Lab 8. Speed Control of a D.C. motor. The Motor Drive

Transcription

1 Lb 8. Speed Control of D.C. motor The Motor Drive

2 Motor Speed Control Project 1. Generte PWM wveform 2. Amplify the wveform to drive the motor 3. Mesure motor speed 4. Mesure motor prmeters 5. Control speed with computer lgorithm 12 v dc Motor c Tchometer 9 v Power Supply Amplifier microcontroller Signl Conditioning (Frequency or Amplitude)

3 Buehler 12 volt permnent-mgnet dc motor with tchometer output Electricl Connections yellow/green -- tchometer output blue/red -- motor winding Note: Tchometer wires my not hve two colors on some units.

4 Exploded view

5 Motor electro-mechnicl models R rmture winding resistnce L rmture winding inductnce i rmture current V t terminl voltge e bck emf T m developed torque T L torque needed for lod ω rottionl speed B friction coefficient J moment of inerti

6 Motor Electricl Dynmics v e t = = R i Kω m + L di dt + e e = bck emf (electromotive force) generted within rmture windings Note: bck emf = 0 t stndstill nd increses linerly with motor speed

7 Mechnicl Dynmics Anlogous to Electricl Circuits! Equtions for these systems hve similr form.

8 Motor Mechnicl Dynmics T T m m dω = J + B ω + T dt = K i L T m = developed torque increses with current J = motor moment of inerti B = motor friction coeffient ω = ngulr velocity of the motor T L = torque required to drive the lod

9 Lplce Trnsformed Equtions Electricl Mechnicl ) ( ) ( ) ( ) ( s K s si L s I R s V t Ω + + = ) ( ) ( ) ( ) ( s T s B s s J s I K L + Ω + Ω =

10 Stedy stte nlysis (s=0) Electricl stedy stte V Mechnicl stedy stte t = R I + K Ω K I = B Ω + T L Solve for speed Ω R K = TL RBm + K RBm + K V t

11 Motor speed vs. lod torque Speed is relted to lod torque nd terminl voltge Ω speed 2 speed 1 operting points incresingv t lod 1 lod 2 T L

12 Trnsient response experiment Mesure V motor, V R, nd V tch I motor = V R (becuse R = 1)

13 Experimentl results Current reches 1 mp during strtup!

14 Wht we now know: For given lod, motor speed is proportionl to voltge pplied to its terminls Use of PWM signl llows the verge voltge of the signl to be vried by vrying duty cycle V vg = V digitl T1 T1+ T 2 We hve 12v dc motor (mx. terminl voltge is 12v) A 3 volt signl will be insufficient to produce full speed, PLUS Motor my drw mps of current, wheres digitl chip outputs cn typiclly supply only millimperes Ide: Use single trnsistor switch to mplify the digitl PWM signl to drive the motor T1 = ON time T2 = OFF time

15 Bsic Trnsistor Switch (idel models)

16 Switching n Inductive Lod (motor winding) Inductor voltge-current lw: When current i C is switched off, di C /dt is lrge nd negtive Inductor voltge is lrge nd negtive Collector voltge > V cc Q my be destroyed!

17 Switching n Inductive Lod (need to protect switch Q) Use nti-prllel diode D!!! reverse bised when Q is ON gives lternte current pth when Q switches OFF (when inductor voltge becomes negtive) protects Q Collector voltge is clmped to V cc +V diode.k.. freewheeling diode

18 Drive design model

19 Drive Design prmeters Mximum lod current, I LOAD Trnsistor current gin, h FE Trnsistor voltge V BE(st) in sturtion mode Microcontroller output voltge, V high

20 Design Equtions Requirement for bse current in the ON stte I >> B I h LOAD FE Clculte bse series resistnce, R

21 EE Bord vrible power supply Positive Supply VP+ output voltge & current limit VP+ ON Actul VP+ Current Wveforms Power Supply Window

22 Connect grounds of multiple power supplies

23 Lb Procedure Verify proper PWM signl genertion Mesure c tchometer output (yellow/green leds) t multiple non-zero speeds Plot motor speed vs. PWM signl duty cycle Repet for severl PWM signl frequencies, over rnge of vlues Find the best frequency (produces most liner plot)

24 Choice of devices Trnsistor (Q) 2N3904 is chep but under-rted for current 2N2222 hs higher current rting Both my be destroyed if motor is stlled Diode (D) 1N4001 is rectifier diode: bit slow, hs lrge dimeter leds 1N4148 (or 1N914) is switching diode: fster, but hs low current rting (but is not expensive)

25 2N2222 NPN trnsistor dt Absolute Mximum Rtings Symbol Prmeter Vlue Unit V CEO Collector-emitter voltge (bse open) 40 V V CBO Collector-bse voltge (emitter open) 75 V V EBO Emitter-bse voltge (collector open) 6 V I C Collector current 1 A Electricl Chrcteristics Symbol Prmeter Conditions min mx Unit h FE Dc current gin I C = 150 ma, V CE = 1 V 50 V CE(st) V BE(st) Collector-emitter sturtion voltge Bse-emitter sturtion voltge Source: Firchild Semiconductor I C = 150 ma, I B = 15 ma 0.3 V I C = 150 ma, I B = 15 ma V

26 2N3904 NPN trnsistor dt Absolute Mximum Rtings Symbol Prmeter Vlue Unit V CEO Collector-emitter voltge (bse open) 40 V V CBO Collector-bse voltge (emitter open) 60 V V EBO Emitter-bse voltge (collector open) 6 V I C Collector current 200 ma Electricl Chrcteristics Symbol Prmeter Conditions min mx Unit h FE Dc current gin I C = 100 ma, V CE = 1 V 30 V CE(st) V BE(st) Collector-emitter sturtion voltge Bse-emitter sturtion voltge Source: Firchild Semiconductor I C = 50 ma, I B = 5 ma 0.3 V I C = 150 ma, I B = 5 ma 0.95 V

27 1N4148 switching diode dt Absolute Mximum Rtings Symbol Prmeter Vlue Unit V RRM Mximum repetitive reverse voltge 100 V I O Averge rectified forwrd current 200 ma I F Dc forwrd current 300 ma I C Collector current 200 ma Electricl Chrcteristics Symbol Prmeter Conditions min mx Unit V F Forwrd voltge I F = 100 ma 1 V I R Reverse lekge V R = 20 V µa t rr Reverse recovery time I F = 10 ma, V R = 6 V, I rr = 1 ma, R L = 100 ohm Source: Firchild Semiconductor 4 ns