If you use a permanent magnet motor the magnetic field is fixed and what you describe will work, i.e. high enough RPM and current flow back into batteries (assuming your controller will let it). You can also do some tricks with the controller, basically switch the controller to boost mode. Then you can even get power out of the motor when it's spinning slower.
With a seperately excited motor you can crank up the field power and get regen easily (just like above only with a variable field) Series wound, gets complicated. Back EMF on series wound isn't exactly straight forward. The magnetic field isn't fixed, it's strength depends on the amount of current flowing through the field coils. So as your motor spins up, the current falls, the magnetic field weakens, and the back emf gets smaller. Get it up to the point where no current flows into the motor and you get no field and therefor no back emf. Series wound regen controllers are special beasts. They require contactors to rewire the motor, there are a couple different ways to do this. Curtis controllers wire the controller directly to the field and the armature to the batteries through a diode (so current can only go from motor to battteries). It then acts like a seperately excited motor and the controller directly controlls the current through the field (and therefor the strength of the magnetic field). More current and you either get stronger regen and/or regen at a lower RPM untill you get to the point where you can't boost the field any more. Problem is that the fioeld winding has very little resistance and this method requires quite a bit of current to work, in fact it requires a large portion of the generated current and creats quite a bit of heat. Zapi controllers get really wierd and rewire the motor for reverse and then (I'm a little hazy on it here) pulse the motor and get regen from the collapsing field (inductive kick). There are some indications that Zapi controlelrs might be more efficient at regen than curtis controllers. Plus you can still buy new Zapi regen controllers./ Curtis won't sell regen controlers to individuals so you have to hunt around for a used/rebuilt one. FWIW: regen with brushed motors has another problem. Brushed motors typically advance the brushes for optimum efficiency. Unfoirtunately what is advanced for motoring is in the wrong direction for regen. So you either need a moveable brush rigging (complicated) or you need to put the brushed near to a neutral position. If you don't then you get brush arcing which can eat up your motor. Neutral position means less efficiency while motoring and less than optimal efficiency while in regen. So now you motor is at maybe 70-75% efficient when motoring and perhaps even less when regen. Add in transmission losses, tire/rolling resistance losses, etc. And you only end up with maybe 10-20% of the extra power put into accelerating being returned to the batteries. This means that regen on series wound motors is great for slowing down, but not very good at extending your range. P.S. no matter what kind of motor/contrller you use you can't recover ANY of the energy used to overcome losses (aero drag, rolling resistance, I2R, friction, etc.). The reason they are called losses is because the energy is lost, changed into heat;etc., bye-bye, gone for good. Seth Murray wrote: > hehe, guess I don't know too much about motors now do I?? =) here's a > dumb question - if our motor here is making all this back EMF, if we > are coasting down a hill and keep on winding the motor up and up and > up until the BEMF exceeds the input voltage, would we not have regen? > My understanding was that series motors are tricky to do regen with. > Why would that be, if we have this apparently straightforward EMF > thing going on? Thanks for the info. > > "Us racers swear at them..." someday, Rich, I will give you a run for > your money (or a new PFC) =) > > Seth > > > > On Saturday, September 28, 2002, at 01:12 PM, Rich Rudman wrote: > >> 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 >> >> >> HOooooH ha ha HHHHEEEE! Yea rather obvious is the BACK EMF of the motor. >> As it spins up it becomes a generator, you need to over come this with >> more volts or your motor will spin up to a certain speed and then wind >> out no futrther!!!! >> Basic V/F ratio. volts to Frequency or RPM same thing. >> All motors have rpm and torque curves. You are just seeing them . >> Us racers swear at them and try to manipulate them to our best uses. >> >> Check the motor curves from Advanced DC and from Rod at EVparts, and Ken >> Koch at KTA. >> >> >> -- >> Rich Rudman >> Manzanita Micro >> www.manzanitamicro.com >> 1-360-297-7383,Cell 1-360-620-6266 >> >> >> >> > > > > -- > 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 > > >
