Equal motor heating in 2wd?

[wil]

I'm building a 2wd ebike at the moment, with a bafang/8fun SWXK5 in front, and a bafang/8fun SWXH in the rear, identical winds and gear ratios, both will be oil cooled and run well above rated power, and as such I want to install a temperature sensor.

However CAv3's only have 1 temperature sensor pad, and the rear SWXH has cabling through shaft and so adding a sensor will be difficult. The SWXK5 on the other hand has a large bearing and through face plate wiring, and will be easy to add a sensor to, and so I would like to just be able to install a sensor in the front and call it a day.

I would assume heating would be equal (or the front would heat slightly faster, due to the lower mass and lower surface area to radiate heat outwards), and so monitoring just the one motor would be sufficient to ensure I don't cook them, although I don't have any experience with 2wd bikes, or the contention that might be experienced.

Can anyone with some experience chime in on the issue?

 

[gogo]

I have no experience, but would predict unequal heating. There are too many variables for it to be exactly equal, but it could be close enough?

Maybe measure both simultaneously with the same type of method to determine which is at more risk?

 

[teklektik]

I think you have some issues drawing conclusions about the temperature of the unmonitored motor because of the different motor construction and the oil fill. Both of these will change the heat flow and temperature. I would expect oil loss to have a pretty dramatic effect and this seems like a failure that you would be looking to detect with unusual temperatures. The no-oil case seems to make this temperature inference idea much more reasonable, but adding the extra variable (oil) appears to make things more dicey. If you are scrupulous about maintaining the oil level, then things may look different, but I think you need to get that second motor monitored - particularly since you intend to flog the little guys.

I monitor only the front motor using axle temp monitoring as described here and find the protection perfectly adequate. This technique might offer a partial solution for your second motor.

I have two identical motors but even so there is some difference in front rear case/axle temperatures (measured externally by infrared, thermocouple, and fingermometer) that most likely derives from differences in controller setups. One controller is running slightly higher amps than the other. This axle method works pretty well for motors with smaller diameter stators and for operational situations where heat buildup isn't going to change materially over a minute or so (heat propagation from windings to axle) - the technique is ineffective for protection against rapid heat rise from climbing a radical hill, etc. For my use, I get heat increases from prolonged running at higher power levels, so the temperature creeps into the red and the V3 works nicely in that case - but certainly not the same quality measurement as direct winding monitoring...

 

[Hugechainring]

Due to the inevitable differences between controllers and applied voltage and current, as well as wheel diameter (wheel diameter is part of the total gear ratio between the motor and the ground), one motor or the other will end up being the primary drive, and one the secondary, due to a combination of subtle differences in the total gear ratio, small differences in each controller/ motor combo, or intentional programming.

Since installing a temp sensor in the front wheel is the option you favor, you could bias the setup by turning up the power on your front wheel slightly. The temp in the front wheel will likely be higher than the back since you are forcing it to work a little harder, everything else being equal.

While the thermal mass and surface area is smaller on the front, you may have increased airflow compared the rear which may make up for that.

Even though your "gearing" is the same, in order to make the true gear ratio front/rear match as close as possible you would have to to run identical tires front and rear with the same wear, and at the same pressure, assuming weight distribution is 50-50.

I agree with teklektik, the oil fill is a variable that is hard to maintain over time, and has the potential to catch you off guard.
"Above rated power" is sort of a conditional statement, depending on whether you are getting that power from higher voltage, higher current, or both. Regardless, high power is going to put more stress on the nylon gears, reducing their service life, whether you choose to oil fill or not. You may want to consider having spare parts in inventory.

 

[wil]

The axle temp sensor is a great, idea, I'm going to be attempting that today, the wheel is already in as well so its easier then disassembling torque arms etc, I will try and fabricate a small rig so I can use a handheld drill and do it in stages, rather then dismounting and using a drill press

Both front and rear rims and tyres are identical, so true ratio is the same and tyre pressures will be kept even, or close to. I'm aiming for a relatively 50/50 weight distribution, or possible slightly rear biased depending on how I end up mounting the battery's.

High power itself will be voltage based, I'm going to be running 18 amp per motor, with phase currents of 35 amp (I haven't totally decided on these limits, they will be determined by actually riding it and seeing if they are reasonable. They are low because higher individual power shouldn't be needed as much with the 2 motors.

I have read that Bafang improved their gears from the 08-09 production run, so I'm hoping the should be ok, although I am trying to source metal gears at the moment in case the nylon does fail (although all suppliers from past threads that I have found seem to have disappeared since)

 

[Hugechainring]

Solid plan. It never hurts to start low and ramp up to what you need. I didn't mention the steel gears because using them makes the entire gear train wear out eventually, basically trashing the motor vs just one set of consumable parts (the nylon gears). Many users don't think this is worth it.

 

[motomech]

Back to Mckeefer, when he over-Volted to MXUS to destruction, the progression of problems went like this, Hall wires, Halls, phase wires and eventually, the windings. Even when the motor was a Crispy Critter, the gears were still fine.
I would suspect if there are benifits to oil-cooling, they would be in these areas.
Is your plan to use the 201 rpm winds @n 72 Volts?

 

[wil]

Is your plan to use the 201 rpm winds @n 72 Volts?

I've got 4s batterys, so the plan is 20s, 80v, although I'm getting everything tuned, worked out and comfrotable at 16s, 64v first. I might move up to 24s 96 volt and undercharging slightly, if eddy currents don't take effect strongly at 80v

I didn't mention the steel gears because using them makes the entire gear train wear out eventually.

They will only go in if I do get fairly serious issues with teeth stripping, and the oil should help reduce the wear somewhat too.