GICHURA ROBERTF17/ 10663/ 2006

UNIVERSITY OF NAIROBI

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

3RD YEAR 2ND SEMESTER LABS.

MACHINES LAB.LOAD CHARACTERISTICS OF A SHUNT GENERATOR

EXPERIMENT: LOAD CHARACTERISTICS OF A SHUNT GENERATOR

Objective:To determine the characteristics of a separately excited shunt generator and to determine

the armature resistance using the d.c drop test.

Apparatus:Crempton Parkinson Motor Generator set0 – 150V D.C voltmeter0 – 10A D.C ammeter0 – 30A D.C ammeter5 kw loading resistanceVariable resistorsShorting switchD.C power source

Theory:To obtain dc electricity, one may prefer an available ac source with an electronic rectifier

circuit. Another possibility is to generate dc electricity directly. Although the latter method is becoming obsolete, it is still important to understand how a dc generator works. This section provides a brief discussion of the basic issues associated with dc generators.

Characteristics of D.C generators

The following are the three most important characteristics of d.c shunt generators;1.) External characteristic, V/I

Also called the voltage regulating curve, which gives the relationship between the terminal voltage, V and the load current, I.

2.) Internal or Total characteristic, E/Ia

Which gives the relationship between the e.m.f E actually induced in the armature and the armature current, Ia.

3.) No-load saturation characteristic, E0/If

Also known as the open-circuit characteristic (O.C.C) or the magnetic characteristic. It shows the relationship between the no-load generated e.m.f in armature, E0 and the field or exciting current If at a given fixed speed.

A generator can be self or separately excited. In this experiment, a separately excited generator is used.

Separately-excited generatora. Internal and External characteristics For a separately excited generator giving its rated no-load voltage of E0 for a certain constant field current, if there were no armature reaction and armature voltage drop, then this voltage would remain constant. But when the generator is loaded, the voltage falls due to these two causes thus giving slightly dropping characteristics.

b. No-load saturation characteristic, E0/If The circiut diagram below can be used to obtain the experimental data for the test;

The exciting or field current If can be obtained from an external independent source. It can also be varied from zero upwards by use of a potentiometer and its value read by an ammeter connected in the field circuit as shown above.The voltage equation for a d.c generator is;Eg = ΦZN/60 * P/A [V] so that for a constant speed the relation above can be written as E = kΦ. When If is increased from its initial small value, the flux, Φ and hence the generated e.m.f. increases directly as current as long as the poles are unsaturated.

Load saturation curve, V/If The load saturation curve refers to a curve that shows the relation between the terminal voltage and the field current when the generator is loaded. The curve can be deduced from the no-load saturation curve provided the values of the armature reaction and armature resistance are known. While considering this, account is taken of the demagnetizing effect of armature reaction and the voltage drop in armature which are practically absent under no-load condition.

Critical Resistance for Shunt GeneratorThe field windings are connected back to the armature and the machine run. Due to

residual magnetism, some initial e.m.f and hence current would be generated. This current while passing through the field coils will strengthen the magnetism of the poles. This will increase the pole flux which will further increase the generated e.m.f. increased e.m.f means more current which further increases flux and so on.

The mutual reinforcement of e.m.f and flux proceeds on till equilibrium is reached at some point like p. the point lies on the resistance line OA of the field winding. The voltage OL corresponding to the point p represents the maximum voltage to which the machine will build up with R as the field resistance. OB represents a smaller resistance and the corresponding voltage OM is slightly greater than OL.

In case the field resistance is increased, the slope of the resistance line increases hence the maximum voltage to which the generator will build up at a given speed decreases. If R is increased so much that the resistance line does not cut the O.C.C at all then the machine will obviously fail to excite i.e. there will be no “build up” of the voltage. If the resistance line just lies along the slope, then with the value of field resistance the machine will just excite. The value of the resistance represented by the tangent to the curve is known as the critical resistance Rc for a given speed.

Procedure:The circuit was connected as shown below;

The motor was then started with field initially at minimum resistance position and its speed adjusted to 1500rpm. The speed was kept constant throughout the experiment. With the field connected in the build up position, the generator was switched on with the field resistance at maximum resistance position.

The generator was setup up at 110V at no load by adjusting the field resistance and left at this position through out the experiment. The load was then switched on, adjusting the current to a low value. The voltage, field and line currents were noted.

The load was increased in steps while taking down the parameters. At maximum load, the variable resistance was adjusted to maximum resistance and the load switched on. More readings were taking for output voltage, field current and line current, cutting the resistance in stages down to a complete short.

To the set the variable resistance was first switched off, then the other loads gradually switched off and finally shorted down

Results:

Table1: Terminal voltage, line current and field current and varying values of load power for a d.c shunt generator

Load power[Kw]

Voltage[V]

Line current[A]

Field current[A]

0 110 0 1.70.5 102 3.8 7.31.0 92 7.3 1.41.5 95 11 1.42.0 88 14.5 1.32.5 82 16.5 1.23.0 80 19.8 1.23.5 75 21.6 1.14.0 72 23.7 1.14.5 67 25.2 1.05.0 65 26.8 1.0

Table2: Terminal voltage, line current and field current with shorting resistancesVoltage [V] 13 15 9 5 0Line current [A] 11 13.8 11.6 10.5 9.5Field current [A] 0.2 0.2 0.2 0.1 0

The armature resistance Ra for the D.C drop test was Ra = 0.477Ω

Discussion:

External characteristicsA plot of terminal voltage against line current was made from the results in Table1 to

establish the external characteristics.

Internal characteristics This was determined by plotting a graph of generated voltage, E against armature

current.The following relations were used to obtain the values of armature current

Ia = I + If

E = V + IaRa

Table3: Internal characteristics

E [V] 111.5 105 96 101 96 90 90 86 84 80 78Ia [A] 1.7 5.4 8.7 12.4 15.8 17.7 21.0 22.7 24.8 26.2 27.0

The armature resistance obtained from the d.c drop test was Ra = 0.488Ω

Conclusion:The objective of the experiment was achieved because the characteristics of a separately

excited shunt generator were determined. The terminal voltage/ line current characteristics were found to be linear whereas the generated e.m.f / armature current characteristics were found to be non-linear with increasing armature currents, reasons of which were cited in the theory section as being due to armature reaction and armature voltage drop. The armature resistance was graphically determined as Ra = 0.488Ω.

References:1. G. McPherson, An Introduction to Electrical Machines and Transformers, New York:

Wiley, 1981.2. S. J. Chapman, Electric Machinery Fundamentals, New York: McGraw-Hill, 1991.3. Edward Hughes, Electrical Technology; 4th Edition4. University of Nairobi “Machines lab. Manual”, Dept. of Electrical and Electronics

Engineering, 2005.