The short story
What makes a bike motor get too hot?
As the motor slows down, it draws more current (up to a limit set by the controller – about 15A which it reaches at about 17km/h).
As it slows down further, the motor gets less efficient, so less of the electrical power leaves the motor as horsepower, and more of the electrical power becomes heat in the copper windings (according to some data I’ve seen, at some point under 10km/h half of the power is going to heat).
In a 250w Bafang hub motor going less than 15km/h at full throttle, the winding temperature is rising fast (I measure about 0.5°C/sec).
If the motor is laboured like this for more than a couple of minutes, the motor windings will reach high temperatures which risk damaging the wire insulation.
You can’t tell how hot the motor windings are from the outside: the temperature of the outside of the hub motor can be lukewarm to touch, when the windings are over 160°C.
If the winding insulation is damaged, the electric current can start taking shortcuts between wire loops, making the windings even hotter, losing power, and cooking your motor until the wires smoke.
How do you avoid cooking your motor?
If you are riding your ebike at full throttle, under 15km/h, for more than about 2 minutes, give your bike a break. Take a break about every minute of riding if the hill is long.
Walking while motoring your bike works fine: the motor will keep cooling as you go. Sometimes this is the best solution (especially when carrying passengers – let them walk). There is no shame in walking a bike up a steep hill!
The motor windings cool very quickly once you stop, especially if the motor hasn’t been hot for long.
The long story
A run of burnt motors
My biggest problem with e-bikes lately has been cooked motors. I think I now have some sort of grasp of the problem:
why this is happening: riding too slow for too long up too-steep hills,
how to prevent it: stopping and givng the motor (and rider) frequent rests on steep hills
how to fix it: rewinding burnt motors – I hope!
I now have 5 motors in the storeroom which have failed through overheated windings – actually 6 if I count a non-Bafang motor which seems to have failed through a combination of water damage and burnt windings. All of these motors have failed after use on our mountain, with long steep climbs.
We do have exceptionally difficult conditions for riding bikes where we live. The mountain roads include inclines of 15% and more, which mean riding at speeds down to around 10km/h with our 250w Bafang motors and hard pedalling, especially when carrying loads. The biggest challenge is the “PEI Road” (stands for Public Estate Improvement: the road was built as an employment program during the Depression). The PEI Road climbs up to Mount Glorious from Highvale (coming from Samford), and its stand-out feature is a continuous incline of 15% - 17% for about 2km. I tend to maintain between 10 and 12km/h on this section, which takes over 10 minutes of hard pushing at full motor throttle if you don’t stop (these days I stop…).
My son Jasper rides his Dahon (Bafang SWXH 250w rear motor) home from school up the PEI road most school days (about 17km each way), pushing hard and not stopping till he gets to the top of the main incline before taking a rest (to prepare for the next huge hill…). Jasper has burnt out 2 SWXH motors this year, each after 2500km of this treatment.My Xtracycle’s first rear SWXH hub motor survived a similar distance (about 2700km) of heavy mountain riding. Then it was fully cooked:
I’ve never ridden it up the PEI Road, but I use it to carry our youngest child Luka (now nearly 30kg) to and from Mount Nebo, 11km away. This means travelling slowly for long climbs – which I now clearly understand as the way to kill motors. As Luka has got heavier, I’ve given up trying to carry him up the steep hills, and now routinely let him get off and walk. I now realise this is the only way I’ll get this new motor to last a long time.
Measuring winding temperature
So that I could get some real information on what was happening inside my motors as they heated up, I bought and installed a thermocouple to measure the temperature of the motor windings as I rode.
A thermocouple uses the junction between 2 dissimilar metals to determine temperature. A meter measures the voltage created at the junction, and translates this to a temperature reading. Effective thermocouples can be bought on Ebay for about $6.
The probe which measures temperature is tiny – just the end of a pair of twisted wires.
To measure the temperature of my Bafang SWXK 250w front motor, I drilled a 2.5mm hole in the phasewire cover plate on the RHS of the motor (after checking for the best spot). The probe wire then entered the motor through one of the phase wire holes, and the probe was inserted amongst the windings in the copper motor coils. This way the probe is measuring the actual temperature of the copper wires, rather than the temperature of the iron core, or the aluminium case. The wire temperature is what most matters: this is the heat which will damage the thin layer of insulation on the winding wire (often titled “magnet wire”) and cause current to cross between windings and motor failure.
Here is the probe wire entering the motor:
Here is the thermocouple mounted on the bike frame:
This way I can watch the temperature of the windings as I ride.
The ideal way to measure this would be to log battery current, windings temperature and speed over time. However I think you’d come to the same conclusions.
What temperatures does the motor reach?
Most ebiking involves continuously changing conditions: speeding up, cycling above motor speed without power, slowing down at a hill and turning on the power. The windings heat under power and cool when unpowered or at high speed.
Here are some observations:-->
- In normal undulating conditions the windings rarely exceed 80°C.
- Intermittent climbs where speeds are over 15km/h most of the time, get windings over 100°C, which are fine.
- Once the power is on and the speed has bogged down below 15km/h for extended periods, the temperature climbs by around 0.5°C/sec. Temps will reach 160°C and higher if low speed continues uninterrupted.
- Short stops help a lot, with temps falling fast.
- Walking up a steep hill while powering the bike alongside you is really effective. The motor isn’t working at an efficient speed, but the power is low so only small amounts of heat are generated.
A bigger motor can help with the overheating problem. I've been using a Bafang BPM 350w motor on my Xtracycle for a couple of years. This motor is sold as a hill-climbing motor, and is clearly more comfortable with climbing. However I haven’t tried measuring real temperatures in it, and this wouldn’t be easy with a back motor as the wires enter the motor through the axle. It would be possible though, especially if I removed the 5 hall sensor wires which I don’t use (it works fine in sensorless controller mode.
2-speed hub motors
For a while now I've been using a Xiongda 2-speed hub motor on my Dahon. This can change gears internally so it can climb steep hills without problems (so far). See my post at http://bruceteakle.blogspot.com.au/p/blog-page_20.html
Fixing burnt motors
I have enquired about obtaining replacement armatures for Bafangs (they are easy to remove), but have been told that Bafang doesn't sell them. This is a pity, and leaves rewinding as the only option unless a spare armature can be found.
I've done some careful dismantling of burnt motors, and hope to try rewinding. I plan to report on that in another page.