Thanks, excellent explanation
Seth
On Saturday, September 28, 2002, at 02:39 PM, Peter VanDerWal wrote:
> 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
>>
>>
>>
>
>
>
>
--
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
My EV Album page
http://www.austinev.org/evalbum/387.html
