You kinda answered your own question, only you don't know it. PM motors have a fixed field so they will do exactly what you say. For a given voltage they spin up to a certain RPM with no load and stay there, within certain limits if you double the voltage you pretty much double the RPM.
Series motors however... Let's assume a 9" ADC with no load and lots of voltage (100+VDC). The motor starts out not spinning, you apply the voltage and, since it's not spinning, the only thing that limits current is the motors internal resistance. So the motor draws a couple thousand amps and quickly starts spinning. Now it's spinning because the magnetic field from the armature pushes against the magnetic field from the stators or field coils. The strength of these fields (and the resulting torque) is related to the amount of current flowing throught them. (Side Note: Doubling the current in a series motor doubles the magnetic strength in BOTH stator AND armature, theoretical result is 4X the torque. That's why we like series motors...lots of torque). BEMF is related to both field strength and the speed of the moving coil. As the motor speeds up the BEMF increases, which reduces the current, which reduces the magnetic field strength (in both stator AND armature). Now this is where it gets fun...less field strength means less potential BEMF, so the motor speeds up, the BEMF increases, the current drops, which means less field strength... Rinse and repeat steps 1 through 4. Of course this all happens at the same time so the effect is that, on an unloaded motor, BEMF goes up too slowly to limit the motors top RPM to a safe level. At low voltages the motor's own friction can be enough of a load to stabilize the RPMs at some safe level. Seth Murray wrote: > OK, I knew I had one more question for all you motor experts! Why is > it that a series motor will keep on going faster and faster and > finally explode if given adequate voltage unloaded? Plus, if we have > this back EMF stuff, where does it go if you take the load away? > Wouldn't there be an upper limit on the RPMs because of the BEMF, or > is that upper limit a bit high for our commutator? > > 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 > > >
