Help! Hub Motor Parameter Questions

[Bikes4two]

Hub Motor Parameter Questions

There are so many things I don’t understand about hub motors (Brushless DC aka BLDC and of the geared variety, not direct drive), so apologies for my meanderings but if you good folks could help me out here please?

1. RPM – when the RPM of the motor is given:

> Is that RPM at the axle ie after the motor gearing - I'm assuming it must be but best to confirm?
> if say the RPM is given as 200RPM at 36v, does that mean if you run the same motor at 48v then the RPM is 200*48/36 = 267?
> And conversely a 48v motor stated as 300RPM runs on 36v at 300*36/48 = 225?
> Why might you consider running the motor at a voltage above (or below) it’s stated voltage – is that simply to achieve a specific RPM?

2. Road Speed
> assuming the quoted RPM is at the axle, the max no load road speed is a product of the RPM*wheel circumference with tyre, yes? (obviously after getting the correct time and distance units sorted out first).
> So that’s the no load speed – in practice I’m guessing there’s a bit of a rule of thumb about the most efficient speed – is it around 70% of max RPM or somethingmore complicated?

3. Torque vs Speed
> I see folks in the context of choosing a motor and it’s RPM, mentioning lower RPM for better torque at lower road speeds vs less torque but higher road speeds – what is this all about?
> Some motor manufacturers quote a torque figure – in relation to the RPM, where is this torque exerted?

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In my simple mind, if I imagine a BLDC motor spinning away on a bench under no load conditions, the drive shaft (axle) isn’t exerting any torque and no doubt the motor isn’t drawing much current either.

Conversely, if I grip the drive shaft and attempt to stop it spinning, I can understand that the motor would resist this (and exert torque) and that the motor would draw a lot of current under these stalling conditions.

Is my analogy too simple?

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Apologies again for the dumb questions but until now (as in considering a hub motor) I’ve only really used the TSDZ2 mid drive motor – out old cranks/BB, install the new kit and peddle off into the distance – none of this PAS sensor/controller/display bunkum – just simple plug and play lol!

 

[saneagle]

1. RPM is wheel speed. 48v is 13 cells and 36v is 10 cells, so the ratio of voltages is 1.3, not 48/36.
2. Most efficient speed is approx 75% of maximum RPM
3. Don't take any notice of manufacturer's torque figures. They only relate to the arbitrary control conditions they tested at. Torque is proportional to current, so it depends on how much current you give the motor. Likewise, it depends on voltage. Generally, the bigger the motor, the more current it can handle, so the more torque you can get from it, but another factor is the reduction ratio. The higher the reduction ratio, the more torque the motor produces. Lower speed motors don't make more torque than high speed ones with the same reduction ratio. Instead, they run more efficiently at low RPM.

At max rpm, the back emf is equal to the battery voltage, so no current can flow. When you slow down the wheel with your hand, the back emf goes down so you have some volts left from the battery to make current and torque. The slower the wheel turns, the lower the back emf, so the more volts you have, which makes more current and torque. The controller has to limit the current at low rpm because there is no back emf at zero speed, which would mean the battery voltage would cause massive current. That's why your motor burns when a MOSFET in the controller blows.

 

[Bikes4two]

Thanks @saneagle - a lot of (hub motor) fog has been lifted today.

48v is 13 cells and 36v is 10 cells, so the ratio of voltages is 1.3, not 48/36.

Right, I can see the difference as 48/36 is just using nominal cell voltages for 48v and 36v packs
(48/36 = 1.33 recurring) but I can see that by using the cell count this gives a more precise ratio.