6 Laboratory tasks. Content of the work

6.1 To study the theoretical material by the recommended literature. To study the principle scheme of the stand and features of its functioning.

Purposet, features of structural, functional,schematic construction of three-phase rectifier circuitry ,as well as features of the processes of their operation. Read the description of the laboratory stand presented in Sec. 3.

6.2 Investigate multi-phase rectifier circuits of two types (three-phase single-cycle (k = 1) and three-phase roadway (k = 2)) with the active and inductive loads.

6.2.1 Investigate external (load) characteristics of rectifiers.

6.2.2 Determine the output resistance of rectifiers..

6.2.3 Determine the percentage reduction of output voltage of rectifiers.

6.2.4 Determine the coefficients of usage by power of transformer`s windings ТV1.

6.2.5 Investigate the processes of functioning of rectifiers with active and inductive load:

• switching process(overlapping of phases);

• fluctuations in the output circuit of rectification block and in the circuit of rectifier`s load;

• electrical processes (currents and voltages) in the elements and rectifier circuits.

6.3 To process the experimental data. Make conclusions on the results of laboratory work.

7 Methods of laboratory work processing

7. To the Paragraph 6.1. Program and order of the study of theoretical issues.

To study the purpose, pecularities of structural, functional, schemo-technical constructing of three-phase rectifying devices and processes of their operation based on this laboratory work and recommended literature [2.1 ... 2.5]. To study the features of schemo-technical implementation and operation of the laboratory stand (see point 3). Get test questions by the studied material and prepare answers to them. Check with the teacher form and volume of their presentation in the report. NOTE. Stop further execution of laboratory work without the permission of the teacher. An obligatory condition for permission to perform work is an understanding of features of schemo-technical performance of the laboratory stand (see point3).

7.2 To the Paragraph 6.2.Research of multiphase rectification circuits of two types (three-phase single-cycle (k = 1) and three-phase bridge (k = 2)) with active and inductive loads.

Providing the required scheme of research - a three-phase single-cycle (k = 1 - Fig. 3.2) and three-phase bridge (k = 2 - Fig. 3.3) - by means of switch of selection of researched circuit (see Fig. 3.1). To ensure the required parameters of the circuit elements and operating modes of the investigated rectifier can be used Shortcuts(keys of fast access)“G1”, “ТV1”, “L1”, “R_н”, “SW1”, “SW2” (рiс. 3.1).

7.2.1To the Paragraph 6.2.1 Study of external (load) characteristics of the rectifiers.

The external characteristics of the rectification circuit is a dependence of output voltage U_н of current rectifier I_н U_н = f(I_н) while U₁ = U_C = U_{G1 ном} = const (U_A = U_B = U_C = U_{G1 ном} = const).

Gradually increasing current throught the load to a maximum value, fix in the whole range of change of current throught the load 5 ... 7 of its values, uniformly dividing the range of measurements. Measure for these values of current I_н values R_н, I_а, U_а, U_н, U₁ = U_G1 , I₁ = I_G1. Put the values to the table 7.1(k), где k – type of rectification circuit.

Таble 7.1(k) – Results of researches

№ meas.

Experimental data

Calculated data

RL,

Iн, A

Uн, V

Uа, V

U1, V

I1, A

Iа, A

ΔUн, V

ΔIн, A

S1, VА

rb,

S2, vА

Pн, W

K2

K1

1 2 …

Method for determination of external characteristics is following:

with the help of switch of selection researched circuit (see Fig. 3.1) select the type of rectifier circuit "Three-phase single-cycle" – k = 1, or“Three-phase Bridge” – k = 2);

• edit the parameters of the source G1 (phase voltage U_А = U_В = U_С = U_{G1 ном}, working frequency f_c = f_{c ном}), using the key “G1”on elements panel (see Fig. 3.1) for the appearance of the editing window;

• edit the parameters of the transformer ТV1: establish the necessary transformation coefficient n₁₂ and active resistance

• r₂ = r_{2 ном} of secondary transformer windings ТV1 phase a (r_a), phase b (r_b), phase c (r_с), (r_a = r_b = r_с = r_{2 ном});

• edit the parameters of throttle L1: establish a nominal value of inductance of the throttle L1 = L1_ном ;

• using the switch SW2 (see Fig. 3.1), to ensure the desired character of the load:on the active load switch SW2 is closed to conducting state, on the inductive load - in an open non-conducting state

• switch off the load moving the switch SW1 to the open non-conducting state;

• Begin the simulation by clicking the "Start";

• Measure by voltmeters PV1, PV2 and PV3 of voltages U_А, U_а of primary and secondary transformer windings ТV1 phase А and а and U_{н хх} – voltafe of no-load running on the output of rectifier. The result write in the table 7.1 (k);

• connect the resistance R_L of the load, transferring the switch SW1 in the closed conducting state;

• changing the resistance R_L of the load (using the slider “R_L” - See Fig. 3.1, bottom right corner) from the minimum (maximum) to a maximum (minimum) (eg, R_L = 1, 2, 5, 10, 25, 50 and 100 Оhm), Measure by voltmeters PV1, PV2 and PV3 of voltages U_А , U_а phases А and а of transformer ТV1 and U_L – in the rectifier’s load, ampermeter PА1, PА2 and PА3 of currents I_А and I_а, used from phases А and а, and current I_н. The measurements put in the table 7.1.1(k), where show I₁ = I_G1, U₁ = U_{G1 ном});

• using the results of research to build the external characteristics

7.2.2 1To the Paragraph 6.2.2. Defining of the output impedance of rectifiers

Output resistance of rectifier is defined on the base of measurements p. 7.2.1 (See table 7.1 (k)) by formula:

r_b = U_н/ I_н ;

U_н = U_{н max} – U_н;

I_н = I_{н max} – I_н,

wher U_{н max} , I_{н max} – maximum value of voltage and current on the load (See table 7.1 (k)).

7.2.3 Determination of percentage decreases of the output voltage of rectifiers.

Percentage decreases U_н% of output voltage of the rectifier while changing from no-load running with the U_{н хх} to nominal load with output voltage U_{н ном} is defined by formula:

.

7.2.4 Determination of coefficient of usage by power of transformer’s windings ТV1.

Coefficient of usage by power of transformer K_тр and its primary K₁ and secondary K₂ windings in the rectifier’s circuit:

K_тр = P_н / S_тр; K₁ = P_н / S₁; K₂ = P_н / S₂;

S₁ = 3U₁I₁; S₂ = 3U_aI_a ; S_тр = (S₁+S₂)/2; P_н = I_нU_н ,

wherе P_н – active power in the circuit of rectifier’s load;

S_тр, S₁, S₂ – Overall power of the transformer ТV1 and its primary and secondary windings

Determination of coefficient of usage by power of transformer’s primary and secondary windings, as well as of the whole transformer in each of the schemes of rectifier with active and inductive we need to establish a nominal value of current I_н ном through, measure by PА1, PА2 и PА3, PV1, PV2 и PV3 and put data to the table 7.2.

7.2.5 To the Paragraph 6.2.5Investigation of the functioning of three-phase rectifier circuits.

To study the functioning of rectifier’s schemes it is necessary, using the switch of researched circuit selection, to choose (see Figure 3.1.) the needed circuit (with active or inductive loads). The study of pulsations in the output circuits of the rectification block rectification RB and rectifier’s load For measurement of parameters in variable components at the output of rectification block RB of frequency f_вu and width(размах) U_в~ of voltage u₀(t) and width I_в~ of current i₀(t) it is necessary to switch on (See pic. В.13.(В.1)) the oscilloscope to the control points (see table 3.2): in the first case to “₀,U” – voltage u₀(t) at the output of Rectification block

RB, in the second case to “P₀, I” – current i₀(t) at the output of Rectification block

RB. For measurement of parameters in variable components at the output of rectification block RB of frequency f_нu and width(размах) U_н~ of voltage u_н(t) and width I_н~ of current i_н(t) it is necessary to switch on (See pic. В.13.( В.1)) the oscilloscope to the control points in the first case to “R_н, U” – voltage u_н(t) on the load R_н of rectifier (see table. 3.2), in the second case to “R_н, I” – current i_н(t) through the load R_н of rectifier.

Experimental values of frequencies f_вu and f_нu of researched processes are difined by the oscilogramms u₀(t) and u_н(t). For this you need to measure on a mask length of time oscilloscope the duration of a period Т of observed curve, using the values of n multiplier "Time/Division" of the switch of the oscilloscope with the same name. Measured time interval Т is defined by multiplications of two valuesТ = n : lengths of measured time interval on the screen of the oscilloscope horizontally in DEVISIONS and the value n of time for a division in switch "Time/Division". In the given case

.

Таble7.2 – Results of researches

Parameters(measured or calculated)	Three-phase single-cycle circuit		Three-phase bridge circuit
	Active load	Inductive load	Active load	Inductive load
Iн, А
Uн, V
I1 = IG, А
U1 = UG1, V
I2 = Ia, А
U2 = Ua, V
Pн = Uн Iн, W
S1 = 3 U1 I1, VА
S2 = 3 U2 I2, VА
Sтр = (S1+S2)/2,VА
K1 = Pн / S1
K2 = Pн / S2
Kтр = Pн / Sтр

Measuring parameters of variable component (f_вu, U_в~, I_в~, f_нu, U_н~, I_н~) oscilloscope is used in the mode of close input (the switch of constant component SS is switched off– pic. В.13 ( В.1)) of amplifier of vertical amplification.

Method of research of filtering properties of rectifying filters involves the following steps (the results of measurements recorded in the table 7.3(k)):

• using the switch of circuit selection, chose(see pic. 3.1) the needed circuit of rectifier– “Three-phase single-cycle circuit” (k = 1 – рiс. 3.2), “Three-phase bridge circuit” (k = 2 – рiс. 3.3);

• correct the transformer’s parameters ТV1 – transformation coefficient n₁₂ of the transformer n₁₂ = n_{12 ном} (See рiс. 3.1 – key “ТV1”);

• correct the parameters of the source G1 (voltage U_G1 = U_{G1 ном}, needed frequency f_с = f_{с ном}), using the key “G1” on the elements panel (see. рiс. 3.1) window of editing;

• correct the element’s parameters L1 of rectifying filter of the rectifier, using the key “L1”, on the elements panel (see. piс. 3.1), to set the nominal value L1_ном;

• using the switch SW2 (см. рис. 3.1), provide the needed load: in active load the switch SW2 is set in closed conducting state, in inductive load – in opened non-conductive state;

• begin modeling, pressing “Start” – рiс. 3.1;

• set the resistance R_н = R_ном in the load of rectifier, using slider “R_н” (see. рiс. 3.1), providing the value of the current I_н = I_{н ном};

• to take the values by voltmeter PV3 and ampermeter PА3. Results of measurement U_н и I_{н ном} put into the table 7.3 (k);

• measure the width U_в~ and frequency f_вu of voltage pulsation on the output of rectification block RB using the voltage oscilograms u₀(t) (control point Р₀, U from the table 3.2);

• measure the width I_в~ of current pulsation on the output of rectification block RB using the current oscilograms i₀(t) (control point Р₀, I);

• measure the width U_в~ and frequency f_вu of voltage pulsation on the output of rectification block RB using the voltage oscilograms u₀(t) (control point R_н, U from the table 3.2);

• measure the width I_в~ of current pulsation on the output of rectification block RB using the current oscilograms i_н(t) (R_н, I – тable. 3.2);

• to make the same measurements for currents I_н through the load, 0,75I_{н ном}, 0,5I_{н ном} и 0,25I_{н ном}, using slider “R_н” – see pic.. 3.1 (results put to the table. 7.3 (k));

• Research on this method to perform for two versions of the rectifier load: the active and inductive loads;

• for the inductive load perform similar measurements at different values of inductance of throttle L1 – results put to the table 7.4(k);

• using the results of the experiment (Table. 7.3(k) и 7.4(k)), calculate and fill the tables 7.3(k) и 7.4(k) with respect to next formulas:

The experimental results allow to define the normalized frequency:

Coefficient of pulsation on the output of the rectification block RB (current K_пв and voltage K_пвU), in the circuit of the load (current K_пн and voltage K_пнU) of rectifier is defined as

Rectification coefficients by current S_i and by voltage S_U are defined taking into account the coefficient of pulsation:

Results of calculations , K_пв, K_пн, K_пвU, K_пнU, S_i, S_U put into tables 7.3(k) and 7.4(k).

Using the results of researches to build characteristics K_пв, K_пн, K_пвU, K_пнU, S_i, S_U = f (I_н, С1, L1).

Research of switching processes (overlapping) of phases

One of the main characteristics of rectifiers with a number of phases of the secondary windings two or more is the switching angle (overlapping) of phases

It is defined with the help of oscilloscope. To determine the angle of overlapping phases must do the following:

начать моделирование;

• start modeling;

• set the nominal current I_{н ном} through the load;

• connect the oscilloscope to the output of rectification block (control point Р₀, U – see the table. 3.2);

Таble 7.3(k) – Results of researches

№

Experimental data

Calculated data

Iн, А

Uн, B

fвu, Гц

∆Uв~, В

∆Iв~, А

fнu, Гц

∆Uн~, В

∆Iн~, А

Kпв

KпвU

Kпн

KпнU

Si

SU

1 2 …

Таble 7.4(k) – Results of researches

№

Experimental data

Calculated data

L1, мГн

Iн, A

Uн, V

fвu, Hz

∆Uв~, V

∆Iв~, А

fнu, Hz

∆Uн~, V

∆Iн~, А

Kпв

KпвU

Kпн

KпнU

Si

SU

1 2 …

• calculate the angle of overlapping phases by formula

• ,

where а_u, a – duration of intervals of work of phase, taken separately,of ideal (while L_b = 0: а_u = 360^º/m) and real (while L_b 0) rectifier; b – duration of the interval of simultaneous work of two phases.

The values of a and b in the formula should be substituted in millimeters or in the number of cells of the measuring grid of the oscillograph (Fig. 7.1).

Рicture 7.1 – Defining of the angle of phases overlapping

Study of electrical processes of three-phase rectification circuits

Using an oscilloscope to observe waveforms of voltage and current in accordance with the notation (see Table. 3.2):

 

 

Предложить лучший вариант перевода

• voltage u_a(t) of secondary transformer`s winding ТV1 phase а;

• current i_a(t) ) of secondary transformer`s winding ТV1 phase а;

• voltage u₀(t) and current i₀(t) on the out put of the rectification block (control point Р₀);

• voltage u_н(t) on the resistence of the load R_н(t);

• voltage u_ya(t) on the output of the control(managing) device CD phase а, etc.

Figures must be depicted in a scale horizontally.

7.3. To the point 6.3. Conclusions on results of rectification circuits

Using the experimental data of tables 7.3 and 7.4, calculate, build and analise:

• Coefficients of usage by power of primary, secondary windings and the transformer as a whole with active and inductive loads;

• external characteristics of the rectification schemes;

• processes of rectifiers functioning.

According to the received results of study of multiphase rectification circuits necessary to formulate conclusions. These conclusions should reflect the features of the processes of functioning of each circuit, results of compared analysis of rectification circuits by the basic parameters obtained experimentally. Conclusions should not repeat the content of the work.