Electric Go Kart Story – Part 6

Building A Motor Controller For Electric Go Kart

Hi,

After just having a push button switch and replacing various other switches as mentioned in Part 5, I decided that I wanted to be able to control the speed of the Electric Go Kart. Battery range and technology took a back seat for the time being. This brought me into the world of Pulse Width Modulation (PWM) technology.

Pulse Width Modulation (PWM) is a method of controlling Electric Motor speeds by switching power on and off to an Electric Motor many times per second. This is like pressing the accelerator right down, then letting go and repeating this cycle. The duration the controller switches on (allows current to the motor) as a percentage of one complete switching on and off is known as duty cycle. So for example, when I created my Motor Controller, it was a simple one that controls DC current to the Electric Motor. As my battery source was 12 volts, if I wanted to apply approximately half speed, I would need to simulate 6 volts or have a 50% duty cycle. So for every switching cycle, the controller would allow current to the Electric Motor for 50% of the time and the other 50%, there would be no current to the motor. So if the frequency was 1Hz, then for half a second, there would be current going to the motor and the other half a second, there would be no current going to the Electric Motor.

If I wanted to have my Electric Motor run at 75% of it’s maximum speed, I would have a duty cycle of 75%. So if I have my Motor Controller frequency at 1Hz, then for 0.75 seconds, the Motor Controller would send current to the Electric Motor and for 0.25 seconds, the Motor Controller would not send current to the Electric Motor. In this case, the Electric Motor would think it is getting 9 volts.

In reality, you would want the Motor Controller to have a PWM frequency a lot higher than 1Hz (usually in the kHz range) so that you don’t notice the Electric Motor stopping and starting. A higher frequency will help provide a smoother ride as the inertia of the Electric Motor will keep the Electric Motor in motion during the time the Motor Controller switches off current to the Electric Motor.

Unfortunately, with my Motor Controller, which was a crude design would not  able to properly switch at the higher frequencies, but that was due to other reasons. I used a 555 timer based circuit to generate square waves which were pulse width modulated. So far so good. But to handle the high currents the Electric Motor was drawing, I could not run the Electric Motor directly from the 555 timer based circuit. So instead, the 555 timer based circuit actuated 2 relays which in turn actuated a contactor  solenoid. That was all fine and it adequately worked until the frequency of the 555 timer based Motor Controller reached a certain frequency (which I have not measured). At this frequency, the relays driven by the 555 timer based circuitry would not be able to switch on and off as quickly as the 555 timer and hence stayed in the on position, giving the Electric Motor full power.

Commercially available Motor Controllers use solid state electronics that can handle the high currents. I did think of using high powered Mosfets and Diodes to replace the relays, but with the cost of the Mosfets getting close to the cost of some commercially available Motor Controllers, I decided not to pursue this path.

Despite the limitation of the crude Motor Controller I built, it generated quite a bit of interest the 2011 Canberra International Electric Vehicle Festival and also amongst other enthusiasts who have seen this. This simple and crude Motor Controller did work, but not to the refinement of a Motor Controller which one can buy for a few hundred dollars.

Although, this crude Motor Controller has served its purpose, it lacks a lot of functionality which a commercially off the shelf Motor Controller offers. Companies such as Kelly Controller, Alltrax, Curtis Instruments etc, allow you to configure the characteristics of the Motor Controller via software such as power, torque and other factors that affect driveability. To configure my crude Motor Controller, I would have to change resistors and capacitors. I’ve been told by people who use Kelly Controllers that they can configure how much current is fed to the Electric Motor and some of these Motor Controllers offer regenerative braking (the ability to recover some charge for your battery whilst the Electric Motor is slowing down or the Electric Vehicle is braking). Regenerative braking is a wonderful thing to have because it you can use that to slow down your Electric Vehicle and put some charge back into your battery.

After going through the process of my making my own Motor Controller, I realized a lot of time and effort had gone into it and from a time management point of view, I would have been better off getting a commercially available  off the shelf Motor Controller which has been tried and tested.

The next evolution of my Electric Go Kart is to shop around for a commercially available off the shelf Motor Controller and to test out using higher voltages.

This is Crazy Al signing out.

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Check out the other parts to the Electric Go Kart Story:

Part 1 – What Inspired Me To Do An Electric Go Kart Conversion

Part 2 – Taking Action – Modifying An Electric Starter Motor

Part 3 – When The Rubber Hits The Ground

Part 4 -Extra Parts Added To Electric Go Kart – Adding A Touch Of Professionalism

Part 5 – Replacing Electrical Switches & Electric Motor

Part 6 – Building A Motor Controller For Electric Go Kart

Part 7 – What I’m Looking For When Upgrading The Motor Controller On The Electric Go Kart

Part 8 – Doubling The Voltage To The Modified Electric Starter Motor To Increase Speed To The Electric Go Kart

 

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Tags: Electric Go Kart, Motor Controller
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