Disassembling and Reassembling the Hub Motor

Removing the stator is quite easy, but getting the cover plate off can be a little more challenging

Introduction

The hub motor – like any motor – is made up of two main parts. The first part is an axle surrounded by a fixed ring of copper coils, called the stator. The second part is a housing into which the axle seated, and where it is allowed to rotate freely. This housing, which part of the rear wheel, is surrounded by a ring of strong magnets that surrounds the ring of coils connected to the axle. The hub motors used by most (but not all) electric bikes also have three ‘hall-effect’ sensors seated in a metal ring surrounding the coils, which relay signals back to the controller.

The phases wires that provide power to the motor and the thinner, sensor wires that feed back to the controller are all housed in a thick, insulated cable that runs through a hole in axle to the inside of the stator. To get access to the workings of the motor you need to remove the stator from the rear wheel, and – if necessary remove the cover plate so that the area inside the coils can be accessed.

1) Remove the bolts securing the stator

The stator is held onto the rear-wheel housing by a ring of allen bolts. A ratchet screwdriver with a suitable attachment will make short work of these. It’s a good idea to put them in a little baggy so you don’t lose any.
The

The stator cover – little alan bolts secure it to the wheel

2) Remove the drum brake assembly and push out the stator

First it’s best to remove the drum brake assembly from the other side of the wheel. You can see the drum brake assembly below. Remove the nut and washer from the end of the axle and the drum brake assembly, including the cover plate, lever and brake pads should just slide off in once piece.

The next bit requires a bit of force, but is quite straightforward. Even though the bolts are off, the stator is still held in place by powerful magnets, and needs a bit of encouragement to release. To remove the stator from the wheel, find a piece of wood and rest the wheel on top of it so that the end of the axle on the underside is firmly braced against the ground. Then push down firmly on both sides of the tyre. If it seems stuck, then put your knees on the tyre and bear down with all your weight. With enough force the stator will pop out and you’ll be able to remove it from the wheel.

Here it is removed. Towards the bottom, you can see the hall sensor wires where they meet the hall effect sensors embedded around the edge of the unit.

3) Remove the stator cover-plate

This is as far as you’ll need to go in taking the motor apart if all you need to do is replace a defective hall sensor, but if you need to repair damaged wiring (like on the unit here) or even replace the phase wires for something thicker then you’ll need to also get that cover plate off the stator.

The only thing holding the plate on is the friction between it and the axles’s bearing, however it’s a very tight fit and can’t easily be removed without specialist equipment. A hub-puller of the right size, or a hydraulic press can be used to push the axle through while the plate is held firmly. In the end I went to my local university’s mechanical engineering workshop, and they popped it off with a big hydraulic press.

Once removed, you can see where the cable emerges from the axle on the other side of the plate, and where short lengths of surplus phase wire are covered in nylon and tied back.

Below you can see the ends of the phase wires once released from the cable-ties and with the bits of nylon sheath removed.  You can see where they are joined to the ends of three thick, copper cables which run lead into the banks of coils surrounding the stator. Once you have access to this part, you can make any repairs to damaged wires, or even replace the thinner phase wires that come with some hub motors with thicker grade wiring that can carry more power.

4) Reassembly

The stator cover

Though the cover plate for the stator may be quite tough to remove, it’s usually a lot easier to get back on. In my case, some gentle help using a wooden-headed mallet was enough to get the cover plate back over the bearing.

Replacing the stator

As for putting the stator assembly back into the wheel, this needs to be done with a certain amount of care, as once the stator is far enough into the rear wheel housing, the magnets will slam the stator back into place quite firmly, so MIND YOUR FINGERS!

You will also need to bear in mind that the stator needs to be properly aligned so that all the holes in the case meet up with those in the housing. If not you’ll have to remove and reseat it again until it’s properly aligned. To help with this, I poked a screwdriver through on of the holes in the case and its corresponding hole in the housing so that the stator slid into place reasonably well aligned. Once the stator was in place, I replaced the bolts, diagonally from one another and eventy spaced, tightening them up alternately to ensure that the stator went back in properly aligned.

Testing the Hub Motor

Though hub motors tend to be fairly well behaved and can run for years without problems, they do occasionally go wrong. If the bike suddenly stops working or develops problems, you often end up in a position where you need to find out if it’s the controller or the motor that’s at fault.

Many enthusiasts who’ve done a lot of tinkering with their bikes usually end up with at least one spare controller lying around, in which case it’s just a case of swapping it out to see if that fixes the problem. Most people, however, do not have spares available and may need to do some specific testing to rule out any issues with the motor.

1) Access the Phase Wires and Hall Connector

The first thing to do is to remove the seat to get access to the junction where the phase wires are connected (bottom) and the hall connector socket can be found (top left). The phase wires are the three, thick blue, green and yellow wires. The hall sensor wires are five thin wires: black, red, blue, green and yellow that run from the motor cable to a 6-way mini-connector. On some bikes the phase wires are part of a plug instead, in which case you’ll have to adapt your test accordingly. For this test you will need to detach the phase wires on the motor side from the connection block so that they are isolated from the controller. Leave the hall connector connected, though.

Phase wires and Hall Connector

The phase wires are the wires that provide power to the motor, magnetising sequences of coils that generate the rotational motion. The hall sensors are three little transistor-like devices embedded along the inside edge of the motor, which send information about the motor movement back to the controller.

2) Test the Phase Wires

First, with the bike on its stand (and the bike switched off), test that the rear wheel spins freely. If it doesn’t you most likely have a short between the phase wires, though this is rare. The next thing to do is to remove the phase wires from the block and short them one pair at a time (e.g: yellow/green, green/blue, blue/yellow). As you hold them shorted together, try turning the wheel again. This time, you should find resistance. If you don’t get resistance when any pair of wires is shorted, then there is a break somewhere between the phase wire and the motor.

Phase wires and hall connector, detached

3) Test the Hall Sensors

For the next test keep the phase wires disconnected but make sure that they are kept properly separated. Preferably put a bit of insulation tape on the ends to prevent mishaps. Again, keep the hall connector connected.

Next, you will need to turn the ignition on, and take voltage readings from the wires in the hall connector. Since the connector is plugged in, you will need to access it from the rear, sticking the multimeter probes into the recesses and touching the crimps holding the wires in place.

The hall sensor wires

Take voltage readings across the black wire and – in turn – the blue, green and yellow wires with the back wheel turning slowly. It’s best to get someone to help out with turning the wheel for this part, while you take readings. As the wheel turns, the signal should flip between 0V and about 4.5V, at a rate of about 23 cycles per rotation of the wheel. If you get this reading from each of the wires, then all is well, otherwise, you have a blown hall sensor and will need to open the motor to replace it.

If you have a bit of pocket money spare, and want to make life easier, you could invest in one of these very handy motor/controller tester gadgets that will do it all for you. You just plug in the hall connector, attach clips to the phase wires, and the blinky lights give you the answers straight away! The controller-tester part of the unit, however, is only compatible with controllers up to 60V, though, so it can test the motor – but not the controller –  if you have a 72V system.

Replacing a Hall Sensor

Hall effect sensors sometimes fail, but are easy enough to fix

Taking the hub motor apart and replacing a hall sensor might sound like a pretty daunting task, but it’s really not that difficult. The first thing you need to do is get hold of a suitable, replacement SS41 hall-effect sensor. One can usually be found on ebay here, or from RS components (but with a pretty steep postage price) here.

Next you remove the stator (the hub motor section of the rear wheel) from the rear wheel itself (see blog entry) so that you can get at the motor’s inner workings. This done, the hall sensors and wiring are exposed and ready to work on.

The sensors are held in little shaped grooves in the metal treads on one side of the stator’s perimeter. Now you need to identify which sensor holds the wire that you detected a fault on. Each sensor has three wires, – two of them are the 4.5V live (red) and GND (black) which serve the sensor, the third is the hall sensor wire which returns a signal to the controller based on its rotational position in the hub motor casing. The third wire for each hall sensor is yellow, green and blue respectively, as shown below.

In my case, it was the yellow wire that was showing no signal, and this one goes to the middle hall sensor on my stator.

The component itself is held in with a bit of epoxy resin and sometimes has bits of silicone sealant gummed about. The epoxy can be softened by warming it a little with the flame from a cigarette lighter.

A gentle tap with a hammer and screwdriver knocks it loose with ease.

Note how one side of the hall effect sensor has bevelled corners. This side faces outwards (on my motor, at least) and is designed so that it can only fit in its socket the right way round. This is important, as it makes it hard to accidentally reverse the wiring sequence.

Here’s the replacement sensor in situ.

You can more clearly here how it fits with the bevelled corners facing outwards.

Next it’s a simple case of soldering the new sensor in. Snip the head of the old sensor off, strip the hall sensor wires, and desolder the legs from the old one, then solder on the legs of the replacement sensor. Don’t forget to put fresh sections of heatshrink on the wires, and push them way up the wires where the soldering iron won’t prematurely shrink them. Then pull the heatshrink down into place over the soldered legs, like with their neighbours.

This done, it’s a case of fitting the sensor back into its socket. Use a little blob of epoxy resin to hold it firmly in place.

All done! Finally, carefully refit the stator to the rear wheel and test the wiring from the hall connector. If you did everything right, then you should once more have a working hub motor.

Torque Talk

The final product! – The Zenid, custom, 12mm, heavy duty torque arm made from laser-cut, 6.3mm steel (alongside the original)

I’ve taken a bit of a break from the bike in recent weeks to catch up on work and to await the delivery of some key parts for my next upgrade. I ordered a nice 18-FET version of Lyen’s controller to replace the 12-FET one I already have, and also had the current one repaired after the FET blew from the torque arm mishap.

Figuring out how to come by a suitable replacement for the rubbish torque arms that come with the hub motors turned out to be something of a journey. First I followed up a link on Endless Sphere to a guy who had custom made some torque rings that looked like they might be suitable, however they were the wrong design for the type of swing arm that the Ego and other electric scooters use.

After enquiring with a local machine shop about the cost of custom machining some suitable parts, and doing some exploring online, I eventually decided that it would be more cost-effective to simply custom design and order my own, and sell the surplus in my shop. The wonders of the internet now mean that there are manufacturers who have pretty much automated the whole process so that you can design, explore cost options, and order whatever you want with little or no human intervention. So I duly figured out how to use some web-integrated CAD software, came up with a design for my dream torque arm, and placed my order.

The CAD design, in 3D preview mode

The only thing that wasn’t automated though, were shipping options. My manufacturer of choice was a US firm (I couldn’t find any UK company who did this, alas), and the only option was for a ridiculously expensive express UPS delivery that cost almost as much as the order itself, so I emailed them and asked if I could have it shipped by a much cheaper (and slower) USPS tracked service. The said it was fine, and told me to just add a note to the CAD drawing for the

I spent the last three weeks occasionally checking the tracking to see how the package was progressing, and it seemed to take a dog’s age to crawl across the planet from from its east-coast origin. When it finally arrived in the UK it spent nearly a full week sitting in customs before finally clearing and continuing its journey. I was none too happy to be landed with an import duty and ‘clearance charge’ of nearly £40 for the privilege of this inconvenience, but duly paid it and eventually ended up with a batch of freshly minted parts on my doorstep this morning.

The real thing, next to the original that came with the motor

Here it is, the real thing, shown next to the inferior original. They’re 62mm x 24mm, and manufactured from 6.3mm laser-cut steel, and have 12mm torque slots, with a 6mm hole for the adjuster bolt, – suitable for the 1400W/1500W motors. The torque slots are slightly wider as the 1500W motor has a 16mm axle on the cable side, as compared to the 14mm on the 1400W motor, though the slot diameter required (on mine at least) is 12mm on both. I’ve also made mine longer, with a wider adjustment slot so that the rear wheel can slide back further to accommodate larger tyres. I got a batch of them, so there’s plenty left to go in my little shop in due course.

There was a moment of panic when I put it on the axle for the 1500W motor – it’s a tight fit and needed a gentle tap with a hammer to get it over the end, but then it slid down the shaft nicely, an almost perfect fit with very little free play! Here it is next to the even worse torque arm that came with the 1500W motor (these are the ones that failed).

The bike is in bits at the moment, while I repair a bracket on the battery box, respray the swing-arm, service the battery pack and fix the wiring on  the hub motor. I’ll get round to this in due course and keep you posted.