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Tech Info - Motor Speed Controllers

A 150Amp 24/36V 4 quadrant controllerThe motor speed controller is a key element in the electrical drive system of an electric vehicle. What does it do? The DC motor speed controller takes the nominally fixed voltage from the power source, usually your battery pack, and outputs a variable voltage supply needed to control the motor speed. Its voltage output to the drive motors changes in response to control signals supplied by the user – from a foot or thumb accelerator or a joystick control say. In modern controllers the main voltage conversion is done very efficiently using PWM techniques, with much much less wasted energy than a variable resistor arrangement and, given a good quality controller, with very smooth speed control results.

Motor controllers for battery electric vehicles also bring other valuable control functions – easily reversed drive, top speed limiting, control over acceleration and deceleration ramps, over-discharge protection for the batteries and in “4 quadrant” units dynamic regenerative braking capabilities in which the power generated by the motors as they slow a vehicle is fed back into the battery pack.


 A 150Amp 24/36V 4 quadrant controller - cover removed

It is possible to make an electric vehicle work without a modern controller but using one provides many advantages in terms of performance, controllability and efficiency.

The latter issue is important because the Achilles Heel of most EV's is the amount of energy that can be stored in a reasonably sized battery pack – loosing a significant proportion of this through inefficient speed control circuitry is just plain daft.

DC battery motor controllers are rated by voltage and current. If you think back to your high school physics you might remember that electrical power (in watts) in a DC circuit can be calculated from P= I x V where I is current in Amps and V is voltage. A typical smallish 24V 75 Amp controller therefore has an electrical power output capacity of 24 x 75 = 1800 W or 1.8 kW, about 2.5 Hp. Similar 75 Amp 24V controller mounted in simple boxcontrollers are available right up to 48V x 300 Amp units with nominal power outputs of 48 x 300 = 14400 Watts, about 20 Hp. Note that these electrical powers are not what's finally available as mechanical work output at the drive wheels. Inexpensive DC motors may have efficiencies around the 75% to 85% mark and some energy will be lost in the mechanical transmission also. Perhaps our 2.5 Hp electrical output will end up as about 1.75 Hp at the drive wheels.

When selecting a controller try to download the full spec sheet or manual from the manufacturer and read the conditions that apply to the current ratings – they are usually time limited and, because in the end it's controller heating that's usually the concern, effective heat sinking can make quite a difference. In my experience however it's much easier to keep a properly rated controller cool than the motors it supplies!

Manufacturers' usually also provide clear instructions on wiring and circuit diagrams - check their web sites.

I use mainly UK built controllers from 4QD and have generally had very good service from them. For the lower power vehicles their low cost Vortex units have worked well - two basic versions are available with 40 & 75 Amp ratings well suited to the 250W and 500W vehicles. For heavier duty we use the 150Amp 4qd series controller shown above with the cover removed. There are several other good manufacturers of similarly rated units - follow the supplier links and have a look around for yourself.


A lower cost alternative can be to use electric scooter controllers. These tend to be made in the far east and can be a lot less expensive although perhaps with fewer features and lower quality. If you go this route check the number of "quadrants" of the design - many scooter controllers do not run in reverse. Check also the requirements for speed control input - they often require a hall effect speed input "pot". Check PartsForScooters for some examples of scooter controllers.


MD01 Speed ControllerA newer experience for me is using smaller H-Bridge type speed controllers on the DIY Motion Cockpit. These don't have many of the EV related features that the 4QD controllers have and are better in rapid or frequent reversing applications. Devantech are one supplier and I've used their 20 Amp MD03 units. These are of lower current capacity than most EV related controllers but are also quite a bit cheaper. As with other types they can be controlled from programmable microcontrollers and are often used for robot applications.  Many offer a number of different ways of supplying the speed and direction demands including straight analogue voltage inputs, PWM voltage inputs and other more sophisticated methods such as Serial communications and Philips I2C bus communications - read up on the technical data to see what's what.


Interestingly it is not uncommon for robot builders to make their own controllers, and if you are interested in more detail there are some good sites which can help give insight into how they work and how they can be put together - here's just a few:


And the Open Source Motor Controller Project here


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