CRO Figure Quiz and Flashcards

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Questions and Answers

When the frequencies of two voltage sets are different, what happens to the phasor difference (VA1 - VA2)?

It becomes zero

It remains constant

Its magnitude varies in time (correct)

It oscillates between two fixed values

What happens to lamp L1 when the voltage across it passes through zero?

It remains unchanged

It becomes dimmer

It becomes brighter

It turns off (correct)

What is the advantage of the cross-connected synchroscope method?

It shows the exact frequency difference

It determines the phase sequence

It measures the armature voltage

It shows whether the incoming machine is running above or below synchronous speed (correct)

What is the purpose of the cross-connected lamp synchroscope?

To check for synchronization Signup and view all the answers

What is the result of the cross-connected lamp synchroscope when the lamps are arranged in a cyclic manner?

The lamps light up sequentially Signup and view all the answers

What happens to the brightness of lamp L1 due to the difference in frequencies?

It changes in time Signup and view all the answers

What is a necessary condition to obtain a Lissajous figure on the CRO screen?

Connecting an attenuator (probe) to the X and Y plates Signup and view all the answers

What is required to ensure condition (a) is satisfied?

A simple measurement Signup and view all the answers

What is the purpose of closing the synchronizing switch?

To observe a straight line with a slope less than 90º Signup and view all the answers

What is the voltage across lamp L1 represented by?

V A1 - V A2 Signup and view all the answers

What is the frequency of the phasor (V A1 - V A2)?

f Signup and view all the answers

What is the purpose of a synchroscope?

To ensure conditions (a) and (b) are satisfied Signup and view all the answers

What is the primary function of a synchroscope?

To synchronize a generator with an infinite bus-bar system Signup and view all the answers

What is the purpose of a single-phase variac in the synchronous machine starter circuit?

To control the armature voltage of the synchronous motor Signup and view all the answers

What is the significance of the phasor diagram in Fig.1(b)?

It corresponds to the straight-connected lamp synchroscope Signup and view all the answers

What is the purpose of the synchronizing switch (S2) in Fig.3?

To connect the synchronous machine to the infinite bus-bar system Signup and view all the answers

What is the Lissajous figure method used for in synchronization?

To determine the synchronizing conditions Signup and view all the answers

What is the significance of the cross-connected lamp synchroscope?

It provides a visual indication of synchronism Signup and view all the answers

Study Notes

Voltage Phasors and Lamp Synchronization

When voltages have different frequencies (f1 and f2), their phasor difference (VA1 - VA2) varies over time.
Brightness of lamp L1 fluctuates due to this varying phasor difference.
The phasors overlap (w1 - w2)/2π times per second, causing voltage across L1 to reach zero intermittently, turning L1 off.
Closing the switch can achieve synchronization when phase voltages coincide.

Cross-Connected Lamp Synchroscope

Lamps L1 and L2 are cross-connected while L3 is straight-connected in a synchroscope setup.
Phasor diagram analysis shows cyclic variation in voltages across the lamps.
Sequential lighting of the lamps occurs, indicating the relative speed of the incoming machine compared to synchronous speed.
This configuration creates an optical effect of light rotation when lamps are arranged in a cycle.

Advantages

Provides clear visual feedback on whether the incoming machine is above or below synchronous speed.
The sequential lighting effect can easily indicate phase matching.

Disadvantages

Complexity increases with cross-connection versus straight connection.
Requires careful arrangement and synchronizing conditions to function correctly.

Lissajous Figure Synchronization

An attenuator must be connected to X and Y plates for Lissajous figures displayed on a CRO screen.
A rotating Lissajous figure appears before synchronization.
Connecting both X and Y plates to a common phase helps check the resulting figure.
A straight line with a slope less than 90º on the CRO signals readiness to close the synchronizing switch.

Operating Principle of Straight-Connected Lamp Synchroscope

Instantaneous voltage across lamps corresponds to the difference of the two voltage phasors.
The voltage across lamp L1 is expressed as VL1 = VA1 - VA2.
If VA1 and VA2 rotate at the same frequency, their difference remains a constant phasor, generating consistent display behavior in the synchronization process.

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Description

This quiz assesses your understanding of Lissajous figures on a CRO screen, including synchronization and phase connections. Learn how to obtain a Lissajous figure and identify the shape of the figure on the screen. Test your knowledge of CRO operations and Lissajous figures.