Speed Controllers

By Paul Williams.
Article posted 12th March 2009.

Modern electronic speed controllers for brushless motors are quite remarkably sophisticated devices. Not only does a speed controller take a direct current feed from the cell pack and turn it into 3-phase supply to the motor, a brushless speed controller also handles motor commutation and timing. Brushed motor speed controllers control motor speed simply by pulsing the current on and off very quickly; there's an awful lot more going on inside a brushless controller.

As with most things fast electric nowadays, when it comes to buying an electronic speed controller you have a choice of a handful of well known, established brands like Castle Creations, MGM-Compro, Schulze, Hacker and Jeti, newer players like Etti and Feigao, and a vast range of unknown "ebay brands". The same caveat applies when buying anything of unknown brand from a supplier on the other side of the world: what backup have you got from the manufacturer or vendor?

Vendor/manufacturer backup is a very serious issue where speed controllers are concerned, as the speed controller is now the weak link in a fast electric boat. Lipo cells can deliver huge continuous current, and brushless motors can consume this current and drive big propellers, but the speed controller has to control the process, and do so in an environment that could not be more hostile to delicate, complex electronic components.

Heat, vibration and water - all these are present to some degree in a fast electric boat, and mean you will break speed controllers, no matter how expensive. Unless you buy cheap controllers from ebay and treat them as "one-shot" disposible items that you run until they blow up then discard, you need to know that if you break a speed controller the manufacturer is willing to support their product and fix your controller. All the established names have a good reputation in this respect, mostly charging reasonable fees for fixing quite badly damaged controllers and doing so quickly.

Apart from the obvious precautions, like keeping the inside of your boats as dry as possible, there are other things you can do to make life as easy as possible for your speed controllers. Firstly, long cables kill speed controllers. The high frequencies that brushless motors and speed controllers operate at mean long, thin power cables impose a very high load on the capacitors on a speed controller. Use heavy gauge, thick cable, decent quality gold-plated connectors, and keep all cable runs as short as possible. You may not want to put the isolation loop on the side of your boat, prefering perhaps to hide it on the transom, but you may have no choice if putting it on the transom results in excessive lengths of cable and blown speed controllers.

Watercooling

The other thing that kills speed controllers is heat. Most boat-specific controllers are now fitted with effective water cooling, and those that are not can be cooled using an aftermarket water cooling plate that can be fixed to the FET banks on the controller. Use a thermal paste like Servisol to eliminate air gaps and couple the cooling plate to the FETs to maximise the cooling effect. Route water from the water inlet to the speed controller first and then to the motor; the speed controller needs first go of the cold water, the motor isn't so critical.

Bear in mind that the stated operating current limits only apply if you keep the controller cool - if the controller heats up, its ability to handle high current disappears, and you risk breaking the controller.

Overrating

Another cause of speed controller failure is running a controller at too high a current. All speed controllers have a maximum continuous operating current limit and a maximum peak current limit that the controller can tolerate for very, very brief intervals. Step over these limits and you will break the controller. Fast electric boats do not run at a nice even current level. The motor will be constantly loading up and then free-running as the boat moves across the water and propeller immersion changes. A boat that is well trimmed and keeps a more or less constant state of propeller immersion, and thus motor load, will be kinder to its speed controller.

Use of a data logger to record average and peak operating current consumption will give you a detailed picture of what is happening inside your boat, allowing you to see if you're overstepping the limits of the controller, perhaps through using too big a propeller.

Eagle Tree V3 MicroPower e-Logger.

The first few runs of a new model should be done with small propellers, not just to limit the current drawn by the motor. A smaller propeller will cause less torque effects and will make initial trimming much easier. Once the boat has had a few runs, you can begin working up to the desired propeller size, all the while checking the temperature of speed controller and motor. Building a big, powerful, high voltage setup and jumping straight in with monster propellers is a recipe for charcoal speed controllers, ballooned lipos and blown motors.

Buying A Speed Controller

A speed controller will be rated in terms of the voltage range it can handle - ie., the minimum and maximum number of cells you can use - and the maximum level of current it can safely draw from the cellpack.

When buying a speed controller, always err on the generous side where current limits and voltages are concerned. Your boat may draw, say, an average of 50 amps, but current spikes could be three times that. A new controller may have six or eight inches of cable on the motor and cellpack sides, but that does not mean you should use all of it. Chop the cables back to the minimum required to fit in the boat, and use good quality gold plugs of a sensible size for the type of model - ie., 3.5mm for mini oval, 6mm for a 10S monster.

Don't be tempted to save money buying less controller than you really need. If you blow it up through running too high a voltage or current, the manufacturer may well refuse to honour the warranty. They're not stupid, and can immediately tell if a controller has been destroyed by excessive current or voltage.

Make sure, if using lipo cells, that you buy speed controllers with a low voltage shutdown mode for lipos. Deep discharging lipo cells can, and probably will, fatally damage them, so you need controllers that will shut down the motor when the cellpack voltage falls too low, thus protecting the cellpack from deep discharge.

Controller Programming

Most brushless speed controllers have a range of setup parameters the user can modify to suit a particular setup. The most critical of these is the number of lipo cells you are using, as this determines at what voltage the controller shuts down to prevent deep discharge of the cellpack.

Other settings can include motor timing advance, as motors with different numbers of poles and Kv ratings will require differing degrees of advance for the most efficient operation.

Some manufacturers, for example Castle Creations, offer a PC interface via USB to allow the user to program their controllers using a computer. Some use a "programming card", others, like Etti, require the user to program their products via the radio transmitter using a system of beeps and tunes to represent a menu system which is navigated using throttle stick position. Needless to say, using a PC interface with keyboard and mouse, or a plug-in programming card, is very much easier than programming using the throttle stick.

Waterproofing

Some controllers, for example the Etti Race model, come as standard with a waterproof coating and will, or should, survive getting dunked. Other controllers are marketed as "splashproof" and will most likely break if immersed in water when live. Unless you can guarantee that your boats will never, ever get flooded (I can't!) then you should look at trying to waterproof controllers that are not waterproofed by the manufacturer. Try covering the controller in some clear heatshrink sleeving, then sealing the ends with acid-free silicone sealant - this can be very effective at keeping electronics and water separate.

Conclusion

No other type of electric-powered model stresses speed controllers quite like fast electric powerboats. The ever present threat from water, the constant jolting of running at speed over water and the heat build-up in a sealed boat hull all conspire to make life very hard for the electronics in a fast electric boat. A boat speed controller must tolerate a constantly changing load and extreme current spikes - nothing like the easy life of an aeroplane controller with a constant, smooth load, dry conditions and plenty of air cooling.

For this reason, buy one of the many boat-specific controllers now on the market in preference to something intended for aircraft or cars. Car controllers are notorious for being very short-lived in a powerboat. Ideally, buy a controller which is already waterproofed by the manufacturer, and always buy a bigger controller than you think you need and run it comfortably inside its limits.

Broken speed controllers are a fact of life when running fast electric boats. Keeping your controllers as dry and cool as possible, and running them within their rated capacity reduces the chances of a failure as much as is possible.

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Last modified: 08th July 2010 @ 09:06