Motors

It is anticipated that most teams will use standard low voltage direct-current (DC) motors. However there are other types that can also be used.

The STAWA Stealth II uses a 24 volt brushed DC electric wheelchair motor rated at 300 watts. This is sufficient to propel the Stealth II along at about 25 km/h and with two 12 volt 24 Ah batteries (giving a total energy storage of 576 watt-hours) the distance travelled is about 20 - 25 kilometres.

If the motor uses only 300 watts shouldn't it be able to go 48 km ({[576 Wh / 300 W] x 25 km/h } = 48 km) ?? No, not really. Have a look at the batteries page for a brief explanation.

Which way to go? Speed and Power or Endurance?

It is very important to realise that by putting in a big, powerful motor, your eV may run much faster - but not for long. You will discharge the battery so quickly that the distance that you can drive will be very limited. Using a very small motor may allow you to go a long way but at a very slow speed, but if you are too slow then you won't go very far in the set time.

Rather than a 'hare vs tortoise' race, you will need to carefully balance the hare's speed and power against the tortoise's endurance and find a compromise where you have enough endurance to last the race but that you use only enough power so that you don't prematurely flatten the battery but still get to go the greatest distance.

Types of Motors

Electric wheelchair motors are one option. Electric wheelchair motors are usually 12 or 24 volts and rated at about 300 watts. They have a reduction gearbox built onto them making them an excellent and cheap choice. However they are not usually designed for efficiency and other, more efficient motors may provide a better chance of winning. On the other hand, high-efficiency motors are more expensive and may blow the team's budget. It may be wiser to get a cheap motor and spend the money on other aspects such as a good electronic motor controller or better wheels and tyres, all of which also affects the eV's overall efficiency and performance. One of the teams' main tasks will be to examine these issues and come up with the best compromise.

Another option is to use second-hand computer drive motors. Large 'mainframe' computers often have huge diskdrives or tapedrives that need 50 - 500 watt DC motors to spin the 18 inch disks or tape drives. These can often be purchased from computer recyclers or scrap dealers for $20 - $100. These types of motors are often efficient, reliable and very well made and can be used to run small electric vehicles.

Motor vehicle windscreen wiper motors can also be used as can some motors from some types of cordless electric power tools.

Specially designed hub-motors are also available from companies such as Toprun, Elebike and Heinemann or from some local bike shops. However these are often more expensive - $500 to $2000 each - but are a very good solution as they often have high efficiency and don't need chains and sprockets to drive the wheel. Instead, the motor is actually built into the hub of the wheel itself. There are a few bike shops that sell them - but be careful that you don't blow the budget! Remember that as part of the rules the total cost of the vehicle - including the value of gifts, donations and other support - must not exceed $1000 and proof of the eV's cost may be requested during the scrutineering for the event!

Right - A hub motor from Toprun in China. Left - a mountain bike with a hub motor fitted in the rear wheel.

Internally, motors can take many different form. However when it comes to direct current motors, there are basically two types - brushed dc motors and brushless dc motors.

All rotary electric motors have two main components - the stator (which stays stationary and is usually on the outside of the motor) and the rotor (which rotates and is on the inside) and 99.999999% of electric motors use a rotating electric field (from the rotor) to generate a rotating magnetic field. This rotating magnetic field pushes against another rotating or stationary magnetic field (from the stator) and this makes the rotor spin. The rotor and stator are usually made from winding copper wire onto an iron core. Some motors have one of the windings (usually the stator) replaced with permanent magnets. This can make the motor smaller and more efficient.

One of the main problems with an electric motor - any electric motor - is how to get the current to the rotor which is spinning around. In a brushed motor, graphite brushes 'brush' against copper contact on the motor shaft (which the rotor is on) and the current then then travels via a wire to the rotor windings. While simple and easy to make, these motors are not very efficient - the brushes cause friction on the motor shaft and also have resistance losses. This limits the maximum efficiency of a brushed motor to about 85%. The brushes also slowly wear away and need to be replaced every so often.

Brushless motors were developed to overcome these problems and a number of varieties exist. One type for example, uses a permanent magnet rotor - so no current needs to be fed to any moving parts. However this means that the current needs to be fed to the stationary windings in a very precise order and timing to the stator windings. This means that fairly complicated electronics are needed to sense the speed, direction and load of the motor so that the right amount of current can be fed to the motor at the right time. The advantage is that the motor can be much more efficient that a brushless motor can be - up to 97% efficient.

IMPORTANT NOTE:

Preslite Drive Technologies, manufacturers of electric motors, has offered to provide 12 and 24 Vdc motors, suitable for the eV Challenge to teams at heavily discounted prices. The motors have an in-built reduction gearbox and are rated at approximately 350 or 400 watts output. The three models are the left-hand drive 14000059-Y and the right-hand drive 14000060-Y motor/gearbox assembly which are 73% efficient at full rating and cost $240 + GST + postage/delivery and the right-hand drive 14000053-Y assembly which is about 80% efficient at full rating. This one costs $270 + GST + postage/delivery. These motors are used in electric wheelchairs and similar devices and are very heavy duty. The difference between the l/h and r/h versions is where the output shaft is located - the shaft sticks out at a right angle to the motor. Have a look at www.preslite.com.au. Look under 'products' and then 'Industrial Products'. If using these motors, it is important to first run them for an hour or so with no load so that the brushes are run in. Failure to do this may result in damage to the motors!

For more information contact: Claude Costantin, email costantinc@preslite.com.au or telephone 03 9460 6566.