Experiment 1. Speed control of a DC motor with an inner current loop

Transcription

1 he University of New South Wales School of Electrical Engineering & elecommunications ELEC463 - ELECRIC RIVE SYSEMS Experiment. Speed control of a C motor with an inner current loop. Introduction In this experiment, the speed of a C motor will be controlled by using a pulse-widthmodulated (PWM), four-quadrant, H-bridge switching amplifier. An inner (minor) current loop is used to control the torque continuously and an outer speed loop is used to reduce the speed error. he control system for the motor is indicated in figure. he H-bridge consists of four IGB transistors with reverse connected diodes. he bridge is supplied from a 3-phase diode rectifier terminated by an LC filter. he AC supply to the rectifier is from a three-phase variac (variable autotransformer). 200V Speed Current P Power I f Speed ref + + W M Converter CM E 0V Figure. 3 3 V d A i a B 4 4 v o 2 2 Figure 2. he H-bridge power converter he four transistors in the H-bridge are switched from a pulse-width modulator and a crossover protection logic circuit. he PWM circuit uses a high-frequency triangular waveform v tri and compares it with a modulating waveform v c which is the output from the closed-loop current controller, as indicated in figure 3. he output signals from the modulator are converted into two Experiment F. Rahman ELEC463 Electric rive Systems August 204

2 non-overlapping bipolar outputs each of which drive two diagonal switches of the bridge. If the switching pulses from one of the diagonal elements are removed, the drive then operates in one quadrant only. he pwm switching frequency initially set for this experiment is 5 khz. e c P W M v c v tri t High for &2 ON Low for 3&4 ON Figure 3 t he current controller is usually a high-gain amplifier so that it overcomes the back emf disturbance of the motor armature adequately. Figure 4 shows the current control system representation in terms of a PI controller transfer function and the motor transfer function. he action of the high-gain current controller is to produce a transfer characteristic as indicated in figure 5. C u r r e n t C o n t r o l l e r Armature L o a d I a r e f + K ( + s c ) + E b / R a +s a I a + K L J s + s K E Ia Figure 4. he current controlled system I a r e f I a K + s i Js + Figure 5. he current controlled system with a high gain Experiment 2 F. Rahman ELEC463 Electric rive Systems August 204

3 he speed control system then becomes as indicated in figure 6. he inner current loop of figure 4 is included in it. he speed controller is a P or a PI type controller. Speed ref + +s i K J s + Figure 6. he speed control system with the inner current loop In the above two-loop hierarchical control structure, the inner current loop is the torque loop (since = K I a ) and the outer speed loop design can usually be considered in isolation. his is possible when the current loop has a cut-off frequency at least two octaves higher than the speed loop. he current and the speed controllers can be P or PI type, as desired. hese are implemented in SP software. In each controller software, you can adjust the proportional gain and integral time constants by using appropriate sliding cursors. he adjustment of KPI corresponds to adjusting the proportional gain of the current controller. Adjustment of the KII corresponds to adjustment of the integration time constant for the current controller. Similarly, KPSP and KISP adjustments are for the speed controller. Note that for a P controller, the integration adjustment must be reduced to zero. In this experiment, the response of the current and the speed control systems will be studied for different controller parameters. 2 Equipment wo IGB modules forming the H-bridge, including the protection diodes One PWM module One crossover protection module One isolated current transducer module One three-phase rectifier module One LC filter for the rectifier One dc motor-generator set with encoder and tacho-generator One 24 mh inductance for the armature circuit One filter capacitor discharge resistor One three-phase variac One 4 channel storage CRO One PC with a SP board, interface module and printer Experiment 3 F. Rahman ELEC463 Electric rive Systems August 204

4 3.0 Experiment Familiarise yourself with the experimental set-up (figure 7), which has been pre-wired. CM Filter 4 Idc 4 Vdc CM L 3 2 Idc C 3 2 dc Rectifier Variac 3 PHASE 45V,50Hz SUPPLY 2 3 LOA 200VC E 4 V/Amp i P W M SP BOAR INERFACE IBM PC COCKPI CONROL PANEL ref Speed + + i Current MS320C3 SP BASE CONROLLER Figure 7. he experimental set up he C supply voltage to the H-bridge power converter must not be adjusted to a voltage higher than 200 VC. [his voltage limit is due to the rating of the filter capacitor. he variac must be so adjusted that the C voltage never exceeds 200V]. he speed feedback is from an incremental encoder of 5000 pulses/rev. he current feedback is from an isolated current sensor giving V/Amp. he power circuit of the experiment is pre-wired. o not adjust or alter any part of the power circuit in the course of this experiment. If the drive ever becomes unstable at any time, reduce the C supply voltage to the H-bridge to zero promptly by adjusting the variac. IN HE FOLLOWING, ALLWAYS REMEMBER O REUCE HE C SUPPLY VOLAGE O ZERO BEFORE YOU GO FROM ONE SECION O ANOHER. Experiment 4 F. Rahman ELEC463 Electric rive Systems August 204

5 3. Initial set-up and checking 3. Identify all circuit elements and the voltage and current meters for the C supply, the motor and the C generator which is used for loading the motor. Before start, your lab supervisor will start the SP board and make sure that all signals are connected with proper polarity. Initially, make sure that the C supply voltage is set to zero. Load the SP program C Motor rive from the esktop. Open Control esk and Open Experiment from File menu. From Instrumentation menu, select Animation mode. You may now start the experiment. he field supply to the C motor should be off initially. Increase the C supply voltage to about 25V. Now change the reference slider and observe the corresponding change in the armature current. Check that the polarities of both are the same. he current loop may now be closed. Ask your supervisor to check this part. Current controller 3.2 Step response test: 3.2. Reduce the C supply to zero. Run the C-rive 3.2 program and the corresponding animation. Increase C supply voltage to 200V. A 2 volt peak-peak square-wave signal is used as reference to the current controller. Observe the armature current waveform on the CRO. Sketch a few reference and armature current waveforms for V dc of 00V and 200V, and for various KPI and KII settings in your logbook Select suitable controller parameters for a satisfactory current response. Check your results with your supervisor. Record the selected values of KPI and KII which will be used later in your logbook. 3.3 Frequency response test: 3.3. o obtain the frequency response of the current control system which you have chosen, run the C-rive 3.3 program and the corresponding animation. An V peak-peak sinusoidal signal is used to perform a frequency response test on the current loop. Adjust the frequency of the sinusoidal current reference using the cursor and record, and plot in your logbook, data of peak-peak armature current values against frequency slightly beyond the 3dB frequency. Repeat this test for a VC of 00V. Adjust the C supply to zero. Speed 3.4 Steady-state performance test: 3.4. Run the C-rive 3.4 program and the corresponding animation. he speed loop is now closed. Adjust the speed reference using the cursor and set it to zero. Choose proportional speed controller (ie KISP = 0) and set the current limit to 0A. his is the maximum allowed armature current. Switch the field supply to the motor ON and then gradually increase the C supply to the bridge to 200V C. Set the best current controller parameters KIP and KII which you found previously. Check this with your supervisor Experiment 5 F. Rahman ELEC463 Electric rive Systems August 204

6 before closing the loop. Adjust the V dc to 200V and run the motor with various speed references Set the speed reference so that the motor runs at 000 rev/min. Load the motor via the C generator and the loading rheostat and check how speed is held against load variation. Load the C generator using the rheostat and check the speed holding for a few other speeds and loads. Adjust the proportional gain KPSP of the speed regulator and record its effect on the steady-state speed error. Also record the speed error with Vs load torque. he load torque L that you apply may be found by dividing the C generator output power by the speed in rad/sec. Record a few readings of the motor armature current and speed for a few load settings and a fixed speed controller gain KPSP Introduce some integration in the controller and observe its effect on the motor speed and steady-state speed error. Repeat for this controller for a few loads. Record and plot, a few readings of the armature current and speed for the same load settings as in Reduce the V dc to zero. 3.5 Step response test: 3.5. Run the C-rive 3.5 program and the corresponding animation. Adjust the signal generator for a square-wave output at about 0.3 Hz. Select the previous settings for current limit and current controller. Increase the C supply voltage to 200V. he drive should now be accelerating and decelerating between 800 rev/min, without any load connected to the generator. Adjust the speed controller gains KPSP and KISP for a satisfactory response Record a few speed references and corresponding speed and motor current responses Identify sections of the armature current which relate to motor acceleration, steady-state running and deceleration Set the CRO in the XY recorder mode (ask the lab supervisor for help). Connect the armature current and motor speed signals to the X and Y inputs of the CRO. Adjust the gains of channels X and Y of the CRO so that you get the display covering a significant square area of the CRO screen. Identify the quadrants of operation of the drive as it accelerates and decelerates in both directions with and without load. Record two XY plots and clearly identify operating quadrants with armature current (torque) and speed Run the H-bridge in one quadrant only. Observe quadrant operation of the drive in the CRO screen in XY recorder mode. Record this result Return the CRO to the normal mode and drive the machine in one quadrant only for a few speed references and the corresponding speed and motor current responses. Associate sections of the armature current responses to motor acceleration, steady-state running and deceleration. 5. Results and Report:. At the end of the laboratory, produce your logbook with all plots of experimental data (your own drawing or from the CRO) to your lab demonstrator. He will mark the logbook according to the data presented in the logbook and your answers to any question that he may ask. Experiment 6 F. Rahman ELEC463 Electric rive Systems August 204

7 2. If you are assigned to write your lab report on this experiment, you should note the following: Your lab report on this experiment should include brief theory, main objectives of the experiment and what you expected to learn from the experiment. You should indicate the formulae you used and sample calculations you performed. Your explanation of the experimental data and what you actually found and learned through the experiment, compared to what you actually expected should be included. Highlighting of any differences from the expected results, if any, and your own explanations for these would be highly regarded. You are also expected to make some conclusions on the experimental data you obtained and the machine/drive behaviour. You are also expected to include in your lab report your answers to the following: 5. Obtain suitable parameters for the current controllers if the desired current controller bandwidth is 600 rad/sec. 5.2 Plot the steady-state current error vs K p for the proportionally controlled current loop. 5.3 Plot Bode plots for the current controller you chose for V dc = 00 V and V dc = 200 V. What are the 3db frequencies (the bandwidths) for the two voltages? Calculate the parameters of the PI regulators for the chosen current controller. 5.4 Comment on the bandwidth of the current loops for C supplies of 00V and 200V. 5.5 Comment on the effects of the PWM switching frequency on the drive. 5.6 Plot the steady-state speed error vs K p and load torque L for the proportional controlled speed loop. 5.7 Comment on the effect of the P and PI controllers on the steady-state and transient responses of current and speed controllers. 5.8 Identify and mark the motoring and regenerative braking currents on the current waveforms obtained in Also identify the transistors and diodes which carried these currents during motoring, braking and steady-state running. 5.9 Compare the dynamic responses under one and four quadrant drives. Explain the difference in the response that you may have noticed from the results of sections and Machine parameters: Motor R a = 5.7, L a = 52 mh, K E =.27 V/rad/sec J = kgm 2, = Nm/rad/sec Calculate the motor poles for the V transfer characteristic. Generator P = HP = 746W; V dc = 80V; I a = 4.9A; N = 750 rev/min Experiment 7 F. Rahman ELEC463 Electric rive Systems August 204