I recently purchased a 2015 EM. It had minor battery problems and a click in the transmission, but was in excellent condition. It had two BMS upgrades and would cut out occasionally. I had trained as an engineer in London and had built three electric bicycles including triangular battery packs to fit in the bicycle frame. I set about building a spare battery pack and resuscitating this six year old battery. I shall document each item rather than give a chronology. My purpose is to bring new life to all the other aging EMs around the world. Please email me on my errors and omissions. a n d y (at) c h a l k l e y . i d . a u
This is a very nice Golden Motor 5kw, commonly described as an “axial flux motor”. This ‘axial flux motor’ consists of a metal disk of about six inches diameter, from memory, with strong flat magnets stuck on one face. The magnets are like the very strong magnets in a computer hard-drive. Facing the magnets is a stator with numerous coils. There are three fat power wires from the stator down which three-phase alternating current is supplied. The stator is held to the fan side of the casing next to your right knee by three 8mm Allen bolts. The the fan is missing. You will need that white thermal paste from an electronics shop before assembly. When disassembled, you will not be able to get the disk of magnets from the stator because the magnetism is stronger than you or me. If you can pull a Gas Gas to pieces and get it back together and still have a functioning motorcycle, you will manage the servicing of this Golden Motor. The only serviceable parts are the bearings with the usual trials problem of dirt ingress on the drive side. Axial flux motors are great. They are narrow, cheap, and reliable. The main issues are:
- getting heat out of them.
- the air gap between the rotating magnets and the stator.
- totally sealed unit.
In trials, we tend to have short bursts of high output and prolonged periods of moderate speed. The same model comes with water cooling, but it appears unnecessary. Similar motors from other manufacturers have air cooling holes, much like the casing of an automotive alternator. On some ebike hub motors, enthusiasts cut casing holes, whilst others use ‘fero fluid’. The fero fluid appears to have very fine iron particles in tiny quantity of a light oil. It clings to the stator in the air gap allowing greater cooling. It may also give better flux in the ‘air gap’. I don’t think the trials engine warrants this and the gap on the axial motor is adjustable. You may wish to take a temperature reading and let me know what you get.
Heat is generated in the windings due to the resistance of the copper wire. So the grade and diameter of the copper is of significance. The controller appears to have a rating of 150 amps peak and the fuse is 300A. I have yet to determine what current we use when demanding peak current. What is fascinating about these motors is that maximum current is at low revolutions. The 48 volt pack works against the winding resistance of the copper coils along with the ‘back emf’ that is generated as the motor turns. Maximum current is determined by ohms law and occurs when you open the throttle to full at zero rpm. Current drops off as the motor spins and is low when the motor get close to maximum rpm. Don’t expect to learn that immediately. So heat is generated when it is most difficult to get rid of it.
The motor casing has ribs to assist convection cooling. As one might expect, the interior has no finning. Heat is a waste product of the motor and comes almost entirely from winding resistance. The heat path would be from copper, through the varnish coating, through more copper wires and varnish, through any insulating layer into the iron laminated stator, to the fan side cover of the motor. Some will find its way to the main body of the motor at the outer edge of the fan cover. Some heat will travel from the copper windings to the air in the motor, whence it should pass to the casing of the motor, then to the passing air. Under a dramatic increase in load for a prolonged time, heat will be generated and pass as as in the above list. If the heat generation is prolonged, all items will become hot and cooling will be impeded. Thus, there is a log between high load and over heating. The time scale is not known. Heat generation will be greatest at high amperage which tends to be a full throttle and low engine revolution speed. The characteristic of these engines is that they take less current as the rotational speed increases.
The motor uses regular ‘ball journal bearings’ with double seals like trials wheel bearings. Some axial motors, use an ‘axial ball journal bearing’ on the non-drive-side as that bearing takes the thrust of the magnetic attraction between stator and rotor. There is a suitable, bearing of the same size, but it does not have internal seals. There is no provision for alternative sealing. On some similar motors, a roller bearing is used on the drive side. I think the ball journal bearings are quite adequate.
I believe that the grease inside the bearing is suitable for the high speed operation of the motor. I did not pack them with water-proof grease as I do with wheel bearings. I will for warn you, the non-drive-side bearing has to be adjusted to give the correct ‘air gap’ between rotor and stator. I shall call the non-drive-side bearing the fan side bearing, although there is no fan. The fan side bearing has a threaded adjuster the diameter of the bearing. To lock it in place, it has a couple of small countersunk screws that clamp it in place by means of a split that is not visible. Stage one is to put two centre punch marks or drill marks on the adjusting ring and the case. Two marks will tell you that it was the original location. Subsequent marks can be with single marks. The adjuster ring has two holes to engage a double prong tool similar to the centre of an angle grinder. Using these is not a wise move. The force is likely to damage the threads in the aluminum case. Better is to make a puller. 6mm threaded rod is more than sufficient. Next time I pull the motor out, I will tap two 6mm holes in the thick aluminum plate of the transmission to take a home made puller. This can be a piece of 20mm square tube with a bolt and nut in the middle. Even a piece of wood would probably suffice.
This is contentious and appears to be a guarded secret. After many hours of search over many days, I can find no definitive measurement for the air gap. The adjusting thread on the retainer is 1mm. I estimate that the air gap was 0.8mm or 1.0mm before disassembly. I believe I now have mine set to 0.5mm and have completed one trial with this air gap. If the air gap is too small, catastrophe could occur. If the air gap is large, there is less magnetic flux and less torque, although the motor may spin a little more freely. I don’t know my original setting as I marked the location with a felt pen. The lines subsequently disappeared when I cleaned the items! The lesson: Centre punch the original setting with two marks so that it is obviously the original setting.
Besides major items such as rewinds, the only modifications that one might look at are:
- Weight. The casing is not strong enough to start removing metal. The bearing retainer for the drive side bearing appears to be superfluous as we are not using it in a helicopter or aircraft. The fan fins could be trimmed.
- Air gap. Feel free to experiment. With a rudimentary puller, this could be done in the field.
- Increased battery voltage.
- Some experts talk about the phase difference between the hall sensors and the magnetic field from the stator.
With magnetism, a north pole attracts a south pole, a south pole attracts a north pole, but south repels south and north repels north. The rotor is a flat disk with powerful magnets stuck to the surface such that the surface has north south north south north south and so on. The stator has a similar number of poles each with copper coils around them. The controller supplies current such that the poles are magnetised south north south north south and so on in a manner such that a north on the stator is slightly ahead of the corresponding south magnet on the rotor. With all the souths slightly advanced on the norths and all the norths slightly advanced on the souths, the rotor is pulled round to get norths to meet the souths. However, the clever controller, moves all the poles on the stator round to the next poles to give continuous motion to the rotor. This is why the controller can cost as much as the motor and plays an important role in motor characteristics and performance. One might also see that the ‘hall effect sensors’ detect the position of the rotor and thus determine the ‘advancement’ of the magnetic field in the stator. For a tuner, the behaviour of the hall effect sensors is of interest and alternative arrangements may be possible.
Even more complex is the following. When the armature with its magnets is rotated, it produces a voltage in the windings. It thus acts as a generator like the alternator in your car. The voltage tends to be dependent on the speed of rotation. This generated voltage is commonly called ‘back emf’. In most situations, this back emf is not great enough to exceed the battery voltage. So when the throttle is closed, the motor tends to freewheel with just minor losses, giving minimal retardation. However, clever engineers played some tricks with the controller circuitry. If you press a ‘regen’ button, the motor acts as a generator giving significant braking power. Some controllers have on/off regen and some appear to have a progressive function like a reverse throttle. To make this work, the controller collects the available voltage and boosts it so that it can charge the battery, hence performing work and adding drag and braking force. One distraction is that the regen braking tends to drop out below a certain revs. Controllers can give you control over this and many other settings.
The Strip Down