Hi
@saneagle - my ebike knowledge is limited (but growing every day thanks to this forum) and as a TSDZ2 kit person I've never had to understand BLDC hub motors, the different RPMs and voltages etc. but seeing as I might have to get a hub motor someday, I'd like to learn more.
Any chance of an idiots guide to why RPM/voltage matters or a pointer to a useful web page/YT vid please?
I don't know of any good explanations, but it's not difficult. This is a simplified explanation. Please no nerds quoting science facts.
1. A motor will reach a maximum speed that's proportional to voltage given to it.
2. A 48v battery (13S) has 30% more volts than a 36v one(10S), so a motor that spins up to 100 rpm at 36v will go 130 rpm with 48v. Don't forget that batteries change voltage as they run down, so your 100 rpm 36v motor will do 116 rpm when the battery is fully charged (42v) and 86 rpm when the battery is run down (31v)
3. When a motor spins, it generates voltage that cancels out the battery voltage. Using the above example, the motor will generate 36v at 100 rpm, so the net voltage is zero at that speed, and there's nothing to make the motor spin any faster, which is why it has this theoretical max speed. Obviously, the practical max speed is a bit less than 100 rpm because you still need something to keep it spinning.
4. The motor will make good power up to about 75% of its max rpm because at that speed (75 rpm) it's generating 27v, leaving 9v to push current through the motor, which is still enough to get meaningful power out of it. Below that speed, the controller regulates the power (cuts it) anyway.
5. 15 mph is approx 201 rpm with a 26" wheel.
Putting it all together, if you have a bike with 26" wheels and a 36v 201 rpm motor. The wheel will spin to 15 mph off the ground when the battery is half discharged. It will make good power on the road up to about 11 mph, which some might find a bit slow. Ideally, if you want good power up to 15mph with a half discharged battery, you'd need a 268 rpm motor, and even then, you'd feel the power weaken off a bit as the battery goes down from 36v. Nearly all EC compliant 36v ebike hub-motors are around 260-270 rpm.
When you run a 36v 201 rpm motor at 48v, it becomes a 261 rpm one because the 30% more volts make it spin 30% faster.
There's one extra advantage of 48 v. The controller sends pulses of power to the motor at high frequency. The pulse widths are the current and the height is the voltage. The power in each pulse is the height times the width, so you get more power in each pulse with 48v compared with 36v. That has the effect of increasing the torque, so with the same current, you get more torque running at 48v compared with 36v. In other words, your 36v 260 rpm motor will run at the same speed as a 36v 201 rpm one running with 48v, but when using the same current, the 48v one makes 30% more torque.
One more consideration and we're done. The efficiency of the motor is dependent on the actual speed it's running compared with it's maximum speed. Once you go below 50% of maximum speed, efficiency drops off rapidly, the lost power is converted to heat and things start to become very hot, which is why you can't have a very fast motor. The ideal motor has a max speed of roughly 33% more than your modal riding speed. That will give you good power and efficiency.
To summarise the last couple of points, lets consider two bikes. One has the 36v 260 rpm motor a 15A controller and a 36v 10Ah battery, and the other has a 36v 201 rpm motor, a 15A controller and a 48v 10Ah battery. With the wheels off the ground, they'll both spin up to the same speed. They'll both have the same efficiency at the same speeds. The 48v one will have 30% more torque, so will be able to accelerate faster and climb hills steeper hills or the same hills faster, and faster means better efficiency, so will be more efficient overall. The 48v one will get slightly more range due to its better overall efficiency, but bear in mind it's starting with 30% more capacity in the battery, and the battery would be 30% more expensive because it has 30% more cells in it.
