"Bridge B2HZ" For The Control of A DC Motor [PDF]

Zitiervorschau

EXERCISE No. TITLE:

NECESSARY COMPONENTS:

30 “Bridge B2HZ” for the control of a DC motor 1 Microprocessor module M5R 1 Module of Power Circuits M6R + Mask 1 (BSP) 1 Signal acquisition module MDAQ 1 DC motor M-1/EV 1 Tachogenerator M-16/EV 2 True RMS multimeters 1 Multimeter (recommended for the measurement of RPM on the tachogenerator; thus avoinding the use of a specific instrument) 1 Dual-trace oscilloscope Power supply unit: mod. AEP-1/EV

TARGETS: 1. Measurement of the voltages and currents crossing the power circuit with resistive load and with load represented by a motor in open-loop and closed-loop configuration. 2. Analysis of voltage and current waveforms. STARTING PROCEDURES : A specific starting procedure is applied to each experiment and it is available at the beginning of the description. EXPERIMENT No. 1: MEASUREMENT OF VOLTAGES and OF CURRENTS and ANALYSIS OF WAVERFORMS WITH RESISTIVE LOAD This experiment with resistive load enables to obtain “canonical” waveforms. Then the resulting data will be compared with the theoretical values. Therefore this experiment will be taken as reference for the other experiments where the load is represented by a DC motor. Differently from modules M3R and M4R described in the Second and Third parts of the handbook, where the main parameter was the firing angle of SCRs and it varied from 0 to 180°, the “TRIGGERING” parameter of this Experiment is not expressed in degrees, but in percentage and it varies from 0% to 100% (where 0% corresponds to a firing angle of 180°). STARTING PROCEDURE : 1. Arrange the modules on the vertical support. Insert Mask 1 on module M6R. 2. The expected resistive load is of 100 Ω. 3. Connect the jumpers with the power supply unit to assemble the circuit shown in Exercise 30 Fig. 2. Adjust the variac of the power supply unit to attain a max. voltage of 70 Vac across input L1-N of Module M6R. 4. Connect the white jumper of mask identification. 5. Connect the white jumpers with the Gates of the two SCRs. 6. Carry out the following preliminary operations: - Switch Module M5R on. - Enable the component pressing the ON key of Module M5R (on the right of the display). After the preliminary operations have been carried out correctly, implement the following measurements: 1. Measure the AC voltage across input L1-N of Module M6R. Write down the value on Table 1. 26

2. Set the tester to measure the average value Udc of the voltage across the load. Turn the potentiometer of module M5R, following the indications of Table 2, and observe the waveforms of voltage for different values of the TRIGGERING parameter. Write down the data on Table 2. 3. Use the tester as an ammeter A (open the circuit and connect the Tester), to measure the average value Idc of the current crossing the load. Turn the potentiometer of module M5R, following the indications of Table 2, and observe the waveforms of voltage for different values of the TRIGGERING parameter. Write down the data on Table 2. 4. Plot the curve of the data UdAV versus the TRIGGERING parameter / firing angle, and compare with the theoretical curve. At the end of the exercise push the OFF button of module M5R (on the right of the display) to switch the control circuit off. EXPERIMENT No. 2: MEASUREMENT OF VOLTAGES AND CURRENTS AND ANALYSIS OF THE WAVEFORMS WITH A LOAD CONSISTING OF A DC MOTOR WITH SEPARATE EXCITATION (OPEN CONTROL LOOP) In this experiment the load is represented by a motor, consequently, as the open loop configuration is used, the tachogenerator is not necessary. STARTING PROCEDURE : 1. Arrange the corresponding modules on the vertical support. Insert Mask 1 (BSP) on module M6R. 2. Connect the jumpers with the power supply unit to assemble the circuit shown in Exercise 30 Fig. 2. Adjust the variac of the power supply unit to attain a max. voltage of 160 Vac across input L1-N of Module M6R. 3. Connect the white jumper of mask identification. 4. Connect the white jumpers with the Gates of the two SCRs. 5. Carry out the following preliminary operations: • Switch Module M5R on. • Pressing the arrow key ( ↓ ) move to SETUP (buttons of the central area of Module M5R). • Pressing the arrow key (→ ) move to FEEDBACK (buttons of the central area of Module M5R). • Pressing the keys (+ and -), near the display, select OL (open loop) for the test without feedback (open loop). • Confirm OK pressing the central button of Module M5R. • IMPORTANT WARNING: BEFORE POWERING THE MOTOR ARMATURE, MAKE SURE TO HAVE POWERED THE FIELD (160 Vdc). After the preliminary operations have been carried out correctly, implement the following measurements: 1. Set the tester to measure the average value Udc of the voltage across the motor. Turn the potentiometer of module M5R, following the indications of Table 3, and observe the waveforms of voltage for different values of the TRIGGERING parameter. Write down the data on Table 3. 2. Use the tester as an ammeter A (open the circuit and connect the Tester), to measure the average value Idc of the current crossing the load. Turn the potentiometer of module M5R, following the indications of Table 3, and observe the waveforms of voltage for different values of the TRIGGERING parameter. Write down the data on Table 3. 3. Considering the same values for the triggering parameter, compare the values and waveforms of this experiment with the values obtained before with the resistive load. 4. Plot the curve of motor RPM versus the parameter of TRIGGERING / firing angle. At the end of the exercise push the OFF button of module M5R (on the right of the display) to switch the control circuit off. MOREOVER, DISCONNECT THE EXCITATION VOLTAGE OF 160 Vdc FROM THE MOTOR ARMATURE. 27

EXPERIMENT No. 3: MEASUREMENT OF VOLTAGES AND CURRENTS AND ANALYSIS OF THE WAVEFORMS WITH A LOAD CONSISTING OF A DC MOTOR WITH SEPARATE EXCITATION (CLOSED CONTROL LOOP) In this experiment the load is represented by a motor, consequently, as the closed loop configuration is used, the tachogenerator will be necessary. STARTING PROCEDURE : a. Arrange the corresponding modules on the vertical support. Insert Mask 1 (BSP) on module M6R. b. Connect the jumpers with the power supply unit to assemble the circuit shown in Exercise 30 - Fig. 2. Adjust the variac of the power supply unit to attain a max. voltage of 160 Vac across input L1-N of Module M6R. c. Connect the white jumper of mask identification. d. Connect the white jumpers with the Gates of the two SCRs. e. Carry out the following preliminary operations: • Switch Module M5R on. • Pressing the arrow key ( ↓ ) move to SETUP (buttons of the central area of Module M5R). • Pressing the arrow key (→ ) move to FEEDBACK (buttons of the central area of Module M5R). • Pressing the keys (+ and -), near the display, select TG (tachogenerator) for the test with speed feedback by tachogenerator (closed loop). • Confirm OK pressing the central button of Module M5R. • IMPORTANT WARNING: BEFORE POWERING THE MOTOR ARMATURE, MAKE SURE TO HAVE POWERED THE FIELD (160 Vdc). After the preliminary operations have been carried out correctly, implement the following measurements: 1. Set the tester to measure the average value Udc of the voltage across the motor. Turn the potentiometer of module M5R, following the indications of Table 4, and observe the waveforms of voltage for different values of the TRIGGERING parameter. Write down the data on Table 4. 2. Use the tester as an ammeter A (open the circuit and connect the Tester), to measure the average value Idc of the current crossing the load. Turn the potentiometer of module M5R, following the indications of Table 4, and observe the waveforms of voltage for different values of the TRIGGERING parameter. Write down the data on Table 4. 3. While the motor is running, disconnect a terminal of tachogenerator TG. Reading the values on the tester connected with the tachogenerator, observe that RPM increase because the motor is not controlled any more. Connect the terminal of tachogenerator again and observe that motor speed is reset to the set value. 5. Considering the same values for the triggering parameter, compare the values and waveforms of this experiment with the values obtained before with the motor in open loop configuration. 4. Plot the curve of motor RPM versus the parameter of TRIGGERING / firing angle. Compare with the curve resulting from the previous test where the motor was controlled in open loop configuration. At the end of the exercise push the OFF button of module M5R (on the right of the display) to switch the control circuit off. MOREOVER, DISCONNECT THE EXCITATION VOLTAGE OF 160 Vdc FROM THE MOTOR ARMATURE.

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TABLE 1: SUPPLY VOLTAGE OF THE MODULE (Vac)

AC voltage across L1 – N of Module M6R:

TABLE 2: VALUES AND PHOTOS FOR THE RESISTIVE LOAD TRIGGER. (%)

α (°)

100

0

75

UdAV (VDC of tester) (V)

CH1 MDAQ Terminal

IdAV (IDC of tester) (A)

CH2 MDAQ Terminal

See Photo

1

12B (1V/div)

12A (1V/div)

45

12B (1V/div)

12A (1V/div)

2

50

90

12B (1V/div)

12A (1V/div)

3

25

135

12B (1V/div)

12A (1V/div)

4

0

180

12B (1V/div)

12A (1V/div)

29

5

Exercise 30 - Fig. 1 Study of a Half controlled Bridge with resistive load and connected instruments

Exercise 30 - Fig. 2 Connection diagram of the Modules with resistive load 30

FORMULA FOR CALCULATING THE VOLTAGE ACROSS THE RESISTIVE LOAD

Udc / Udc(0 °) = 0.5 * (1 + cos α )

Function Udc/Udc (0 °) of this circuit complies with the following law: Udc/Udc (0 °) = 0.5 * [1+ cos α]

α (°) 0 45 90 135 180

TRIGGER. (%) 100 75 50 25 0

0° ≤ α ≤ 180°

MEASURED VALUES Udc Udc/Udc (0°) (V across the load) blue

THEORETICAL VALUES FORMULA Udc/Udc (0 °) red 0.5* [1+ cos α]] 0.5* [1+ cos α] 0.5* [1+ cos α] 0.5* [1+ cos α ] 0.5* [1+ cos α]

THE CURVES OF MEASURED DATA AND OF THE THEORETICAL VALUES ARE SUPERIMPOSED.

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EXERCISE 30 - PHOTOS OF PURE RESISTIVE LOAD

Exercise 30 – Photo 1 - α = 0°

Exercise 30 – Photo 2 - α = 45°

Exercise 30 – Photo 3 - α = 90°

Exercise 30 – Photo 4 - α = 135°

Exercise 30 – Photo 5 - α = 180°

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TABLE 3:VALUES AND PHOTOS WITH MOTOR IN OPEN LOOP CONFIGURATION AS LOAD

AC voltage across L1 – N of Module M6R:

UdAV (VDC of tester) (V)

CH1 MDAQ Terminal

IdAV (IDC of tester) (A)

CH2 MDAQ Terminal

U TG (V)

MOTOR RPM

See Photo

TRIGGER. (%)

α (°)

0

180

12B (1V/div)

12A (1V/div)

6

25

135

12B (1V/div)

12A (1V/div)

7

50

90

12B (1V/div)

12A (1V/div)

60

72

12B (1V/div)

12A (1V/div)

75

45

12B (1V/div)

12A (1V/div)

100

0

12B (1V/div)

12A (1V/div)

Tachogenerator mod. M-16/EV: • • •

K TG = 2 mV/RPM MOTOR RPM = U TG / K TG Connecting a multimeter with the terminals of tachogenerator mod. M-16/EV enables to measure UTG

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8 -------9 -------

EXERCISE 30 – LOAD: DC MOTOR – SEPARATE EXCITATION BRIDGE B2HZ KTG = 2 mV/RPM VCA (L1-N) = 160 V

TRIGGER. (%)

α (°)

UdAV

U TG

MOTOR

(VDC of tester)

(V)

RPM

(V) 0

180

25

135

50

90

60

72

75

45

100

0

34

Exercise 30 - Fig. 3 Study of a Half controlled Bridge with load represented by a DC motor (open loop), and with connected instruments

Exercise 30 - Fig. 4 Connection diagram of the Modules with load represented by a DC motor (open loop) 35

EXERCISE 30 - PHOTOS OF THE LOAD WITH MOTOR (OPEN LOOP)

Exercise 30 – Photo 6 - α =180°

Exercise 30 – Photo 7 - α = 135°

Exercise 30 – Photo 8 - α = 90°

Exercise 30 – Photo 9 - α = 45°

CONCLUSIONS: The current crossing the motor is almost constant because the motor runs in no-load condition. The current shows an intermittent trend. That provokes a damage to the motor because magnetic losses inxcrease and the torque is not kept constant.. Voltage increases as soon as the firing angle decreases. The voltage waveform is deformed with respect to the case of pure resistive load. That is due to the inductance of motor armature.

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TABLE 4: VALUES AND PHOTOS WITH A DC MOTOR (CLOSED LOOP) AS LOAD

AC voltage across L1 – N of Module M6R:

TRIGGER. (%)

α (°)

0

180

25

135

50

UdAV (VDC of tester) (V)

CH1 MDAQ Terminal

IdAV (IDC of tester) (A)

CH2 MDAQ Terminal

U TG (V)

MOTOR RPM

See Photo

12B (2V/div)

12A (1V/div)

12B (2V/div)

12A (1V/div)

90

12B (2V/div)

12A (1V/div)

12

60

72

12B (2V/div)

12A (1V/div)

13

75

45

12B (2V/div)

12A (1V/div)

100

0

12B (2V/div)

12A (1V/div)

Tachogenerator mod. M-16/EV: • • •

K TG = 2 mV/RPM MOTOR RPM = U TG / K TG Connecting a multimeter with the terminals of tachogenerator mod. M-16/EV enables to measure UTG

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10 11

14 15

EXERCISE 30 - BRIDGE B2HZ LOAD WITH DC MOTOR - SEPARATE EXCITATION -CLOSED LOOP KTG = 2 mV/RPM VCA (L1-N) = 160 V

TRIGGER. (%)

α (°)

UdAV

U TG

MOTOR

(VDC of tester)

(V)

RPM

(V) 0

180

25

135

50

90

60

72

75

45

100

0

38

BRIDGE WITH LOAD CONSISTING OF A DC MOTOR (CLOSED LOOP)

Exercise 30 - Fig. 5 Study of a Half controlled bridge with load consisting of a motor (closed loop), and with connected instruments

Exercise 30 - Fig. 6 Connection diagram of the Modules with load represented by a motor in closed loop configuration

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PHOTOS WITH LOAD CONSISTING OF A DC MOTOR (CLOSED LOOP)

Exercise 30 – Photo 10 - α =180°

Exercise 30 – Photo 11 - α = 135°

Exercise 30 – Photo 12 - α = 90°

Exercise 30 – Photo 13 - α = 72°

Exercise 30 – Photo 14 - α = 45°

Exercise 30 – Photo 5 - α = 0° 40

EXERCISE 30 - BRIDGE B2HZ LOAD: DC MOTOR –SEPARATE EXCITATIONCOMPARING OPEN/CLOSED LOOP CONFIGURATIONS KTG = 2 mV/RPM Vac (L1-N) = 160 V

TRIGGER. (%) 0 25 50 60 75 100

α (°)

MOTOR

MOTOR

RPM

RPM

OPEN LOOP

CLOSED LOOP

180 135 90 72 45 0

CONCLUSIONS: • THE CURVE OF VOLTAGE VS. FIRING ANGLE OF OPEN LOOP CONTROL IS NOT LINEAR, BUT ITS TREND DEPENDS ON (1 + COS(α)). IN FACT A MOTOR IS NOT A PURE RESITIVE LOAD, BUT IT ALSO INCLUDES SOME INDUCTIVE COMPONENTS. • ON THE CONTRARY THE CURVE OF VOLTAGE VS. FIRING ANGLE IN CLOSED LOOP CONTROL IS LINEATR. IN FACT, THE PID CONTROLLER FOLLOWS THE RPM REFERENCE OPTIMIZING THE RESPONSE SO THAT IT BECOMES LINEAR.

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