While it is one of the most important parts of any modern electric vehicle, the speed controller is not always completely understood. The aim of this article is to explain the basic functions of the speed controller. Considerations regarding the correct implementation of the controller will be discussed as well, including choosing the correct controller for a specific application and what factors can affect this choice. It is hoped that after reading this overview the reader will have a better understanding of the function of the speed controller and is able to appreciate the importance of choosing the correct controller for their vehicle.
Importance of a Speed Controller
As its name implies, one of the crucial functions of a speed controller is to allow the operator to manage the vehicle’s speed. Operating a traditional DC (direct current) motor without a speed controller means that the operator can only apply the full battery voltage and current to the motor. This approach will cause a “neck snapping” take-off and tremendous stress on the mechanical drive components. The vehicle will accelerate to top speed only – good for the drag strip but not much use for the school safety zone.
In the case of newer AC or Brushless DC motor technologies, the motor will not function at all without a speed controller. These motors require multiple electrical signals which reverse in direction periodically in a manner which is precisely timed with the motor rotation. A critical function of the speed controller for these motors is to generate these precisely timed reversing (alternating currents, or “AC”) signals.
Instead of a full-on DC motor or (what would be) a useless AC motor, a speed controller meters out the battery voltage and current to the motor based on some type of operator input such as a throttle, twist grip, or pedal force (in the case of bicycles). This allows for the operator to control the rate of acceleration and deceleration as well as the rate of speed the vehicle moves at.
Basic Functions of a Speed Controller
In addition to speed control some other basic functions that the speed controller may offer, regardless of motor type, include:
- Motor current limiting – protects the motor and controller from damage if stalled or overloaded
- Battery current limiting – protects the battery from excessive discharge, and reduces or “folds-back” the available battery current as it becomes depleted to prevent cell damage
- Battery state of charge and health monitoring
- Thermal protection – protects the controller from overheating under adverse conditions
- Acceleration rate – changes how fast the vehicle accelerates when full throttle is quickly applied
- Throttle mapping – changes the response profile of the speed input device so that the operator could have finer control at very low speeds and less fine control at mid speed, for example
In the case of the newer AC motor technologies, the speed controller may offer additional features:
- Motor parameter setups – these motor controllers typically require specific technical information about the motor so the controller can operate them in an efficient manner
- Speed of position feedback device support – AC motors often require some kind of sensor so that the controller can maintain smooth and efficient operation
- Field weakening or phase advance information – AC motors can be operated at higher speeds than the nominal speed. The controller will need setup information to implement this feature efficiently
Beyond the Basics: Programmability
Beyond these functions controllers are offered with a multitude of various adjustments, feature sets and programmability. Most modern controllers can be programmed by either a handheld programming pendant or via connection to a computer. This allows the user to access a wide variety of settings to customize the operation of the controller and, in some cases, tune the performance of the controller to match the motor being used. If this procedure is done incorrectly the operation of the vehicle can range anywhere from poor to dangerous. Some vehicles use more than one controller and these are often tied together with some type of onboard communications network.
Before the programming can be considered, however, the correct controller must be chosen for the vehicle in question. This choice is based on of a number of factors. The first place to start is the type of motor being used. Today’s options include series wound, separately excited, brushed permanent magnet, induction AC, and brushless DC or permanent magnet AC. Each has its own strengths and weaknesses and its own specific controller requirements. Secondly, the controller needs to be able to operate with the rated battery pack voltage. Most small electric vehicles operate with 24V, 36V, 48V, 72V, or even 96V battery packs. Larger vehicles and on road vehicles tend to operate at even higher battery voltages.
Importance of Determining Needed Power Rating
Once the type of motor and battery voltage has been determined the current or power rating of the controller needs to be considered. This is based on the size and weight of the vehicle (including the load), the speed the vehicle needs to move at, and the environment the vehicle is intended to operate in. The amount of power the controller must provide to the motor will obviously be different between a small, 3 wheeled mobility scooter operated in a shopping mall compared to a 4 wheel drive off road vehicle pulling a loaded wagon up a mountain road.
In many cases careful needs analysis needs to be done to properly match the speed controller to the vehicle. This needs analysis typically includes understanding how the vehicle will be driven – what kinds of typical and maximum loads, grades, and operating environments will be encountered. This analysis is necessary to select and validate components which are sized so they are the smallest possible without overheating during the required operating conditions.
This analysis can be relatively straightforward when changing from one brand or model of controller to another in an existing electric vehicle design. If a manufacturer of an Internal Combustion Engine (ICE) vehicle is looking to convert to electric more work is often required. Frequently the vehicle manufacturer may excel in mechanical design but has little experience with power electronics and control systems. In this case it is definitely to the advantage of the vehicle manufacturer to work with a controller company that is able to step in and assist in the electrical design stage. A good controller provider can make sure the demands of the end users of the vehicle are met and often can help to source and integrate other related components such as throttles, batteries, contactors, etc.
End User’s Perspective
From the perspective of the end user it is important to understand how a vehicle is going to be used. If the vehicle is to be used basically for transporting 1 or 2 people or light loads on smooth roads or flat ground, a small 2-5 horsepower motor and lower current controller around 200-300 Amps will often perform adequately. Many vehicles in this category operate with 24-36 Volt batteries. When operating a larger vehicle designed for carrying 4 or more passengers, heavier loads, or for operating in rough or hilly terrain a more powerful electric drive train would be in order. This could be represented by motors in the 5-10 horsepower range, a controller capable of delivering 300-600 Amps, and 36-48 Volt battery packs. Some of these vehicles utilize 2 motors and 2 controllers for 4 wheel drive. Industrial vehicles such as forklifts and heavy equipment tugs will use even larger motors and require more current. Often battery pack voltages in these vehicles are increased to 72 Volts or higher. Specific vehicle designs and applications may call for higher powered motors and controllers than the ranges listed.
In summary, it is clear that the electric speed controller is truly the heart of any modern electric vehicle. When properly selected and implemented the controller will allow the vehicle to operate in a smooth, safe manner. Many variables go into the correct choice of controller and if chosen wrong the whole vehicle can be impacted negatively. In designing or redesigning an electric vehicle it is wise to partner with a controller company that is able to work closely with the vehicle manufacturer. When specifying an electric vehicle for a certain application it is important for the end user to match the intended use to the vehicle’s control system and motor. This approach can ensure both the vehicle manufacturer and the end user a positive, safe, and trouble free experience.