground at the outlet end of the pipe.

explosion vent

In transformers that have both windings wound in the same direction with respect to the core, the polarity is changed from subtractive to additive by -----

interchanging the position of the two X leads

An insulation ------ test is performed to measure the leakage current though the coil insulation.

resistance

A ------ draws liquid from the lowest poing in the transformer tank.

sampling valve

The OCPD in an autotransformer ----- be wired in series with the shared winding.

should not

2 common methods for sizing a buck boost transformer.

- determine load poer and calculate the needed size\n- determin load current and buck or boost voltage and calculate the needed size

A ----- is a device that indicates whether two AC sources to be connected in parallel are in the correct phase relationship.

synchroscope

A ------- winding is a thrid winding that is often used in power transformations to provide station power requirements or a tie with synchronous condensers.

tertiary

All phases of a e phase transforemer have ----- relative polarity

the same

The power in a single phase circuit drops to zero ----- times every cycle.

two

A ------ is a transformer with no electrical connection between the primary and the secondary.

two winding transformer

What is the difference in measurements taken with a hipot tester and a megohmmeter?

- hipot test leakage current\n-megohmmeter tests insulation resistance

In a wye-connected system, the only current that flows in the neutral wire is the ----- current.

unbalanced

A ----- transformer is a continuously adjustable autotransformer consisting of a single layer of wire wound on a toroidal core, and a carbon brush that traverses this winding

variable

A ----- connection is a 3 phase connection that has one end of each coil connected together and the other end of each coil left open for external connections

wye

What are 2 significant limitations of autotransformers?

- low impedance\n- lack of isolation from the primary and secondary (harmonics, etc. can leak through)

3 applications for autotransformers.

- reduced voltage motor starting\n- voltage regulators\n-variable transformers

4 tests that need to be run when checking liquid from a liquid-immersed transformer.

-dielectric\n-moisture\n-acidity\n-dissolved gas analysis

What are the 2 main objecties accomplished by guarding a transformer?

-protect people\n-protect equipment

An OCPD must be provided to cause the main switch or common OCPD to open is the load reaches ------ of it's rating

125%

When an autotransformer has current of 9A or more, an OCPD is allowed to be rated at ----- of the full load current

125%

In a balanced 3 phase ----- connected system the line voltage is equal to ------ times the voltage in one of the coils

wye\n1.73

Can a wye to delta connection may be made with 3 single phase transformers?

yes

When using an open delta connaction, is it possible to remove one unit for repair and keep the system up and running?

yes

What does grounded mean?

(Earthed) Connected to earth or to some conducting body which serves in place of the earth.

A transformer has a temperature-rise rating of 105C. what is the maximum temperature the windings are permitted to be continuously subjected to

105 + 40= 145

To prevent injury to personnel when servicing any equipment connected to a current transformer, the secondary winding must be ____ - _____.

SHORT CIRCUITED: Unless the secondary winding is short-circuited, a high open circuit voltage that can be dangerous will exist.

If the transformer is used as a _____ transformer, the low voltage winding is the primary.

STEP-UP

---- is the ability of an inductor in a circuit to generate inductive reactance, which opposes change in the circuit.

Self-induction

What is a flux?

The magnetic lines of force that flow from the north pole of a magnet and return to the south pole.

Why are taps necessry on some transformers?

The taps allow the turns ratio of a transformer to be modified in the field to compenste or a low primary voltage (voltage drop)

Why do designers laminate the core of a transformer?

To reduce eddy currents

A magnetic field changing with respect to the conductor will generate a _____ in the conductor.

VOLTAGE

List the magnetic units of measure that are analogous to voltage, current, resistance.

voltage = magnetomotive force\ncurrent = magnetic flux\nresistance = reluctance

Is magnetic permeability nonlinear?

yes

What is an ammeter?

An instrument used to measure electrical current.

A ----- is an electric device that uses electromagnetism to change voltage from one level to another or to isolate one voltage from another.

transformer

What does "rating" mean?

A designated limit of operating characteristics based on definite conditions.

What is an Electromagnet?

A magnet made by winding a coil of wire around a soft iron core. Current passed through this winding produces magnetic lines of force in the coil.

What is a fuse?

A part of a circuit made of a low melting point material so that it will melt and break the circuit when a specified current exceeded. Always the weakest point in the circuit.

What is a rotation meter?

A small three-phase meter used to determine the direction of phase rotation on circuits or transformer banks.

What is a transformer?

A stationary device for transferring alternating current electrical energy from one circuit to another circuit by electromagnetic means.

What is a grounded system?

A system of conductors in which at least one conductors or point (usually the middle wire or neutral point of a transformer or generator windings) is intentionally grounded either solididly or through a current-limiting device.

What is a wigginton?

A test instrument used to indicate low voltage potentials in the range of 110 to 600 volts either alternating or direct current. (slang term: wiggy)

What is a step down transformer?

A transformer in which the primary is the high voltage winding and the secondary is the low voltage winding.

What is a step up transformer?

A transformer in which the primary is the low voltage winding and the secondary is the high voltage winding.

What is a distribution transformer?

A transformer rated at 500KVA and below used for voltage transformation on the distribuition system.

What is a short circuit?

An abnormal connection of relative low resistance, whether made accidentally or intentionally, between two point of different potential in a circuit.

Transformer losses are classified as _____ and _____.

COPPER LOSSES; CORE LOSSES

Some losses occur in all transformers. These losses are classified as copper losses and _____ losses.

CORE: Core losses are caused by eddy currents set up by the magnetic field.

The output rating of a transformer is based upon the maximum _____ the transformer is capable of carrying without exceeding a certain temperature rise.

CURRENT

Two situations in which an electrician would need to know the percent impedance of a transformer.

Calculating fault currents whether or not we can parallel.

_____ currents are reduced by laminating the core.

EDDY

A transformer is a device that has no moving parts and that transfers energy from one circuit to another by _____.

ELECTROMAGNETIC INDUCTION.

When an alternating current is applied to the high-voltage (H) winding of a transformer with no load on the secondary (X) winding, a small current, called the _____ current, flows in the primary.

EXCITING

Name the two main core losses

Eddy Current Hysteresis

When a buck boost transformer has current less than 9A, an OVPD is allowed to be rated at not more than ----- of the input current.

167%

What are the 3 most common wye connection voltages?

208Y/120\n480Y/277\n600Y/347

Today, the majority of dwellings are wired with ----- service

3 wire

Wye to delta connected transformers with properly marked leads always have an angular phase displacement of ----

30 degrees

In a wye-delta connection of 3 single phase transformers, one unit may be disconnected from the circuit and service maintained with the secondary operating in an open delta at ------ of normal bank capacity

57.7

A buck boost transformer is typically used to change voltage by between ---- and ---- percent

5\n25

For autotransformers the NEC makes a distinction between those rated at less than ----- and those rated higher than ------

9 A\n9 A

Transformers typically have efficiencies in the range of

96% to 99%

ANSI standardization rules state that high-voltage leads brought outside the transformer case are to be marked with a ----- and low voltage leads are to be marked with a ------

H\nX

When runnin a turns ratio test, a load tap changer must be ------

cycled through all its possible positions

A ----- is any transformer in which the primary and secondary circuits have a portion of their two windings in common

autotransformer

A ------ is a small transformer designed to lower or raise line voltage.

buck-boost transformer

When 2 or more conductors are near each other, charge builds up in the insulation between the wired and acts like a -----

capacitor

Transformers are rated by their capacity in -----

kilovolt amps (kVA)

A(n) ----- connection may also be called a V connection because only 2 sides on the delta triangle are included.

open delta

The ------ is the ratio of the 10 minute reading to the 1 minute reading taken on a dielectric absorption test

polarization index

A ----- is a transistion device between underground cable and lines

pothead