AC : BASIC CLASS MATERIALS AND LABORATORY PROJECTS WITH DC MOTORS IN AN INTRODUCTORY UNDERGRADUATE ECE CLASS FOR NON-MAJORS

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1 AC : BASC CLASS MATERALS AND LABORATORY PROJECTS WTH DC MOTORS N AN NTRODUCTORY UNDERGRADUATE ECE CLASS FOR NON-MAJORS Sergey N. Mkrov, Worcester Polytechnic nstitute Submitting uthor: Sergey N. Mkrov erned his B.S./M.S./Ph.D./Dr. Sci. degrees t the Stte University St. Petersburg (Leningrd), Russin Federtion Fculty of Mthemtics nd Mechnics. Dr. Mkrov joined nstitute of Mthemtics nd Mechnics t Stte St. Petersburg University in 1986 s resercher nd then joined the Fculty of Stte St. Petersburg University where he becme full professor in n 2000 he joined the Fculty of Deprtment of Electricl nd Computer Engineering t Worcester Polytechnic nstitute, MA. His current teching interests include fundmentl ECE clsses. Reinhold Ludwig, Worcester Polytechnic nstitute Reinhold Ludwig is Professor of Electricl nd Computer Engineering with joint ppointments in Mechnicl nd Biomedicl Engineering. His reserch nd teching is relted to high frequency circuit design nd electromgnetics. Kung Myt Win, WP ECE Dept. Kung Myt Win. Sophomore Electricl nd Computer Engineering student t Worcester Polytechnic nstitute. Undergrdute ssistnt in ECE Deprtment t WP. c Americn Society for Engineering Eduction, 2011

2 Bsic clss mterils nd lbortory projects with DC motors in n introductory undergrdute ECE clss for non-mjors Most electricl nd computer engineering deprtments in the United Sttes nd brod typiclly offer fundmentl one or two-semester course in ECE for non-mjor students. Sometimes, this course is offered to both mjors nd non-mjors. n generl, it is very difficult tsk to tech complex electricl engineering concepts, including circuit theory, semiconductor fundmentls, nd digitl fundmentls in single course. Therefore, the intro clss for non-mjors (or for both mjors nd non-mjors) is frequently devoted to circuit (nd digitl) fundmentls only. This scenrio cretes visible disstisfction, especilly mong ME nd CE mjors, who wish to be exposed to power electronics - motors, even in their first ECE clss. The sme trend is observed for ECE mjors, when both clsses re combined together. The exposure to power electronics is not only beneficil for the intro clss, but it my lso stimulte further long-term interest in ECE nd Robotics. On the other hnd, the systemtic study of motor fundmentls (including AC motors 1-4 ) in the intro clss my not be possible, s it would require significnt extension of the lredy tight syllbus. Wht cn be done to resolve this dilemm? This pper reports on our pedgogic pproch of introducing the motor-relted mterils into the clss syllbus (3-4 lectures) nd into one or two clss lbortories in n engging yet informtive mnner tht is prticulrly ppeling to non-mjor students. The course is tught to both mjors nd non-mjors. Course objectives include studying bsic DC circuits, circuit equivlents, circuit lws, the bsics of power trnsfer, the DC model of opertionl mplifiers, simple switching circuits, nd the bsics of A to D conversion. Throughout their studies in this course, students will be introduced to the commercilly populr mthemticl computtion progrm known s MATLAB. Mjor course outcomes include students confidence nd fluency with DC circuit nlysis, s well s the bility to immeditely pply their knowledge to rel-world scenrios. The supporting clss mterils on DC motors re prepred bsed on the first principles. They re resonbly different from some common populr guidelines 5. At the sme time, these mterils hve the following (simplifying) fetures: 1. Only n introductory nlysis of DC motors nd DC genertors operting in stedy stte is given, strting with the Lorentz force concept nd liner motors. 2. DC motor resistnce, the torque constnt, nd the voltge constnt re then introduced nd studied together. 3. Next, it is suggested tht the clss lern how to find nd interpret motor constnts from the dtsheet is suggested next. 4. Afterwrds, the torque-speed curves re studied using four mesurble motor quntities:,, T, nd re studied fterwrds. 5. Finlly, the nlysis leds to n exmple - selection of DC motor for prticulr ppliction, including both stedy-stte opertion nd ccelertion. The supporting lbortory syllbus currently hs the following fetures: 1. n the first lbortory, motor ( 12 surplus Mbuchi model for ~$5) is introduced s lod device. Students mesure R M,,nd. 2. n the following (or the sme) lbortory, students connect the motor to hlf H-bridge nd subsequently to the full H-bridge nd observe four different motor sttes: (i) forwrd

3 mode; (ii) reverse mode; (iii) free run to stop nd; (iv) brke. Students hve the option of pplying PWM to this circuit to vry motor speed. We shll describe our experience with this scenrio in n introductory course offered during Fll 2010/Spring 2011 (bout 250 students totl), nd the corresponding ssessment results. 1. Lorentz force - liner motor/genertor/brke concept (one lecture) The lecture sequence is outlined using the three figures tht follow. We only consider liner motors since understnding the Lorentz force effect requires cre. Corresponding exmples re presented in clss. 1.1 First step - Motor ction: F Nf l Anq B l B Fig. 1. Electromgnetic ccelertor or liner motor bsed on the Lorentz force effect. E W / q lb 1.2 Second step - Genertor ction: Fig. 2. Liner genertor (liner dynmo) bsed on the Lorentz force effect Third step - Brke ction: F l B l 2 B R b b / Fig. 3. Brking effect of the Lorentz force - motor (or genertor) setup.

4 2. Rotting DC mchine. Torque constnt nd voltge constnt (one lecture) The lecture sequence is outlined using four figures tht follow. We first derive the torque constnt, then introduce the torque nd the current. Finlly, we proceed to the equivlent circuit model of motor nd to the voltge constnt. This lecture ends with the set of three motor equtions t stedy stte. 2AB 2.1 Torque constnt. de of DC coil rotor: T T KT KT Fig. 4. Two-pole DC mchine with the permnent mgnets nd one-turn rmture. S T 2.2 Model of DC motor t :, K T RM Here we introduce motor (rmture/brush) resistnce nd conditions. Finding torque constnt from conditions is explined in clss nd illustrted by rel-life exmples. Fig. 5. Motor model t (no rottion). 2.3 Model of DC motor t stedy stte nd power conservtion lw: Power conservtion lw leds to: E K T ron losses led to: E K, T E KT K KT K Fig. 6. Motor model for 12 DC motor t stedy-stte rottion with ngulr speed.

5 2.4 Finding voltge constnt from no-lod conditions: K Exmples my include ny of the DC motors from Tble 1: E S R M Tble 1. Motor constnts nd their respective rtios for five DC motors. Motor prt # 3 1.4W Mbuchi DC motor FC- 130RA W Mbuchi DC motor RC- 280SA W Mbuchi DC motor RC- 280SA W Mbuchi DC motor RS- 380PH-3270* 12 96W higher-power Mbuchi DC motor RS-550C-7525* K T from conditions, N m/a K from no-lod conditions, /(rd/sec) Rtio K / K T 3. Torque-speed curves. Motor selection (one or two lectures): The lecture sequence is outlined using the three figures tht follow. First, we define the lod torque, nd then we show how to crete the torque-speed curves from the motor dtsheet. Finlly, we select the motor operting point, nd discuss the motor selection bsed on the power criterion. 3.1 Lod torque: T T lod B f Fig. 7. Blnce between the lod torque nd the friction torque t different motor speeds. 3.2 Torque-speed curves from motor dtsheet: use no-lod speed in rpm (revolutions per minute) or in rd/sec t rted voltge; no-lod motor current in mperes t rted voltge;

6 current in mperes t rted voltge; nd torque of the motor T in N m t rted voltge. Torque-current line: Tlod T Torque-speed line: Torque-power curve: Torque-efficiency curve: T lod 1 T T p M Tlod Tlod 1 T TlodT Tlod T T T S lod lod [ lod ] Fig. 8. Generic torque-speed curves. 3.3 Selecting motor operting point: Fig. 9. Selecting proper operting point either for cruiser (left) or for ccelertion (right) opertion.

7 4. Lbortory One suggested lbortory syllbus is explined below. This is 3-hour long lbortory. 4.1 Mesuring motor prmeters No-lod speed ( ) mesured with the strobe light illuminting the shft notch: - Ask TA for cler power LED nd connect the LED directly to the function genertor, then plce the LED with your hnd perpendiculr to the notch of the motor shft, t the distnce of pproximtely Slowly vry the frequency of the function genertor in the 70Hz - 100Hz rnge by rotting the frequency knob of the function genertor. You should be ble to see the light from the LED reflecting bck from the flt side of the motor shft periodiclly, forming delyed rottion figure. As you chnge the frequency, you will see the notch rotting slower or fster. This is clled the strobe effect. - At one specific frequency, you will see the notch of the motor pper to. Record this vlue. This is exctly the frequency of the motor shft rottion in Hz Motor Resistnce ( R M ) mesured with the clibrted DMM: Tble. 2. Set of mesurements for motor resistnce ( wrm motor). Fcing direction (South) (West) (North) (Est) R totl (Ω) Averge R M (Ω) R M (Ω) Then, the current is found using Ohm s lw t No-lod current ( no_lod ) mesured with the DMM: Fig. 10. No-lod current mesurements Stll toque The torque is found using the motor torque constnt vlue given to students nd the (indirectly) mesured current.

8 4.2 Generting torque-speed curves nd finding mximum motor efficiency: 1. Open MATLAB from the desktop or from Strt All Progrms MATLAB R2010 MATLAB R Then click FileNewScript or click the New Script button to open new MATLAB m-file. 3. Copy nd pste the smple MATLAB script, substitute your mesured vlues. 4. Put your nme nd your prtner s nme t the beginning of the script 5. Sve the script s lb2.m. Run the script. 6. Observe the figure nd keep it on the screen. 7. Using the textbox option of the figure window, put lbel ner ech curve (current, speed, power, efficiency). 8. Using the cursor option of the figure, determine (pproximtely) the mximum possible efficiency of the motor, nd ll of the other relted motor prmeters (torque, current, speed, power) t its mximum efficiency. 9. Using the cursor option of the figure, determine the mximum possible output power of the motor nd ll of the other relted motor prmeters (torque, current, speed) t its mximum power. After completion of this ssignment, students will hve creted complete dtsheet for the nonme motor. 4.3 Prctice question: determining unknown torque for motor lod As model for low-torque motor lod, we use white plstic disk (shown in Fig. 11 tht follows). The disk rottes t rted lod voltge. Fig. 11. An rtificil low-torque lod with n unknown torque. When rotting fst, the disk experiences significnt ir friction, which is dditionlly enhnced by the four inner slots. The lod torque is the torque tht is necessry to overcome this friction, similr to the lod torque of propeller. The students gol is to find the lod torque t the rted voltge. They hve ll of the prerequisites to do so, nd re required to do only one extr mesurement.

9 5. Conclusions nd ssessment Tble 3 lists preliminry dt: student ssessment (two cses in point - stimultion of interest in ECE nd mount of lerned mteril) for two identicl ECE intro clsses with non-mjor prticiption: one with nd one without the specific motor content tught in the Fll of Ech clss is one-qurter long. Clsses were tught by two different instructors. About 65% students hve responded in every cse. The students hve tken the course only once. Tble 3. Assessment results for two nerly identicl non-mjor clsses (Fll of 2010). Grde from 0 to About 70 students per clss hve responded. The instructor stimulted my interest in the subject mtter (compred to other clsses) The mount lerned from the course ws (compred to other clsses) Without motor content With motor content More ssessment results will be reported bsed on the ongoing clsses. Certin elements of our clssroom teching strtegy cn esily be dopted by professors nd instructors who re ssigned to tech introductory ECE clsses for non-mjors. References [1]. A. Hmbley, Electricl Engineering: Principles nd Applictions, 5th Edition, Jn. 2010, Prentice Hll, Upper Sddle River, NJ, 912 p. [2]. J. W. Nilsson nd S. Riedel, Electric Circuits, 8th Edition, My 2007, Prentice Hll, Upper Sddle River, NJ, 880 p. [3]. G. Rizzoni, Fundmentls of Electricl Engineering, 1st Edition, Feb. 8th 2008, McGrw-Hill, 736p. [4]. R. C. Dorf nd J. A. Svobod, ntroduction to Electric Circuits, 8th edition, Jn. 7th 2010, Wiley, New York, 886 p. [5]. Robotics, Wikibook, Oct