you probable need to go to the Motor/Controls forum to get the full low down of motor speed control.
Most pure EVs ..Bolt, Spark, Leaf, BMW I3, Tesla, Rimac , Koenigsegg (?) etc etc all use "single speed" geared reduction drive.
There was an issue at the time with the controller providing the amps necessary to get the desired performance. As I recall, there were issues with the transmission prototype, and then there was a breakthrough with the controller components. Once higher temporary peak amps were available, the roadster was able to achieve the performance goals with a single-speed reduction (8.2:1 between motor and wheel). Much simpler, more compact, and less weight, less cost.
Motors with permanent magnet rotors or fields can only change speed with a change in applied stator or armature voltage. An induction machine can (with a good controller) play with the flux in the stator and achieve a approximately 3:1 range of field weakening. I.E. the motor can spin at ~ 3 faster than the speed available with full voltage and full flux. If a synchronous motor is used where the flux is controlled by a separate DC controller just for the rotor field, then up to a ~9:1 range is possible. Of course if you reduce the flux then the torque available is also reduced in proportion. You then have to use more stator current to get the torque back up. So if you want not to use a gearbox then an induction or synchronous motor starts to make a lot of sense.
Motors with permanent magnet rotors or fields can only change speed with a change in applied stator or armature voltage. An induction machine can (with a good controller) play with the flux in the stator and achieve a approximately 3:1 range of field weakening.
Ah well ! that last comment surprised me. Field weaken permanent magnet motors ? To be sure I contacted someone who is a real authority on this stuff. Sure enough you can do a few tricks when you have significant stator current flowing, but that's likely to come and bite you when the current is removed as the no load back EMF will over Volt the drive module and probably kill it. So in the context of the discussion you're not going to get extra speed range with a permanent magnet rotor you have to use an induction motor or synchronous motor to do that.
A gearless system for electric makes alot of sence eliminating the gears added extra friction and mass but when it comes to regen you lose the mechanical advantage gained that's why the regen paddle on most cars even full on is not that strong.
Any gear that's used to increase the regen capability would need to be strong enough to take the torque of the electric motor multiplyed by the gearing so it would need to be a beast and that's not a viable option I believe the answer needs to be in the motor design, but how do you add stronger variable regen braking to a motor without ruining it's simplicity.
Ah well ! that last comment surprised me. Field weaken permanent magnet motors ? To be sure I contacted someone who is a real authority on this stuff. Sure enough you can do a few tricks when you have significant stator current flowing, but that's likely to come and bite you when the current is removed as the no load back EMF will over Volt the drive module and probably kill it.
?? The back EMF is channeled by the body diodes of the FET back into the battery. That's why if you go faster than base speed with a PM motor, you see regen even if you're not doing anything actively with the controller. Try it; take your 25mph base speed PM motor up to 30mph and see if the inverter dies (it won't.)
So in the context of the discussion you're not going to get extra speed range with a permanent magnet rotor you have to use an induction motor or synchronous motor to do that.
I'm sure there are some research papers written on it with lots of ugly math. In a brushless PM motor, you can get field weakening by essentially advancing the timing. The commutation occurs before full BEMF is reached so the motor can go faster before it matches the source.
I have a controller that does it too. A cheap one. But it still gives an impressive speed increase over no weakening.
One question: when in field weakening the current and Vbackemf are no longer in phase, is this the cause of the inefficiency (and motor heating?) associated with field weakening? Is it simply the same as a poor power factor with a traditional AC induction motor?
One question: when in field weakening the current and Vbackemf are no longer in phase, is this the cause of the inefficiency (and motor heating?) associated with field weakening? Is it simply the same as a poor power factor with a traditional AC induction motor?
It is similar to think about like intentionally getting bad power factor for RPM. What amazed me was how little of an efficiency penalty it really is, typically just a couple percent added inefficiency to push +30-40% RPM range (depending on the motor design and some other variables.)
Wow, that's a lot less than I thought and I can understand why it's popular now.
It had been explained to me before as energizing the field coils out of time in order to create a magnetic field that would partially oppose the magnets and so effectively reduce their strength and raise Kv and that this was a significant amount of entirely wasted energy. I'm glad to learn this the case.
Wow, that's a lot less than I thought and I can understand why it's popular now.
It had been explained to me before as energizing the field coils out of time in order to create a magnetic field that would partially oppose the magnets and so effectively reduce their strength and raise Kv and that this was a significant amount of entirely wasted energy. I'm glad to learn this the case.
It had been explained to me before as energizing the field coils out of time in order to create a magnetic field that would partially oppose the magnets and so effectively reduce their strength and raise Kv.
The explanation above does not explain why it also works with external inductors (that do not reduce the magnets field, as they are outside the motor)
I know this is the popular believe but basically it is wrong. A better explanation would be that the motor inductors are used to build a boost (dcdc) converter, to obtain a higher effective voltage to drive the motor.
So you guys don't count the 10:1 tesla gear reduction as a gear? 10:1 is an awesome steep gear. very cool, but not gearless. it is shiftless, or a 1 speed trans/rear, that is true.
Heres a read up i found that explains it there are two varients i can find one uses the bemf the other uses the halls timing so both versions are correct below is an explination of the hall sensor version ad we are all familiar with the Bemf version on all cheap controllers giving 120% output.
Brushless direct current (BLDC) motor is widely used in small and medium sized electric vehicles as it exhibit highest specific power and thermal efficiency as compared to the induction motor. Permanent magnets BLDC rotor create a constant magnetic flux, which limit the motor top speed. As the back electromotive force (EMF) voltage increases proportionally with motor rotational speed and it approaches the amplitude of the input voltage, the phase current amplitude will reach zero. By advancing the phase current, it is possible to extend the maximum speed of the BLDC motor beyond the rated top speed. This will allow smaller BLDC motor to be used in small electric vehicles (EV) and in larger applications will allow the use of BLDC motor without the use of multispeed transmission unit for high speed operation. However, increasing the speed of BLDC will affect the torque speed response. The torque output will decrease as speed increases. Adjusting the phase angle will affect the speed of the motor as each coil is energized earlier than the corresponding rise in the back emf of the coil. This paper discusses the phase advance strategy of Brushless DC motor by phase angle manipulation approaches using external hall sensors. Tests have been performed at different phase advance angles in advance and retard positions for different voltage levels applied. The objective is to create the external hall sensor system to commutate the BLDC motor, to establish the phase advance of the BLDC by varying the phase angle through external hall sensor manipulation, observe the respond of the motor while applying the phase advance by hall sensor adjustment.
