Open Circuit Test On Three Phase Alternator 352x5r

PRE LAB TEST 1)

The power factor of an alternator under short circuit conditions is almost near

zero leading zero lagging unity zero leading or zero lagging depending upon the type of alternator

2)

In a 3-phase cylindrical-rotor alternator,

field mmf leads the ait-gap flux and the ait-gap flux leads the armature mmf the armature mmf leads the ait-gap flux and the ait-gap flux leads the field flux the armature mmf leads the ait-gap flux and the ait-gap flux lags the field flux field mmf leads the ait-gap flux and the ait-gap flux lags the armature mmf

3)

A cylindrical-rotor synchronous generator will deliver maximum power output when

load angle = synchronous impedance angle load angle = internal power-factor angle 1,2,4load angle = 90⁰ none of above input power factor is unity

4)

5)

THEORY

A 3-phase synchronous generator is operating at a constant load while the excitation is unity pf current. If the excitation is now increased, the pf will

become leading

become lagging

remain at unity

become zero

6. A synchronous motor with negligible armature resistance runs at a load angle of 20⁰ at If supply frequency is increased by 10%, keeping other parameters constant, the new load (b) (c) (d)

16⁰

18⁰

20⁰

22⁰

The most commonly used machine for generation of electrical power for commercial purpose is the synchronous generator or alternator. An alternator works as a generator when its rotor carrying the field system is rotated by a prime-mover which in this case is DC shunt motor. The terminal voltage of an alternator changes with load. Alternators are by far the most important source of electric energy. Alternators generate an AC voltage whose frequency depends entirely upon the speed of rotation. The generated voltage value depends upon the speed, the dc field excitation and the power factor of the load. As the DC field excitation of an alternator is increased, its speed being held constant, the magnetic flux, and hence, the output voltage, will also increase in direct proportion to the current. However, with progressive increases in DC field current, the flux will eventually reach a high enough value to saturate the iron in the alternator. Saturation in the iron means that there will be a smaller increase in flux for a given increase in DC field current. Because the generated voltage is directly related to the magnetic flux intensity, it can be used as a measure of the degree of saturation. When an alternator delivering full rated output voltage is suddenly subjected to a short-circuit, very large currents will initially flow. However, these large short-circuit currents drop off rapidly to safe values if the short-circuit is maintained. The output voltage of an alternator depends essentially upon the total flux in the air-gap. At no load this flux is established and determined exclusively by the DC field excitation. Under load, however, the air-gap flux is determined by the ampere-turns of the rotor and the ampere-turns of the stator. The latter may aid or oppose the MMF (magnetomotive force) of the rotor depending upon the power factor of the load. Leading power factors assist the rotor, and lagging power factors oppose it. The open-circuit test or the no-load test, is performed by driving the generator at its rated speed while the armature winding is left open. The field current is varied in suitable steps and the corresponding values of the open-circuit voltage varied in suitable steps and corresponding values of the open-circuit voltage between any two pair of terminals of the armature windings are recorded. The OCC follows a straight-line relation as long as the magnetic circuit of the synchronous generator does not saturate. In the linear region, most of the applied mmf is consumed by the air-gap; the straight line is appropriately called the airgap line. As the saturation sets in, the OCC starts deviating from the air-gap line.

PROCEDURE: 1)Close the DPST (Double Pole Single Throw) switch connecting the armature winding of DC shunt motor to supply. Rheostat is used as a potential divider. It is used to increase or decrease the stator voltage of DC shunt motor.

2) Bring the DC shunt motor to rated speed (1500rpm) by clicking the arrows on rheostat. 3) Now, even though field voltage of the generator is zero, we can observe some value at generator output due to residual magnetism. 4) Alternator is brought to synchronous speed (1500rpm) by using DC shunt motor. 5) As DC shunt motor is connected mechanically to alternator, So both machines rotate with same speed. Therefore we can observe speed of alternator in 6) Close the DPST switch connecting the alternator to supply through rheostat. 7) Now increase the field current of alternator by using the rheostat connected to field winding of the alternator. 8) Observe the values of ammeter and voltmeter. 9) Store this data by clicking “Start Storing Data” 10) Go on repeating this procedure till sufficient readings are stored. 11) Now display the data by clicking “Show data”. For Ex it will display data as shown Vfa=field voltage of alternator Ifa=field current of alternator Vin=input voltage DC motor Iin=input current of DC motor Vt=terminal voltage 12) Now select one of the graph from available graphs and use “Generate graph” to plot the graph. For Ex. 13) Slowly decrease the excitation of DC shunt motor to zero. 14) Switch off supply.

POST Lab Test 1)

A cylindrical-rotor alternator on load draws a current at a leading pf angle Ѳ and with an i Ψ. Under this operating condition, electrical angle between fiend axis and armature-reactio

90 + Ψ

90 + Ѳ

90 - Ѳ

90 – Ψ

2)

3)

4)

Which of the following will change in a 3-phase synchronous motor, as a consequen variations? 1. Pull-out torque 2. Torque angle 3. Magnitude and power factor of stator cu power Select the correct answer using the codes given below :

1 and 3

2 and 4

2, 3 and 4

1, 2 and 3

A synchronous motor is operating on no load at unity power factor. If the field current is i will become

leading and the current will decrease

lagging and the curren

lagging and the current will decrease

leading and the curren

The power-angle characteristics of a salient-pole synchronous machine contains a second because

the air-gap flux density has a second harmonic component the frequency has a second harmonic component of the slot harmonics of the difference in reluctance of air-gap in the direct and quadrature axes.

5)

In a round-rotor alternator, reactive power is maximum at a load angle of

90⁰ 0⁰

180⁰