6 Laboratory tasks.

Content of the work

6.1 To study the theoretical material about the features of schematic implementation of a bridge circuit of rectification, processes of operation, areas of application by the recommended literature. To study the principle scheme of the stand and features of its functioning.

6.2 To calculate the electrical parameters of the scheme working on the load with capacitive response.

6.3 Calculate the electrical parameters of the scheme working on the load with an inductive response.

6.4 Investigate the scheme of single-phase bridge rectifier for different types k of load of the rectification block RF : on active load (k = 1); on inductive load (k = 2); on capacitive load (k = 3); on Г-type LC-filter (k = 4); in П-type CLC-filter (k = 5).

6.4.1 Investigate external (load) characteristics of the rectifier

6.4.2 Determine the output resistance of the rectifier.

6.4.3  Determine the percentage reduction of output voltage rectifier.

6.4.4 Investigate filtering properties of rectifying filters of the rectifier: K_пв, K_пн, K_пвU, K_пнU, S_i, S_U = f (I_н).

6.4.5 Investigate the processes of the functioning of the rectifier.

6.5 To make the conclusions on the results of the performed work.

7 Methods of laboratory work processing

7.1 To the point 6.1. Program and order to the study of theoretical issues.

To study the purpose, peculiarities of structural, functional, schemo-technical constructing of 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 point 6.2 Calculation of electrical parameters of the rectification circuit with its work load with capacitive response

Calculation of a bridge circuit goes to analytic determination of the values of voltage and current of elements of rectifier’s scheme.

Initial data:

1) constant component of the rectified current І₀ = I_н = 0,5... 5 А and the constant component of the rectified voltage U₀ = ±U_н = 20...30 В (given by teacher);

2) transformation coefficient n₁₂ = 5…15 of transformer Т1;

3) frequency of the feeding net f_c = 50…400 Hz;

4) Effective voltage of feeding net U_с = U_G1 = 198… 242 В.

As a result of calculation we determined the following:

1) Satisfying the condition of the rectification circuit work on the load with capacitive response, ie,.

С_н >> 1/4π f_cmR_н, С_н >> 10/4π f_cmR_н;

2) A main calculated coefficient A, which connects a cut-off angle of the valve with the parameters of the rectification circuit, in the form

А = tgθ – θ = πr_Ф / mR_н,

where r_Ф –active resistance of the phase of rectification scheme, in this case r_Ф = 120 Оhm; m = 2 – phase character of the rectification circuit;

3) effective voltage U₂ of secondary winding W2 of transformer TV1 – (pic. 3.3):

,

where B = f (A) is defined by В = 0,75 + 1,2А while А 0,5 or by the graph [2.4, p.130; 2.2, p.118];

4) average value of the current of the phase of the rectification circuit

I_cp = I₀/m;

5) effective (rms) value of the current valve

I_B = DI_ср,

whеre D = f (A) is defined as D =2 + 1/27 А while А 0,5 or by the graph [2.4, p.130; 2.2, p.119];

6) effective value of current I₂ of secondary winding of the transformer

.

7) maximum (peak) value of current of the diode and the secondary winding of the transformer:

I_2m = FI_cp,

wherе F = f (A) is defined as F = 5 + 0,25 А while А 0,5 or by the graph [2.4, p.132; 2.2, p.119];

8) effective value of current I_с = I_G1, consumed from the power source and the current I₁ of primary winding of the transformer

I_G1 = I₁ = I_с = n₂₁ I₂ = I₂ /n₁₂;

9) peak reverse voltage appearing across valve

U_{обр m} = 0,5(I+B )U₀;

10) coefficient of valve usage by the k_U and current k_i:

k_U = U_обр /U₀; k_i = I_2m / I_B;

11) coefficient of the scheme

k_cx = U₀ / U₂;

12) coefficient of transformer usage by power

K_тр = P₀/S_тр = 2K₁K₂/(K₁+K₂) = /BD,

wherе K₁ = P₀/S₁, K₂ = P₀/S₂ – coefficient of usage of primary and secondary windings of transformer correspondingly;

S₁ = I_сU_с, S₂ = I₂U₂, S_тр = 0,5(S₁ + S₂) – total (overall) power of the primary, secondary windings and the transformer, respectively, ВА;

P₀ = U₀ I₀ = U_н I_н – полезная мощность на выходе выпрямителя, W.

Fill out the following calculation of the relevant part of the table 7.1.

The results of calculation to reflect in the table 7.1.

7.3. to point 6.3Calculation of electrical parameters of the rectification circuit while it is working under load with an inductive response.

The calculation scheme is carried out on the following initial data:

1. the constant component of the rectified current I₀ = 0,5...5 А (given by teacher);

Table 7.1 – Results of the research

Values of loads

I0, А

U0, В

Icр, А

IB, А

I2m, А

Iс, А

Uс, В

U2, В

I2, А

A

B

D

F

θ

kU

ki

Kсх

Kтр

Capacitive load

Calc.

220

Exp.

220

Inductive load

Calc.

-

-

-

-

220

Exp.

-

-

-

-

220

2. the constant component of the rectified voltage U₀ = 20...30 В (given by teacher);

3. transformation coefficient of the transformer n₁₂ = 5…15;

4. active resistance of the inductance L of the filter is R_Ф= … Ом; frequency of feeding net f_с = 50…400 Hz,

5. voltage U_с = 198…242 В.

As a result of calculation we determined the following:

1) voltage U₂ of secondary winding of the transformer:

,

where – drop of the constant component of the rectified voltage on the active resistance of the inductance of L filter; g – coefficient of rectifying scheme;

2) voltage U₁ of primary winding of the transformer

U₁ = U_c = U_G1 = U₂ / n₂₁ = U₂n₁₂ ;

3) effective value of the current through the valve

I_B = I₀ / ;

4) effective value of the current I₂ of secondary winding of the transformer

I₂ = I_B;

5) effective value of the current а I₁ of primary winding of the transformer, current consumed from the networkI₁ = I_с = I_G1 = n₂₁ ;

6) maximum reverse voltage across valve ;

7) coefficient of usage of the valve by voltage k_U and current k_i:

k_U = π / 2; k_i = 0,707;

8) coefficients of scheme

k_сх = U₀ / U₂;

9) coefficient of usage of the transformer by power:

/S₁; /S₂; /S ,

where .

Results put to the table 7.1.

7.4 To the Paragraph 6.4. Investigation of the work of the bridge rectifier for different types k of load of rectification block RB: on active load(k = 1); on inductive load (k = 2); on capacitive load (k = 3); on Г-type LC-filter (k = 4); on П-type CLC-filter (k = 5).

Providing of needed variant k of the load of rectification block RB of the bridge rectifier is done by the switch of researched circuit selection (see pic. 3.1).

To ensure the required parameters of the circuit elements and modes of work of rectifier circuits can be used Shortcuts (keys for fast access)“G1”, “TV1”, “L1”, “C1”, “C2”, “ R_н”, “SW1” (see pic. 3.1).

7.4.1 To the Paragraph 6.4.1 Research of external (load) characteristics of the rectifier.

external characteristics – is a dependence of the voltage U_н in the load of the rectifier on current I_н through the load U_н = f (I_н).

Methods of external characteristics studying involves the following steps: choose by selection switch the needed researched circuit (see pic. 3.1) – “Active load” (k = 1– see pic. 3.2), “Inductive load”(k = 2 – рiс. 3.3);

• to control the source’s parameters G1 (voltage U_G1 = U_{G1 ном}, needed frequency f_с), using the key “G1” on the instrument panel (see pic. 3.1) for appearance of editing window;

• to control the transformer’s parameters ТV1 – the transformation coefficient of the transformer n₁₂ (see pic. 3.1 – key “ТV1”);

• correct the element’s parameters L1, C1, C2 of rectifying filter of the rectifier, using the keys(see pic. 3.1) “L1”, “С1”, “C2”, set them to nominal value L1_ном, С1_ном, C2_ном;

• using the switch SW1, switch off the load R_н of two-semi-cycle circuit of rectification;

• begin modeling, pressing “Start”;

• measure by voltmeter PV3 the output voltage U_{н хх} in the non-conductive mode, results put into the table 7.2 (k);

• switch the load on R_н;

• changing resistance R_н of the load from maximum (minimum) to minimum (maximum), to measure the output voltage U_н and current I_н of the load R_н. Measured parameters put into the table 7.2 (k).

Using the results of researches build the characteristics.

Таble 7.2 (k) – Results of researches

Rн, Оhm	¥	100	50	25	15	10	5	3	1
Uн, V
Iн, А
ΔUн, V
ΔIн, А
rв, Оhm

7.4.2To the paragraph 6.4.2. Determination of the rectifier output resistance.

Output resistances r_в of the rectifier are defined using the table of measurements. 7.4.1 (see таble 7.2 (k)):

r_в = ∆U_н /∆I_н;

∆U_н = U_{н max} – U_н; ∆I_н = I_{н max} – I_н,

wherе U_{н max} , I_{н max} – maximum values of voltage and current of the load (see table. 7.2 (k)).

Results of calculations ΔU_н, ΔI_н, r_в put into the table 7.2 (k).

7.4.3 2To the paragraph 6.4.3. Determination of percentage decreases of the output voltage of rectifier.

Percentage decreases ∆U_н% of the output voltage of rectifier while switching from non-conducting mode with voltage U_{н хх} to nominal load with output voltage U_н ном is defined using the results of paragraph 7.4.1 (see table. 7.2 (k)):

.

7.4.4 To paragraph 6.4.4. Investigation of filtering properties of rectifying filters of the bridge rectifier.

For measurement of parameters of variables components at the output of rectification block RB, frequency f_{в  и} and width ∆U_в~ of voltage_в(t) and width ∆I_в~ of current i_в(t) it is necessary to connect (see pic. В.13 paragraph В.1) oscilloscope to control points: in the first case to “P₀,U” – voltage u_в(t) on the output of rectification block (see table. 3.3), in the second case to “P₀, I” (current i_в(t) on the output of rectification block – see table. 3.3).

For measurement of parameters of variables components in the load of rectifier of frequency f_{н u} and with ∆U_н~ of voltage u_н(t) and width ∆I_н~ of current i_н(t) it is necessary to connect (see pic. В.13 paragraph В.1) oscilloscope to control points: in the first case to “R_н, U” – voltage u_н(t) of the load R_н of rectifier (see table 3.3), in the second case to “R_н, I” – current i_н(t) in the load R_н of rectifier.

Experimental values of frequencies f_{в и} and f_{н и} of investigated processes are defined by oscillograms u_в(t) and u_н(t).

For this you need to measure by a mask of oscilloscope the period duration Т of observed curve using the values n of multiplier "Time/Division". Measured time interval Т is defined by two values Т = 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

.

Measuring parameters of variable component (f_{в u}, U_в~, I_в~, f_{н и}, 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 paragraph. 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– “Active load” (k = 1– рiс. 3.2), “Inductive load” (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, C1, C2 of rectifying filter of the rectifier, using the key “L1”, “C1”, “C2” on the elements panel (see. piс. 3.1), to set the nominal value L1_ном, С1_ном, C2_ном;

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

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

• measure the resistance R_н, of voltmeter PV3 and ampermeter PА3. Results R_н, U_н and I_{н ном} put into the table 7.3 (k);

• measure the width U_в~ and frequency f_{в и} of voltage pulsation on the output of rectification block RB using the voltage oscillograms u_в(t) (control point Р₀, U from the table 3.3);

• measure the width I_в~ of current pulsation on the output of rectification block RB using the current oscillograms i_в(t) (Р₀, I);

• measure the width U_н~ and frequency f_{н и} of voltage pulsation in the load of rectifier using the voltage oscillograms u_н(t) (control point R_н, U from the table 3.3);

• change the width I_н~ of current pulsation in the load of rectifier using the oscillogram i_н(t) (R_н, I – таble 3.3);

• to make the same measurements for currents I_н through the load, 0,75I_{н ном}, 0,5I_{н ном} and 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;

• Make these researches for all types k of loads of rectification block (k = 1, 2,…, 5);

• perform similar measurements at different values of capacity С1 for k = 3, 4, 5 – results of measurements recorded in the table 7.4 (k);

• perform similar measurements at different values of inductance of throttle L1 k = 2, 4, 5 – results put into table7.3 (k);

• using the results of the experiment (table 7.3 (k), 7.4 (k), 7.5 (k)), calculate and fill the tables 7.3 (k), 7.4 (k), 7.5 (k).

Таble 7.3 (k) – Results of researches

№

Experimental data

Calculated data

Iн, А

Uн, V

fвu, Hz

∆Uв~, V

∆Iв~, А

fнu, Hz

∆Uн~, V

∆Iн~, А

Kпв

KпвU

Kпн

KпнU

Si

SU

1 2 …

Results of experiment let us to define normalized frequencies:

The coefficients of pulsation on the output of the rectification block RB (current K_пв and voltage K_пвU), in the load circuit (current K_пн and voltage K_пнU ) of the rectifier:

Coefficients of rectification by current S_i and voltage S_U are defined using the coefficients of pulsation:

Results of calculations , K_пв, K_пн, K_пвU, K_пнU, S_i, S_U put inti the table 7.3 (k), 7.4 (k), 7.5 (k).

Using the results build the working characteristics K_пвU, K_пнU, S_i, S_U = f (I_н, С1, L1).

7.4.5 To paragraph 6.4.5. Investigation of operation of the rectifier.

Switch (see pic. 3.1) the needed rectifier’s circuit – “Active load” (k = 1– рiс. 3.2), “Inductive load” (k = 2 – рiс. 3.3),

Set, using slider “R_н” – рiс. 3.1) load resistances R_н, current I_н, equal to the nominal value I_н =I_{н ном.} Draw the oscolograms of voltage u₁(t) of primary transformer winding ТV1, current i₁(t) of primary transformer winding ТV1, currents i_VD1(t), i_VD2(t), i_VD3(t), i_VD4(t) of diodsVD1, VD2, VD3, VD4, output current i_в(t) of voltage rectifier u_н(t) on the load R_н etc. (according to table. 3.3). Drawing oscillograms use equal scale by time axes, the coordinates beginnings should be on one vertical line.

Таble 7.4(k) – Results of researches

№

Experimental data

Calculated data

C1, мкФ

Iн, A

Uн, V

fвu, Hz

∆Uв~,,V

∆Iв~, А

fнu, Hz

∆Uн~, V

∆Iн~, А

Kпв

KпвU

Kпн

KпнU

Si

SU

1 2 …

Таble 7.5 (k) – Results of researches

№

Experimental data

Calculated data

L1, мГн

Iн, A

Uн, V

fвu, Hz

∆Uв~, V

∆Iв~, А

fнu, Hz

∆Uн~V

∆I~, А

Kпв

KпвU

Kпн

KпнU

Si

SU

1 2 …

Save the amplitude and time correlations, drawings the oscillograms, as this is essential for the analysis. Duration of the oscilloscope division is selected such that at the working part of the screen was placed 1,5 ... 2 periods of the observed waveforms. Under each image of oscillogram should indicate to what physical quantity it belongs.

Cutoff angle of current θ in the capacitive load is defined in calculated data by the relations A = tgθ–θ, and in experimental part – using current oscillogram i_в(t) at the output of rectification block. Using the oscillograme find the value Т – period of pulses of current in line units of oscilloscope screen– and value t_и – pulse duration of current – in same line units. After define the value θ, using the formula

θ = 360 t_u/2T, эл. град.

To find А experimentally, we should use radian units of θ. (θ = 360 t_u/2T, эл. град.)

Parameters B, D, F are defined by explanations to paragraph. 6.2 using the obtained value of А. Values I_в (effective value of current of valve), I_2m (maximum value of the current of valve), K_тр (coefficients of transformer’s usage by power) is calculated using the experimental data, using relations from paragraph 7.2 (or7.3).

Comparison of calculation and experiment is carried out by comparing the values measured or taken experimentally. The results of measurements and calculations are summarized in table

7.1 (while I_{н ном} = I₀ and U_{н ном} = U₀).

7.5 To paragraph 6.5. Conclusions on the work.

Design report with regard to the requirements of section 6 of this methodological development. 6.4, plot in one coordinate system, the external characteristics of the rectifier U_н = f (I_н).

Conclusions of done work should contain a shirt and clear points, which reveal the essence of the phenomena, observed or received graphic dependencies. Conclusions should not state the external signs, but to explain to them, establishing the unity of theory and experiment, explaining the fundamental difference between one case from another (eg, the difference in the form of current flow through the valve at the different character of the load, nonlinearity, or conversely, the linearity of the external characteristics and so on .). The conclusions should show comprehension of the results of laboratory studies. Between the answers to test questions and the conclusions of the work is a direct link. Conclusions should not repeat the content of the work (section 6).