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AEM & Devices Lab Classes 7 02 2011.docx
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Table of report contents

1. Nominal data of transformer.

2. Chart of carrying out tests and list of devices which are used with their metrology descriptions.

3. Responses, built under experiments data and calculations results.

4. Equivalent circuit of transformer with pointing of resistance values, and also vector diagrams at cosφ2 = 1; cosφ2 = 0, at

((φ2 = + π/2), cosφ2 = 0, at (φ2 = − π/2).

5. Short conclusions which explain the results got in-process.

Control questions

1. How is a single-phase transformer arranged?

  1. What do the values of windings EMF of transformer depend on?

  2. What is called the transformation ratio?

  3. What mode of transformer operation permits to define most exactly its transformation ratio?

  4. What losses of energy do take place in a transformer and what do they depend on?

  5. In what case the efficiency of transformer does reach a maximal value?

  6. Why do the parameters of magnetizing branch of equivalent circuit of a transformer determine from experience of idling?

  7. What is called a voltage of short circuit of transformer?

Laboratory work 2

Research of dc generator of parallel excitation Aim of work

To familiarize with the design of DC generator, characterize its characteristics and study the features of service.

Order of work implementation

  1. To familiarize with the generator of parallel excitation, equipment, measuring devices and write down their technical characteristics in a table 2.1:

Table 2.1

Name of device

Conditional denotation on a chart

Measuring system

Class of exactness

Borders of measuring

Cost of division of scale

To write down the nominal parameters of the under test generator in a table 2.2:

Table 2.2

Nominal parameters of generator

Preset parameter

Calculated and observable parameters

Uno,

V

Pnom

W

nnom,

rpm

Inom,

A

Rarm,

Ω

Iexc, nom,

A

Iarm,no,

A

Rexc,nom,

Ω

Enom, V

η

ksat

2. To calculate the value of nominal load current of generator Inom under the formula Inom = Pnom/Unom and write down it in a table 2.2.

3. To measure a resistance of armature circuit of immobile rotor Rarm by the method of ammeter and voltmeter. Results write down in a table 2.2.

4. To assemble a plant for research of DC generator G with the parallel excitation winding EW according to a chart on a fig. 2.1,

Fig. 2.1

where M is a DC motor with the centrifugal stabilizator of speed;

EW, S1, resistor R1, rheostat R2, Aex is an excitation winding, resistor, switch and ammeter of the excitation circuit;

V, AL, S2, R2 is a voltmeter, ammeter, switch and rheostat of load circuit;

5. To define the value of nominal excitation current Iexc,nom by means of experiment, for what to switch on a Sex, turn off the switch S2 and, setting maximal resistances of resistor R1 and rheostat R2, to start up a motor M by means of switch S1. After selfexcitation of generator to switch on S2 and by means of resistor R1 and rheostat of R2 to set the basic values of generator voltage Unom and load current IL. To define the excitation current Iexc,nom with the help of ammeter.

To define basic values: armature current Iarm,nom, resistances of excitation circuit Rexc,nom, EMF Enom and efficiency factor ηnom under formulas:

Iarm, nom = IL − Iexc, nom; Rexc, nom = Unom/ Iexc, nom; Enom = Unom + Rarm Iarm, nom; ηnom = 1 RarmI2arm,nom / [(0,5÷0,7) Unom Iarm, nom].

To write down the results of measuring and calculations in a table. 2.2.

To remove load and excitation of generator, putting the resistor R1 and rheostat R2, in previous position turning off the switches S2 and Sex.

6.To characterize a characteristic of open circuit of generator E = f(Iexc) at Ω = Ωnom.

To measure voltage of generator, which corresponds EMF, excited by a resigual magnetic flux. Smoothly increasing the excitation current, to characterize the rising branch of o.c. characteristic. After achievement of maximal voltage of Umax = 1,2Unom, which corresponds to EMF Emax, smoothly diminishing the excitation current, to characterize the descending branch E = f(Iexc).

Attention! To change the value of excitation current only one-way.

To write down data of measuring in a table. 2.3.

Table 2.3

Experimental characteristic

Averaged characteristic

Rising branch

Descending branch

Iexc, A

U, V

E, V

Iexc, A

U, V

E, V

Iexc, A

E, V

To calculate EMF of armature winding under the formula of E = U + RarmIexc and average out the calculated values of EMF of both branches at the chosen values of excitation current. To write down results in a table. 2. Under data of table. 2 to build averaged characteristic of o.c. E = f(Iexc) and by it to define the saturation coefficient of magnetic chain of machine ksat. To write down results in a table. 1.

7.To characterize an external characteristic of generator U = f (I) in such sequence:

а) to set the nominal mode of operations of generator;

b) to unload a generator to the o.c. mode;

с) to load a generator to the critical point of characteristic;

d) to load a generator to the s.c. mode.

For this purpose to switch on Sex, after selfexcitation of generator switch on S2 and by means of resistor R1 and rheostat R2 to set the basic values of generator voltage U and load current I. Further, not changing position of regulator handle of resistor R1, gradually to increase resistance of rheostat R2 to the maximal value, taking data of measuring devices. Whereupon to turn off S2 and write down the data of measuring devices for the plotting of initial point of characteristic which corresponds to o.c. characteristic of generator.

Because after achievement of critical current Icr the generator voltage arbitrarily falls to the zero, then should beforehand to define by experimental path the approximate value of critical current, and then to continue to characterize a characteristic.

After it to switch on S2 and set the nominal load current IL. Then, gradually to diminish resistance of rheostat R2 to the value at which the load current will attain a critical value Icr, taking data of measuring devices. After achievement by the current I of critical value Icr to decrease resistance of rheostat R2 to the zero and measure the current of short circuit Isc.

After measuring to transfer a generator in the o.c. mode, for what to turn off the switch S2 and increase resistance of rheostat R2 to the maximal value.

To write down the results of measuring in a table. 2.4.

Table 2.4

Measuring

Calculations

U, V

I, A

Iexc, A

Icr, A

Isc, A

ΔU, %

Icr/Inom

Isc/Inom

To calculate the change of generator voltage in percents of nominal voltage ΔU, % under the formula ΔU, % = (Uoc − Unom )∙100/ Unom, and also ratios of currents Icr/Inom, Isc/Inom.

To write down the results of calculations in a table 3. From data of table 3 to build external characteristic of generator U = f (I).

8. To characterize a regulation characteristic Iexc = f(I) at U = Unom. For this purpose at o.c. of generator by means of resistor R1 to set nominal voltage Unom and measure the excitation current. After it to switch on S2 and gradually diminishing resistance of rheostat R2 and keeping up a generator voltage unchanging by means of resistor R1, to take data of measuring devices. The load current it is here necessary to increase until generator voltage can be supported by unchanging.

To write down the results of measuring in a table. 2.5.

Table. 2.5

U = Unom, V

I, A

Iexc, A

Under data of table. 2.5 build regulation characteristic of generator U = f (I).

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