Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What is the difference between 24 and 48 volt motors.

  1. Oct 10, 2011 #1
    So Ive been trying to make an electric bike. Ive read that to overcome steeper hills you need higher amperage, but in looking for motors I found a 1000 watt 48 volt motor that is rated for 28 amps and a 600 watt 24 volt motor rated for 32 amps. Which motor creates more torque? If anyone could help me find a good motor for an electric bike that would be great.
  2. jcsd
  3. Oct 10, 2011 #2


    User Avatar
    Homework Helper

    The torque doesn't really matter, since gearing can be used to compensate. Assuming both motors are optimally geared for a specific speed range, the higher powered motor will produce more rear wheel torque.
  4. Oct 11, 2011 #3
    I set up a 4 to 1 gear ratio from the motor to the lower bracket then a 1.25 to 1 gear ratio to the back wheel. Is there anyway I can create more power out of the motor keeping the same gear system?
  5. Oct 11, 2011 #4
    Power comes from the motor operating under particular conditions, e.g. rpm - so the optimum gearing for a particular speed is the one that allows you to operate the motor under optimal conditions. In a typical motorcycle, peak power is way up near redline, so ideally you arrange gearing so that you can always operate up near redline at any speed (the limit of this is a tiny, peaky 2-stroke engine running a gearbox with a dozen or more gears). I'm not sure how power vs. rpm varies with your electric motor, but looking at that curve would give you an idea how to gear it optimally for powering a bike.
  6. Oct 11, 2011 #5


    User Avatar
    Gold Member

    Generally you can say that two motors which is rated 1000W, the one with highest voltage and lowest ampére rating usually have the highest efficiency. The batteries are less efficient if the motors require more ampéres.

    I experience this with my RC plane motors, which is rated 2kW both. One require a higher voltage (22.2V) than the other (18.5V), and also have the greatest torque, and the battery last longer even if I compensate with higher battery capacity on the "low voltage" motor.

    So the battery pack you use will be, in general, the main factor which determine torque and efficiency. In theory both motors will produce the same torque with ideal wiring and batteries.


  7. Oct 11, 2011 #6


    User Avatar

    Remember about safety issues.
    24V is generally considered to be safe (although there were some electric shock accidents reported in extreme conditions), while 48V is very likely to cause possibly lethal electric shock if you, for exemple touch a mass of the bike with your leg and hot wire with hand while your skin is wet.
    Under European regulations 24V installations are not restricted, but 48V installation must fulfill electric standards and then be certified, otherwise you may be accused for bringing danger to someone who uses it.
    Last edited: Oct 11, 2011
  8. Oct 13, 2011 #7


    User Avatar
    Gold Member

    Those regulations might be the case in some countries, but that doesnt change the fact that a higher voltage and lower current increase the over all efficiency.
    I do not remember how this is in Norway, but what I learned at school 25 years ago is that AC at 50V is the limit, but DC is much higher. You will burn your fingers at high DC voltage, but isn't as dangerous as AC. AC can cause heart failure if the current through the body is high enough. But I would not advice to put your tounge on 50V DC....
    Last edited: Oct 13, 2011
  9. Oct 13, 2011 #8


    User Avatar

    In Poland 30V DC is a limit for unrestricted installations (which is set such just to cover "24V" lead battery chargers), for higher voltages installations must be build in conformance with standards and certified by qualified electrician inspector.
    In Austria and Italy the limits are similar: 48V DC power distribution installation for telecom equipment must be certified for electric safety.
    So I think it is a kind of EU standard regulation - but maybe the limits vary?

    I believe 48VDC may be deadly in some conditions. Actually there were several deaths reported even for 24V DC - all of them were miners, who performed hard physical work in hot deep mines where 24VDC is used to power auxiliary systems (lights, detectors, control systems, etc.), so their hearts were exhausted, and skin well conducting with sweat.
  10. Oct 13, 2011 #9


    User Avatar
    Science Advisor
    Gold Member

    So you have a choice - use 200V or higher, giving you even better efficiency - or you go by the accepted safety standards and have one less thing to worry about whilst you are riding your bike. What actual numerical difference in efficienty is involved?

    On the subject of RC planes. 2kw at 20V is 100A. How long does a small battery (airborne?) last with that sort of drain? A pretty short flight I imagine.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook