ECE 628 Experiment 9

ECE 628 Experiment 9 - Experi‘men‘b q PVVM DC Motor...

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Unformatted text preview: Experi‘men‘b q PVVM DC Motor Drive Using Surface Mount Power MOSFETs BB 628 (Power Electronics Lab) Department of Electrical Engineering Ohio State University I. Introduction DC motor drives with pulsewidth—modulation (PVVM) are very popular for applications in speed and position control. In this lab, we study twoquadrant DC motor control by PWM surface mount power MOSFETS. II. DC hdotor Control A. DC Motor Basics For many variable speed applications, DC motors have long being the best choice, since a DC motor is the simplest motor to control electronically, comparing with brushless DC, stepper, AC induction and switched reluctance motors. ' In a DC motor, the field flux is established by the stator field winding or permanent magnets, and the electromagnetic torque is produced by the interaction of this flux with the rotor armature current. The armature winding is connected to the commutator segments which maintains the DC voltage and current through one or more pairs of carbon brushes. An electrical model of a shunt-wound DC motor that we will control is shown in Fig. 1. Note that the back Electromagnetic Force (EMF) is proportional to the motor speed. The electromagnetic torque can be derived as . . L FV: L Tc : LAfzjza : Thus the electromagnetic torque produced is a function of both the armature applied voltage and the rotor speed. Fig. 2 shows a typical torquespeed characteristics for a shunt-wound DC motor, which indicates that for a constant armature voltage, the rotor speed is approximately fixed regardless of the torque. Therefore, by controlling the armature voltage, the motor speed can be conveniently controlled. At zero speed, the armature current can be large due to the zero backsEMF and the small armature resistance. Thus the applied armature voltage should be reduced in motor starting. This is particularly true for large rating motors. Figure 1. E u’lxaient Circuit oi Shunt-wound Figure 2. Torque—Speed Characteristics otor B. Two Quadrant Pit/Ill DC Motor Speed Control DC motor control can be classified by the quadrants of operation in terms of the torque and the motor speed, as illustrated by Fig. 3. A single quadrant control, which operates only in the first quadrant with positive torque and positive speed, usually uses a single transistor and a single clamp diode, as shown in Fig. 4. For the DC motor drives, by using a H-bridge, dc reversing capability is obtained. Two-quadrant operation, in quadrants lst and 3rd, is a popular operation mode for the H—bridge DC motor drives. A simple H-bridge two~quadrant DC motor drive circuit is shown in Fig. 5. One of the bottom transistors is pu15e~width-modulated (PWM’ed) and the diagonally opposite transistor is turned on ,(thus used for steering). To reverse the direction of the motor, the active steering transistor is turned off while the other chmring; transistor is turned on, and the other bottom transistor is now PWM’ed. A single PWM signal is used to control either bottom transistor and a second signal “FWD” is used to steer the direction Fast motor acceleration and a stalled or locked motor may cause a high current; meanwhile, the current carrying capability of the transistors is limited. A cycle-l>y»cycle current limit is then added, as shown in Fig. 6. The small current sense resistor at the bottom of the H—bridge is essential for the disabling of the PWM signal when the current limit is exceeded. When the PWM input. is switched lou‘ the Reset—Set latch will be reset and the PWM signal will be enabled to turn on the bottom transistor on the next cycle. Thus the peak current is limited on a cycleby—cycle basis. VM Torque First Quadrant - positive speed. positive torque, “forward-accelerating“ I .- ‘V Speed J Second Quadrant negative speed; positive torque, "reverse-braking" ii iii Third Quadrant Fourth Quadrant negative speed. negative positive speed. negative torque, "reverse-accelerating" torque, "onward-braking" T Figure 3. Torque/Speed Quadrant: 0! Operation Figure 4. Singiefluadrani DC Motor Drive Circuit -: [351V] 1/ .. BQZiCU. C' l} ‘JC 'L/\ ’ 9015+ 512,!ch nature “’ mo gar a o v Hng 5. Woman! DC Motor Orin Ckcult V00 N73 Figaro B. Two—Oupdunt DC Hotov Drive (2ka with Current Um" III. hdotorola Surface Mount MOSFETS Surface mount technology has been utilized for small disk drive with peak currents of 1A or 2A. The Motorola motor drive with low err—resistance surface mount power MOSFETS achieve current handling capability of 5A. The motor drive has the foll0wing attractive features: 0 Low on~resistance 0 Increased current capability 0 MOSFET advantages 0 Cycle-by—cycle current limit 0 Simplified control a Advanced packaging technology The power MOSFETS in the Motorola motor drive are configured as a standard H—bridge, with P- channel MOSFETS in the upper half legs and N-channel MOSFBTs in the bottom half legs. Other major components include logic devices (NANDs and logic gate drives), linear devices such as linear comparators, resistors and filter capacitors. The complete circuit using surface mount components is shown in Fig. 7. It is an extension of the basic Hsbridge configuration with current limit shown in Fig. 6. In the circuit, the logic gate drives provide logic level inputs to the MOSFETs, and previde under- voltage lock-out and high current outputs. The P-channel gate resistors are optimized for an optimal diode reverse recovery. The NANDs provides control logic and over current latch. Salient facts about the power MOSFETS can be summarized in the following: turned on by gatessource voltages; turns on/off fast (majority carrier device, no stored charge); on—state resistance has positive temperature coefficient; large safevoperation-area (SOA), thus less need for snubber circuits. The P- channel MOSFET and the N-channel MOSFET have an on-resistance of 120m§2 and 40mf2 respectively. The internal diode of the P-channel MOSFET has fast enough reverse revovery of 200ns but high forward voltage. The maximum forward voltage of 2V at a current of 5A would create an unacceptable 10W of power dissipation. Thus a Schottky diode is put in parallel with each of the P-channel MOSFET, to decrease the onevoltage to about SOOmV. The use of the Schottky diodes also reduces diode clearing losses and switching noise The surface mount package (DQPAK) is one of leadformed, with a single heat-spreading copper area for each half»bridge, and many through-hole vias to the back of the board for additional heatsinking. IV. Special Considerations 1) The armature inductance and the field inductance are important to the operation of the PWM control. For a given switching frequency, the higher the inductances the lower the current ripples. For the motor controlled in this lab, the inductances are so small that. the DC current is highly discontinuous, which contributes to undesirable torque pulsation and armature heating. An increase in switching fre- quency would normally reduces the current ripples, but not necessarily in our case with small inductances. Another undesirable efiect is that for the H-bridge DC motor drive, the relationship between the control voltage and the average output voltage becomes nonlinear because of discontinuous current operation. 2) The reversing capability of the motor drive is limited to “static reversal” only. That is, the direction of the motor can not be changed while the motor is still moving. Otherwise, for a “dynamic reversal", a high current will be produced if the back EMF of the motor is high, Which does not flow through the sense resistor, resulting in a possible damage of the P-channel MOSFETs. V. Procedures Before proceeding with the following procedures, review DC motor control and study the Motorola surface mount MOSFETS drive circuit. ' 1. Set the function generator to 'produce a PWM signal with 0 to 5V peak-to-peak, lkHz and a duty-cycle of about 15%. 2. Turn off the function generator. Connect the circuit composing of the DC motor drive, the DC motor, the function generator and the drive pewer supplies, referring to Fig. 8. Supply the direction signal “FWD” of the motor drive with high (5V). V . I" 4 luv/ii ‘1\ ;:‘3C} {LU \f‘ CW1) the L 3. Double check the circuit connection carefully. Turn on the power of the breadboard; then turn on the function generator to supply the PWM signal. Check the speed and direction of the DC motor. Observe and plot the drive output voltage and the output current. 4. Increase the PVVM signal duty-cycle to about 50%. Observe the change in motor speed. Observe and plot the drive output voltage and the output current. Then increase the PWM signal switching frequency to about 5kHz. Observe the change in current ripples and acoustic noise. 95. Turn off the PVVM signal and disconnect the function generator from the circuit. Adjust the PWM duty—cycle to about 15% at lkEz. Supply the direction signal “FWD” of the motor drive with low (0V, i.e. open). Turn off the PWM signal and connect the function generator to thecircuit again. Repeat the previous steps 3 and 4. In the report, summarize in your own words the control of shunt-wound DC motor by the PWM power MOSFETS, then carefully explain all the phenomena and design methods to achieve better performance. References [l] N. Mohan, T. M. Undeland and W. P. Robbins, “ Power Electronics: Converters, Applications and Design”, John \Viley 8.: Sons, 1989 [2] RC. Krause, “Analysis of Electric Machinery", McGraw Hill, 1986 [3] K. Berringer, “High-Current DC Motor Drive Uses Low On-Resistance Surface Mount MOSFETS”, Motorola Semiconductor Application Note AN1317, Motorola lnc., 1992 12v cm: 1 “'7 Isolation Nl Transf. 5A Surface Mount DC Motor Drive ‘Funcfion ’\I .__‘ Generator Oscillo‘ scope Laboratory Breadboard Figure 8. 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ECE 628 Experiment 9 - Experi‘men‘b q PVVM DC Motor...

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