# Electric Bikes - basic physics question

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View Full Version : basic physics question

mrscooterboy
09-26-08, 10:41 AM
Suppose you have an electric-assist motor that can propel you at 15 mph by itself on level ground. You would like to ride at 25 mph so you start pedaling. Will your speed be equal to the sum of the forces or will the motor actually start to be a drag at some point?

If you want to ride (and pedal) at 25 mph do you need to start with a motor that will turn the wheels unloaded at 25 mph or more?

heather1980
09-26-08, 10:59 AM
My husband and I ride amped kits mine runs between 18 and 19 mph. When I pedal along with it ever so softly I get 21-22 mph. If I try to pedal faster the most I can get is around 25 mph. I cant sustain this speed. I understand what you are asking I dont think it creates resistance by traveling faster.
Hope this helps

stevo1
09-26-08, 12:43 PM
I asked a similar question on the Golden Motor forum.
http://goldenmotor.com/SMF/index.php?topic=337.0

I think the simple answer is yes, the motor will start to be a drag at some point

unime
09-26-08, 01:14 PM
Since you asked a physics question, I'll give you a physics answer.

Electric motors work by forcing current through coils of wire to create magnetic fields. As the motor turns, the coils are energized with alternating voltages (positive-negative-positive-negative...), causing the magnetic fields to alternate, which in turn apply force to the stator magnets in the motor, creating torque.

Electric coils have a property called inductance which is a resistance to change in current flow. Thus the magnetic field lingers for some time after the applied voltage reverses. This settling time limits the upper speed on a motor, and, if forced past the motor's natural speed (for a given driving voltage) will add drag instead of power to your bicycle.

Higher voltages will increase a motor's top speed, by forcing a quicker chance in current through the coils.

In practice, I would bet that this is not an issue for a properly designed electric bicycle.

09-26-08, 01:58 PM
Unime, is it just the voltage or does the current (amps) also increase the motor's top speed?

Bionx has a PL350 and a PL500HS. Both use a 36V battery, but the PL500HS use 500W vs 350W for the other. And the PL500HS has a no load speed of about 42mph while the PL350 has a no load speed of about 25 I believe.

mrscooterboy
09-26-08, 02:11 PM
So a motor does have a natural no-load speed for a given voltage, is that correct?

I'm guessing that you could used a more powerful battery (more volts) to increase the no-load speed but there is probably some limit there as well. Won't the motor burn up if too many volts are applied?

Then what does the controller (throttle) control - volts available to the motor?

It sounds as though you need a motor capable of a no-load speed of 25 mph if you want to travel (including pedaling) at 22 mph.

unime
09-26-08, 04:39 PM
Current (measured in amperes) is what powers a motor by inducing magnetic fields, but voltage is required to get the current flowing. Brushless motors (like the familiar hub motors) have a Kv rating giving the motors natural speed in RPM/volt. Technically, the Kv rating specifies a line in the speed-voltage space along which the "back emf" (voltage generated internally by the spinning motor) equals the driving voltage.

The controller is constantly turning the power on and off at high speed, providing an average voltage to drive the motor, and the current through each coil corresponds to that voltage. Increasing the throttle tells the controller to increase its duty cycle (ratio of "on" to "off" time), providing more power to the motor. (for brushless motors, the controller does this for each of three phases and also maintains motor timing.)

Overheating is the biggest risk from running at high power by increasing voltage. Heat can ruin magnets, melt insulation, burn lubricants, and change mechanical tolerances in a motor. In theory, a controller could limit current at lower speeds, limiting total power regardless of its input voltage, yet still allow that full voltage input at high speeds when the motor is spinning too fast to draw peak power. Stepper motor controllers operate this way, but I don't know about the features of available brushless controllers.

Lastly, I agree - you'll need a motor with a higher no load speed higher than you plan to travel. Power will start to drop quickly as you approach the no load speed. Your best bet is to ask other riders about their experience with combinations similar to the ones you are considering.

lasix
09-26-08, 08:43 PM
Suppose you have an electric-assist motor that can propel you at 15 mph by itself on level ground. You would like to ride at 25 mph so you start pedaling. Will your speed be equal to the sum of the forces or will the motor actually start to be a drag at some point?

If you want to ride (and pedal) at 25 mph do you need to start with a motor that will turn the wheels unloaded at 25 mph or more?

Assuming the system you are using is not chain driven and did not have freewheel, then Yes you will creat drag.

It takes work to move conductive metal through a magnetic field. If the the electric current can give Xrpm in ideal conditions, to get you to 25 and you want to go faster, then the extra, would come from your legs. And YOU would be turning the motor faster than the electric would allow. Then you would be forcing a metal object through a magnetic field, and yes that would create drag...

In theory you would be creating electricity!

gmouchawar
09-27-08, 10:53 AM
If you use a geared hub motor (with internal freewheel) or a freewheel like the cyclone system this won't be an issue. However, your bicycle gearing can you your limit.