Transformer - EEE 2187 Electrical Machines Flashcards

100 fill-in-the-blank flashcards covering transformer theory, construction, types, losses, and calculations from the provided lecture notes.

Transformers are __________ electrical devices used to convert AC voltage from one level to another.

static

A transformer transforms AC voltage from high to low or low to high with the __________ frequency.

same

The fundamental principle behind transformer operation is __________ induction.

electromagnetic

A transformer consists of two windings, the __________ and secondary, wound on a common laminated magnetic core.

primary

The winding connected to the AC source is called the __________ winding.

primary

The winding connected to the load is called the __________ winding.

secondary

The alternating voltage whose magnitude is to be changed is applied to the __________ winding.

primary

The induced EMF in the secondary causes a secondary __________ denoted as $I_2$.

current

According to __________, the induced EMF in a conductor is equal to the rate of change of flux linkages.

Faraday’s law

A transformer uses the principle of __________ inductance to create AC voltage in the secondary coil.

mutual

The efficiency of energy transfer between coils is improved by using a __________ to concentrate the magnetic field.

core

When $N_2 > N_1$, then $E_2 > E_1$ and the device is a __________ transformer.

step-up

When $N_2 < N_1$, then $E_2 < E_1$ and the device is a __________ transformer.

step-down

AC power is transferred from primary to secondary through __________ flux.

magnetic

Core losses consist of __________ current and hysteresis losses.

eddy

Copper losses occur in the __________ of the windings.

resistance

The primary winding generates the required magnetic flux whenever an active AC source is connected across it, making it the __________ section.

input

The magnetic core is generally made of __________ or laminated silicon steel.

soft iron

The magnetic core provides a __________ reluctance path for the generated magnetic fluxes.

low

In a __________ transformer, half of the primary and secondary windings are placed around each limb.

core-type

In a __________ transformer, both windings are placed around the central limb with a double magnetic circuit.

shell-type

Core-type construction is generally more suitable for __________ voltage and small output.

high

Shell-type construction is generally more suitable for __________ voltage and high output.

low

In small transformers below $50\,kVA$, __________ cooling is usually employed.

natural air

In medium size power transformers, __________ is used to carry heat to the tank surface and insulate windings.

oil

The maximum value of the induced EMF in the primary is expressed as $E_{m1} = 2\pi f N_1 \phi_m$ or __________.

$$4.44 f N_1 \phi_m$$

The constant $K = \frac{N_2}{N_1}$ is called the __________ ratio.

voltage transformation

For an ideal transformer, the volt-amperes input to the primary is __________ to the output volt-amperes.

equal

Currents in a transformer are in the __________ ratio of the voltage transformation ratio.

inverse

The primary no-load current $I_0$ lags the applied voltage $V_1$ by an angle $\phi_0 < 90^\circ$ because of __________.

iron losses

The no-load input power $W_0$ is practically equal to the __________ in the transformer.

iron loss

In the no-load test, the magnetising current is calculated as $I_m = I_0 \sin(\phi_0)$ and the core loss current is __________ .

$$I_w = I_0 \cos(\phi_0)$$

When transferring resistance from secondary to primary, you must __________ the value by $K^2$.

divide

When transferring reactance from primary to secondary, you must __________ the value by $K^2$.

multiply

Total primary current $I_1$ is the phasor sum of the no-load current $I_0$ and the __________ current $I'_2$.

load component

The loss-free coil passing the magnetising current $I_m$ is represented by the inductive reactance __________ in the equivalent circuit.

$$X_0$$

Voltage regulation is a measure of the change in voltage magnitude between the sending and __________ end.

receiving

Voltage regulation is defined as the percentage difference between __________ and full load voltages.

no load

The formula for percentage voltage regulation is $V.R. = \frac{E_2 - V_2}{E_2} \times 100$ where $E_2$ is the __________ voltage.

no load secondary

Voltage regulation at a __________ power factor is expressed as $\frac{I_2 R_2 \cos(\phi_2) + I_2 X_2 \sin(\phi_2)}{V_2} \times 100$.

lagging

Voltage regulation at a __________ power factor may involve subtracting the reactance drop term.

leading

The __________ circuit test is conducted to determine iron losses and parameters $R_0$ and $X_0$.

open

In an open-circuit test, the __________ voltage is applied to the primary while the secondary is open.

rated

The __________ circuit test determines the full-load copper losses and equivalent impedance.

short

In a short-circuit test, the secondary is short-circuited and a __________ voltage is applied to the primary.

variable low

Hysteresis loss is given by the formula $W_h = k_h f B_m^{1.6}$ and occurs in the __________ due to alternating flux.

core

Eddy current loss is given by the formula $W_e = k_e f^2 B_m^2 t^2$ and can be reduced by using __________ .

thin laminations

Total losses in a transformer equal the sum of constant losses and __________ losses.

variable

Copper losses account for about __________ percent of total losses in a transformer.

90

The condition for maximum efficiency is when iron loss equals __________.

copper loss

The rating of a transformer is expressed in __________ instead of kW.

kVA

The total loss in a transformer is independent of the load __________ factor.

power

An __________ has a single winding on an iron core where part of the winding is common to both circuits.

autotransformer

In an autotransformer, power is transferred to the secondary both __________ and inductively.

conductively

An autotransformer requires __________ copper than a two-winding transformer of similar rating.

less

A disadvantage of the autotransformer is that the output is no longer __________ from the input.

electrically isolated

In an autotransformer, if an open circuit develops in the common portion, the __________ voltage appears across the load.

full primary

Autotransformers used to compensate for voltage drops in lines are known as __________ transformers.

booster

A three-phase transformer can be built by connecting a bank of three __________ transformers.

single-phase

The four standard connections for three-phase transformers are $Y-Y, \Delta-\Delta, Y-\Delta$ and __________.

$$\Delta-Y$$

The __________ connection is often avoided because it can create serious disturbances in communication circuits.

Y-Y

The __________ connection allows the bank to continue operating in open-delta if one transformer is removed.

$$\Delta-\Delta$$

An open-delta connection has a capacity reduced to __________ percent of the original bank capacity.

57.7

For parallel operation, transformers must be connected properly with regard to their __________.

polarities

In parallel operation of 3-phase transformers, the secondaries must have the same phase __________.

sequence

Signals whose frequencies are integral multiples of the fundamental frequency are called __________.

harmonics

Current harmonics originate from __________ loads like inverters, UPS, and PCs.

non-linear

Voltage harmonics result from current harmonics flowing through the system __________.

impedance

Harmonic distortion is measured as __________, which is the ratio of all harmonic components to the fundamental.

Total Harmonic Distortion (THD)

Harmonic currents increase both core loss and __________ loss, leading to temperature rise.

copper

The maximal instantaneous input current drawn when a device is first turned on is called __________ current.

inrush

Inrush current can be up to __________ to 15 times larger than the rated current.

10

One solution for reducing inrush current is switching on the transformer at the voltage __________.

peak

A __________ gas-actuated relay, indicated as item 6 in the parts list, is used for protection in oil-immersed transformers.

Buchholz

The __________ tank (item 5) allows for oil expansion and contraction during temperature changes.

conservator

The component used to extract moisture from the air entering the transformer is called the __________.

breather

The component that increases the cooling surface of the oil-filled tank is the __________.

radiator

The primary resistance is denoted as $R_1$ and the primary leakage reactance as __________.

$$X_1$$

The induced EMF is proportional to the number of __________ in the winding.

turns

In an ideal transformer, the voltage ratio $V_2/V_1$ is equal to the turns ratio __________.

$$N_2/N_1$$

The no-load current $I_0$ usually ranges from 2– __________ percent of the rated current.

10

If an impedance $Z_2$ in the secondary is transferred to the primary, it becomes __________ .

$$Z_2/K^2$$

The copper loss depends on the __________ of the load current.

square

Direct loading to determine efficiency is avoided because transformer efficiency is very __________ .

high

Hysteresis loss can be minimized by using steel of high __________ content.

silicon

The iron loss of a transformer is considered __________ at all loads because the frequency and flux are constant.

constant

In three-phase transformers, a star-connected winding usually has __________ leads brought out.

four

In a delta-connected winding, __________ leads are brought out.

three

A three-phase transformer costs about __________ percent less than a bank of three single-phase transformers.

20

The secondary line voltage in a $Y-\Delta$ connection is $K \times \frac{V}{\sqrt{3}}$, representing a phase shift and __________ .

step-down

The ratio of secondary phase voltage to primary phase voltage is known as the phase __________ ratio.

transformation

Inrush current transients generally last for only a few __________ .

cycles

Magnetic leakage gives rise to leakage __________ .

reactance

The primary power factor is the cosine of the angle between input voltage and __________ .

primary current

In a transformer, the primary __________ allows the calculation of input power as $P_1 = V_1 I_1 \cos(\phi_1)$.

power factor

The __________ circuit is used to represent core losses in the equivalent circuit.

parallel

Transformer windings are typically made of __________ wires.

copper

High magnetizing inrush current can necessitate the __________ of fuses or breakers.

over-sizing

For large transformers, cooling fans may be used to blow air over the __________.

radiators

The impedance ratio $\frac{Z_2}{Z_1}$ is equal to the __________ of the voltage transformation ratio.

square