An original article by John Emms

You would think that it might be possible to come up with a standard for the classification of electric motors. After all, everyone appears to be born with the understanding of what a .20 engine, or a .40 engine is – so why is it not the same with electric motors? Just where do we start?

I guess we start with companies like Mabuchi and Johnson producing electric motors for toys and industry. If you are producing a motor to fit in an industrial item it is useful to provide a measurement of the item. So, a 380 motor would be 38mm long, a 540 would be 54mm long. Graupner called their longer, higher torque 540 the “Jumbo 550”, so 380, 540, and 550 became familiar numbers for those of us experimenting with early electric flight. With the introduction of Graupner’s new Speed range in the late 1980s the numbers were rounded, the numbers of 400, 500, and 600 for electric motors were born, and quite effectively too. It appears that any 380 motor from any company will have been referred to as a Speed 400 by someone at some time!

Astro Flight in the USA had an interesting idea. Astro Flight developed a range of high quality cobalt magnet motors that are ranked alongside names like Rolls Royce by electric flyers. Astro Flight had the problem that they had to sell electric motors to people who were already flying with glow engines, and they classified their motors as an equivalent to glow engines when used as the manufacturer suggested, so an 05 is equivalent to an early .05 glow engine, a 15, a .15 glow engine, a 25 equivalent to a .25 glow engine etc. Of course the flexibility of electric motors means that things are never that simple, so a geared motor is consider by Astro Flight to be equivalent to the next size up in a four stroke – for example, a standard 15 geared motor is considered to be the equivalent of a .25 four stroke, but then add a high ratio gearbox (Superbox) with the motor operating at high voltage and the rules change again!

Among manufacturers of electric motors specifically for electric flight in Europe there was a new system developing. A common number system is used by Plettenberg in Germany, along with (almost) all of the Czech motor manufacturers including Mega, Model Motors and MP Jet. For all of these manufacturers the first number refers to the diameter of the rotor, the second figure refers to the length of the rotor, and the last figure refers to the wind of the motors (the number of turns of copper wire on the armature). When the outrunner motors were developed for model aeroplane use it was natural for these figures to be used in exactly the same way, so for an outrunner the numbers refer to the diameter of the armature, the length of the armature, and the third figure the wind. Of course all these figures refer to the INTERNAL sizes of the motor, and the external dimensions normally fit within the same size (and mounting) as the standard size 400, 500, 600 and 700 motors. Variations? For their smaller outrunner motors, Mega are using the physical case size along with the armature diameter and wind, eg, RC 400/15/7. Kontronik in Germany also use the external dimensions of their motors, so Fun 400, Fun 480, Fun 500 etc, refer to their physical size when compared to low cost 400, 500, and 600 motors. The last figure for Kontronik is very useful as it gives us the kv or speed of the motor, so for example, a Fun 400/36 has a kv or no load speed of 3 600 revs for every volt applied.

Just a little more chaos and then all will become clear – well, a lot more chaos, and then it should start to become clear!

Perhaps out of the European systems a common system could have developed but then the “world factory” had their own ideas. The motors specifically produced for electric flight in the Far East can be difficult to identify because of the different names applied, but all appear to use the name of a wind or moving air in UK, so for example Tornado, Cyclone, Typhoon and Vortex are names that use different systems. Motors manufactured in the Far East tend to use number system that appears similar to the European system, except the first figure can refer to the diameter of the case, and not the rotor, and the second figure can refer to either the length of rotor or the case length. Typhoon motors ignore the diameter, but use the length of armature for the first figure, along with the wind, and they can also include the number of poles or magnets on the rotor which also changes the physical characteristics of a motor.

I have been asked why it is not possible to simply classify a motor by the input power, this is now being done by HVP of the Czech Republic, however, this is not quite so simple as it might at first appear. An electric motor uses Watts for power, and is the product of the voltage available to the motor under load, and the current (amps) drawn by the motor (Watts = the voltage multiplied by the Amps drawn). A motor that I have seen advertised as a 400W motor (10v at 40A), I would use at up to 350W (10v at 35A), but could be used at up to 480W (12v at 40A). Another motor might be used direct drive as a 280W motor (8v at 35A), as a 90 mm fan motor at 880W (16v at 55A), or geared at the manufacturers rated limit of 30 cells and 70A for over 2000W – for a VERY short time. For sports flying it is well worth staying well within the manufacturer’s limits for a motor, and if in doubt use a motor of higher capability than might be needed – it is always possible to run on lower voltage, with a smaller prop, or some of us might even be tempted to reduce the throttle setting to reduce power and provide longer flight times!

Just as the number of cycles, the bore and stroke, the inlet and exhaust timing change the characteristics of an internal combustion engine, the physical size, method of winding, number of poles (magnetic poles), magnet strength, and number of winds change the operational characteristics of an electric motor. Designers will produce a wide range of motors because they can, but there is a huge overlap in the characteristics of electric motors, and even within the one motor range over half a dozen different motors could be suitable for powering the same model - multiply this by 5 motor ranges and the choice appears endless! Perhaps I am as guilty as anyone for adding to the confusion by asking manufacturers for motors to meet specific requirements – and this will continue as we meet new requirements, for example in EDF applications where we effectively have to match a motor for use with a fixed load.

The important bit:

How do we choose a motor? When I started multi channel RC flying using a whole months pay to buy my first 4 channel set in 1974 (after limited success with single channel) I bought a Kamco Kadet kit, the box top told me the range of motors that the model was suitable for, and I chose the OS Max 25 as a reliable middle of the road engine for this model. When I looked at the instructions for the engine it told me how to care for the engine, what propellers to use, what fuel to use, and what glow plug to use, and I had access to my dealer as well as experienced modellers for help and advice. Selecting an electric power system is similar. The box top or instructions for a good electric model will say what motors are suitable, and the motor instructions of a good motor will tell you about care of the motor, and what propellers are suitable with what cell counts (this information could be available on manufacturers or dealers web sites). If in doubt ASK! Your dealer should be able to give you the right advice, and if in doubt there are specialist electric flight suppliers – just as there are specialist suppliers for helicopters, gas turbines, etc. Please do seek advice from the supplier you trust and is going to be supplying your power system. As a general rule you will be using between 60 and 100 W/lb or 130 to 220 W/kg, but good performance can be achieved on relatively low power by using efficient motors and larger, more efficient propellers.

One last plea:

Unlike an IC engine, keep putting on larger props or keep increasing the voltage and an electric motor will keep on drawing more power to turn the prop at it’s natural speed. Just like an IC engine, if you put on too large a prop or use too much nitro and something is going to break at some time. Please do follow the manufacturer’s instructions on cell counts and propeller sizes and resist the temptation to enjoy ever increasing performance from abusing your motor. If you need more power, as a general rule it is better to use a bigger motor (though we can use gearing to increase torque), and please do resist the temptation to blame your dealer or manufacturer if you ignore the instructions or advice from your specialist dealer and choose to abuse an electric motor.

Thank you for sticking with me to the end, and happy (quiet) flying!

John Emms