Transformers and 3 Phase Power

Why are laminated core types preferred?

because a thin layer of oxide forms on the surface of each laminate and acts as an insulator to reduce the formation of eddy currents inside the core metal.

Subtrative polarity is often referred to a _______ connection.

buck connection

In an isolation transformer, how are the windings coupled?

by the magnetic field

Constant current transformers are also known as _____.

current regulators

______ are often used as instrumentation transformers in high-power circuits.

current transformers

How do you calculate turns ratio?

divide higher voltage by lower voltage

_____ is the amount of magnetic flux lines that do not follow the metal core and are lost to the surrounding air.

flux leakage

What is a instrument transformer?

high accuracy class electrical devices used to isolate or transform voltage or current levels.

Where is an H-type core typically found?

high voltage distribution transformers

Where is a tape-wound core (toroid core) typically used?

in current transformers as it allows measurement of current being used.

_____ is the amount of current that flows when power is initially applied to a circuit.

inrush current

A control transformer and a distribution transformer are what types of transformers?

isolation transformers

What is the advantage of having a multiple-tapped secondary winding?

it allows the transformer to be a combination of a step-up and step-down transformer.

The secondary winding of a transformer is the ______.

load winding (also called output winding)

When a center tap on a multi-tapped winding is grounded, it becomes the _____ (also known as ______)

neutral conductor common conductor

Connecting the primary windings of a control circuit has the effect of increasing _____.

number of turns on the primary

What do the dots shown beside the terminal leads of a transformer represent on a schematic?

polarity connections (north/south poles)

Which winding on a transformer is the power winding?

primary winding

The _____ of an instrumentation transformer is connected to the high voltage or high current circuit, and the _____ or _____ is connected to the secondary circuit.

primary winding meter or relay

On a control transformer, what is the common voltage rating for each winding?

primary winding: 240 V secondary winding: 120 V

Which winding on a transformer is the load winding?

secondary winding

What is the the disadvantage to using an autotransformer?

since it only has one winding, voltage spikes that occur on the supply line side will pass through to the secondary load side.

A transformer with a secondary lower than the primary is what type of transformer?

step-down transformer

A transformer with a secondary greater than the primary is what type of transformer?

step-up transformer

With a subtractive connection, voltage across the entire connection would _____

subtract

If H1 and X1 are directly across from each other, the polarity type is ______.

subtractive polarity

What are magnetic domains?

the area in magnetic materials where you have a strong area of magnetic flux

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

A transformer used for isolation purposes ONLY has a turns ration of ______.

1 to 1 (1:1)

List three classifications of transformers

1) isolation transformers 2) autotransformers 3) current transformers

The most common type of control transformer contains how many primary and secondary windings?

2 primary; 1 secondary

Current transformers used for instrumentation purposes have a standard secondary current of _____ amps.

5 amps

In a 60 hertz AC circuit, how many times does polarity change direction?

60 times per second

Transformer efficiencies range from ______ % to ______ %.

90% to 99% under full load

What is impedance?

AC resistance

What purpose does the oil serve on an autotransformer?

It acts as a coolant and prevents moisture from forming in the windings.

what is the advantage of an isolation transformer?

Line separation; it greatly reduces voltage spikes that originate on the supply side (primary) before they are transferred to the load side (secondary) of transformer.

What is the core design difference between the shell-type transformer and a standard core-type transformer?

Shell-type has a metal core piece through the middle of the window with the windings wrapped around the center of the core as opposed to around the outer core.

What is the difference between a shell-type transformer and an H-type transformer?

The H-core surrounds the windings on four sides instead of two.

T/F: A distribution transformer is a type of isolation transformer.

True

T/F: As a general rule, transformer secondary windings should not be connected in parallel.

True

T/F: As current changes in magnitude and direction, the polarity changes too.

True

T/F: in a multiple-tapped winding, the primary can be connected to different voltages without changing the the value of the secondary.

True

How is the primary and secondary windings of a voltage transformer and a current transformer connected?

Voltage transformer: connected directly across the power line. Current transformer: connected in series with the load

What is the general placement of the windings to the core?

Windings are placed around each end of the core. low-voltage winding closest to core; high-voltage winding placed over the low-voltage winding.

What is a transformer?

a magnetically operated machine that can change the values of current, voltage and impedance without changing frequency.

What is the difference between a voltage transformer and a current transformer?

the primary windings of a voltage transform is connected directly across the power line; the primary winding of a current transformer is connected in series with the load.

What is the most common usage of instrument transformers?

to operate instruments or metering from high voltage or high current circuits, safely isolating secondary control circuitry from the high voltages or currents.

what are constant-current transformers used for?

to provide power to series connected loads

What is an autotransformer?

transformers that have only one winding that is used as both primary and secondary.

All values of a transformer are proportional to its _______.

turns ratio

The values of the voltage and current of the isolation transformer are determined by the ________.

turns ratio

The values of voltage and current of the isolation transformer are determined by ______.

turns ratio (remember, all values of a transformer are proportional to their turns ratio!_

The type of transformer whose turns ratio can be adjusted continuously is referred to as a ______ transformer.

variac or powerstat

When the primary windings are connected in parallel, the same _____ is applied across both windings.

voltage

How is the tape-wound core (aka toroid core) constructed?

with one long tightly wound continuous silicon steel tape into a spiral.

T/F: On a control transformer, when the primary windings are connected in parallel, the same voltage is applied across both windings.

True

T/F: The secondary winding of a current transformer must never be opened when the main power is turned on.

True

The amount of induced voltage is proportional to _____.

rate of change of current (meaning, as fast as voltage increases, the opposing applied voltage will increase)

What generally determines the amount of magnetic coupling between the windings and the efficiency of the transformer?

the type and shape of the core

Most transformer cores are made from ______.

thin steel punchings laminated together to form a solid metal core

What causes the characteristic differences between a reactor and a transformer?

type of core used

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

1) amount of applied voltage 2) resistance of the wire in primary winding 3) flux change of the magnetic field in the core

What is the difference in the connection between a voltage transformer and a current transformer?

With a current transformer, the primary winding is connected in series across the load. With a potential (voltage) transformer, the primary winding is connected in parallel across the line voltage (power line).

A good rule for transformers is that _______.

power in must equal power out

The primary winding of a transformer is the ______.

power input winding

_____ is an inductor used to add inductance to the circuit.

reactor

What is the control transformer used for?

used to reduce the line voltage to the value needed to operate control circuits.

Why does an isolation transformer greatly reduce voltage spikes before they reach the secondary?

Because a voltage spike completes before the current can build up in the inductor.

Where are autotransformers typically used?

By power companies to provide small increases and decreases to the line voltage.

What is a distribution transformer?

A transformer that changes the high voltage of the power company distribution lines to the common 240/120 volts used in homes and businesses.

What is an isolation transformer?

A transformer whose primary and secondary windings are physically and electrically isolated from one another

With an additive connection, voltage across the entire connection would ______.

add

What two polarity connection types can a transformer be connected as?

additive connection subtractive connection

If H1 and X1 polarities are diagonal from each other, the polarity type is ______.

additive polarity

What are air gaps?

air used to separate the steel laminations.

What will happen if the secondary windings in a current transformer are not shorted?

an extremely high voltage will transfer to the secondary winding which will likely be fatal if contacted.

What does air gap do?

breaks the magnetic path through the core material causing the domains to return to their neutral positions once the magnetizing force is removed.

T/F: The tape-type core (toroid cord) is one of the most efficient core designs available.

True

T/F: There is no electrical connection between the primary and secondary windings in an isolation transformer.

True

Additive polarity is often referred to as a ______ connection.

boost connection

transformers with windings that have more than one set of lead wires connected to the primary and seconary are called _____.

multiple-tapped windings

On a transformer, what is meant by "inrush current?"

the instantaneous high input current drawn by a power supply or electrical equipment when it is first turned on

When using turn ratios, the side of a transformer with the lowest voltage will always have _____ and be written on which side of the ratio colon?

the lowest number in the ratio

What must we know about transformers?

the number of turns for each of their windings

What is the formula for computing turn ratio?

Divide the primary turns by the secondary turns OR by dividing the high voltage by the low voltage

When is the secondary voltage at its maximum?

During the periods that the primary is changing the most (0, 180, and 360 degrees)

What is the formula for computing winding current?

Ep divided by Es = Is divided by Ip

What is the standard secondary current rating of a current transformer used for instrumentation?

5 amps

What is the main difference between a reactor and a transformer?

A reactor is connected in series with a low-impedance load to prevent inrush current from becoming excessive. Transformers can produce extremely high inrush currents when power is first applied to the primary.

What is the formula to compute winding voltage?

Ep divided by Es = Np divided by Ns

What is the formula for computing winding current when voltage and impedance is known?

I = Voltage divided by impedance

The induced voltage from the primary to the secondary when there is primary voltage but no secondary load is proportional to _____.

Rate of change of current