Michael Potts


FOWLER SOLAR ELECTRIC camera ready 3.5 inches wide by 4.4 inches high

Above: Electric car controller evolution: on the left is an early transistorized PWM controller; on the right is a modern

MOSFET PWM controller and potbox. Photo by Shari Prange

Electric Car Speed Control Systems

Shari Prange

©1993 Shari Prange

The controller is to the electric car what the carburetor is to the gas car. It meters out the "fuel" to the motor according to the demand, as signalled by the throttle pedal.

Controllers have advanced more than any other part of the electric car over the years. The most primitive system involved series-parallel switching. In this method, the batteries could be run in two configurations: all wired together in series for full battery pack voltage, or split into two equal half-packs connected in parallel. This system had two speeds, which combined with the manual transmission to yield a confusing set of choices. It had complex wiring and required flipping some type of switch as well as shifting gears. Series-parallel switching provided little speed control, and performance was jerky.

A second type of controller used resistors. This type could have several speeds. In low speeds, the unneeded energy from the batteries was burned off as heat by banks of resistors. As each bank of resistors was eliminated from the loop, more energy made it to the motor and speed increased. As you can guess, this system was extremely inefficient, because the batteries were essentially always at full-throttle, but most of the energy was siphoned off and wasted. Resistor type controllers were also a terrible fire hazard.

SCR controlling

An enormous improvement was the Silicon Controlled Rectifier (SCR) controller. It controlled speed by rapidly turning the battery voltage on and off by means of a silicon controlled rectifier. This type of controller is called a "chopper". The power is actually full on or full off at any given moment, but the pulses happen very rapidly, so the effect is an averaging out of the power.

The SCR controller simplified the wiring and gave a smoother and more complete range of speeds. However, the controller still lost efficiency through heat, and required a bypass contactor to achieve full throttle.

The SCR controller was both frequency and pulse width modulated. At higher speeds, it functioned by varying the duration of the "on" part of the cycle — the pulse width — while holding the frequency constant. At lower speeds, however, it also needed to vary the number of times — the frequency — that it turned on and off each second. This ranged from about 0.02 kHz (kiloHertz, or thousand cycles per second) to about 0.4 kHz. This frequency range is audible as a growling sound.

Unfortunately, the SCR controllers had some efficiency losses, and they tended to be worst right in the middle of the performance range where most real life driving is done.

PWM controller

The next step in the evolution of the controller was the transistorized pulse width modulated (PWM) chopper. When this type of controller was introduced by Frank Willey, and then further developed by Curtis PMC, it quickly dominated the electric vehicle market.

The transistorized PWM controller varied speed by varying only the pulse width, operating at a constant 2 kHz frequency. This was much higher than that of the SCR, and reduced noise to a slight whistle.

The transistorized PWM controller was smaller, lighter, quieter, smoother, more efficient, more reliable, and simpler to install than the SCR controller. Later models came in sealed weatherproof aluminum cases with extruded heatsink fins.

Since then, the PWM has evolved again. The Darlington transistors inside it have been replaced with MOSFETs (metal oxide semiconductor field effect transistors). The result is a more streamlined package without the heatsink fins, a broader range of input voltages, and a higher frequency of operation (15 kHz), making it virtually silent.

The choice

A PWM controller is the best choice, (preferably the MOSFET version) but an SCR controller is acceptable. In fact, SCR controllers are still used on very large motor applications, such as diesel/electric trains and electric transit trains. If you have an SCR unit, you will need to accomodate its larger bulk. Since its contactors and components are all exposed, this controller should have some kind of weatherproof enclosure, while still allowing cooling airflow.

Resistor and series-parallel battery systems are not acceptable for high power applications such as EVs.

Installing the controller

The controller should be mounted with its terminals as convenient to the motor terminals as space permits. On the PWM controller, the terminals can face in any direction except straight up. In that position, it is possible for moisture to collect and seep along the terminals into the controller and create a short circuit.


Temperature control is the most critical factor for controller performance. If the controller position does not achieve good airflow in its installed position, duct air to the back of the plate from elsewhere. For a high performance race car, or a car with a rigorous duty cycle including long steep hills, use a finned heatsink plate.

The MOSFET PWM controller heatsinks through its bottom, and must be mounted on an aluminum plate measuring 12 inches by 12 inches by 1/4 inch minimum. The plate need not be square, so long as it has an equivalent quantity of aluminum in it. Coat the bottom of the controller completely and evenly with heatsink grease before mounting it on the plate. This plate needs to be positioned in the airflow for cooling.

One of our customers installed his controller vertically on standoffs on the firewall behind the driver's seat of a Porsche 914. The bottom inch of the plate extended under the car into the airstream. In motion, this caught the air and funnelled it up between the plate and the firewall, providing very effective cooling.

AC controllers

All of these controller apply to DC motor systems. The ac motor has an entirely different controller. An ac controller needs to be, in effect, three DC controllers synchronized together. For this reason, it is also very expensive and bulky, and not generally used by hobbyists.


The other part of the speed control system is the potbox or potentiometer (variable resistor). This is the unit that connects to the throttle pedal. It is typically a 0-5 kiloOhm (kfl) unit much like a dimmer knob on a light switch. Depending on how much the throttle is depressed, it sends a resistance signal ranging from 0-5 kfl to the controller. The controller interprets this signal and varies the duration of the energy pulses proportionately.

The potbox must be mounted rigidly to be effective. Adjusting the throttle linkage is one of the trickiest and most critical operations for good performance. If the potbox does not achieve full "on", the car will never have full performance.

If it does not achieve full "off" position, there are other problems. The PMC potbox has a high-pedal lockout safety feature. This means that the controller will not operate if you try to start the car with a partially depressed throttle. This protects against an abrupt and unexpected lurch if the car is turned on with the throttle accidentally depressed. However, if the potbox cannot return to the full "off" position, the car can't be started.

The potbox lever arm has several holes spaced along it for the throttle linkage, to allow different travel distances. If none of the existing holes works for your throttle, you can build an extension for the arm and add more holes.

Incidentally, the PMC potbox contains one of many emergency disconnects that should be in the system. It is a microswitch that functions as a "deadman" switch. Any time the throttle is released, the microswitch opens the main contactor, shutting off all power to the system.

The three main qualities to look for in an electric vehicle speed control are efficiency, performance, and safety. If you choose a good system and install it carefully, you'll never need to think about it again.


Shari Prange, Electro Automotive, POB 1113, Felton, CA 95018 • 408-429-1989 , the incredible

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