Electric Go Kart Story – Part 7

| December 25th, 2011 | Posted in Electric Go Kart, Motor Controller

Electric Go Kart Story – Part 7

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

VW Beetle Electric Car Conversion-Electric Motor, Controller,Battery setup

Electric Go Kart looking to have a similar set up as this VW Beetle Electric Car Conversion. Obviously, location of Electric Motor, Controller and Batteries will be different and looking for a Motor Controller that is compatible for a Permanent Magnet Brushed DC Electric Motor

Hi,

After building my crude Motor Controller as described in Electric Go Kart Story – Part 6, I have started to look into replacing it with a commercially available off the shelf Motor Controller. The time frame of selecting and installing the Motor Controller will be well into 2012, after this Electric Go Kart has been displayed at the 2012 Summernats Car Festival in Canberra.

However, to accommodate a commercially available off the shelf Motor Controller, further modification to the Modified Electric Starter Motor is required, in particular, to isolate the negative terminals from the chassis of the Electric Motor. That is because the Motor Controllers for Permanent Magnet brushed DC Electric Motors I have seen from Kelly Controller and Alltrax, typically have at least these 3 terminals :

B+/M+ (goes to the positive terminal of the Electric Motor and positive terminal [via fuses, contactors etc] of the Battery Pack),
M- (goes to the negative terminal of the Electric Motor) and
B- (goes to the negative terminal [via a fuse] of the Battery Pack)

There are some other terminals on the Motor Controller, but not relevant for this article.

The criteria I’m using to work out which Motor Controller I’m looking to buy to match this Permanent Magnet Brushed DC Electric Motor are:

ability to handle up to 500 amps;
ability to supply 12 to 48 volts to the Electric Motor;
Regenerative Braking, although with the one way clutch on this Electric Starter Motor, the Regenerative Braking may not be as effective;
compatible with Permanent Magnet Brushed DC Electric Motors;
programing capability or adjustment capability – I would like to be able to adjust how much amps is fed to the Electric Motor and how much voltage the Electric Motor sees.

These above requirements may have to be compromised, but it provides something to work with when specifying Motor Controller requirements.

It should also be noted that with Permanent Magnet Brushed DC Electric Motors, that current is roughly proportional to torque and voltage is roughly proportional to speed. This means that if you want to increase your acceleration, you increase the current so your torque goes up. If you want to increase speed, you need to increase the voltage the Electric Motor sees.

However, as you increase current, the wires need to be thicker to handle it, so you can only increase current up to a point. If you exceed that point, you run the risk of burning your motor out.

Typically, with Electric Motors, you can increase the voltage up to a certain point. That point is determined by the insulation of the Electric Motor windings. If you exceed that voltage, the insulation breaks down or is penetrated and you get a short circuit in the Electric Motor.

Hence, by using a commercially available off the shelf Motor Controller, I hope to be able to limit current, but also have higher voltages fed to the Electric Motor. Currently, this Electric Motor is running at 12 volts (assuming no voltage sag from the batteries) and the current draw has exceeded 300 amps (my ammeter only catered to up to 300 amps, so the current draw could easily be a lot higher than 300 amps). Theoretically, this Electric Motor is outputting over 3.6kW.

If I can double the voltage to 24 volts and half the current to 150 amps to this Permanent Magnet Brushed DC Electric Motor, I will get 3.6kW, but my acceleration will drop as the torque of the Electric Motor drops, the top speed will increase and the windings in the Electric Motor will not get as hot.

If I can double the voltage to 48 volts and half the current to 75 amps to this Permanent Magnet Brushed DC Electric Motor, I still get 3.6kW, but my acceleration will drop further again, the top speed will increase again and the windings will stay cooler. This will also mean that I will not have to run thicker cables to the Electric Motor and by staying cooler, I reduce the risk of the Electric Motor burning out.

It should be noted that there are some Electronic Speed Controllers (ESCs) used in radio controlled airplanes capable of handling 300 amps and high volts. I’ve seen websites that sell them state that they are not suitable for Electric Bikes, Electric Go Karts or Electric Vehicles. No reason was given, but I have been told that the reason why they are not suitable for Electric Bikes, Electric Go Karts or Electric Vehicles is because apparently, they don’t have a current cut off capability which means if you suddenly draw more current (to say go up a hill) you run a great risk of blowing the ESC.

It should be noted, that if I later decide to use a different Electric Starter Motor such as those with field windings instead of Permanent Magnets, the criteria for the Motor Controller will change again. That is why it is important to match the Motor Controller to the Electric Motor you are going to use. Getting a Motor Controller that is incompatible with your Electric Motors is a pitfall in Electric Car Conversions, Electric Go Kart Conversion and Electric Vehicle Conversions.