Motor Control tutorial

I suppose you could just plug the battery in and fly on full power until it runs out but it is more normal to want some control over the power delivery.

The simplest form of control is a switch. The motor is either off or full on. Such a switch can be mechanical, a microswitch activated by the servo arm. This used to be quite common but there are not many of these setups left, particularly on aircraft. You can also buy electronic switches which plug into the throttle channel on your receiver and control the motor in the same way (off or full on). These have some use in electric gliders where the requirement is to climb, glide for a while then switch the motor back on to gain more height but again are not very common any more and are certainly not practical for sport flying where some real throttle control is required.

If you want 'proper' throttle control you will need an Electronic Speed Control (usually called an ESC). These devices are controlled from the throttle channel of the radio and operate the motor much like an I/C engine throttle, from tick-over to full throttle and all points between. Modern ESCs cover a wide range of applications and offer a sometimes bewildering range of features and facilities including BEC, Brakes and various startup safety features (more on these later).

An ESC will generally have 4 heavy wires, 2 red and black go to the battery (red +ve), the other 2 often yellow go to the motor. The positive lead to the motor will be marked. There will be another smaller cable, with 3 wires like a servo cable, which is plugged into the throttle channel on the receiver. Note that this is always plugged into the throttle channel even if the speed controller has the BEC feature and so is providing the power to the radio receiver.

ESC ratings

The major things to look for when buying a speed control are the current rating, voltage rating and features. The various features are individually covered below so let's have a look at the 2 main ratings.

First on the list is the maximum current rating. Typically this will be given as 2 figures e.g. 24/35A, the first is the current which the ESC will take continuously and the second is the short term current allowed normally for no more than 30 seconds. So in the example you could run at 24A for ever and use up to 35A for short periods e.g. at take off.

By the way, if you are used to RC car speed controls the figures for aircraft ESCs will look very low. Car controls are rated in a rather strange way usually as the amount of current you could put through them for about a millionth of a second before melting holes in the output devices. So just because your car ESC claims a current rating of several thousand amps don't be fooled into thinking it is higher quality than a 'proper' aircraft ESC. Often the opposite is true. If you wonder if perhaps I am making this up, just take a look at the cheap plastic connectors used on most RC cars and imagine a few thousand amps going through them !

The other main ESC rating is the maximum voltage, more commonly expressed as a number of cells. This is pretty straightforward. If you try to use the ESC with more cells it will break. It's also worth noting that many speed controls also give a minimum voltage or number of cells. This is because the controller is itself an electronic circuit (usually these days microcontroller based) and needs ata least some voltage to keep itself working.

ESC features

BEC

BEC stands for Battery Elimination Circuit. It is a facility which allows the radio receiver and servos to run off the main motor battery (within certain conditions) so that you do not need a separate receiver battery. There are certain limits associated with BEC circuits that you need to keep in mind. BEC works by reducing the motor battery voltage to down to the 5V needed by the receiver. Doing this creates heat . Because of this it will only work with a main battery of up to some specified number of cells often 10 cells (or 12V) and also with a specified load often 1 or 1.5A. The load is sometimes expressed as a number of servos and may reduce as the number of main battery cells goes up. For example it may allow 3 servos up to 8 cells and only 2 servos 8-10 cells, with no BEC over 10 cells.

Motor cut off

This feature is always associated with BEC. It cuts power to the motor before the battery is completely exhausted so that you still have power to the radio to get to a safe landing. This is one of the features that distinguishes proper aircraft speed controls from those intended for cars. Car controls often have BEC but do not usually have a cut off. This makes them dangerous to use in aircraft. When the battery goes down the radio can stop working and you may find yourself with a plane a few hundred feet up and with absolutely no control.

Brake

Just as it sounds. When the throttle is at zero it applies a braking effort to the motor to stop it turning. This is to allow folding propellers to fold neatly rather than windmilling round creating lots of drag. Most used on gliders and old-timers which typically use the motor to get them up and then thermal around, sometimes for ages.

Opto-isolation

This feature electrically isolates the signal from the radio throttle channel from the ESC. Doing this can dramatically reduce the level of radio interference which can be created especially with very high currents. You cannot have both opto-isolation and BEC working at once in an ESC though quite a few allow you to select at installation which of the 2 features you want to use.

High rate control

The control of motor speed is obtained by switching the power to the motor on and off in various ratios e.g. maximum throttle is permanently on, half throttle is on half time, off half time etc. This switching on and off is done many times a second. The speed at which the switching takes place has a large effect on overall efficiency. Early speed controls used what is known as "frame rate" switching, which means that they switched approximately 50 times a second, the same rate as frames of information are delivered over the radio. Most modern ESCs switch at a much higher rate which makes them much more efficient i.e. they lose less power as heat in the controller. Switching rates around 3000 Hz (times a second) are about optimum. Anywhere between 1000 Hz and 5000Hz is acceptable.

Safety interlock/switch on control

Not long ago most ESCs had a separate "arming switch" which had to be operated before the ESC would deliver power to the motor. With the arrival of microcontroller based controls these switches are no longer provided. Instead the controller relies on either a throttle position (usually throttle off for the obvious reason) or sequence of positions to arm it before it delivers power to the motor. This eliminates the extra weight and unreliability of a separate switch but it does mean that there is nothing physical you can check to see if the motor is going to start up the next time the throttle is touched.

One difficulty with the arming procedures is that the manufacturers have not agreed on any one way of doing things. If, like me, you use controllers from several different manufacturers you may find yourself trying to remember the different safety sequences for each model. Not an ideal situation for safety.