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REMOTE CONTROL BY BOB YOUNG Motors for electric flight models This month, we will discuss the electric motors used in model aircraft & look at controller requirements. In coming months, this will lead to a construction article for an electronic speed controller. Two prototypes of the electronic speed controller are being developed and these will be test flown in a twin Partenavia P68 Victor. This aircraft is shown a photo accompanying this article, along with my old mate and builder of the model, Wes Fisher. Before we can begin to design this unit however, we must have a full understanding of what is expected from an electronic speed controller for modern electric motors. Electric propulsion is an old dream of modellers and appeals especially to those in the modelling fraternity who abhor the noise and mess of internal combustion motors. Burnt castor oil is one of the most obnoxious substances one will meet in the hobby game. It is almost impossible to remove completely from your hands or models and over a period of time will gradually soak into a balsa airframe, rendering it very difficult to repair. Internal combustion technology has greatly improved over the past few years, with synthetic oils which allow much higher methanol/oil ratios .and are very easy to clean off. Fourstroke motors are now available too. These use much less oil, deposit less oil on the model and are much quieter in operation. We also have super glues which work well for bonding oily surfaces and we now have fibreglass which is impervious to the ingress of all oils . However, the appeal of electric propulsion remains undimmed by this impressive catalog of technological wizardry. If anything, it is enhanced (or some would say even finally made possible) by a similarly impressive catalog of technological wizardry peculiar to the field of converting electricity into mechanical energy. At the very heart of this mini technological revolution are dramatic improvements in battery and magnet Shown here is a reversing speed controller made technology. Thus, we some years ago by Sunlux. It was a 12V device rated 15-20 amps. now find ourselves in an era in which model electric motors are capable of delivering staggering performance by the standards of not too long ago. What's more, we also now have batteries that are more than capable of supporting this demanding performance. And now the relentless demand for technology has reached out and encompassed the electronic speed controller. Thus, we are faced with the difficult task of supporting these electron gulping monsters with their seemingly unlimited source of current, via a variable speed controller, capable of delivering up to 70 amps at 30 volts. This amounts to a total power dissipation of 2.1 kilowatts, all inside a flimsy model aircraft. Speed controller development Broadly speaking, speed controllers fall into two categories: forward speed only and reversible. The forward speed type is used mainly in. aircraft, track cars and electric boats. All of these applications place weight and voltage delivered to the motor above versatility. For applications where reversing the motor is a must, a reversing controller is used. These controllers feature a bridge output or a relay to effect the polarity change. Thus, they are heavier and usually more expensive than the forward only controllers. Typical applications are off-road buggies, model ships and most sports models where versatility and realism are th,e key points. The forward-only controller is often fitted with dynamic braking, a feature easily achieved in electric propulsion by simply placing a dead short across the motor terminals. As with all things, speed controllers had a fairly primitive beginning and a typical controller of the 1970s DECEMBER 1991 53 ,+5V + 33 GND~ 330k 10k D1 1S2473 -0.1 + 180k 7 SIGNAL IN 4 11 10k I + 2.7k IC1 BA607A .OSI 09 S1P +24V MAX ... 18k 47k-8.2k + 1+ .05 05 2SMi42 12 6 .05 MOTOR 10 D11 S1P ~ .01! .,. .05! This is the circuit of one of the most popular speed controllers of the late 1970s, made by Futaba. It used a relay for reversing, was rated at up to 10 amps & fitted into a case about the size of a cigarette pack. would deliver about 10 amps in a reversing controller and about 20 amps in a forward-only controller. Keep in .mind here that the motors they were required to drive were equally as primitive and typical current consumptions were in the order of 4-10 amps at 12V. The reversing controller usually used a relay to effect the reversal and these speed controllers were not small. Shown in one of the photos with this article is a finned unit made by Sunlux. This is a reversing 15/20A 12V device. One ofthe most popular speed controls of the late 1970s was the Futaba, giving a reversible 10 amps at 12V and again featuring a relay. About the size of a cigarette pack and built without a heatsink, it was a little prone to overheating when pushed near its limits but gave good service for many years when used with the more conventional motors. Here lies the key issue in speed control design: the ON resistance of 54 SILICON CHIP the switching semiconductors. At high currents, even a small resistance results in a significant voltage drop across the output stage. This lost voltage is critical in a racing application and is the key factor in deciding as to which unit you choose for your application. Luckily for us, the relentless quest for improved battery and magnet performance has been matched by the semiconductor manufacturers who have given us that magical little device known as a FET. However, even a very good FET such as the IRFZ44 has an ON resistance of around .025Q. Therefore, at 50 amps, the voltage drop will be about 1.25V, giving a power dissipation of 62.5W. This power is wasted as heat inside the model, heating the electronics and stressing the output stages. It would, of course, be better if it were delivered to the motor where it would provide extra propulsive force. Thus, we can draw several very significant conclusions immediately from this simple observation: (1) Nothing is better than a stout piece of wire for connecting the motor to the battery. (2) A relay is the next best thing for switching the motor ON and OFF. (3) If you must use semiconductors for switching, use those with the lowest ON resistance that you can buy and use plenty of them in parallel. This reduces the effective ON resistance and therefore reduces the total dissipation. It is quite common to see anything up to eight FETs in parallel in some commercial controllers. These units are very small and very expensive - as much as $350 each. The figures quoted for these modern commercial FET speed controllers are breathtaking, such as 250 amps sustained and 1000 amps instantaneous for forward only controllers. Reversing units are quoted at 150 amps sustained, with a peak instantaneous rating of 450 amps, while dynamic braking on the unidirectional models is quoted at 60 amps. Just how accurate these figures are, I cannot attest to, however I suspect they are fairly close to the mark. What's more, some It may be hard to believe but electric propulsion is being used for progressively larger and heavier models. This twin Partenavia P68 Victor, shown here in the construction stage, will be used as a test bed for an electronic speed controller currently under development by the author. of these units are quite small and most do not feature a heatsink - another technological marvel. Therefore, we now have our first design parameter for our proposed speed controller. The output stages will feature FETs - several of them in parallel. How many, what type and in what arrangement will not be resolved until we examine exactly what the modern electric motor demands of the proposed controller. High power cells and motors For those who have been away from R/C modelling, the world of electric propulsion is a complete revelation. Gone are the days of button cell nicads which melt down if too much curr.ent is pulled from them and which explode if too much current is pushed back into them. In their place stands a glittering array of batteries, some quite capable of melting down battery chargers or welding the connecting wires to the car chassis if one is not very careful. What we have now is a source of electrons of almost zero internal impedance, in case sizes which even as little as five years ago were just a dream. Electric flight is the main beneficiary of these advances, although most models benefit from weight reductions . . However, it is the motors themselves which stagger the imagination. Technology has invaded the world of electric motors in a big way and everything from new magnetic materials to class 10 bearings have been brought into play to squeeze out every last drop of performance. The result is a bewildering array of motors and accessories described in a language which is as mysterious as any ancient Middle Eastern dialect. What does it all mean? Well you might ask, and we will spend the remainder of this column and all of next month's to unravel the mysteries of electric motor language. Motor construction A typical model motor consists of three major sections: the motor can, which houses the front bearing and magnets; the armature and commutator; and the endbell containing the rear bearing and motor brushes. The motor cans are arranged in a hierarchical order, the classification of which seems to defy all but the most expert electric enthusiasts. On the face of it, there appear to be two popular classification systems, one American and one European, the most logical being the American system. The American motors range in size from 020 and 035 (which usually use 4-cell battery packs) to the 05, 075, 10, 15 , 25, 40 and, less usual, the 60. These numbers approximate the output power of an internal combustion motor expressed in cubic inches. The first surprise for tyro electric fan (if you will pardon the pun) is the size of the prop these motors swing and the speed at which they spin it. A 10 x 6-inch prop spinning at 13,000 RPM is really quite ordinary! Thus a 15 size electric motor delivers about the same output power as a 0.15 cubic inch glow plug motor. These figures are usually but not always based on 6-cell battery packs. The Japanese call the 05 housing the 540 series and these are remarkably similar in construction to the American motors. Armature winds range from 6-27 turns . The European system seems to be based on the armature winding and the can sizes seem to be confined to 3 7mm, 42mm and 45mm diameters. The length of the motor varies considerably in various models and the brush housings are quite different in construction from the American and Japanese systems. The Europeans offer armatures of various lengths and windings to accommodate the number of battery cells used in any particular application. Thus, armature winds range from 320 turns to accommodate 7-30 cells. They also use very large commutator areas for adequate brush cooling, a very big item in high power, high revving motors. This also contributes to armature length. Figures of up to 50,000 RPM can be achieved from some motors and brush/commutator heating becomes a serious problem. The magnets are the heart of the modern motor and the rare earth magnets have revolutionised this area of motor design. Low-cost motors are still available with the old ferrite magnets but some of the more exotic rare earth magnets such as samarium cobalt and neodymium are used extensively in the more expensive high-performance motors. Neodymium magnets are, however, prone to demagnetisation at high temperatures and suffer badly in some ii;istallations. A well-ventilated model aircraft is their best environment. Motors of this type are expensive and run at about $370-400. Compare this to a good, fun motor (Speed 600,075, 7-8 cell) at $21.95. And that's it for this month. Stand by for r..ext month's exciting developments. SC 0ECE/11BE R 1991 55