Motor Controller and Throttle

With the motor and batteries in place, they could be cabled up, and the motor would spin, but how would you control the speed?
That’s what the motor controller does.

Direct Current (DC) motor controllers typically all work the same way. They “chop” the current from the batteries to the motor. Essentially, they are a big fancy switch that turns power to the motor on and off very quickly, typically thousands of times per second. The controller varies how long the circuit is on vs. off, depending on the signal it gets from the electronic throttle, or “potentiometer”.

The technique of turning the motor full on and off quickly is called PWM or Pulse Width Modulation. Besides controlling the speed of DC motors, it’s also used to dim LEDs on sports scoreboards and digital billboards, and is also used in many other facets of industry and electronics.

By always providing the full voltage to the motor, but turning it on and off quickly, the motor has full torque (OOOOOMF!) at any speed. The speed of a DC motor is directly proportional to the voltage provided to it. More voltage (such as having more batteries in series) makes the motor spin faster. With PWM, the AVERAGE on/off of the pulse width modulation controls the speed. But there’s still full voltage, and voltage times amperage = HORSEPOWER. Using a modern electronic PWM controller gives an amazing effect of all the power you want at even very low speeds.

There are other ways to control speed on battery powered DC motors. One older technique was to mechanically switch the series/parallel connections of the batteries to the motor in such a way that that it might be 12/24/48 volts. A bit like having “3-gears”. These systems made a lot of clacking noise, and the contactors (heavy-duty power switches)  needed maintenance somewhat regularly. The speed control was fairly basic. Vehicles like the Citicar made use of this type of system.

Voltage to the motor could also be controlled by running the current through a variable resistor. Trouble is, you need a BIG resistor! They get hot, and would need a LOT of forced air cooling. It would be avery inefficient means of control, as the only time all of the power goes to the wheels would be when you were driving full speed. (The rest of the time at least part of the current is being wasted as heat.)

PWM is a nice and efficient means of controlling speed, conserving energy, AND giving you excellent speed control. If it sounds complicated, don’t worry, there’s a PWM controller in nearly every electric golf cart out there.

In fact, a used golf cart motor controller from E-Bay might be a great place to start! Golf cart controllers are typically 36-48 volt, and are so mass produced, that they tend to be rather affordable. You will want to make sure to get one that has a high enough amperage rating to make the cycle fun.

Choosing a Controller
The two most popular brands of motor controllers are Curtis and Alltrax. I’m using an Alltrax AXE 4834 controller.

Because the motor controller is wired up between the batteries and motor, it is common for it to be one of the limiting factors of your electric vehicle’s performance. The model of controller you want will depends on your system voltage (how many batteries you have) and the current you want to be able to pull. You typically want to minimize current while cruising, so that you are “sipping” power from your batteries, and in turn have a long range per charge. However, you want to have high current available to you for quick acceleration (half the fun of an electric motorcycle!) and powering up hills.

My motorcycle has a 48V system, so I purchased a motor controller that can run on anywhere from 24-48 volts. If you want to build a motorcycle at 48V and think you MIGHT want to upgrade in the future to 72V, you could get a motor controller that will operate from 48-72V, but it will cost you a little more up front.
My motor controller can pass up to 300 amps of current to the motor. The motor is only rated for 150 amps continuous, but can briefly take much more than that. The batteries themselves can produce nearly 900 amps (briefly), but the electric motor simply can’t pull that much power.
If you just want a moped type vehicle for around town use, a 175 amp used golf cart controller will be fine. If you want to have good acceleration, get a 300 amp controller. A 450 amp controller should give enough acceleration to keep pretty much anyone happy!

Mounting the controller
The motor controller needs to be mounted  solidly to the frame of the cycle, near the batteries and motor. It does produce a small amount of heat, so ideally the controller should be either out in the airflow, or if you have an aluminum frame, just pressed right up against that.

On my cycle, the best place for the controller was behind the batteries and above the motor. This kept the power cables short and everything was still easily accessed. It also shows the controller off nicely, as people often ask me how the vehicle works, and it’s nice to point out the various components.

I used a scrap aluminum plate to the mount the controller. Aluminum makes a nice heat-sink, and it’s lightweight and easy to cut and drill. The controller has four mounting holes on its base. I marked and drilled matching holes on the mounting plate, and attached the controller with typical nuts and bolts. Adding that plate to hold the controller also gave me room on the OTHER side of the plate to mount the“balance of system” components.

The throttle is a “potentiometer” – a variable resistor that sends a signal to the controller, based on its rotation.
I used a Magura Twist-grip, a popular throttle that replaces the right-hand grip on a scooter or motorcycle. On my cycle frame, the original throttle was rusted and the throttle cable was broken. I removed the original throttle and slid on the Magura Twist-Grip. It easily installs just by sliding it on and then tightening a pair of screws to snug it onto the handlebar. (Had my original throttle been in good condition, I could have just connected the throttle cable to a different style of potentiometer, such as aPB-6.)

The throttle comes with three bare wires, but there are only two connections for throttle on the controller! What do you do!? Well, since you just mail-ordered this part, you can call the dealer and ask which two wire you use. Otherwise, you can test them with the OHMs setting on a multimeter. Potentiometers have three connectors or wires. The center one is the “wiper” which changes as you turn the potentiometer. The other two wires are the “ends” of the range the potentiometer covers. One is high and one is low. Connect the Ohmmeter to two of the wires, and twist the throttle and see if the reading changes. When it reads 0 ohm when just connected, and 5000 ohms when fully twisted, you got the right two wires. Crimp a 1/4″ insulated spade connector to those two wires. Cut or fold back the third wire and cover it with electrical tape or shrink tube.

Route the throttle cable from the handle-bars, along the body of the cycle (leave slack for steering!) and back to the controller. Plug both wires into the connectors marked THROTTLE. Polarity doesn’t matter, plug either wire onto either connector. Make sure they are on securely. As a safety feature of the controller, if the throttle ever becomes disconnected, the controller shuts down.

Wiring up the controller power cables to the motor and batteries is fairly straight forward. The motor controller comes with a manual that includes the wiring diagram. The Alltrax Document Depot is a great resource for all sorts of information on controllers, batteries, and motors.

After the cycle is completely finished and test-driven, you will want to tweak the controller. Some controllers have small potentiometers on the side that are adjusted, and others are computer reprogrammed. The Alltrax AXE lineup has a computer port. You simply plug a cable from the controller to your computer, download a small program, and change parameters through a simple interface. On most controllers, you can control throttle response, limit maximum amperage, and control voltage shut-off points. On an electric motorcycle the “feel” of the throttle is based on how the controller is tweaked.

NEXT UP: Balance of System components —>

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