A mini eV with plenty of go!

Reprinted from Renew 67, January-March 1999

Lance Turner shows you how to make an electric go-cart using principles and components that can be applied to larger electric vehicles

(click here to see a larger image then use the 'Back' button to come back here.)

Having wanted to build some sort of electric vehicle for a while, I got the opportunity when I bought a go-cart rolling chassis (basically a frame, seat and wheels) to 'do up' for my two eldest daughters, aged 8 and 6. I had looked at buying one of the electric 'kids cars' that you see in toys stores, but prices for these started at about $400 and went up to nearly $1,000 for the size of vehicle required for our girls. This is ridiculous, considering that these are really cheaply made moulded plastic toys that would not last long in the hands of most kids so I decided to build a go-cart instead. I found a suitable rolling chassis for $400 in the Trading Post. It was a standard junior racing cart, de signed to be fitted with a petrol motor and used on a race track, but because of the pollution, danger and health risks of using a petrol wanted to make it an electric go-cart instead.

Electromotive power

The first thing I did was to find a suitable motor for the cart. This was in the form of a second-hand computer tape drive motor. It was rated at 36 volts DC and measured around 20Omm long, 100 mm in diameter and weighed several kilos! This was a big computer!

The speed controller

My next step was to design and build the speed controller for this motor. There was no way I was going to have a simple on-off switch with this cart, I consider this idea to be too dangerous, especially for use by children. The best option was for a fully variable controller, and I decided to use one similar to my design that appeared in issue 55 of Soft Technology.

However, that design was meant to be used for 12 or 24 volts systems only, and I wanted to use a 72 volt system, as I was not sure of the power the motor would provide, and the maximum power is easily limited by an internal trim pot in the controller. I also wanted to keep the 72 volt pack electrically isolated from the 12 volt system, which was used to drive the control circuitry. These requirements mean the addition of an opto-isolator board to the controller. This consisted of nothing more than five extra components. The circuit diagram for the controller can be seen in figure 1.

However, after a bit of experimenting with the motor at various voltages, I realised that a 72 volt system voltage was not required, and that 36 volts would be adequate.

Once I had built the controller, the next step was to fit the batteries into their battery boxes. The batteries were 12 volt, 7 amp-hour sealed-lead-acid units purchased from Oatley Electronics in Sydney, and they cost about $25 each. There were seven batteries in all. Six were wired as two series strings of three batteries, with the two strings connected in parallel to double capacity. These powered the drive motor, while the remaining one supplies the 12 volt power for the controller and ancillary equipment, such as the horn.

The battery boxes were made from plywood, and there were four batteries in one box, and three in the other.

Next to be made was the 'pot box, which is basically just a small plastic box which contains a slider-style potentiometer (like those used on most stereo systems, where you move a knob backward and forward, rather than rotating it), VR1, and a return spring assembly so that when the accelerator pedal is released the pot returns to the off position. This box turned out to be a bit strange looking, as it 'wasn't long enough and the spring had to extend through a hole in one end of it to a bracket mounted on the end of the box.

The dashboard was next, and this featured a key switch to activate the drive system, a large power relay, which is activated by the switch, a power-on light and the horn buzzer. The dashboard was also made from plywood and was attached to the cart with a metal bracket to one of the steering column mounts.

Other parts to be made were the accelerator and brake pedals, the reversing switch box and bracket, the motor adaptor bracket, and a large base board that has most of the other parts mounted on it, including the racing seat that came with the cart.

I also had trouble with the racing wheels and tyres that came with the cart-two of them would just not stay inflated, and their width made the go-cart very hard to steer. Also, because the cart has a solid rear axle with both rear wheels fixed to it, they tend to scrub when going around corners. With these racing tyres the go-cart was almost unmoveable with the wheels in anything other than the straight ahead position.

For these reasons, I decided to change the wheels to something a bit simpler and narrower. I found the perfect solution at my local hardware store, in the form of two sets of off-the-shelf wheels designed for use on wheelbarrows and trolleys. These have very high load ratings and are quite robust.

The front wheels just slipped onto the stub axles in place of the original rims, though the axles did need to be modified slightly, while the rear wheels had to be machined and drilled to suit the axle hubs.

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Figure 1. The circuit diagram for the speed controller of the go-cart. The 4N25 is the optocoupler, and contains an LED and photo-transistor in the one package.

(click here to see a larger image then use the 'Back' button to come back here.)

Figure 2. This is the wiring diagram for the whole go-cart. The circuit breakers are inside the battery boxes, while the key switch, relay, power-on light and horn are mounted inside the dashboard.

Assembly

Now I started fitting everything together. The baseboard was screwed onto the large centre section of the frame, and the larger of the two battery boxes fitted to it behind the seat position. The pot box was mounted just in front of this, on the left side of the seat, and connected to the accelerator pedal by a piece of bike brake cable.

To the right of the scat I fitted the reversing switch box on the end of a long stalk to bring it up to a comfortable position.

The motor was fitted to the original petrol motor bracket by the adaptor. I fitted a 2 inch pulley to the motor, and a 5 inch pulley to the 25mm rear axle shaft. The hole in this pulley was too small when I bought it, so I carefully filed it out to the required size. While the alignment was not perfect, it was good enough for this purpose. The motor was connected to the axle pulley via a short fan belt. Tension is kept on the belt by a strong spring mounted to pull the motor away from the rear axle.

The second battery box was mounted just behind the rear axle on a length of 25 x 5Omm rectangular steel tube welded across the back of the cart frame, and wired into the first battery box.

The brakes were made from a standard bike caliper brake mounted on the same rectangular tube as the second battery box and connected to the brake pedal by a standard brake cable. The caliper grabs the 5 inch pulley on the rear axle. While the brakes are not fantastic, they do slow the go-cart fairly quickly, though I would like to improve them later on. I have heard that more expensive brake pads will make a substantial difference to braking power.

The brake and accelerator pedals were made from 10 mm thick plywood and mounted at the front of the go-cart on purpose-made metal brackets. I then proceeded to wire all of the components together with twin-core, cable. Once the connections were complete, I tested the operation of the electrical system and then tidied up the wiring with split-loom tubing and insulation tape. All-in-all, the machine came out looking quite good.

Once everything had been made and fitted to the cart, and the wiring was complete, I gave the go-cart to the girls to test. They rode it up and down the street for the first test, and the biggest problem that occurred was the argument about who was supposed to be behind the wheel!

For the second test, we took it to the local school on a weekend and they raced it around the school grounds for an hour or so, on and off. We left when the batteries started to get a bit tired, but there was still enough power available for them to drive the go-cart home again. The top speed seems to be between 25 and 30km/h, which is fast enough for kids this age. It was a good thing I didn't use the 72 volt system, as it would have been capable of much greater speeds.

The final stage was to pull the whole machine apart to finish the painting of the frame and other bits. The frame was painted in Ultra Blue epoxy paint, and the wooden bits were painted with two coats of undercoat and then a final coat of black epoxy. When everything was dry, it had to be re-assembled and the power system tested again.

Charging system

Having two different battery voltages, the go-cart requires a dual charging system.

To charge the 36 volt battery pack from a 12 volt power supply, I used a simple step-up circuit sold in kit form by Oatley Electronics. It is designed to provide a regulated 14.2 volts from a car's battery for charging other small 12 volt batteries. I changed a single component in the circuit to give me a nice regulated 42.6 volts (three times 14.2 volts) for the 36 volt pack. The 12 volt battery in the go-cart is charged directly from the 12 volt power source.

The 12 volt power comes from one of two sources. Most of the time the go-cart is charged from my workshop solar power system, but occasionally t is connected to a 4 amp, 13.8 volt regulated mains power supply.

The controller

For a full explanation of how the controller works you can refer back to the article in issue #55. The extra bits add for the high-voltage side of the system work in the following way.

The square wave output from pin 1 of the LM358 drive the LED inside the N25, an optocoupler or optoisolator. The light from the LED turns the phototransistor inside the optocoupler on and off, which drives the MOSFET on and off, thus rapidly switching the motor.

The maximum voltage that the MOSFET can be driven with is 20 volts so the 36 volts needs to be regulated down. This is done using R7 and a 15 volt Zener diode as a simple regulator with C4 there to act as a filter and reservoir. R6 turns the MOSFET off when the optocoupter is off, draining charge stored in the gate of the MOSFET.

The 1000 uF capacitor, C5, is used to smooth out the pulses of current being pulled from the battery which helps reduce the risk of battery damage.

Above - Inside the speed controller. The control board is at the bottom left with the opto board just above it. The 1000 uF capacitor is to the right of the boards with D1 at the bottom right hand corner of the box, mounted on a small heatsink.

Above - The business end of the go-cart. The motor is on the right, with the battery boxes on the left. The controller is the box with the heatsink mounted on top of one of the battery boxes. (click here for full size image)

Above - The front end of the go-cart. Note how the inner wheel turns more sharply than the outer wheel, just like a car! The dashboard can be seen at the top of the picture, with the starting key clearly visible. (click here for full size image)

Safety

There were a number of safety features built into the go-cart. The first was the aforementioned brakes, which many home-made carts don't have. Another less obvious safety feature is the frame itself. It is a fully welded steel frame designed for racing at much higher speeds than the electric cart is capable of, and is therefore very strong and not likely to fail in any way. The scat, too, is designed for racing, and is a very snug fit around the body.

When seated in the cart, it is almost impossible to come out of it accidentally. However, a seat belt will be added soon. Two other safety features are the key switch and the reversing switch, which has a centre opposition. Turning either of these switches off will cut all power to the motor and allow the cart to be easily stopped by the brake. This is a just-in-case measure, should the MOSFET in the controller short-circuit, though this is highly unlikely given the device used is a 200 volt, 50 amp motor-control unit.

The go-cart is also fitted with a horn to alert anyone who may be in its path that they are about to be run over! The button for the horn is mounted in the steering wheel, just below the centre.

And finally, while it is very hard to get them to wear one, we do try to make the girls wear a helmet when they are at the wheel of the cart, just in case they manage to hit something unmoveable at high speed.

Problems

There were a few problems with this project. The only hitches were the not-so fantastic brakes, and also a minor problem with the 10 amp circuit breakers in each battery box tripping when the accelerator pedal is 'floored', as happens some times. The finished go-cart looks quite neat, and I am more than happy with it, my first human-carrying electric vehicle.

Other uses

The electrical drive system used in this go-cart, while being quite simple, is actually very similar to the systems used in full-sized vehicles. Indeed, this system could easily drive a small road-capable vehicle, such as a moped or small motor cycle, with very few modifications, though I would suggest a higher voltage such as 72 or 96 volts be used.

Another use for this controller would be for controlling the flow of power in solar-powered vehicle, such as those used in the various solar vehicle races held in Australia and overseas each year. While the controllers used in many of the cars are microprocessor-controlled electronic marvels, this system could be well worth using as a basis for experimentation.

The controller used on the go-cart can control up to 40 amps of current for short periods, and will easily handle 15 amps continuously without getting too hot. At 96 volts, this equates to nearly 4 kilowatts (over 5 horsepower) for short bursts and about 1.4 kilowatts (2 horsepower) continuously. This is enough power to push

many small vehicles along, making this controller a very versatile unit. With the addition of some more MOSFETs in parallel and a larger diode and capacitor, quite a large vehicle could be driven, even a car or small van.