This is caused by Back EMF. Quick and dirty explanation: Move a coil through a magnetic field you get an induced electrical current (this is how generators work). Take the same coil and magnetic field but push electricity through the coil instead and the coil moves on it's own (this is how motors work). What's this prove? Just that motors and generators are exactly the same thing, it just depends on how you use them.
Now for the fun stuff, every motor while motoring is also functioning as a generator at exactly the same time. You are pushing electricity through the coils and that's making them move. The moving coils inside the magnetic field are generating an electrical current/voltage. This voltage is in opposition to the applied voltage. The faster the motor spins the higher the generated voltage (called Back Electro Motive Force or Back EMF or BEMF). The amount of current flowing through the motor is related to the difference in applied voltage and Back EMF. So for a given applied voltage the faster the motor spins the less current it draws. (P.S. move the coil much faster than the applied voltage drives it and it switches to being a generator, this is how Regen works) There is also the controllers current limit to consider. For the follwoing example we will assume a drag racing type acceleration, pedal to the floor the whole way. At low RPMs the motor wants to draw lots of current (Could be thousands of Amps). Most controllers limit this current to a few hundred amps. In order to do this the controller must reduce the apparent voltage applied to the motor. So the controller might draw 120V @ 60 amps from the batteries but applies 12V @ 600 Amps to the motor (assuming a 600 Amp current limit) As the motor speeds up it needs more voltage applied to contrinue to draw 600 amps so the controler sends say 24V @ 600 amps and draws 120 Amps from the batteries. This keeps going untill the controller is drawing 600 amps from the batteries and applying 120V to the motor. At this point the controller comes out of current limit. The motor is now self-limiting current, it has battery voltage applied to it and it's rpm is high enough that it won't draw any more current. The motor continues to accelerate so the Back EMF continues to go up and the current draw starts dropping. When you mix the motor with it's variable current draw (depending on RPM and applied voltage) with a controller that can vary the applied voltage and therefor current, you end up with a combination that can draw vastly different currents from the batteries at a given RPM depending on throttle position. Clear as mud? Seth Murray wrote: > Peter VanDerWal wrote: > >> the resistance through the brushes on a moving comutator is different >> than on a stationary one > > > I have been wondering about this. Lets say I am driving in second > gear. When I first start my EV moving, the motor wants lots of amps. > Once it gets up to 4,000 rpms or so, the motor seems to have a limit > on its current draw, even if I put my foot on the floor (current well > under the current my batteries and controller can deliver). As soon > as I shift into 3rd, though, the motor will take all the current my > poor batteries and controller dish out. The only thing I could thing > of was that the resistance through the motor must change depending on > rpms, which because of Ohm's law would allow the motor greater current > at lower speeds. I know there are other factors involved, but do I > have any clue of what's going on? =) > > Seth > > > -- > QUESTION INTERNAL COMBUSTION > > My electric truck page, with lots of photos and a 25 page conversion > journal. Check it out! > http://www.wpi.edu/~sethm (NO MORE POPUPS!!) > > My EV Album page > http://www.austinev.org/evalbum/387.html > > >
