Tests of Synchronous Machine MCQ Quiz - Objective Question with Answer for Tests of Synchronous Machine - Download Free PDF

Flow of Reactive Power in Synchronous Machine 

A synchronous machine can operate either as a generator or a motor, and its reactive power behavior depends on its excitation level:

Overexcited Synchronous Machine:

• When a synchronous generator is overexcited, it behaves like a capacitor and delivers lagging reactive power (i.e., it supplies reactive power to the system).
• This means it delivers positive VARs (Volt-Ampere Reactive) to the grid.

Hence, option 1 is correct.

Underexcited Synchronous Machine:

• When a synchronous generator is underexcited, it behaves like an inductor and draws reactive power from the system.
• It absorbs lagging reactive power (i.e., takes in reactive power from the grid).

Hence, option 4 can also be correct.

The correct answer is option 4):(95.12)

Concept:

The ratio of work done by the machine to the work done on the machine is known as the efficiency of the machine.

Efficiency =  × 100

Calculation

Given 

Output = 10 000 KVA

Output in KW = KVA × 0.8

= 8000

Efficiency =  × 100

=   × 100

= 95.12

Concept:

There are two axes in a salient pole synchronous generator, the direct axis (d axis) and quadrature axis (q axis).

The reactance along d-axis is maximum and is given by:

The reactance along the q-axis is minimum and is given by:

Calculation:

Given, V_max = 2700 V and V_min = 2650 V

I_max = 360 A and I_min = 300 A

Slip test of Salient Pole Synchronous Machine:

• Slip test is performed on a salient pole synchronous machine to determine the direct axis reactance (Xd) and quadrature axis reactance (Xq).
• As in a salient pole synchronous machine, an air gap is not constant like in cylindrical type rotor reactance around quadrature axis (X_q) is different from reactance along the direct axis (X_d). 
• Normally X_q d Reactance depends upon the air gap.
• As the air gap is non-uniform, the reactance offered also varies, and hence current drawn the armature also varies cyclically.
• The r.m.s. current is minimum when machine reactance is  X_d and it is maximum when machine reactance is  X_q. As the reactance offered varies due to the non-uniform air gap, the voltage drops also vary. Hence the impedance of the alternator also varies.
• The terminal voltage also varies cyclically. The voltage at terminals is maximum when current and various drops are minimum, while the voltage at terminals is minimum when current and various drops are maximum.

Concept:

Open Circuit Test:

• This test is used for determining the synchronous impedance.
• The alternator is running at the rated synchronous speed, and the load terminals are kept open.
• The excitation current may be increased to get 25% more than the rated voltage i.e. up to 125% of the rated voltage.
• A graph is drawn between the open circuit phase voltage E_g and the field current I_f. The curve so obtained is called Open Circuit Characteristic (O.C.C).
• The linear portion of the O.C.C is extended to form an air gap line.

The Open Circuit Characteristic (O.C.C) and the air gap line is shown in the figure below

Additional information:

Short circuit test:

• The machine is driven at rated synchronous speed and armature terminals are short-circuited through an ammeter.
• Now, field current gradually increased from zero, until the short-circuit armature current reached its safe maximum value, equal to about 125 to 150% of the rated current.
• Latter readings may be taken in a short time to avoid armature overheating.
• A graph is drawn between short-circuit current i_sc and field current i_f as shown in the figure.

The correct answer is option 4):(95.12)

Concept:

The ratio of work done by the machine to the work done on the machine is known as the efficiency of the machine.

Efficiency =  × 100

Calculation

Given 

Output = 10 000 KVA

Output in KW = KVA × 0.8

= 8000

Efficiency =  × 100

=   × 100

= 95.12

Concept:

Open Circuit Test:

• This test is used for determining the synchronous impedance.
• The alternator is running at the rated synchronous speed, and the load terminals are kept open.
• The excitation current may be increased to get 25% more than the rated voltage i.e. up to 125% of the rated voltage.
• A graph is drawn between the open circuit phase voltage E_g and the field current I_f. The curve so obtained is called Open Circuit Characteristic (O.C.C).
• The linear portion of the O.C.C is extended to form an air gap line.

The Open Circuit Characteristic (O.C.C) and the air gap line is shown in the figure below

Additional information:

Short circuit test:

• The machine is driven at rated synchronous speed and armature terminals are short-circuited through an ammeter.
• Now, field current gradually increased from zero, until the short-circuit armature current reached its safe maximum value, equal to about 125 to 150% of the rated current.
• Latter readings may be taken in a short time to avoid armature overheating.
• A graph is drawn between short-circuit current i_sc and field current i_f as shown in the figure.

Concept:

There are two axes in a salient pole synchronous generator, the direct axis (d axis) and quadrature axis (q axis).

The reactance along d-axis is maximum and is given by:

The reactance along the q-axis is minimum and is given by:

Calculation:

Given, V_max = 2700 V and V_min = 2650 V

I_max = 360 A and I_min = 300 A

Flow of Reactive Power in Synchronous Machine 

A synchronous machine can operate either as a generator or a motor, and its reactive power behavior depends on its excitation level:

Overexcited Synchronous Machine:

• When a synchronous generator is overexcited, it behaves like a capacitor and delivers lagging reactive power (i.e., it supplies reactive power to the system).
• This means it delivers positive VARs (Volt-Ampere Reactive) to the grid.

Hence, option 1 is correct.

Underexcited Synchronous Machine:

• When a synchronous generator is underexcited, it behaves like an inductor and draws reactive power from the system.
• It absorbs lagging reactive power (i.e., takes in reactive power from the grid).

Hence, option 4 can also be correct.

The correct answer is option 4):(95.12)

Concept:

The ratio of work done by the machine to the work done on the machine is known as the efficiency of the machine.

Efficiency =  × 100

Calculation

Given 

Output = 10 000 KVA

Output in KW = KVA × 0.8

= 8000

Efficiency =  × 100

=   × 100

= 95.12

Concept:

Open Circuit Test:

• This test is used for determining the synchronous impedance.
• The alternator is running at the rated synchronous speed, and the load terminals are kept open.
• The excitation current may be increased to get 25% more than the rated voltage i.e. up to 125% of the rated voltage.
• A graph is drawn between the open circuit phase voltage E_g and the field current I_f. The curve so obtained is called Open Circuit Characteristic (O.C.C).
• The linear portion of the O.C.C is extended to form an air gap line.

The Open Circuit Characteristic (O.C.C) and the air gap line is shown in the figure below

Additional information:

Short circuit test:

• The machine is driven at rated synchronous speed and armature terminals are short-circuited through an ammeter.
• Now, field current gradually increased from zero, until the short-circuit armature current reached its safe maximum value, equal to about 125 to 150% of the rated current.
• Latter readings may be taken in a short time to avoid armature overheating.
• A graph is drawn between short-circuit current i_sc and field current i_f as shown in the figure.

Slip test of Salient Pole Synchronous Machine:

• Slip test is performed on a salient pole synchronous machine to determine the direct axis reactance (Xd) and quadrature axis reactance (Xq).
• As in a salient pole synchronous machine, an air gap is not constant like in cylindrical type rotor reactance around quadrature axis (X_q) is different from reactance along the direct axis (X_d). 
• Normally X_q d Reactance depends upon the air gap.
• As the air gap is non-uniform, the reactance offered also varies, and hence current drawn the armature also varies cyclically.
• The r.m.s. current is minimum when machine reactance is  X_d and it is maximum when machine reactance is  X_q. As the reactance offered varies due to the non-uniform air gap, the voltage drops also vary. Hence the impedance of the alternator also varies.
• The terminal voltage also varies cyclically. The voltage at terminals is maximum when current and various drops are minimum, while the voltage at terminals is minimum when current and various drops are maximum.

Concept:

There are two axes in a salient pole synchronous generator, the direct axis (d axis) and quadrature axis (q axis).

The reactance along d-axis is maximum and is given by:

The reactance along the q-axis is minimum and is given by:

Calculation:

Given, V_max = 2700 V and V_min = 2650 V

I_max = 360 A and I_min = 300 A

Concept:

Slip test:

Direct and quadrature axis reactance can be determined by this test.

During this test, the synchronous machine is driven by the separate prime mover at slightly different from synchronous speed.

The field winding is open-circuited and rated frequency and reduced voltage is applied across the terminal.

Under these conditions, the relative velocity between the field pole and the rotating armature m.m.f wave is equal to the difference between synchronous speed and rotor speed.

Explanation:

For maximum current, during slip test on a Synchronous machine, the armature MMF aligns along the q-axis.

Explanation:

Slip Test

Definition: The slip test is an experimental method used to determine the direct-axis (d-axis) and quadrature-axis (q-axis) reactances of synchronous machines, such as synchronous generators or synchronous motors. These reactances are essential for understanding the machine's behavior under different load conditions and for analyzing its stability and operational characteristics.

Working Principle: In a synchronous machine, the reactances in the d-axis and q-axis are different due to the construction of the rotor. The slip test is conducted by rotating the rotor at a speed slightly different (slip) from the synchronous speed. This creates a relative motion between the stator magnetic field and the rotor, allowing the measurement of the reactances.

Procedure:

1. The synchronous machine is run as a motor at a speed slightly below or above the synchronous speed (introducing slip).
2. A balanced three-phase voltage is applied to the stator, and the resulting currents are measured.
3. The voltage and current waveforms are analyzed to determine the values of the d-axis and q-axis reactances. These values are derived based on the impedance seen by the stator windings during the test.

Advantages:

• Provides accurate values for both d-axis and q-axis reactances.
• Essential for designing and analyzing synchronous machines, especially for transient and steady-state stability studies.

Disadvantages:

• Requires specialized equipment and careful setup to ensure accurate measurements.
• Not suitable for machines with significant imperfections or unbalanced windings.

Applications: The slip test is widely used in the design and testing of synchronous machines, including generators in power plants and large industrial motors. It helps in determining machine parameters for stability analysis and control system design.

Correct Option Analysis:

The correct option is:

Option 2: Both, q-axis and d-axis reactance

This option is correct because the slip test is explicitly designed to determine both the direct-axis (d-axis) and quadrature-axis (q-axis) reactances of a synchronous machine. These reactances are critical for understanding the machine's operation under different loading conditions, and the slip test provides a reliable method for their measurement.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: q-axis reactance

This option is partially correct as the slip test does determine the q-axis reactance. However, it is not the complete answer because the slip test also determines the d-axis reactance. The test measures the impedance in both axes to fully characterize the machine's reactances.

Option 3: d-axis reactance

Similar to option 1, this option is partially correct. The slip test does determine the d-axis reactance, but it also determines the q-axis reactance. The complete purpose of the slip test is to measure both reactances for a comprehensive analysis of the machine.

Option 4: Armature resistance

This option is incorrect. The slip test is not used to measure the armature resistance of a synchronous machine. The armature resistance is typically determined using a separate test, such as the DC resistance test, which involves applying a DC voltage to the stator windings and measuring the resulting current.

Conclusion:

The slip test is a vital method for determining the direct-axis (d-axis) and quadrature-axis (q-axis) reactances of synchronous machines. These parameters are essential for the design, analysis, and operation of these machines. While options 1 and 3 are partially correct, they do not capture the complete purpose of the slip test. Option 2 is the correct answer because it accurately reflects the test's ability to measure both q-axis and d-axis reactances, which are crucial for understanding the machine's performance under various operating conditions.