Voltage Regulation of synchronous generator by EMF and MMF method.

Experiment No.: – 05

Name of the Experiment:

Voltage Regulation of synchronous generator by EMF and MMF method.

Apparatus Required:

Sl. No.NameSpecificationQuantity
1DC Motor Coupled with Alternator1 no.
2Ammeter(0-5)A, MC1 no.
3Ammeter(0-10)A, MI1 no.
4Voltmeter(0-150)V, MC1 no.
5Voltmeter(0-500)V, MI1 no.
6Rheostat(0-80)Ω, 5A1 no.
7TachmeterDigital type, 1 no.
8Connecting WiresPVC Insulated CoperAs per required.
Table No. 4.1

Circuit Diagram: –

Circuit diagram for EMF and MMF Method

Theory: –

The regulation of a three-phase alternator can be predetermined by conducting the open circuit (OC) test and short circuit (SC) test. The methods employed for determination of regulation are EMF or synchronous impedance method. MMF Method is also known as Ampere Turn Method.

Open Circuit Test-

The open-circuit test, also known as the no-load test or the saturation test, is an important procedure conducted on alternators to evaluate their performance under no-load conditions and to determine key parameters such as core losses and magnetizing impedance.

Short Circuit Test-

The short-circuit test, also known as the locked-rotor test or the impedance test, is a crucial procedure conducted on alternators to determine their impedance characteristics and evaluate their performance under short-circuit conditions.

It is obtained by short circuiting the armature (Stator) through a low resistance or ammeter. The excitation is adjusted to obtain 1.25 times of the full load current. During this test the speed is kept constant which need not be synchronous.

Voltage Regulation-

The regulation is defined as the change in output voltage from no-load to full-load.

\[\color\red{\%Voltage\; regulation= \frac{E_0-V}{V} X \;100}\] \[\color\red{Where\;E_0\;is\;No-load \;voltage\;and\;V\;is\;Full-load\;voltage.}\] \[\color\red{E_0=\sqrt{{(Vcos\;\phi+IR_a)^2+(Vsin\;\phi\pm IX_s)^2}}}\]

Note: use ‘+’ sign for lagging power factor and use ‘-‘ sign for leading power factor.

Procedures: –

For Open Circuit Test-

  1. Make all the connections as per the circuit diagram.
  2. Set its field potential divider to zero output position.
  3. Set the field resistance of motor to minimum value.
  4. Switch ON the DC supply and start the motor with the help of the starter.
  5. Adjust the speed of motor to the synchronous speed of alternator with the help of the field rheostat and note the meter readings.
  6. Switch ON the Supply to alternator field and increase excitation in steps and note the corresponding meter readings. take upto 10% more than the rated voltage.

For Short-Circuit Test-

  1. Make all the connections as per the circuit diagram.
  2. Switch ON the supply and start the motor with the help of starter. Keep its field rheostat in minimum resistance position.
  3. Adjust the speed of the motor to the synchronous speed with the help of field rheostat.
  4. Switch ON the supply to the alternator field. note the ammeter readings for different values of excitation. take the readings upto 1.25 times of rated armature current.

Measurement of Armature Resistance: –

Measure the armature resistance of all the three phases separately by ammeter and voltmeter method or by ohmmeter accurately.

Observation Table: –

For Open Circuit Test: –

Sl. No.Field Current (IF) in Amp.Induced Voltage (VI) in Volts.
1
2
3
4
5
6
Table No. 4.2

For Short Circuit Test-

Sl. No.Field Current (IF) in Amp.Short Circuit Current (Isc) in Amp.
1
Table No. 4.3

Calculations: –

\[\color\red{Z_s=\frac{open\;circuit\;voltage}{Short \;circuit \;armature \;current}}\] \[\color\red{X_s=\sqrt{(Z_s)^2-(R_a)^2}}\] \[\color\red{R_a=r_{ac}=1.3*r_{dc}}\] \[\color\red{r_{dc}=\frac{R}{2}}\] \[\color\red{E_0=\sqrt{{(Vcos\;\phi+IR_a)^2+(Vsin\;\phi\pm IX_s)^2}}}\] \[\color\red{\%Voltage\;Regulation=\frac{(E_0-V)}{V}*100}\]

Precautions: –

  1. The motor field resistance should be kept in the minimum position.
  2. The alternator field excitation should be kept in minimum position.
  3. Initially keep all switches are in open position.

Conclusion: –

To be written by Student.

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For Your Reference:

In synchronous generators, voltage regulation is crucial to maintain a stable output voltage under varying load conditions. Both EMF and MMF methods are utilized for voltage regulation in synchronous generators.

  1. EMF Method:
  • In the EMF method, voltage regulation is achieved by adjusting the excitation current supplied to the generator’s field winding.
  • The induced electromotive force (EMF) in the stator windings is directly proportional to the rate of change of magnetic flux linking these windings. By varying the excitation current, the strength of the magnetic field produced by the rotor can be controlled.
  • Increasing the excitation current increases the magnetic field strength, resulting in higher induced EMF and consequently higher terminal voltage. Conversely, reducing the excitation current decreases the terminal voltage.
  • Therefore, by controlling the excitation current, the terminal voltage of the synchronous generator can be regulated to maintain it within acceptable limits under different load conditions.
  1. MMF Method:
  • In the MMF method, voltage regulation is achieved by adjusting the magnetomotive force (MMF) produced by the generator’s field winding.
  • The MMF is directly proportional to the excitation current flowing through the field winding and the number of turns in the winding.
  • By adjusting the excitation current, the MMF can be controlled, which in turn affects the strength of the magnetic field produced by the generator.
  • Similar to the EMF method, increasing the excitation current increases the MMF and hence the magnetic field strength, resulting in higher terminal voltage. Decreasing the excitation current decreases the terminal voltage.
  • Thus, by controlling the excitation current and hence the MMF, the terminal voltage of the synchronous generator can be regulated.

In both methods, the objective is to adjust the excitation current to maintain the terminal voltage of the synchronous generator within acceptable limits as per the load requirements. Proper voltage regulation ensures the stable operation of electrical systems powered by synchronous generators and prevents voltage fluctuations that could damage equipment or disrupt operations.

Some Viva-Voce Questions related to this Experiment.

What is EMF & MMF method ?

Answer: See above discussion.

Draw the circuit Diagram.

Answer:

Circuit diagram for EMF and MMF Method

What are the ratings of Apparatus & Machines ?

Answer: See as per your experiment.

How to perform Open circuit test and Short circuit test ?

Answer: Open Circuit Test (OC Test)

  1. Run the alternator at rated speed with no load.
  2. Gradually increase field current and measure open-circuit voltage (Voc).
  3. Plot the OCC curve (Voltage vs. Field Current).

Short Circuit Test (SC Test)

  • Run the alternator at rated speed.
  • Short-circuit the armature terminals through an ammeter.
  • Increase field current and record short-circuit current (Isc).
  • Plot the SCC curve (Current vs. Field Current)

Write the formula of Voltage Regulation.

Answer:

\[\color\green{\%\;Voltage\;Regulation\;=\;}\color\red{\dfrac{E_{ph}\; −\;V_{ph}}{V_{ph}}\times100}\]

where:

  • Eph​ = No-load (open-circuit) voltage per phase
  • Vph​ = Full-load terminal voltage per phase

What is meant by voltage regulation ?

Answer: Voltage regulation of an alternator refers to the change in terminal voltage when the load varies from full load to no load, while keeping the field current and speed constant. It indicates how well the alternator maintains voltage under different load conditions.

\[\color\green{\%\;Voltage\; Regulation\;=\;}\color\red{\dfrac{E_{ph}\; −\;V_{ph}}{V_{ph}}\times100}\]

What is meant by Synchronous Impedance ?

Answer: Synchronous Impedance is the equivalent impedance of an alternator when operating under synchronous conditions. It accounts for the combined effects of armature resistance (Ra​) and synchronous reactance (Xs​), given by:

\[\color\green{Z_s\;=\;R_a\;+\;jX_s}\]

What is OC test ?

Answer: The Open Circuit Test (OC Test) is conducted to determine the open-circuit characteristics (OCC) of an alternator, which helps in calculating synchronous impedance and understanding the no-load behavior.

What is SC test ?

Answer: The Short Circuit Test (SC Test) is performed on an alternator to determine its synchronous impedance and armature reaction effect under short-circuit conditions.

What is meant by MMF or field ampere turns ?

Answer: MMF (Magnetomotive Force) or Field Ampere-Turns refers to the magnetic potential required to establish flux in a magnetic circuit. It is the product of field current (I) and the number of turns (N) in the winding, given by MMF = N × I (Ampere-Turns), driving magnetic flux in machines.

What are the advantages and disadvantages of estimating the voltage regulation of an
Alternator by EMF method ?

Answer:

Advantages of EMF Method:
  1. Simple and Easy: Requires only open-circuit and short-circuit tests.
  2. No Load Required: Can estimate voltage regulation without actual load conditions.
  3. Quick Calculation: Uses basic equations and impedance calculations.
  4. Widely Used: Common in practical applications due to its simplicity.
Disadvantages of EMF Method:
  1. Less Accurate: Overestimates voltage regulation.
  2. Ignores Armature Reaction Effects: Assumes armature reaction behaves like reactance.
  3. Assumes Constant Synchronous Impedance: Not valid for all loads.
  4. Overly Conservative: Leads to higher-than-actual regulation values.

Why voltage regulation on alternator is negative for leading power factor ?

Answer: Negative voltage regulation in an alternator occurs at a leading power factor because the armature reaction is magnetizing, strengthening the main field. This increases the induced voltage (Eph​), making the full-load terminal voltage (Vph​) higher than the no-load voltage, resulting in negative regulation (Vph>Eph).

What is meant by armature reaction ?

Answer: The interaction between flux set up by the current carrying armature and the main is defined as the armature reaction.

What are the causes of changes in terminal voltage of Alternators when loaded ?

Answer:

  • Voltage variation due to the resistance of the winding, R
  • Voltage variation due to the leakage reactance of the winding, Xt
  • Voltage variation due to the armature reaction effect, Xa

Name the various methods for predetermining the voltage regulation of 3-phase
Alternator.

Answer:

  • Synchronous impedance / EMF method
  • Ampere-turn / MMF method
  • Potier / ZPF method

What is the necessity for predetermination of voltage regulation ?

Answer: Most of the Alternators are manufactured with large power rating, hundreds of kW or MW, and also with large voltage rating up to 33kV. For Alternators of such power and voltage ratings conducting load test is not possible. Hence other indirect methods of testing are used and the performance like voltage regulation then can be predetermined at any desired load currents and power factors.

Define the term voltage regulation of alternator.

Answer: The voltage regulation of an Alternator is defined as the change in terminal voltage from no-load to full-load condition expressed as a fraction or percentage of terminal voltage at load condition, the speed and excitation remaining same.