Tech Info: Controllers


The motor controller is the device which controls the flow of power from the batteries to the motor, allowing you to vary the speed and acceleration of the vehicle. Many years ago motor controllers consisted of contactors and/or resistor networks, but all modern motor controllers are based on transistors amplifiers controlled by microprocessors.

Topology of a Modern Motor Speed Controller

The diagram below shows a typical topology for a motor speedcontroller. They use a microprocessor to drive a preamplifier and then power stage amplifier which controls the flow of power from the battery to the motor, with various feedback sensors to monitor the system's operation. Red arrows indicate pulse width modulated outputs, green indicates feedback sensors.


What is Pulse Width Modulation?

All modern motor controllers use some form of Pulse Width Modulation (PWM) to vary the amount of power being delivered to the motor from the power source (e.g batteries).

PWM is the process of switching the connection between the source and load very rapidly to simulate a lower average voltage. The frequency of switching is usually around 20 000 times per second (20kHz) to keep it outside the human hearing range.

(Due to the inductive nature of the motor, most controllers effectively behave somewhat like a buck converters – for more information, check out wikipedia's entry on buck converters.)


Microprocessor and Sensors

The microprocessor's primary responsibility is generating an appropriate PWM signal for the power electronics based on the status of the various sensors. Some controllers will use dual microprocessors for safety (so if either processors fault, the system will shut down gracefully). As well as the throttle input, most controllers should support the following sensors:

  • Voltage: Too high a supply voltage can damage the motor and/or power stage. Too low a supply voltage can mean the batteries are over-discharged, which can cause them irreversible damage.
  • Current: Most controllers will include a variable current limit to protect components.
  • Temperature: To avoid damage to the power stage due to thermal overload. Some controllers may also support thermal sensors on the motor.
  • Speed: Useful for detection of stall conditions or overspeed, which can both damage the motor. Can also be used for cruise control on some controllers.


Power Stage

The heart of the power stage is a bank of transistor amplifiers – usually either MOSFETs or IGBTs.

  • MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor. These are more common among lower voltage controllers since low voltage MOSFETs tend to have very low on resistance (and hence power loss). MOSFET controllers are very efficient at low power levels, since power loss in a MOSFET is proportional to the square of the current. Their resistance also increases as they heat up, so when used in parallel they tend to automatically balance the load.
  • IGBT stands for Insulated Gate Bipolar Transistor, and it somewhat like a hybrid between bipolar transistors and field effect transistors. IGBTs have a constant voltage drop which makes them more efficient than MOSFETs at high power levels, though often less efficient for low power applications. One disadvantage with IGBTs is that, like all bipolar transistors, they are prone to thermal runaway and imbalances when used in parallel. As such IGBT controllers require good cooling systems (liquid cooling is common) and/or matched transistors to avoid imbalance.

Major manufacturers

  • Curtis Instruments make a range of DC motor speed controllers with power ranging up to 144V 500A (72kW). The Curtis 1231C is probably the most commonly used controller in EVs around the world.
  • Cafe Electric make controllers ranging from 156V 1000A (156kW) to 350V 2000A (700kW!), CafeElectric Zillas are a popular choice for many high performance EVs using series DC motors.
  • Siemens are the main supplier of controllers (and motors) for AC induction drive systems. AC equipment is still kind of pricey at present.
  • ZEVA currently produces an ecomonical 144V 600A (86kW) series DC motor speed controller for use in small road-going cars.

Curtis 1231C

Cafe Electric Zilla2K

Siemens Simotion