MagicPie-3 16" wheel -> max speed @48V

[Just]

Hi All,

What's the maximum speed of 16" MagicPie-3 wheel with 48V battery?

What's the weight of 16" PM-3?

Where can I see a spec of the wheel?

Are there any plans for MP-4?

Please response.

Thank you!

[Just]

What speed should I expect with 16" motor wheel and 60V battery? 72V battery?

[Bikemad]

What speed should I expect with 16" motor wheel and 60V battery? 72V battery?

Without knowing the true maximum speed of the 16" MPIII on 48V (which I don't have details of) it's impossible to give any real estimation of the expected speeds on those voltages but hopefully the following will help.

Having said that, if you were to use the standard internal controller with a 60V or 72V battery, the motor's maximum speed would be 0 mph as the voltage would be much too high for the controller to work.

However, if you were to use a suitable external controller (capable of running on those voltages) the unloaded wheel speed with a 60V pack should be 25% faster than the 48V battery, and 50% faster on a 72V pack.

Unfortunately, the force needed to overcome the aerodynamic drag (wind resistance) increases dramatically as the bike's speed increases, so the difference in maximum road speeds under load will be less than the increases gained in the unloaded wheel speed.

Although the 72V pack would produce a 50% increase in the unloaded speed, it would also require 237.5% more power to achieve a 50% increase in the actual road speed in order to overcome the additional force generated by the increased aerodynamic drag.

The power needed to overcome the drag increases as the cube of the velocity, so a 2x speed increase would require 8x the amount of power ( 2³ = 2 x 2 x 2 = 8 )

Example
If 1000 Watts of power (20.83 Amps @ 48V) is required to achieve a top speed of 24 mph on the level without any head/tailwind etc. then:

• 1,953 Watts (~27.13 Amps@60V) would be required to reach 30 mph
• 3,375 Watts (~46.88 Amps@72V) would be required to reach 36 mph
• 8,000 Watts (~83.33 Amps@96V) would be required to reach 48 mph

Dmitryl, I realise this is probably not the simple answer you were hoping for, but if you are able to understand what I have tried to explain, you will realise why the answer to your question is not as straightforward as you originally thought it would be.

Alan
 

[Just]

Thanks Alan for your great comments!

According to your notes, increasing a speed by 50%, the power consumption will be increased by 3 times (1.5^3=~3.4). Correct? Is the above calculation correct for ALL speeds?

Is ratio between speed and voltage constant for ALL speeds/voltages? Let's  if applying 1V on a certain unloaded motor will run it at 1kmh then applying 50V on the same motor will run it at 50kmh accordingly?

Thank you!
 

[Aliasssss]

If we were to live in a perfect world with no air and bearings friction at all that sentence would be true ... almost... but we don't live in a perfect world so one can only hope... Thus if we are to omit the paradox of breaking even the simplest rules of physics , in theory the speed at 50v would be at most 50km/h, but in practice the speed would be way lower than that...

PS - Even with no load at a 50x factor you don't get to see the same multiplication on the RPM...

[Just]

As for a bicycle, anyone could run 50kmh or should I buy some special one? I mean a bicycle converted to e-bike and driven by electric motor.

[Bikemad]

The example I posted were based on information that is readily available regarding Power and aerodynamic drag:

The power required to overcome the aerodynamic drag is given by:


where

F_d is the drag force, which is by definition the force component in the direction of the flow velocity
p is the mass density of the fluid (Air)
v is the velocity of the object relative to the fluid (Air)
A is the reference area, and
C_d is the drag coefficient – a dimensionless coefficient related to the object's geometry and taking into account both skin friction and form drag.

Note that the power needed to push an object through a fluid (Air) increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, four times the work done in half the time requires eight times the power.

Even with no load at a 50x factor you don't get to see the same multiplication on the RPM...

That's not the way I see it.

Brushless motors are usually defined by their KV (rpm per Volt) so if you double the voltage you should double the unloaded rpm of the motor as the ratio between speed and unloaded rpm should be linear (within reasonable limits).
The switching speed of the controller's electronics will eventually limit the motor's maximum rpm if the voltage is increased too far. The motor's maximum rpm limit will also be affected by the number of poles/magnets etc. as a higher number of poles means more switching operations per revolution, which will inevitably cause the controller's switching limit to be reached at a lower motor rpm.

Motor velocity constant, back EMF constant
Kv is the motor velocity constant, measured in RPM per volt (not to be confused with kV, the abbreviation for kilovolt).
The Kv rating of a brushless motor is the ratio of the motor's unloaded RPM to the peak (not RMS) voltage on the wires connected to the coils (the back-EMF). For example, a motor of Kv, 5,700 RPM/V, supplied with 11.1 V, will run at a nominal 63,270 RPM (5,700 RPM/V × 11.1 V).

An inverse measure is also sometimes used, which may be referred to as the speed constant.

By Lenz's law, a running motor generates a back-EMF proportional to the RPM. Once the motor's rotational velocity is such that the back-EMF is equal to the battery voltage (also called DC line voltage), the motor reaches its limit speed.

As for a bicycle, anyone could run 50kmh or should I buy some special one? I mean a bicycle converted to e-bike and driven by electric motor.

Most bikes could be persuaded to go that fast, but a poor quality bikes with smaller wheels and no suspension might not be able to cope with the additional stresses that would be encountered if it was constantly ridden at higher speeds.

Alan
 

[Aliasssss]

So ... "within reasonable limits" means that you got my point, not to say that a motor wounded to run at 50v wont turn at 1v, nor one wounded to run at 1v won't withstand 50v without blowing up(bearings calibrated to run at 50 times lower rpm, or phases shorting out).

In MP III boundries if you run at 24v unloaded you get half the RPM you would get if runing the MP III at 48v, but that is only an x2 multiplication factor. The RPM/v multiplication rate is not that linear once you reach the 2 figures numbers because of the friction with air and within the bearings. I'm an RC guy, I know my way around BLDC motors a bit