Motors tutorial

There are a number of different classes of motor used in electric flight.

Brushed motors which consist of
- Can or Ferrite motors (by far the most common type)
- Cobalt (or other rare earth) motors
Brushless motors which can be subdivided into
- Standard brushless
- Sensorless
- Outrunners - the latest variant, currently all sensorless but with a difference

There are also things called "coreless" motors which are actually a special type of brushed motor generally only available in very small sizes as used in servos and perhaps for indoor flight. More about these in the Brushed Motor section.

Brushed vs Brushless (what's the difference) ?

You don't really NEED to know how these things work so if you just want to get on with the flying skip to the next section. If you are interested it may be worth printing this section to read at your leisure. It can get a bit confusing to take in at once.

All motors have magnets and coils of wire (windings). If you remember any of your high-school physics this is where Faraday's Law comes in. Basically a coil of wire moving in a magnetic field will create an electric current. The reverse also works so if you can get a varying current through a coil of wire it will move relative to the magnetic field. I.e. it goes round and round, a motor.

I'm only going to cover the main differences between brushed and brushless motors here. There are a number of good tutorials on how motors of both types work in considerable detail on the Web if you're interested.

Thanks to Mathew Orme at Aveox http://www.aveox.com for permission to use the excellent diagrams below.

Brushed motors

A typical brushed motor is shown above. The magnets are attached to the case and the windings (coils of wire) are wrapped onto an iron armature and shaft. The power is fed in through brushes which together with the "commutator" changes the direction of the current through the motor's windings as the armature rotates.

There are a few problems with this layout. The windings are pretty heavy to rotate and they are in the middle of everything so they have trouble getting rid of any waste heat that is created. The brushes make imperfect contact with the commutator especially as currents and speeds get higher. There are quite a lot of losses in this setup. In practise a brushed motor will rarely be more than about 65-70% efficient and many are difficult to get more than 50% efficient in our sort of high-power application.

Coreless motors are very similar to the above except that the coils are not wrapped round a metal armature but are simply fastened into shape with glue, generally epoxy. This makes the armature much lighter and hence faster to accelerate which makes them good for servos. On the other hand it means they will not stand sustained high speed (revs/min) or loads without falling apart. Coreless motors are generally very small, low speed, low power devices. As flight motors they are only of any use for small indoor planes. E.g. Wes-Technic have a range of coreless motors for just that purpose.

Normal brushed motors do come in different qualities. The main difference is in the material of the magnets. Standard (also called can or ferrite) motors have magnets made of ferric oxide. These include probably the commonest type of motor used for electric flight, the Graupner Speed xxx series (where xxx denotes the size, 400/500/600 etc). These come in a wide range of sizes, styles (plain bearing, ball bearing, race etc) and different voltage ratings. Note that the voltage ratings refer to how the motor is wound. For electric flight we invariably use a higher voltage than nominal and therefore use more power than the "official" design powers. E.g. a Speed 400 7.2V motor will usually be run on 7 or 8 cells (8.4V or 9.6V), sometimes more.

The better motors as made by AstroFlight, Plettenburg etc have what are known as "rare earth" magnets. The commonest "rare earth" material is cobalt but there are others e.g. neodymium. These motors are usually made specifically for model use and are of generally better construction than can motors. Note : just to confuse matters the term cobalt motors is often used as a general term for such motors even where the magnets are some rare earth material other than cobalt.

Motor designations

You may find the numbering systems used to describe brushed motors a little confusing. There are 2 quite different schemes, one used by most of the world and based loosely on the designation used by Mabuchi the company who make most of the electric motors used anywhere for anything and a second devised by Bob Boucher of AstroFlight which is mainly used by American companies for the specialist rare earth motors.

One of the difficulties is that the vast majority of motors used in electric flight are not specially built for us but are simply selected versions of standard motors produced in their millions for all sorts of things. So we mainly use the designations used by the manufacturers.

That's the first numbering scheme, used by the electric car guys and with some minor changes by most of the electric flight people. It is very simply based on the physical size of the motors. The most basic sizes are 360/380 and 540/550. These motors are 36 or 38 or 54 or 55 millimeters long. Easy ? I believe the German company Graupner coined the Speed xxx naming convention and in the process rounded things up a bit. But basically if you measure the length of a standard ferrite electric motor in mm and add a 0 you have its approximate Speed xxx designation (in fact most Speed 400s are actually Mabuchi 380s but that's near enough). These are now widely used as generic names so a Speed 400 motor tends to mean any of that size, not necessarily one from Graupner.

In a sense these work a bit like glow engine sizes, 400 is smaller than 600 and 700 is bigger still. But, like choosing between an OS40LA, a 40FX and a Jett 40, you still need to know more than that to find out what planes they are good for.

The other naming convention used by Astro and others, 05/15/25 etc, was originally intended to refer to the equivalent glow motor size. I.e. 25 is in theory equivalent to a glow .25. The trouble with this is that electric motors are much more versatile than glow motors, which makes it a very vague designation e.g. an Astro 25 will fly most planes designed for a standard .40 motor. These designations are not based on physical size (e.g. 25 and 40 are the same size) but on the amount of power the motor is designed for.

Brushless motors

The conventional brushless motor on the other hand has the windings attached to the case and the magnets attached to the rotating part. Brushless motors work by electronically switching the motor current on and off in the different windings so there is no commutator and no brushes to bounce and loose efficiency. This is why brushless motors need special controllers.

Because the coils are in contact with the case they can get rid of the waste heat better. This allows the brushless motor to use more power and run faster. The brushless motor is both more efficient (using more of the power in the pack for flying and less for heat) and able to work efficiently over a greater range of cells and currents.

The two main sub-divisions of brushless motors refer to how the current through the windings is sensed and controlled. The original motors had small sensors inside to sense the position and movement of the armature and allow the electronics to control the current to the windings. These have typically 3 main heavy duty wires which carry the drive current and additionally a set of small wires (often 5 or 6) connected to the internal sensors. They generally work only with specific controllers from the same maufacturer.

Advances in electronics now allow the current to be controlled without the need for these sensors, which are relatively fragile and take up space which could otherwise be used for magnets or windings. You will be surprised to hear that this newer type of motor is known as "sensorless". This technology allows you to select the controller and motor separately again (for best of breed). There used to be a considerable cost to this. The sensorless controllers were VERY expensive but the latest improvments in software and electronics have made them a lot more affordable. Almost all the current production brushless motors are sensorless. In fact a sensorless controller can also be used with a conventional brushless motor (you just don't connect the sensor harness).

The latest type of brushless motor available is the so-called "outrunner" like the Model Motors AXI types. At first sight these are rather odd. They are arranged the same way round as a brushed motor with the coils in the centre and the magnets on the can. But...it is the CAN which rotates NOT the centre armature. This means they are a bit tricky to mount since you obviously can't just clamp them down but it does have one BIG advantage. These motors generate much more torque than a conventional arrangement. In practise what this means is that they will turn a much larger and more efficient propeller without needing a gearbox. Gearboxes of course add complexity, cost and weight so that's a real advantage.

As far as the motor designations go there are no standards for brushless motors. Each of the manufacturers has their own style. You need either to be able to read and understand motor constants or, better yet, to ask the manufacturer/seller. Since these motor/controller combinations are still relatively expensive you can expect to get individual attention to your queries from the better suppliers in a way that is not possible for the cheap "can" motors.

What are they good for

It's never that simple. It depends what your requirements are. The best motor in an absolute sense would be a brushless but in some applications this would be a bit like putting a Ferrari engine in a Mini (or perhaps a Rolls Royce Merlin engine in a Cub). There is no doubt that if you have to have the ultimate in performance for competition work then you need a brushless motor. For any other application it's a bit more debatable.

If all motors and controllers cost the same we would all be flying brushless. Two or three years ago a typical brushless motor and controller cost around 300 pounds (or dollars) whereas a small ferrite brushed motor and controller might cost under 30. The can motor costs are still very similar but at the time of writing (2003) you can get a brushless motor and controller for under 100 pounds. As a result more and more people are flying brushless motors. The outrunners are getting ever more popular.

Evaluating motors - Motor constants

So how do you tell how a particular motor will operate ? The easiest way is to ask around, perhaps on one of the many Web forums (I'd recommend the RCGroups Ezone forums) or to try it out in one of the eflight calculation programs like Motocalc or Electricalc (all can be found on my Links page).

But if you don't mind a bit of fairly simple maths you can work out quite a lot for yourself. You need to look up (or measure for yourself) some of the motor's characteristics, known as motor constants. For most purposes there are only 3 constants needed to allow you to sort out almost everything you want to know about an electric motor. These are the Voltage constant (Kv), the no-load current (Io) and the motor equivalent resistance (Rm).

Kv gives the rpms produced by a motor per volt applied, i.e . if the motor has a Kv of 2000 and you run it on 6V it will turn at 12000 rpm. If the motor was 100% efficient this constant would be all you needed but there are losses in the motor that make this impossible. That's what the other 2 constants Note that this is a particularly useful constant because it is a basic characteristic of electric motors that they try to maintain a constant speed given by Kv x V. If you force them to turn a load they simply apply more and more power by increasing the current to maintain this speed until either they achieve it or they burn up.

If Kv is known, then we can determine another constant called Kt. Kt is the torque produced per amp. Kv and Kt are related by the simple equation :
Kt = 1355 / Kv
This relationship between Kt and Kv is true for any motor.

Io, is the no-load current. This represents the minimum current required to turn the motor itself without any additional load. It is essentially the amount of the applied current which does not contribute to driving the load. So if you have an Io of 2A and you are running at 20A, effectively only 18A of that is driving the propeller, the other 2A is just turning the intrernals of the motor.

Rm is called the "equivalent resistance" or "terminal resistance" of the motor. This is basically the internal losses inside the motor due to the wiring in the armature. Rm is a constant which represents a loss of power due to imperfect materials. The lost power turns into heat.

Measuring the motor constants?

Note: for brushed motors only. Brushless motors are much trickier to measure.

Of the 3 motor constants Kv and Io are very easy to measure.

Kv : With the motor shaft in a drill press running at a known speed measure the voltage at the motor terminals. Kv = speed / voltage i.e if the speed is 6000 rpm and the voltage is 3V
Kv = 6000/3 = 2000 rpm/V

Io : Simply run the motor with no load (no propeller)and measure the current taken. That's Io. You can use almost any voltage because the current does not vary with voltage. However the motor will still be turning at the voltage defined by V x Kv so don't go too mad. I usually use 5V.

Both Kv and Io should be measured with neutral timing. If your motor has adjustable timing and you're not sure how it is set you can rotate the end-bell while you are measuring Io (having first marked where it is now). Neutral timing will be when the Io value is lowest.

Rm takes a little more work. The motor and shaft must be held so that neither can move i.e. the motor is stalled. You then need to apply a voltage through a limiting resistor and measure the current through the motor and the voltage at the motor terminals. Note it must be directly at the motor terminals NOT the power supply. You will need to do this quickly as the current may be quite high and the motor will get hot as it is not getting the usual cooling from turning. Rm is the voltage / current.
E.g. if you measure 5A and 1.2V then
Rm will be 1.2/5 = 0.24 Ohms, a fairly typical Speed 400 value.