Electricity Transformers Quiz

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Questions and Answers

What did Russian engineer Yablochkov invent in 1876?

Power transformer for distribution systems

Windings wrapped around a laminated square core

Transformer used at a substation for voltage stepping

Lighting system based on a set of induction coils (correct)

What is the purpose of wrapping windings on top of each other in a transformer?

To prevent the transformer from functioning properly

To increase flux leakage and decrease efficiency

To make the transformer lighter in weight

To decrease flux leakage and increase efficiency (correct)

What is the role of a substation transformer?

Generate electricity from a power source

Step down voltage to the distribution level (correct)

Step up voltage to the transmission level

Connect to the output of a generator

How are power transformers used in power distribution systems referred to?

Unit transformers Signup and view all the answers

What type of transformer steps voltage up to the transmission level?

Unit transformer Signup and view all the answers

In what configuration are windings wrapped around a laminated core?

Around two sides of a square core Signup and view all the answers

Which of the following statements about an ideal transformer is true?

The input and output reactive powers are equal. Signup and view all the answers

What is the relationship between the input and output currents of an ideal transformer?

The input current is equal to the output current multiplied by the turns ratio. Signup and view all the answers

If the input voltage of an ideal transformer is 220V and the turns ratio is 10:1, what is the output voltage?

22V Signup and view all the answers

Which of the following is true for an ideal transformer?

The input and output apparent powers are equal. Signup and view all the answers

What is the significance of the statement 'Pout = Pin' for an ideal transformer?

Both (a) and (b). Signup and view all the answers

What is the primary function of a transformer?

To convert one AC voltage to another AC voltage at the same frequency Signup and view all the answers

What is the key component that allows a transformer to function?

Ferromagnetic core Signup and view all the answers

What was the historical significance of Faraday's work on transformers?

He demonstrated the principle of electromagnetic induction, which is the basis of transformer operation Signup and view all the answers

What is the relationship between the primary and secondary windings of a transformer?

They are connected through the common magnetic flux confined to the core Signup and view all the answers

What is the purpose of the primary winding in a transformer?

To connect the transformer to a source of AC power Signup and view all the answers

What is the purpose of the secondary winding in a transformer?

To step down the voltage Signup and view all the answers

In an ideal transformer, if the voltage at the dotted end of the primary winding is positive, what will be the polarity of the voltage at the dotted end of the secondary winding?

Positive Signup and view all the answers

What is the relationship between the direction of primary and secondary currents in an ideal transformer?

If the primary current flows into the dotted end, the secondary current will flow out of the dotted end Signup and view all the answers

In an ideal transformer, if $\theta_p$ and $\theta_s$ are the angles between voltages and currents on the primary and secondary windings respectively, what is the relationship between $\theta_p$ and $\theta_s$?

$\theta_p = \theta_s$ Signup and view all the answers

If $P_{in}$ and $P_{out}$ are the input and output powers of an ideal transformer, and $V_p$, $I_p$, $V_s$, and $I_s$ are the primary voltage, primary current, secondary voltage, and secondary current respectively, what is the relationship between $P_{in}$ and $P_{out}$?

$P_{in} = P_{out}$ Signup and view all the answers

In an ideal transformer, if the primary winding has $N_p$ turns and the secondary winding has $N_s$ turns, what is the relationship between the primary and secondary voltages, $V_p$ and $V_s$?

$V_p = \frac{N_p}{N_s} V_s$ Signup and view all the answers

In an ideal transformer, if the primary current is $I_p$ and the secondary current is $I_s$, what is the relationship between $I_p$ and $I_s$?

$I_p = \frac{N_s}{N_p} I_s$ Signup and view all the answers

What is the relationship between the primary and secondary voltages of an ideal transformer?

$v_p(t) = av_s(t)$ Signup and view all the answers

Which of the following statements about the phase angles of an ideal transformer is correct?

The phase angles of the primary and secondary voltages and currents are the same. Signup and view all the answers

What is the relationship between the primary and secondary currents of an ideal transformer?

$i_p(t) = \rac{i_s(t)}{a}$ Signup and view all the answers

Which of the following is NOT a type of transformer mentioned in the text?

Step-up transformer Signup and view all the answers

What is the main purpose of a distribution transformer?

To convert the distribution voltage to the final level (110 V, 220 V, etc.) Signup and view all the answers

What is the assumption made about the transformer considered in the text?

Both (a) and (b) are correct. Signup and view all the answers

Study Notes

Ideal Transformer

The output power of an ideal transformer equals its input power, with no loss assumed.
For an ideal transformer, the following holds: Vs = Vp, Pout = Vs Is cos θ = Vp Ip cos θ = Pin, Qout = Vs Is sin θ = Vp Ip sin θ = Qin, Sout = Vs Is = Vp Ip = Sin.

History of Transformers

Russian engineer Yablochkov invented a lighting system based on a set of induction coils, which acted as a transformer, in 1876.
The invention of a transformer can be attributed to Faraday, who demonstrated electromagnetic induction in 1831.

Types and Construction of Power Transformers

Core form: windings are wrapped around two sides of a laminated square core.
Shell form: windings are wrapped around the center leg of a laminated core.
Windings are usually wrapped on top of each other to decrease flux leakage and increase efficiency.

Power Transformers

A power transformer connected to the output of a generator and used to step its voltage up to the transmission level (110 kV and higher) is called a unit transformer.
A transformer used at a substation to step the voltage from the transmission level down to the distribution level (2.3 … 34.5 kV) is called a substation transformer.
A transformer converting the distribution voltage down to the final level (110 V, 220 V, etc.) is called a distribution transformer.

Preliminary Considerations

A transformer is a device that converts one AC voltage to another AC voltage at the same frequency.
It consists of one or more coil(s) of wire wrapped around a common ferromagnetic core.
The coils are usually not connected electrically together, but are connected through the common magnetic flux confined to the core.
Assuming that the transformer has at least two windings, one of them (primary) is connected to a source of AC power, and the other (secondary) is connected to the loads.

Polarity of Voltages and Currents

One winding's terminal is usually marked by a dot used to determine the polarity of voltages and currents.
If the voltage is positive at the dotted end of the primary winding at some moment of time, the voltage at the dotted end of the secondary winding will also be positive at the same time instance.
If the primary current flows into the dotted end of the primary winding, the secondary current will flow out of the dotted end of the secondary winding.

Power in an Ideal Transformer

The power supplied to the transformer by the primary circuit is: Pin = Vp Ip cos θp.
The power supplied to the output circuits is: Pout = Vs Is cos θs.
Since ideal transformers do not affect angles between voltages and currents: θp = θs.

Transformer Equations

The relationship between the voltage applied to the primary winding vp(t) and the voltage produced on the secondary winding vs(t) is: vp(t) / vs(t) = Np / Ns = a.
The relationship between the primary ip(t) and secondary is(t) currents is: ip(t) / is(t) = 1 / a.
In the phasor notation: Vp / Vs = a, Ip / Is = 1 / a.
The phase angles of primary and secondary voltages and currents are the same.

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Description

Test your knowledge about electricity transformers including their scope, history, types, and construction. Learn about the concept of ideal transformers and their role in voltage conversion. Thank you and get ready for some questions!