Measuring Magneto Energy Output and Inductance Revision 1

Transcription

1 Measurig Mageto Eergy Output ad Iductace evisio Itroductio A mageto is fudametally a iductor that is mechaically charged with a iitial curret value. That iitial curret is produced by movemet of the rotor while the primary coil is shorted by mechaical (or electroic) igitio poits. Iductors ted to maitai curret flow whe the poits ope, the stored iductive eergy discharges ito a spark gap, igitig a combustible fuel mixture. Iductor curret steadily decreases as eergy is delivered to the spark from its magetic field. Durig the spark the output voltage is determied by the ature of the spark; it does ot deped upo the mageto itself. Spark discharge voltage is wildly variable, difficult to measure or to characterize. The istataeous output voltage does determie the rate of chage of output curret. Neither curret or voltage are well eough behaved to easily measure the output eergy directly. Oe method for measurig output eergy is to coect a appropriate resistor load across the spark gap to suppress the spark ad absorb the output eergy. The discharge will the occur i a predictable ad easily measured maer. This works well uder may coditios, but there are complicatios that will be discussed below. Measuremet Method for ow Tesio Mageto Ideally, output eergy of a low-tesio mageto would be measured by the followig method. Coect the mageto coil to quick-opeig iterrupter poits that are sychroized with the rotor. Time the poits to ope at the istat of maximum short circuit curret while the rotor turs at the desired operatig rotor speed. Coect a appropriate resistor to divert the coil curret so that there is o spark discharge at the cotact poits. ecord a table of samples of the discharge pulse with a recordig oscilloscope. Output power ad total eergy are calculated: N v v P = ad W = P, or W t The summatio form is used to calculate the itegral umerically from the table of sampled data. This method has two practical problems that must be dealt with. First is that the output voltage trasiet will be sufficiet to iduce a spark ayway if the rotor speed is high or if the resistor is large. The method is thus limited to low speed testig with relatively small resistace loads. Secod, the turig rotor geerates some voltage durig the discharge period, ad that voltage becomes the domiat part of the output voltage ear the ed of the trasiet decay. The method must therefore be refied to separate the rotor-geerated voltage from that produced by the iductorstored eergy. The schematic diagram below shows the discharge circuit after the poits ope with rotor-geerated voltage, vg, icluded. ow Tesio Mageto with resistor load copyright Doald K Grimm page of 5 May 7, 5

2 P = ( v vg ) i = ( v vg ) v N W = P, or W v ( v vg ) t The variable tables cotaiig v ad vg samples are recorded i two separate data rus with a digital samplig scope. v is measured with the switch operatig. vgis the ope circuit mageto-geerated voltage measured o the same time scale, usig the igitio poits to trigger the measuremet so the measuremet periods are coicidet. The two data files are liked directly to software i a Access program that calculates the eergy usig the uatios above. See the author for details about the Access program implemetatio. This residual geerated voltage, vg udoubtedly cotributes some eergy to the spark output above ad beyod the stored iductor eergy. That icremetal output eergy is ot calculated here. Uless oe ca characterize the time-voltage behavior of the spark, it is ot possible to calculate the magitude of the cotributio. For most situatios it appears to costitute a small bous eergy of relatively isigificat value, although exceptioal cases have bee see, especially with high-tesio magetos havig large coil turs ratios that greatly step up the rotor-geerated voltage. Calculatig ow Tesio Mageto Iductace ad Iductace-stored Eergy Aalysis of the iitial part of the discharge pulse ca provide additioal iformatio, icludig a measuremet of the mageto iductace ad a idepedet calculatio of stored iductor eergy. di v di v vg =, i =, ad = v vg = vg v = at t = T = at = W v i = = ( vg v ) v W t T Voltages v, vg, ad the slope of the discharge curve, the data sample tables recorded by the oscilloscope as above. ( vg v ) ad, ca all be obtaied from the early part of will both be egative the results will be positive. Be aware that oise o the sampled data will give ureliable results to these quatities uless the samples are appropriately smoothed over time. Eyeball estimates of these quatities from oscilloscope pictures ca give useful results. From oe or two examples so far, appears to be fairly costat for values of tthroughout the discharge iterval. vg i v copyright Doald K Grimm page of 5 May 7, 5

3 High Tesio Mageto Measuremets Voltage delivered to the spark plug by a high-tesio mageto geerally exceeds the breakdow voltage of stadard oscilloscope probes. Usig a high voltage probe with sufficiet rage, the measuremet methods developed above ca still be applied. However, as a practical matter, stadard istrumets ad probes may be used with a resistive voltage divider coected as show i the diagram. A atteuatio ratio i the order of : combied with typical hardware voltage limits of 3 volts will accommodate trasiets up to 3kv. As with the low-tesio mageto measuremets, two data files are recorded. The symbol vm i the uatios at right represets the operatig mageto output, ad vb represets the ope circuit voltage with the coil ope. Both files are sychroized by the mageto igitio poits so they have the same time base. The uatios show assume the atteuator cosistig of ad is i place for both measuremets. Software developed for data reductio described above may be used directly by calculatig the value of a "uivalet load resistor," as show at right. Actual measured voltage readigs are processed i the program. No further scalig is ruired. Atteuator output measuremets of the begiig of the output pulse from a high-tesio mageto ca also be used to calculate iductace ad stored eergy. The uatios are derived at right, ad are similar i form to those derived for low-tesio magetos i the previous sectio. As oted before, oise o the sample data files for vm ad vb must be appropriately smoothed to get accurate results. Mageto poits operatig: + + vm = vh vh = vm Mageto poits ope: + + vb = vg vg = vb + P = ( vh vg ) i = ( vm vb) vm + P = ( vm vb) vb + =, or = + P = ( vm vb) vb N W = P, or W vm ( vm vb ) t vg i vh vm, vb vm i = ( ) di d vm, = vh vg = ( + )( vb vm ) di, or + ( vm vb) = at t = T = d( vm) d( vm ) ( vb vm ) vm W t T = at = d vm + vb vm vm W = i = copyright Doald K Grimm page 3 of 5 May 7, 5

4 Example esults from ecet Tests ed Wig ow Tesio Mageto The waveform show o page is from a test of the low-tesio Grimm ed Wig mageto described i Model Egie Builder magazie, Issue 5. The body of this mageto is.75" thick ad.9" wide. The coil core is of octagoal cross sectio made from lamiated trasformer iro, with a area of.67 i. 5 Mageto Output Eergy ed Wig ow Tesio mjoule rpm W rpm mjoule Vietti Cast Frame High Tesio Mageto Data from a recet high-tesio mageto desiged ad built by Joh Vietti is show here. The body of the mageto is.3" wide ad.975 thick, with a coil core cross sectio of.49 i. Body stator thickess is.375". The waveform photo at right shows some evidece of spark plug or igitio poit flashover durig the leadig edge of the pulse, reducig measured output eergy ito the load resistor. Mageto Output Eergy Vietti Molded Body High Tesio mjoule rpm W rpm mjoule copyright Doald K Grimm page 4 of 5 May 7, 5

5 Selectig oad esistor Values Determiig the best resistor value for testig is more art tha sciece. The legth of the discharge pulse is cotrolled by the resistace of the load. High resistace icreases the discharge voltage ad shortes the pulse. Short pulse effects iclude susceptibility to flashover breakdow of the iterrupter poits or the spark plug* (high-tesio mageto). Eergy shuted ito a spark discharge will deduct from the eergy measuremet. Short discharge pulses will also iduce higher eddy curret losses i the mageto iro flux path. owerig the load resistor decreases the pulse voltage ad icreases its duratio. This allows measuremets at higher rpm without sparkig. The prologed curret flow does icrease the copper loss i the mageto widig(s). owerig the pulse voltage also makes the ope circuit geerator voltage of the mageto a more sigificat fractio of the total voltage compesatio for this effect becomes more importat. The practical optio, the, is to choose a resistace value that just suppresses sparkig at the maximum rpm to be tested. It is ot ecessary to suppress all sparkig. The sigle trace optio of the oscilloscope ca be used to select oe good pulse out of umerous bad oes, give some patiece ad persistece. It seems best, however, to avoid reducig the resistace to the poit where the output pulse becomes sigificatly loger tha ms. Most igitio sparks are shorter tha that, ad it is usually aisable to test the mageto with operatig coditios as close to ormal as possible. Most of my T tests were made with a 55 ohm load, ad HT tests were coducted with a value of about 7 kohm. Coclusios Theoretical bases for two methods for measurig mageto output eergy have bee described, alog with oe method of measurig iductace. Detailed implemetatio istructios are ot provided here, but ca be obtaied from the author. Other methods exist. See for a example. This tester discharges the output pulse ito a Zeer diode array that clamps the output voltage to a value ear kv. Power is calculated by multiplyig the pulse curret times the clamped voltage, ad the itegratig over time to compute eergy. It should give accurate results, but is ot easily adapted to testig low-tesio magetos. A commoly used aalysis tool is the calibrated spark gap. Oe form is described i my workig paper, o page 6. This will give approximate values of output eergy whe properly applied. Please cotact the author with suggestios for improvemets or iformatio regardig other methods that may exist. *Best practice is to keep the spark plug i place whe testig a high-tesio mageto to protect the coil from iteral flashover if excessively high voltage spikes should happe to occur. copyright Doald K Grimm page 5 of 5 May 7, 5