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REMOTE CONTROL BY BOB YOUNG Voltage losses in speed controllers Following last month's article introducing the topic of speed controllers for electric motors, we present more figures on motor performance and voltage losses in typical FET circuits. Having looked at the performance of some commercial controllers last month, let us now look at the figures for an unloaded motor running in both directions and driving directly from short leads (no speed controller), to establish some sort of additional benchmark-refer to Table 1. The columns in the table are as follow: Vs = Battery Voltage at the terminals; Vm = Voltage at motor terminals; Is = Instantaneous start up current in amps; Ir = Sustained running current in amps. Table 1 last month - see page 71. In Table 1 below, we are losing 0.02 volts with no speed controller in circuit and a lower current. The 2-digit resolution of the meter, and the fact that the batteries were losing their charge as we measured, made accurate measurements very difficult to achieve but the general thrust of the problem is there. Does this mean that Australian automotive wire is at least twice the resistance of what appeared to be a lighter American wire? On the face of things, it certainly seems so. TABLE 1 Comments Vs Vm Is lr 12.47 12.45 17.80 2.64 Anti-clockwise direction & timing. No load. 12.44 12.42 19.56 1.92 Clockwise, incorrect rotation & timing. No load. The interwiring was 75mm of 10A automotive cable. The small anomalies in the figures are due to the resolution of the digital meter and the use of two separate battery packs. I am at a loss to explain these figures as they are nothing like what I would have expected. Unfortunately, I did not have the facilities to measure the unloaded RPM but at least we have some guide as to current and voltage drop in the leads, the latter being 0.02 volts across 0.0075 76 ohms. Here we have 10-amp automotive cable giving almost twice .the resistance of the wire and speed controller in the Novak installation listed in 78 SILICON CHIP As I said earlier, beware the power of the milliohm. To compound the problem, I would have expected the motor to run at a higher current when running clockwise than in the correctly timed anticlockwise rotation. This was not the case and I would be very interested in retiming this motor at some later date. Paralleled FETs To further illustrate the point, Table 2 lists some system losses with the prototype speed controller fitted. This table was compiled very early in the development of the speed controller and shows the effect of paral- leling FETs to reduce the "ON" resistance of the switching network. If you will recall, in an earlier column I stated that the number of FETs to be used and what type would be decided as a result of this work. Incidentally, although it is not a good idea to parallel bipolar transistors, FETs are just the opposite and it is quite in order to parallel these devices. These figures were taken using 10 x 1.8Ah cells, 10 x 4 Masters propeller and the standard-wind Kyosho 360ST motor, as tested above. This motor was timed to run anticlockwise and was fitted with a 2.5:1 gearbox, the interwiring again being the poor quality 10A automotive cable. Due to the fitting of the gearbox, the motor had to be run in reverse. Column 1 shows the number ofFETs fitted to the speed controller. Vd represents the system losses calculated by subtracting the voltage across the motor from the battery voltage measured under load. These losses include wiring harness resistance, voltage drop across the FETs, contact resistance and meter shunt. The leads in this case were quite long. Two things become very obvious from a quick glance at Table 2. Firstly, the results are not very good at all. Secondly, the FETs are the major source of losses. There is, however, a law of diminishing returns in regard to adding additional FETs to reduce the "ON" resistance. At some point the cost and weight of the additional FETs will outweigh the gains. This is in keeping with Ohm's Law. The same applies to wire thickness. Adding copper beyond the optimum point will add considerable weight for only very insignificant gains. It appears that a better approach here is to use high quality cable, probably This model shows that electric flight can be applied to quite large models. This model of an Australian Air Force Caribou has a wing span of 2.4 metres. It was built a few years ago by David Masterton who also built a model of a B-36 bomber which had six electric engines. test equipm ent cable or some equally high quality cable. Insufficient gate drive As th e FETs used were IRFZ44s and these have one of the lowest "ON" resistances in th e range of readily available FETs (28mQ), there was obviously a problem somewhere in the FET drive circuitry. Measurement of the Gate voltage showed only 7.73V. It is obvious th at this is not enough drive voltage to push the FETs into saturation. As a result of these tests, I decid<:Jd to change th e voltage doubler driving the FET gates to a voltage tripler. This resulted in an increase in Gate drive voltage from 7.73V to 12.21V. The results of this are sh own in Table 3. All other test parameters remain u n changed . Table 3 shows a substantial increase in RPM for a reduction in the number of FETs and FET heating when compared to Table 2. Note also that there TABLE 2 No. Vd l(A) RPM 1 0.62V 12.6A 7100 2 0.42V 12.4A 7200 3 0.38V 14.0A 7600 4 0.33V 14.2A 7800 is still a substantial loss in the interwiring. In the first item on Table 3, we have lost 0.63V (46 mQ) across the wiring which was deliberately made very light. In line 2 of Table 3, there is only a 0.15V (llmQ) loss but it still seems high to my mind. Mind you, there is a 100A meter shunt in this circuit and the wiring harness is longer than that used in a model , but it does illustrate the point in regard to wiring. Do not just grab the nearest piece of wire, even if it does look thick enough. There is little pointin spending $12.50 on each FET if you are going to lose more voltage across a 5-cent piece of wire. Alternative FETs Incidentally, there are a number of very useful FETs which cost a lot less than the IRFZ44 and deliver almost the same p erformance. One in particular which I have been testing gives an ON resistance of just 23mQ. Six of these in parallel will give us the required 4mQ ON resistance required to match the commercial units . More on these in a future issue. Comments There is also no fus eFET very hot holder in this circuit but fusi ng is a must, particuFETs hot larly for aircraft. This is another potential source FETs warm of unwanted milliohms FETs OK and fo r this reason the finished design will feature a PCB track fuse option, to reduce bulk and parasitic resistance. Table 4 gives a final set of figures, to illustrate problems of a completely different type. These figures were taken with a prototype speed control fitted with two FETs only; this time using a Leisure 05 Stock motor driving an 8 x 4 propeller on a direct drive. In th e figures of Table 4, we see the usual problems of insufficient FETs and voltage drop in the interwiring, resulting in a total of 0.74 volts. However, look at the battery voltage under load. Wh y is it so low and why is the voltage drop much higher than usual? The answer here lies in the much higher running current of 25 amps. The direct propeller driver provides a much greater load and thus the armature RPM is lower than the geared version , resulting in the higher current. The low battery voltage is an indication of poor quality batteries. These were sold to us as high current 1.8Ah batteries. When I cut the pack open to find what brand and type they were, I found that they were just simply labelled "Japan: (no brand, no type number). Do not forget that these figures w ill all change in flight as the propeller unloads. I have gone into a great deal of detail in the foregoing material to drive home the point that in high current electric motor applications you cannot just grab the n earest piece of wire , battery and motor. Great care must be exercised if you intend to get the best from your system. Th e one lesson which stands tall from th e rest is do not underestimate the power of the milliohm. As stated earlier, there is great scope in electrics for careful system design and the whole thing can become very absorbing indeed. However, having outlined the foregoing shortcomings , I must say that , all things con sidered, the system shows a lot of promise. By the time I have finished with the development work and installed all six FETs, we will have a good cheap controller with performance comparable to some of the commercial units. Dangers of electric props One last point for tyro electric fans (I love this pun). Do not get casual about the dangers of using electric M ARCH 1992 79 ~ ~ - - The electronics magazine for the enthusiast Four good reasons why you should subscribe to SILICON CHIP 1. You get a 12.5% discount on the newsstand cover price. Recently, we ha d to increase the cover price of the magazine because of r ising costs but we have held the subscription to the old price so you get the benefit. 2 • You'll never miss an issue. Sometimes it can be very hard t o find SILICON CHIP in the newsagents because it sells out early or gets buried under other magazines. By taking out a subscription you don't have to search for it - it arrives in your letter box in mint condition. We wrap it in tough pla stic to make sur e of that. 3• Get a discount on t he binder too. You'll want t o store and protect your issues of SILICON CHIP so when you take out a subscription you get a fur ther discount on the binder. By taking out a 2-year subscription and buying two binders at the same time, you get even bigger savings. And we despatch the binder(s) with your first issue so you have it right fr om t he start. 4 • From time to time, we will have special promotions and offers in t he magazine, like the Bose Lifestyle compet ition featured in the September to December 1990 issues. When you are a subscriber you are automatically eligible for these and you get the chance to benefit. Interested in these savings and benefits? Just fill in the coupon on the following page and send it with your remittance. You'll be glad you did. Why not do it today? I hope to see you as a supporter soon. Leo Simpson, Publisher Just fill in the order form on page 93 ~ 80 SILICON CHIP JUNK MAIL Most magazines sell their subscriber lists to mail order companies, to earn extra income. Be assured that we will not do this. Your privacy will be respected and you will not be deluged with unwanted mail. MAGAZINE BINDERS Speciafly designed binders for SILICON CHIP are always available. Made with a distinctive high quality 2-tone vinyl, you can obtain them at a discount when you take out or renew your subscription . The price details are on the coupon overleaf. TABLE 3 No. RPM l(A) Vd Protect your valuable issues Comments 2 0.77V 13.7A 7200 0.14 V across FET S/D pins. Thin Fig.8 speaker cable used for interwiring. 2 0.3V 14.2A 8000 0.15V across FET S/D pins. 1OA automotive cable used for interwiring. FETs warm motors, particularly with propellers. In more than 30 years of power flying with internal combustion motors, I had very few accidents because I treated those propellers with great respect. However, I did not treat the electric driven propeller with the same respect - at first. In my first week of electric experimenting, I had two nasty accidents. One involved a gearbox which I damaged by switching on the power without adequately securing the motor. As a result, the propeller hit a nearby object and cracked the gearbox housing. This was repaired with superglue and seemed OK - that is, until the aluminium strap holding down the motor began to stretch. At this point, the propeller, which had not been balanced, began to shake the motor and the gearbox disintegrated and the propeller and drive shaft went hurtling around the workshop. It gave me a hell of a fright. The second accident was more serious and occurred when I accidentally connected the positive lead of the motor directly to the positive battery terminal instead of the positive speed controller lead. At this time , the speed controller was switched off but the battery terminal was live and the motor burst into life. I was standing in front of the motor with my wrist just inside the propeller arc. The tip of the propeller hit my metal watch band and skidded off into the flesh of my wrist, slashing it across the arteries. If it had not have hit the watchband first, I may have ended up in hospital that day. As it was, I had a bruised wrist and a cut which drew blood. I saw a friend of mine do the same thing with a 60-size IC motor and he was not so lucky. He did end up in hospital. So the rules are simple. Polish off the flash on plastic propellers, balance them properly, and never work in line with the propeller or with your hands inside the prop arc. It's also a good idea to always switch off the power to both the battery and the speed controller. And finally, because they are only electric motors, do not get casual. They deliver every bit as much power as an internal combustion motor and are just as dangerous. Design features So here we are at the end of another column. As a result of the foregoing work, the design of the completed controller is now beginning to firm up. A brief summary of the major points is now in order. Firstly, the design calls for a simple and cheap controller, which will give forward only speed control. Secondly, all components are to be readily available. This precludes my favourite servo amplifier chip, the NE544 which is now obsolete. Thus, we will be using standard op amps as the active elements. Other features will include a ZkHz switching rate , a voltage tripler for operation from a low cell count, provision for up to six FETs, a PCB track fuse option, and finally a dynamic braking option. All of this will be in surface mount technology.· Thanks to ABC Models at Bexley, Hobbyworld at Hurstville and Moore Park Model in Armidale for the help given in preparing these articles. sc Silicon Chip Binders These beautifully-made binders will protect your copies of S ILICON CHIP. They feature heavy-board covers & are made from a distinctive 2-tone green vinyl. They hold up to 14 issues & will look great on your bookshelf. * High quality * Hold up to 14 issues * 80mm internal width *gold-coloured lettering logo printed in on spine & SILICON CHIP cover Price: $A 11.95 plus $3 p&p each (NZ $6 p&p). Send your order to : Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or fax (02) 979 6503; or ring (02) 979 5644 & quote your credit card number. Use this handy form l ----------Enclosed is my cheque/money order for $_ _ _ or please debit my O Bankcard O Visa O Mastercard Card No: Card Expiry Date __/__ Signature _ _ _ _ __ _ _ _ _ TABLE 4 lr Name _ _ _ _ _ _ _ _ _ __ Vs Vm Is 12.7 12.7 12.0 2.60 No load, no speed controller, 75mm of 1OA auto cable 8.44 7.70 50.0 25.0 Comments 10,800RPM on an 8x4 propellor, with two FETs Address_ _ _ _ _ _ _ _ __ _ _ _ _ __ _ P/code _ _ _ ·-----------· MARC H 1992 81