“Keep the voltage high ”

A series of articles on electric flight by Rob Ireland

MOTORS AND THEIR SELECTION FOR ELECTRIC FLIGHT

Having covered the basics of cells, this chapter deals with motor types useful for electric flight.

I have described previously the multitude of cell types now available to us, and it's no easier a choice with motors. There is a vast selection of motors available, which all have a particular suitability for a particular task.

Like the available cell technology, motor availability has been largely dictated by what industrial sectors are using in vehicle equipment and cordless power tools etc. This has resulted in huge numbers of motors in the Speed 600 and Speed 400 class being available at low prices. These designations have been given to these particular motors by Graupner, and are actually mass-produced Mabuchi motors.

However, the designation used, rightly or otherwise, is all part of the huge array of feature identifiers used on motors.

HOW DOES A MOTOR WORK?

Of course you already know how they work..

The principle is that a current flowing in a wire will produce a field. This field is opposed by another, usually fixed strength field. By switching the field created by the current, the wire can be forced to move indefinitely and if constrained to rotate about an axis we have an electric motor – a very useful device!

That's the basics, but of more interest to us is what parameters do we need to consider to make a motor selection?

Firstly, we need to consider how electric motors fail.

HOW DOES A MOTOR FAIL?

Electric motors fail mainly because they have overheated! Persistent overheating will de-magnetise the magnets, or cause the windings to fuse together.

All of the heat created comes from the current flowing through the windings, which have a resistance. If we can cool a motor sufficiently well, we can pass larger currents through it.

Voltage is irrelevant to this failure mode. However, excessive voltage could cause a rotational speed high enough to throw a winding off the rotor, thus wrecking the motor!

WHAT GOVERNS THE PERFORMANCE OF A MOTOR?

The factors governing a motor's performance are:

Motor constant This is a number stating how many revolutions per volt applied the motor will turn (rpm/Volt). Usually expressed as K with the quoted value being for an unloaded motor. K reduces as load on the motor increases, but I'm not going to worry about that here. Motors referred to as “hot” will have higher K values than “soft” or “eco” motors.

Speed range There is a range of voltages which can be applied to a motor which will run it. There is also a maximum efficiency at a particular rotational speed. Given that ferrite magnet motors are not very efficient (~55%) it can be of some importance to try and match the operating voltage to the motor speed at best efficiency. Cobalt and other rare-earth magnet motors are much more efficient (~75%) and consequently their “flexibility” is greater i.e. there ability to operate in different models using different voltage battery packs. Brushless motors surpass this again, with “outrunner” types (outer body rotates) having efficiencies of 88% or more.

Since brushless motors are now available from many manufactures at much lower prices than when first available, the choice of motor really falls to either a brushed ferrite motor or a brushless motor. This represents the very “cheap” and the very “flexible” end of the market.

An indication of the difference in efficiencies between brushed (ferrite) motors and brushless motors is shown in the following graph;

This not only demonstrates the much higher efficiency possible with a brushless motor, but because the peak brushless efficiency is much flatter than the brushed motor curve, the brushless motor can be loaded up with higher currents without suffering much fall-off in efficiency.

Perhaps a picture of some motors and their parameters would be useful here.

All of the above are ferrite magnet brushed motors, and from left to right they are:

The following picture contains a selection of brushless motors:

From left to right these are:

Motor types (1), (2) and (6) have the same mounting hole pattern in the front flange, and are thus interchangeable between models (remembering that slight weight differences will require a C of G adjustment.)

Ditto (4), (5), (7) and (8). Additionally, (8) is a direct replacement for (4) in weight. (7) is a shorter body version of (8) by 15mm.

Brushless motors have generally adopted a more logical approach to type numbering, usually by making some reference to an overall body diameter or other size parameter, a stator length, and number of winds. It is these physical combinations which end up in the K value (rpm/volt).

For instance;

The Torcman 350/20/13 designation indicates a stator diameter of 35mm, stator length of 20mm, and has 13 winds of wire.

The Jeti Phasor 15/4 (which has a body diameter of 36mm the same as a Speed 600) has a stator length of 15mm and 3 winds of wire fitted.

TORCMAN BRUSHLESS MOTORS

The outrunner type of brushless motor offered by Torcman provides the most flexibility as it can be obtained as a kit. This gives the user the ability to tailor the winding on the motor to suit a particular combination of cells and prop size. Being an outrunner type (the outside can containing the magnets is the rotating part) the wires are easily wound onto the stator.

While this can be useful for competitions and experiments, the kit price is twice that of some other outrunner motors with standard winds, ready built.

Typical Torcman motor kit.

USES

The correct motor to suit the cells, model, flying style etc., must be arrived at after some inevitable compromises.

Instead of adding confusion, here are some of the models I have been flying using motors selected from those mentioned above;

MORANE SAULNIER type L

(speed 400)

GAF NOMAD

(2x speed 480)

Ju 128

(Mega 16/15/4)

FILIP 600

(Speed 600, or Phasor 30/3)

WATTAGE IMPRESS

(Speed 400, geared)

WATTAGE F22 ‘RAPTOR'

(Speed 480)

GRUMMAN WILDCAT

(Speed 600, or Phasor 15/4)

AMPMASTER

(Torcman 350/20/13)

EASY STREET

(Speed 600)

Relevant information can be obtained from the following sites;

www.torcman.de the torcman range of brushless outrunners.

www.motocalc.com/data/motor.html motor constants

www.megamotor.cz/ Mega Motors web page

www.jetimodel.cz/ Jeti web page