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Design and Realisation of Battery Powered Drivetrains

FAQ's Agni Motors
What is the difference between an Agni motor and a Lynch motor?
The Agni motor is an improved version of the Lynch motor, designed by Cedric Lynch who now works for Agni Motors. It offers higher torque per amp and lower speed per volt, which in some applications makes possible a simpler and cheaper transmission. It is also slightly more efficient.
Please could you provide technical information on the motor?
There is quite a lot of technical information on our web site under performance but you need a recent version of Adobe Reader on your computer for some of it to show properly on your screen. You can download a recent version of Adobe Reader from our web site if necessary. Please contact us if you have a query not covered on our site.
What warranty is there?
If the motor goes wrong because of a manufacturing fault within one year, we will replace or repair it or send you the part(s) needed to repair it (whichever is the best/quickest solution to the problem).
Are spare parts available?
Yes, all parts are available as spares. If we make alterations to the motors we will ensure that parts remain available for motors made before the alteration, either by continuing to stock the earlier part or by supplying new parts together with any other part that is needed to make them fit the earlier version motor.
What controller do you recommend?
It depends on the application. In some applications with low inertia such as a hydraulic pump run on 24V or less, or where the motor is run directly from solar panels, you can simply use an on-off switch.
For some other applications, especially ones in which the motor will be run at full speed most of the time but has to start gently, it is possible to use a mechanical switch that first makes contact through a resistance and then cuts the resistance out of circuit in a number of steps. The switch should be made so that it does not lose contact with the contact it is on until it has made contact with the next one (make-before-break). The resistance may be made from steel welding or fencing wire. This technique may even be used on a light vehicle if the drive is through a gear change mechanism such as a cycle derailleur gear, although it makes economic sense only on an amateur-built vehicle because in production quantities an electronic controller will be cheaper. In this case there must be an initial resistance step in which the current is just enough to take up the slack in the chain and freewheel, and a step giving enough power to reach three-quarters of full revs in first gear. Further steps in between will make starting smoother. Once under way the motor would normally be run straight from the battery and the speed controlled by changing gear (moving the starting switch to the initial resistance step during gearchanges).
For many applications including heavier vehicles and heavy boats an electronic controller is necessary. For boats and low-speed vehicles (below 15 to 20 Km/h, 9 to 12 mph) we recommend those made by 4QD, the Curtis 1225 and 1227, the Sevcon Millipak 4QPM (all of which give forward, reverse and regenerative braking) and the Curtis 1204, 1205 and 1209 (single direction but can reverse or brake by use of switch or contactor; 12 volt versions of the 1204 and 1205 are available on request but are not listed in Curtis’s catalogue). The Curtis 1225/1227 and Sevcon are microprocessor-controlled and if you wish to alter their settings you need either a programmer or an adaptor and software to connect them to a personal computer.
For vehicles for higher speeds (and anything else with high inertia, such as light railway traction even at low speeds) we recommend the Brusa MD95 series models, an old design but still best because they control torque directly in proportion to the throttle position and without any time delays. The difference this makes is that if you suddenly apply one-tenth throttle with the Brusa controller in a stationary vehicle you will get one-tenth of maximum acceleration with no definite speed (you will eventually reach full speed if the air resistance is low and you don’t ease off the throttle). If you do this with the other controllers you get full acceleration up to one-tenth of top speed and then the acceleration stops, constituting a substantial jerk! The makers have attempted to mitigate this effect with delay ramps but these are not entirely satisfactory; they can in some circumstances cause acceleration to continue for a moment after you release the throttle. The designer of the Brusa MD95 owns an electric car that he uses every day; we don’t think the designers of the other makes do. The Brusa controller drives in one direction only, but can provide reverse and very well-controlled regenerative braking if used with two double-pole changeover switches or contactors.
Is the motor suitable for my vehicle?
It is suitable for use with a fixed transmission ratio on a road vehicle whose total loaded weight in Kg multiplied by its intended top speed in Km/h come to a figure of 30000 or less, if run on 60V (or 72V, which is safe if the motor is not overdriven above the speed at which it runs without a load). If the weight multiplied by speed come to more than 30000 it will be necessary to use a gearbox (and accept a large reduction of speed when climbing hills) or to use more than one motor. With a fixed transmission ratio it will be possible to maintain near full speed when climbing hills, provided that the battery can supply the necessary power.
(1) a motorcycle weighs 200 Kg loaded and is to have a top speed of 120 Km/h; multiplying these figures gives 24000 and there should be no problems. The reduction ratio needed will be about 2.75:1 with typical size motorcycle wheels (3.00x17, 23 inches overall diameter including tyre) or 2:1 with typical scooter wheels (3.50x10, 17 inches diameter including tyre). These ratios can easily be obtained in a single stage with various types of chain or toothed belt. Note: motorcycle and scooter tyres have quite high rolling resistance and it would be much better to use radial-ply car tyres if you can obtain acceptable handling characteristics with them. They lack camber thrust when leaned from vertical and may need unusual steering geometry for best feel.
(2) a delivery van weighs 1500 Kg loaded and is to have a speed of 60 Km/h; this comes to 90000 and with a fixed ratio it will be necessary to couple three motors (two might be adequate if the vehicle is not to be used in a hilly area) or to use a gearbox, with which the speed on the steepest hills will be reduced to 20 Km/h with one motor or 40 Km/h with two motors. With a fixed ratio, or in top gear with a gearbox, the overall reduction ratio needed will be about 5.7:1 with typical size small van wheels (155/80x13, approx. 23 inches diameter including tyre). (If the drive is through a gearbox the ratio between motor and gearbox will probably be about 1.5:1 reduction).
(3) a small car weighs 750 Kg loaded and is to have a speed of 80 Km/h; this comes to 60000 and will be satisfactory with two motors and fixed ratio or with one motor and a gearbox. With a fixed ratio, or in top gear with a gearbox, the overall reduction ratio needed will be about 3.6:1 with typical size small car wheels (145/70x12, approx. 20 inches diameter including tyre). This ratio can be obtained in a single stage with various types of chain or toothed belt. (If the drive is through a gearbox the ratio between motor and gearbox will be quite close to 1:1).
For vehicles to be used on soft and/or steep ground (trials/motocross motorcycles, golf cars etc) the figure of 30000, quoted above as the product of weight and speed, should be HALVED, and the weight used in the calculation should include any trailer to be towed. Take into account also the power required to drive any accessory, powered by the same motor, which may be fitted to the vehicle. The power for an accessory may be considered separately rather than added to the traction power if the accessory is one (such as a lifting platform) that is only worked with the vehicle stopped
If the motor is to be run on less than 60V the figure of 30000 should be reduced in proportion to the voltage. For example if the motor will be run on 12V the weight multiplied by speed should not exceed 6000. Example: an electrically-assisted cycle-rickshaw has a loaded weight of 350 Kg and the motor, running on 12V, is to assist up to 15 Km/h; 350 x 15 = 5250; the performance will be satisfactory. The reduction ratio needed will be about 5:1 with 28-inch wheels (used on Indian and Bangladeshi cycle rickshaws) or 3.5:1 with 20-inch wheels (used on some British cycle rickshaws). This can be obtained in a single stage with a small-pitch chain (25H duplex, 219, 8mm, 0.375 inch).
For light rail applications gradients and rolling resistance are much lower, and a single motor can pull greater weights. With a design maximum speed of 30 Km/h and maximum gradient of 2.5% (1 in 40) a single motor can pull a total train weight of four or five tonnes. With driving wheels 10 inches diameter the reduction ratio needed is about 5:1, which can be obtained in a single stage with 25H duplex chain (19 teeth on motor, 95 teeth on axle) with sufficient clearance for the axle sprocket to pass over points without fouling. The drive can be transmitted to additional axles by further chains with 1:1 ratio or by coupling rods. Four-wheeled (as opposed to bogie type) vehicles with a long wheelbase should be avoided, because they have high rolling resistance on curves.
Is the motor suitable for my boat?
On a launch, a skiff, a canoe or a sailing boat with auxiliary drive it is possible to use the Agni motor with direct drive to a propeller approx. 13 inches diameter, using 12 or 24 volts. The motor bearings can accept the propeller thrust and transmit it to the hull with no need for a separate thrust bearing.
Using a larger propeller with a reduction by toothed belt, multi-rib belt or chain it is possible to use one Agni motor to power river and canal boats weighing up to about 30 tonnes. With an 18-inch propeller (typical on a UK canal narrowboat) the reduction needed will be in the region of 4:1. (This is the largest propeller that can usually be fitted to a narrowboat, but it is better to use an even larger one if there is room for it, e.g. on some boats originally designed for steam power. On a heavy, slow boat, the larger the propeller the larger the proportion of the motor power that is translated into propulsion of the boat and the smaller the proportion that goes into propelling a jet of water backwards).
Most boats operating at the speed typical of river or canal cruisers can take advantage of the high efficiency of the Agni motor to be operated entirely on solar power if there is a canopy or roof on the boat that can be wholly or partly covered with solar (photovoltaic) panels. This means that there is no worry about finding a place where you can recharge your boat’s battery. A battery is of course necessary so that it is possible to run at night, in a tunnel or under heavy tree cover, but much of the time it is possible to run without it connected!
A recent technical development, the lithium-ion or lithium-polymer battery in large sizes, makes possible an electric speedboat that can maintain planing speed for an hour or more and also an electric aircraft that could fly for 100 to 200 Km on a single charge. The Agni motor’s high power-weight ratio makes it very suitable for projects of this type. The propeller size for direct drive on a speedboat will be approx. 7 to 8 inches, and for direct drive on an aircraft 40 to 50 inches (on 60 to 72 volts). The thrust may be transmitted through the motor without a separate thrust bearing. For these applications the brush-holder should be loosened, turned about 3 degrees in the opposite direction to motor rotation and re-tightened. (3 degrees is equivalent to 3mm measured around the edge of the brush-holder).
What else is it suitable for?
It is very suitable and cost-effective for solar-powered applications such as water pumping. For a power of about 3 KW a conventional DC motor is about 83% efficient, the Agni motor 91 to 93%. This means that for 3 KW mechanical power you need 3.6 KW of electrical power with the conventional motor and 3.3 KW with the Agni motor. At the time of writing (September 2006), the cost of 0.3 KW of solar panels exceeds the full retail price of the Agni motor, making the Agni motor more cost-effective even if the conventional motor were to cost nothing!
The motor is also suitable for lifts, cranes, some machine tools and other industrial machinery.
Is the motor also suitable for use as a generator?
In some circumstances it is suitable. It is good for regenerative braking in a vehicle, with a suitable controller. If driven at about 1500 to 2000 rpm by a petrol or diesel engine or a mains-powered electric motor (with ability to provide seven or eight horsepower without stalling) it will generate an output very suitable for arc-welding with coated electrodes; no electronic controller is needed for this (using a light but powerful petrol engine such as one from a large chainsaw or disc-cutter, with a belt or chain drive giving a reduction of approximately 4:1, you could make a welding outfit that can be carried in a backpack and used to weld heavy steelwork in places not accessible with a normal electric welding outfit). Driven by a petrol or diesel engine it can also be used to charge the battery of an electric vehicle or boat in order to extend its range beyond that available from the battery. Either there must be a diode in the circuit to prevent backflow of current or the current must be controlled (for example to enable the motor to start the engine that will run it as a generator). The charging current must be regulated either manually or automatically so that it conforms to the battery’s requirements and is within the safe capacity of the Agni motor.
The Agni motor is less suitable for use with a wind turbine because there is normally a need for high efficiency at extremely light load, and the friction of the brushes represents a fixed loss of about 2 amps. If mounted by the shaft with the case rotating, brush assembly removed and wires attached to some of the commutator segments to take the output as polyphase AC to a rectifier it would be more suitable for this application.
It is suitable for use as a generator driven by a water wheel or by human or animal power. If used for charging a battery via a diode it will run easily up to the speed at which charging begins, and as more torque is applied the charging current will increase with little further increase in speed.
Can the motor be run immersed in….? What protection does it need?
It will run under water but we cannot recommend this because it will rapidly damage the bearing, and everything else too if the water is not pure. It might be possible to run the motor immersed in certain light oils, but this would have to be the subject of experiment. Normally liquids, especially oily or corrosive liquids or solutions that will leave a sticky deposit (paint, syrup), should not be allowed to enter the motor. Also it must be protected against powders that can melt and re-solidify, such as paint in powder form. Water can be tolerated as splashes that strike the motor only when it is running, as might happen in a vehicle going through a pool of water (so most of it is thrown out and the rest quickly dries up). The motor should be protected against water thrown directly off vehicle wheels by using suitable mudguards.
In industrial applications the motor needs to be protected against cutting fluids, swarf etc, by suitable baffles that will still allow air flow.
For safety reasons this motor must not be used where there could be a flammable or explosive atmosphere.