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
U_no, V	P_nom W	n_nom, rpm	I_nom, A	R_arm, Ω	I_{exc, nom}, A	I_arm,no, A	R_exc,nom, Ω	E_nom, V	η	k_sat

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, S ₁, resistor R ₁, rheostat R ₂, A_ex is an excitation winding, resistor, switch and ammeter of the excitation circuit;

− V, A_L, S ₂, R ₂ 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 S ₂ and, setting maximal resistances of resistor R ₁ and rheostat R ₂, to start up a motor M by means of switch S ₁. After selfexcitation of generator to switch on S ₂ and by means of resistor R ₁ and rheostat of R ₂ 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 R ₁ and rheostat R ₂, in previous position turning off the switches S ₂ 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 = 1,2 U_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
↑ I_exc, A	U, V	E, V	↓ I_exc, A	U, V	E, V	I_exc, 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 S ₂ and by means of resistor R ₁ and rheostat R ₂ to set the basic values of generator voltage U and load current I. Further, not changing position of regulator handle of resistor R ₁, gradually to increase resistance of rheostat R ₂ to the maximal value, taking data of measuring devices. Whereupon to turn off S ₂and 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 S ₂ and set the nominal load current I_L. Then, gradually to diminish resistance of rheostat R ₂ 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 R ₂ 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 S ₂ and increase resistance of rheostat R ₂ 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	I_exc, A	I_cr, A	I_sc, A	ΔU, %	I_cr/I_nom	I_sc/I_nom

To calculate the change of generator voltage in percents of nominal voltage Δ U, % under the formula Δ U, % = (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 R ₁ to set nominal voltage U_nom and measure the excitation current. After it to switch on S ₂ and gradually diminishing resistance of rheostat R ₂ and keeping up a generator voltage unchanging by means of resistor R ₁, to take data of measuring devices. The load current it is here necessary to increase until generator voltage can be supported by unchanging.