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REMOTE CONTROL BY BOB YOUNG Motors for electric flight models, Pt.2 Last month, we discussed the can size and bearings of motors intended for electric flight. This month, we continue with a detailed examination of the armature, brushes and associated items. The armature, from the user's point of view, is the one area in which great changes can be made to improve system performance and also the one that will influence the design of our proposed electronic speed controller the most. The range of options is staggering, with armature winds varying from three turns to 27 in 1-turn steps. The big problem from the motor manufacturer's point of view is that the range of applications is so diverse that it is impossible to provide a true stock motor. The main consideration is the bal- At the point of switch on, the motor armature is stationary and thus the armature winding provides a purely resistive load, the value of which is the DC resistance of the armature coil itself. Thus, a 3-turn armature provides a virtual short circuit. This is an important factor in the design of electronic speed controllers, for the electronics must be capable of delivering the full instantaneous starting current or a very serious complication arises. As a motor begins to turn, the back EMF from the windings rises and starts "The big problem from the motor manufacturer's point of view is that the range of applications is so diverse that it is impossible to provide a true stock motor. The main consideration is the balance between motor output power and run time". ance between motor output power and run time. Four factors - battery capacity and weight, armature winding and run time - must all be considered very carefully and the scope for some clever system design and development is unlimited. This is one of the things about electric propulsion that makes the field so fascinating. Now the really important thing here is to understand how an electric motor works. 78 SILICON CHIP to oppose the voltage applied to the brushes, with the result that the armature current begins to fall. The final running current depends largely upon the voltage applied, armature resistance, timing of the brushes and the motor load or RPM. The lowest running current in a correctly timed motor occurs when it is unloaded and when the revs are at their highest. Loading the motor will begin to slow it down and cause an increase in armature current. Out of this simple observation arises the concept of the correct gear ratio or prop size to suit the application. If we use too low a gear ratio or too fine a pitch on the prop, the revs will be high, current low and thus run time high, but the speed will also be low. Conversely, a high gear ratio or coarse pitch prop will load the motor. Revs will be lower, current higher and thus run times shorter, but the speed of the model will be higher. The motor in this case will also run at a much higher temperature, as will the speed controller. This loading factor also raises the problem of starting current. As stated previously, the maximum current is drawn with the armature stationary. If the load is high, then the time taken for the motor to accelerate to full RPM is lengthened with the result that large amounts of current are used for a considerable length of time. This will heat the motor, batteries and speed controller and considerably reduce run times. Now we can see the importance of gear ratios and prop pitches. If there is constant starting and stopping, average current consumption will be increased dramatically. The correct driver or pilot style also has a great deal of influence. For example, the driver who thinks ahead, never lets the forward speed fall to zero, and who uses minimum throttle changes will always achieve longer run times than his lead-thumbed mate. It is also obvious that an aircraft enjoys a real advantage here, as there are very few rocks, twigs and pranged cars in the sky. Thus, the throttle can be set at one speed and left there for a considerable length of time. Here the prime consideration is the pitch of of the modelling business with lots of scope for the clever and/or innovative modeller. The complications involving armatures do not end here. Delving deeper into the black art of electric motor theory, we find some very interesting factors involved. Multi-wound armatures Fig.1: this Futaba speed controller from the 1970s was rated at 12V & 10A, a flea-power rating by modern standards which require controllers rated up to hundreds of amps. the propeller. Applications calling for constant climb demand a fine pitch prop to keep the revs high. The need for speed calls for a coarse pitch prop with some sacrifice in current at take off (here a variable pitch prop would be really nice) and once at speed, you never allow the nose to go up. There is a second complicating factor in regards to starting which affects the design of the speed controller. If we do not supply the full start-up current required for the stationary armature, then the time to run the armature up to the correct operating speed is extended with attendant heating problems. For this reason, speed controllers are quoted at instantaneous and sustained currents. For example, the state-of-the-art Tekin TSC 41 lP is rated at 1050A maximum current, a staggering figure by previous standards but a necessary one if 3-turn armatures are going to be used to full effect. Compare this to the 1970's era Futaba 12V 10A speed controller in the photo of Fig.1. There is another problem which involves the number of poles on the armature winding. A 7-pole motor provides a greater mechanical advantage at start-up than a 5-pole motor and a 3-pole motor is approaching the bottom of the barrel. This problem is compounded when starting under heavy loads and for this reason the European manufacturers tend to prefer 5 and 7-pole armatures whereas the Americans and Japanese tend to stay with the 3 and 5-pole layout. Now we are beginning see where the enormous complexity in providing a motor to suit all applications begins to arise. For starters, a compromise must be struck between starting torque and cost (3, 5 or 7-pole). From here we move rapidly into a bewildering array of compromises involving armature winds, battery run times, brush material, bearings, and thermal considerations. Again, all of these factors influence the cost. Obviously an application involving lots of starting, stopping and accelerating would tend to call formultipole motors and cost becomes a secondary consideration. Track and offroad vehicles fall into this category. On the other hand, in applications such as aircraft, where the run time is lengthy and the motor RPM never varies, we can live with the slower acceleration of the 3-pole motor. Average current Even here , the position is by no means clear cut for the number of poles also affects the average current for any given load and thus affects the run times (for any given battery size). As a general rule, the greater the number of poles the lower the running current. Which is the most important in your application: acceleration, cost, RPM, run times or any one of a myriad of considerations? As stated before, electric propulsion is an intriguing branch One would think that an armature wind is an armature wind but not so in this mad, highly competitive world, where everybody is looking for that small edge. Trinity (America) quote their armature winds as singles, doubles, triples and quads. What does it all mean? The answer to this little question lies in the problems (or as the positive thinkers would have us believe, challenges) involved in coil winding. Copper wire has mass, volume and resistance and the heat dissipation takes place on the surface of the wire. Now the problem is that a single strand of say 19-gauge wire (American) is very stiff and will not bend easily around the armature contours. The large diameter also leaves diamond shaped spaces between winds, thus wasting valuable volume. This space is vital to another factor involving the magnetic flux density and that is the concept of amp-turns. The magnetic field will increase with a constant current if we increase the number of turns. All of these factors play an important part in the final wound armature. Coil winders have always faced these problems and one simple method of improving efficiency is to use parallel windings of two or more strands of wire which will give the same mass and resistance. Thus, two strands of 22-gauge wire will give the same mass and electrical resistance as a single strand of 19-gauge wire. Triples consist of a 3-strand winding and a quad winding uses four strands of a very fine wire indeed. There are two benefits that accrue from using this method and these are of great interest to the electric motor enthusiast. One is the fact that because the diameter is smaller on each strand, they fill in the spaces between winds much more readily. Thus, there is less wasted volume and this results in a better amp-turns ratio. These finer wires also follow the armature contours more readily, again saving space. JANUARY 1992 79 MOTORS FOR ELECTRIC MODELS - CTD The second factor is that two strands of wire have a greater surface area than an equivalent single strand, thus assisting in the heat dissipation of the armature windings. (Editor's note: the reduction of"skin effect" may also be an important factor. The speeds at which these motors run means that the currents through the armature constitute a relatively high frequency which may be 5kHz or more. Clearly, at the very high currents involved, skin effect could be very important. It would be minimised by trifilar and qµadrifilar windings; ie, triples and quads). Now the importance of doubles, triples and quads becomes crystal clear. There is another factor of importance in this issue and that is the more snugly wound coils using smaller diameter wire tend to throw off armature winds less than the heavier single strand windings. This is an important factor when the RPM of some of these motors is considered. Trinity quote 52,000 RPM for their 9-turn, double wind "Nuclear Assault" 4.9 wet magnet motor. I assume this is unloaded and presently I can offer no explanation of what a "wet magnet" is. "Everybody" can tell me that the "wet magnet" is better than a "dry magnet" but "nobody" can tell me why! Does this mean that if we drop a "dry" magnet into a bucket of water it becomes "wet" and works better? The mystery continues. Stay tuned to this magazine for further episodes of this intriguing little mystery. As you can well imagine, motors spinning at these revs and drawing the amount of current that they do, generate a large amount of heat - so much so that parts of the motor are seriously in danger of melting down. The brushes and motor "endbell" are two such components. High brush wear A complicating factor for the brushes is the fact that most modern motors allow the timing of the commutator to be advanced or retarded. This can result in severe arcing at the com mut ator/b rush junction and brushes will just simply melt or at best wear extraordinarily quickly. 80 SrucoN CHIP For this reason, brush design has become a major factor in modern motor design, so much so that some classes of car racing are almost a motor and brush tweaking competition instead of a drivers' event. There is a bewildering array of brush types available in a variety of materials and physical shapes. As a general rule, a soft brush material will allow a higher RPM but will wear more quickly. The harder materials withstand heating better and thus last longer but wear the commutator more quickly. The usual brush composition is a mixture of copper/graphite which will boil off the copper if they overheat, leaving just the graphite riding on the armature. The resistance of the graph- "The endbells carry the brush housing and rear bearing and can get very hot. The usual composite plastic endbells can actually melt". ite is much higher than copper and this is why the brush goes black at the end and the motor slows noticeably. Trinity offer a special brush alloyed from copper/silver which gives excellent results but wears very quickly. Using this type of brush, the commutator stays cleaner and does not burn at the commutator slots. These brushes are very soft and are usually changed every two runs on modifieds and every three runs on stock motors. Another popular trick is to cut the brushes to reduce the surface area in contact with the commutator. This increases the cooling area of the brush and reduces friction . The shape of the cut also effects the timing of the motor. By cutting one side from -the brush, an effective increase in timing of 2-3 degrees may be achieved if they are inserted the normal way. This will result in an increase in RPM. If they are installed in the reverse mode, an effective retarding of the timing is achieved, resulting in more torque and lower battery drain. One point here is that the brush width to commutator diameter ratio must be kept realistic. Brush timing With regard to the timing, the normal method of timing an electric motor is to advance or retard the brushes so that the motor will deliver equal performance in either direction of rotation. If the timing is advanced or retarded, the motor will become unbalanced and run more efficiently in one direction or the other. As there are not too many races run fully in reverse, it is usual to time the motor to work the way you want it in the forward direction only. The usual timing angle range for modern car motors is from 8-37 degrees. Some motors come pre-timed and others feature a fully adjustable endbell, which allows any timing angle to suit all manner of applications. The endbells carry the brush housing and rear bearing and can get very hot. The usual composite plastic endbells can actually melt. To prevent this and to improve motor cooling and thus efficiency, some manufacturers offer aluminium endbells. Keep in mind here also that magnets do not like getting hot and most will demagnetize very quickly if the heating gets out of hand. One final word on the brushes. Spring tension also plays a major role in establishing the RPM/torque ratio of your motor. Again as a rough rule of thumb, the lighter the spring, the higher the RPM and the less the brush wear. The heavier the spring the higher the torque and the greater the wear on the brushes and commutator. The final word is on shunts (the braid connecting the brush to the battery terminals). Once again, dual and triple shunts are the go. These braids must carry the full motor current and if they are too light, this will result in a loss of power. What you must always keep in mind when working with very high currents is that a lQ resistance in the wiring at 12 amps will result in a 12volt drop. If your supply battery is 12V then there is nothing left for the motor. At 120 amps, we are now talking 0. Hl Just make sure that your wiring is thick and all connections are sound; that is if you want any current to reach your motor. SC