5 ms·

Why Axial Flux Motors Are the Key to Small, Fun EVs

11 points by tomohawk 3y ago

audunw 3y ago
No mention of Koenigsegg’s Quark “Raxial” flux motor?
ActorNightly 3y ago
They aren't the key, they are just a little bit more efficient and more compact. The game to optimizing EVs for a given battery pack is that you want to minimize current. Motor torque is directly proportional to current, and efficiency losses are a function of current^2. So generally you are looking at ways to generate more torque per amp. There are 3 ways to do that. First is to use a low KV motor. KV is RPM/Volt spec of the motor. The other important spec is Torque/Amp, which is KT. KV*KT = constant for permanent magnet motors. So the lower the KV, the higher the KT. There is a configuration of motors called outrunner which places the rotor with perm magnets on the outside shell, giving a bigger torque arm (which is also the case for the axial flux) - this reduces the KV, but increases torque/amp. The second way is to use a gearbox. You eat a little bit of efficiency in the gears, but functionally its the same as trading rpms for torque with the kv/kt motor configuration. Porsche Taycan uses this Third way is to spread out torque generation over multiple motors. If each motor is responsible for 1/4 of the torque, the overall efficiency is much higher. Back in early 2000s, with much older battery tech, there was this car (https://en.wikipedia.org/wiki/Eliica https://en.wikipedia.org/wiki/Eliica), which achieved a range of 200 miles, all because of the 8 wheels with 8 individual motors. IMO the best way to make a small fun EV without exuberant costs is to put an outrunner motor into the hub of each of the 4 wheels. The downside to this is big increase in unsprung mass, but suspension systems are pretty good these days to where dynamic damping adjustment can be done quite easily at very fast response times.
- aidenn0 3y ago
  Wouldn't a lighter motor make spreading torque generation over multiple motors easier (not to mention making hub motors potentially more practical).
  - ActorNightly 3y ago
    It would, but the weight savings would matter more for handling then for power. Motor weight is still a fraction of battery weight.

11 points by tomohawk 3y ago

No mention of Koenigsegg’s Quark “Raxial” flux motor?

They aren't the key, they are just a little bit more efficient and more compact. The game to optimizing EVs for a given battery pack is that you want to minimize current. Motor torque is directly proportional to current, and efficiency losses are a function of current^2. So generally you are looking at ways to generate more torque per amp. There are 3 ways to do that. First is to use a low KV motor. KV is RPM/Volt spec of the motor. The other important spec is Torque/Amp, which is KT. KV*KT = constant for permanent magnet motors. So the lower the KV, the higher the KT. There is a configuration of motors called outrunner which places the rotor with perm magnets on the outside shell, giving a bigger torque arm (which is also the case for the axial flux) - this reduces the KV, but increases torque/amp. The second way is to use a gearbox. You eat a little bit of efficiency in the gears, but functionally its the same as trading rpms for torque with the kv/kt motor configuration. Porsche Taycan uses this Third way is to spread out torque generation over multiple motors. If each motor is responsible for 1/4 of the torque, the overall efficiency is much higher. Back in early 2000s, with much older battery tech, there was this car (https://en.wikipedia.org/wiki/Eliica https://en.wikipedia.org/wiki/Eliica), which achieved a range of 200 miles, all because of the 8 wheels with 8 individual motors. IMO the best way to make a small fun EV without exuberant costs is to put an outrunner motor into the hub of each of the 4 wheels. The downside to this is big increase in unsprung mass, but suspension systems are pretty good these days to where dynamic damping adjustment can be done quite easily at very fast response times.

aidenn0 3y ago
Wouldn't a lighter motor make spreading torque generation over multiple motors easier (not to mention making hub motors potentially more practical).
- ActorNightly 3y ago
  It would, but the weight savings would matter more for handling then for power. Motor weight is still a fraction of battery weight.