Qs 138 90H at 120 volts

[Sattva Ram]

I am pondering the overspinning of this motor by the factor of 1.66 relative to the factory data to gain some efficiency. 1.66 times the rpm means 2.6 times less heat generated in the windings at a given output. Now of course higher rpm means more eddy current losses also more heat in the bearings, but the gain seems to outweigh the losses so this way the motor will produce far less heat overall. Sure the rotor will be hotter but nowhere near as much hotter as the windings will be cooler. Is there any fault with my thinking? Some expert advice is appreciated. Thanks

 

[John in CR]

Since it's an inrunner, then you'd be relying on the motor being sufficiently over-engineered to handle the extra rpm without flying apart.

It's only a good idea depending on how it is used. If you do a lot of relatively high speed cruising, then no it's probably not a good idea, since those extra core losses are created whether you're on the throttle or just coasting. Look at the no load current and multiply that times your new voltage to give you an idea of the losses just to spin the motor.

OTOH, if you ride like I ride my 100mph+ capable ebikes, ie typically far below top speed (typically at only about 50% of what my bikes are capable), then yes, volting up and gearing down pays big dividends.

 

[Sattva Ram]

Hmmm interesting. It spins only 4000 similar inrunners like the tesla m3 spins at 15k so I am absolutely not worried about the structure falling apart especially that it's an ipmsm. The question is rather possible rotor overheating.

However it seems I'll settle for 72volts. If I opt for 134 charge voltage then I'll have to use lower voltage onboard charger which is less efficient so probably I'll lose the whole gain of the higher voltage system - at least in terms of practical range - because of the charger will be slower. Sure sure at 120 volts I might have a more powerful bike, but hey it'll never be a racebike anyway so chuck it

However it's worth noting that overspinning the motor by the factor of 1.6 yields 2.6 times less heat in the windings at a given power output that is 2.6 times less heat produced by the motor. Of course it's an ideal scenario not accounting for the losses due to higher rpm, but still I think it's waaay disproportional sure higher rev will heat the rotor more but nowhere near to the extent as the windings will be cooler relative to stock rpm. So some heat will be "exported" to the rotor from the stator by overspinning, but I doubt it would be 2.6x the heat in the rotor relative to the stock rpm of the motor. So it seems like a net gain to me. Lotta heat saved in the windings for a minimal excess heat in the rotor.

I kinda suspect tho that in lower rpm higher torque scenarios - heavy accelerations - the lower voltage would be more efficient than the higher voltage. So my hunch is similar to what you are saying it all depends on riding style for steady countryside cruising it's the higher rpm version the winner for red to red style driving it might be better sticking to lower revs. At least this is what feels intuitive to me. I might be totally wrong tho.

I hope by ebike you meant e-motorcycle. 100mph+ seems kinda crazy on a bicycle. What motor do you use btw? 100+ aint no joker after all...

 

[ayedurand]

Would you please point me at the math that shows the relationship between rotor speed and heat generation? I'm assuming this is for an equivalent motor power input but would like to understand the assumptions better.