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 I_nom under the formula I_nom = P_nom/U_nom and write down it in a table 2.2.

3. To measure a resistance of armature circuit of immobile rotor R_arm 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, A_ex is an excitation winding, resistor, switch and ammeter of the excitation circuit;

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

5. To define the value of nominal excitation current I_exc,_nom by means of experiment, for what to switch on a S_ex, 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 U_nom and load current I_L. To define the excitation current I_exc,_nom with the help of ammeter.

To define basic values: armature current I_arm,_nom, resistances of excitation circuit R_exc,nom, EMF E_nom and efficiency factor η_nom under formulas:

I_{arm, nom} = I_L − I_{exc, nom}; R_{exc, nom} = U_nom/ I_{exc, nom}; E_nom = U_nom + R_arm I_{arm, nom}; η_nom = 1– R_armI²_arm,nom / [(0,5÷0,7) U_nom I_{arm, 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 S_ex.

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

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 U_max = 1,2U_nom, which corresponds to EMF E_max, smoothly diminishing the excitation current, to characterize the descending branch E = f(I_exc).

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 + R_armI_exc 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(I_exc) and by it to define the saturation coefficient of magnetic chain of machine k_sat. 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 S_ex, 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 I_cr 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 I_L. Then, gradually to diminish resistance of rheostat R₂ to the value at which the load current will attain a critical value I_cr, taking data of measuring devices. After achievement by the current I of critical value I_cr to decrease resistance of rheostat R₂ to the zero and measure the current of short circuit I_sc.

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, % = (U_oc − U_nom )∙100/ U_nom, and also ratios of currents I_cr/I_nom, I_sc/I_nom.

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 I_exc = f(I) at U = U_nom. For this purpose at o.c. of generator by means of resistor R₁ to set nominal voltage U_nom 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).