Go-Kart Sized Electric Vehicle
Project Summary
This project will design, construct, and test a go-kart sized electric vehicle powered by lead-acid batteries, a brushed DC motor, and a brushed DC motor controller. The go-kart frame will be built from aluminum extrusion and use a variety of real-world automotive components.
The final design uses two 12V AGM lead acid batteries, a contactor, a programmable 24V motor controller, and a 24V 500W electric motor. The motor has a 11T sprocket on it that is connected with a #25 chain to a 72T output sprocket on the rear axle.
Project Requirements
- The go-kart should be able to seat a wide range of students comfortably
- The go-kart should operate on a voltage less than 48 V.
- The go-kart should use safe battery technology that can be easily charged and discharged.
- The vehicle should have a top speed less than 20 km / h but be able to accelerate at the traction limit of the rear tires.
- The powertrain system should have a simple, cost-effective design that presents many opportunities for student learning and improvements by future co-ops.
Table of Contents
Go-Kart Electric Vehicle Frame
The vehicle frame is constructed from aluminum extrusion, a modular building material that allows for easy prototyping and stiff structures.
Vehicle Suspension
The EV has a solid rear axle, supported by ball bearings, and independent front suspension. Each front upright, where the wheel hub mounts, is suspended by "A-Arms" or "wishbones" to allow them to safely travel over bumps in the road.
Energy Storage - AGM Lead Acid Batteries
Based on the project requirements, a 24 V motor controller was paired with a 24V motor. Therefore, two twelve volt batteries will be used to supply energy to the go-kart. Currently, two 12V PowerSafe SBS C11 batteries provide power to the go kart, but these are extremely heavy and oversized for the application. They can supply 90A continuously and weigh close to 60 lbs. each. In the future, it would be nice to move to smaller batteries, but still of the AGM / spill-proof type. Lithium Ion batteries could provide better performance and would be technically relevant to modern EVs, but are more dangerous and more difficult to charge. The batteries should be connected to the EV powertrain system with a quick disconnect connector, similar to the 45 A capable Anderson connectors here:Â
45 Amp Unassembled Red/Black Anderson Powerpole Connectors
Throttle Control and Instrumentation
To control the vehicle, the user will depress a throttle pedal which will move a potentiometer, sending a 0 - 5 V analog signal to the motor controller. From the input signal, the motor controller will spin the motor to drive the vehicle forwards. No reverse gear is available. If the user releases the pedal, a strong return spring will pull the potentiometer back to zero output voltage, stopping any further acceleration. On the dashboard, there will be a red status LED that can show fault codes, a vehicle enable / disable button which shuts down the main contactor, a state of charge gauge, analogous to a fuel gauge, which measures the battery voltage, and a amperemeter, which measures the torque output from the motor. Depending on the performance designed by the instructor, the motor speed and torque can be limited in software in the motor controller, by modifying the throttle signal from the pedal potentiometer, or by mechanically limiting the pedal travel.Â
Electric Powertrain Systems
The vehicle uses a Kelly KDS 24050 controller to drive a MY1020 DC brushed motor. As described above, the controller takes a 0 - 5 V input from a potentiometer and outputs a voltage roughly proportional to the throttle position. At slow speeds, such as when the vehicle is starting to move, this increase in voltage greatly increases the torque of the motor because the Back EMF (Electro-Motive-Force) is small. At higher speeds, if the load on the motor is less than the torque it can output, controlling the voltage acts more like a speed control instead of a torque control.
Electric Powertrain System Diagram - Reference Design by Kelly Controllers | Demonstration Video | System Component Ratings | Maximum Voltage  [ V ] | Continuous Current  [ A ] | Maximum Current [ A ] |
|---|---|---|---|---|---|
PowerSafe SBS C11 AGM Cells | 14V, Float voltage 2.30 V, min VPC 1.80 VÂ | 91 A - Capacity 91 Ah (This number should be verified, but seems reasonable) | Large instantaneous currents are possible, but would be limited by the 150A fuse. | ||
CZ150 24V Contactor | 80 V | 150 A | 800 A for 5 milliseconds | ||
100A ANE Fuse | Unspecified | 100 A | Unspecified | ||
Kelly KDS 24050 Controller | 24V Nominal, 8 - 30 V | 30 A | 50 A for 1 minute | ||
| Demo video showing potentiometer throttle and motor velocity control at no load. | MY 1020 DC PM Motor, 24V, 500W | Unspecified | 18 A | 50 A - limited by controller |
CZ150 24V Contactor
A contactor is similar to a relay, in that a small electric current is used to actuate a switch to control large amounts of current. Typically contactors are rated for much higher currents and voltages than commonly available relays. This contactor, the CZ150, can switch voltages as high as 80V and currents as high as 150A. In emergency conditions, the contactor can be relied upon to switch currents as high as 800 A, as shown in the table above. This is a key safety feature of the powertrain, since the contactor is controlled by a simple toggle switch, not software control. In an emergency condition, push and release the black pushbutton to completely shutdown the powertrain system. Another key feature of this implementation is the power resistor connected across the terminals of the contactor. Since the motor controller contains very large capacitors to filter the input and outputs of the controller, it must be connected through a high resistance to slowly charge up the capacitors. If this resistor is not used, extremely high currents will flow into the motor controller, potentially causing damage and arcing. The control contacts on the contactor have a reverse-bias diode that suppresses transient voltage from to the inductance of the control coil.