Getting the Most Out of Ferrite Motors
by Keith Walker
The motors we are going to discuss here are the ones usually referred to as "can" motors. Generally, these have ferrite magnets and are
described as 075, 05 or smaller. There are several major differences in construction but all have fairly similar characteristics. They do have some limitations and shortcomings but most of them can be used to fly model planes quite successfully.
Physically, they vary in several ways. The most significant of these from our point of view is the way the brushes are mounted. We push
the motors fairly hard so the soft carbon brushes wear away quite rapidly. There is not much point in buying a motor that does not have
easily replaceable brushes unless the motor is cheap enough to be thrown away when the brushes wear down. Most of the motors that
do not have replaceable brushes were not designed for heavy duty work. They have fairly hard brushes. This increases their life but makes them unsuitable for the high currents we need to use because they have higher resistance and do not make very good contact with the commutator causing arcing and radio interference.
Some motors do not have any means of adjusting the position of the brushes relative to the magnetic field. For the best efficiency, the
brushes are moved to advance the timing of the commutator which connects the power to the armature. Generally it is no problem if the
timing is fixed, as long as it is set for the direction of rotation you need. It does mean that you can not change your mind at a later date and either add or remove a gear box without changing the motor too.
The bearings on the shafts of these motors are either metal sleeves or ball bearings. Either will work well with a little maintenance, but ball bearings are a little more efficient, especially at higher speeds.
The mechanical power or torque output of a motor is directly proportional to the number of turns of wire on the armature and the
current passing through them. The speed of the motor is proportional to the current but inversely proportional to the number of turns.
This causes a bit of a compromise because there is a very limited space for the windings on the armatures of these small motors. This
means that the more turns of wire, the thinner the wire has to be and the less current can be pushed through it.
Most of the energy losses in these motors is heat, caused by the armature current and the resistance of the windings. To make them really efficient, the number of turns can be reduced and the current increased, but these both cause the speed to increase for a given power out. To run really efficiently, they would have to run at such high speeds that we would have to use ridiculously small and inefficient propellers. The resultant compromise in the windings limits the amount of current that can be safely used without overheating the motor. This is how the power rating of the motors is established. Usually for an 05 size motor, the power limit is about 100 watts. If you push them much more than this, they will get hot enough to de-magnetize the ferrite magnets. Then they only make good paper weights.
There are a couple of ways of getting around this limitation. The first is by using stronger magnets. The stronger magnetic field has the effect of lowering the speed for a given number of ampere turns. This means that a motor with sumarium cobalt magnets, which are about three times stronger than ferrite magnets, can be made with less turns and can therefor be run much more efficiently at higher currents because of the lower winding resistance losses. There is an added bonus, too. Cobalt magnets will retain their magnetic properties at much higher temperatures than fertile, so the motors can run much hotter, increasing their power limitations.
The only snag to using sumarium cobalt magnets is that they are expensive so you will pay four or five times more for a motor using them. Neodymium magnets are even stronger than sumarium cobalt and are much cheaper to make but can not be run at quite such high
temperatures. Unfortunately, the manufacturers have not reflected this in their prices to us. They are generally the same
price or more than motors using sumarium cobalt magnets.
The other way of getting around the power limitations of inexpensive ferrite motors is to use a higher voltage than usual. This results in higher speed and a smatter prop, but if a gear box is used, a larger, more efficient prop can be used. The prop size must be selected to keep the current to about the same as when running on the original specified voltage. If the gear ratio is relatively high, the propeller can be big enough to be very effective and even look right on a scale model. The gain is from the fact that although the current is the same, the voltage is higher. This results in higher power output at the same motor efficiency. This, by the way, is how such wild claims can be substantiated by the company that markets the "emergency surplus" motors. There is nothing really special about the motors. They are just re-specified to run at higher voltages and higher speeds with a high ratio gear box. It really does work!
It is important to maintain these motors regularly. If this is neglected, the performance will deteriorate quite rapidly. The brushes are soft and wear down quickly. This causes most of the loss in performance. The particles of carbon will lodge in the rear bearing. In a sleeve bearing, friction will build up causing wear on the shaft and the bearing. Eventually the bearing will seize, and the motor will be useless. In a ball bearing, the particles will lodge in the races, causing wear and pitting on the balls and the races. Both of these conditions can be avoided by regularly flushing away the carbon particles with a solvent which wilt not damage the plastic parts of the motor. I use old fashioned lighter fluid. The squirt action of the can will clean a motor without taking it apart. Make sure that you lubricate the bearings afterwards because the solvent will dry them out. Use a very thin oil. This will penetrate the pores of a sintered sleeve bearing and will be retained for soma time. Thicker oil or grease just sits on the surface and is quickly Lost. Thin oil will also penetrate the seal of a ball bearing and lubricate it. Thick oil and grease will get thicker with time and clog the bearing. Be careful not to use too much oil or it will get onto the brushes and commutator. This will form an insulating layer on the them which will make the motor run very badly. The only thing you can do then is to start cleaning the motor all over again.
You must regularly check the condition of the brushes as they wear down. Make sure that the brush springs are not near or resting on the bottom of the brush holders because then there is no pressure to hold the brushes against the commutator. Replace the brushes with a new set if this happens. Replacement brushes are generally available from the hobby stores that sell R/C cars. Don't spend a lot of money on them. ALL of them work O.K. for us. Make sure at the same time that there is still plenty of springiness left in the brush springs and that the brushes are free to move in the holders. If there is any binding at all, there will be loss of power and a lot of sparking. If you are using a gear box, lubricate the gear teeth regularly with molybdenum grease, even if they are plastic or composition. This will considerably extend the life of the gear teeth.
Don't buy very expensive car motors with ferrite magnets. Their performance is only marginally better (if at all) than the cheaper ones. If you want to spend that kind of money, you may as well buy a cobalt motor and know that you are getting your money's worth. Don't ignore the larger ferrite motors, either. There are some real bargains out there.