Flux weakening vs High initial Torque

[eng.aj]

I understand the need of field weakening. Say a motor is to produce peak 6Kw and the rated rpm is 3000w (Mid drive). Instead of operating the motor in the rated conditions for peak torque and in the flux weakening region for higher speed, wouldn't it be advisable to build the motor a bit more robust, so that at the rated rpm the motor produces enough torque for top speeds and at for acceleration the peak motor torque is increased (Higher Current via FOC). The reason to do this is because the field weakening region is highly inefficient. I would like your thought on this. (P.S. the average speed is assumed to be 40-60 km/h)

 

[amberwolf]

Depends on your definition of "better" and the specifics of the entire system design.

If you want to use an existing motor design for something it can't quite do given the rest of the system's limitations, FW may let you do it anyway.

If you are designing a complete system from scratch, you can make any part any way you need, if there are no size/weight/cost limitations on the parts that would be exceeded by making the motor the "right" way.

If you are designing a system that has limitations for cost/etc on the motor, or even on the controller or battery, you may have to compromise and use an existing one that can't reach the speeds you want because the battery voltage isn't high enough to reach those speeds, or because the motor winding isn't the right one for those speeds, etc.

If you don't have a limitation on battery voltage for whatever reason, you can just increase that to get the speed you want out of a motor with insufficient kV; no need to change the motor itself.

You can play with the simulator at ebikes.ca to see how various voltages, currents, etc affect a system vs the riding conditions.


FWIW, there isn't a question of "higher current vs FOC"; FOC isn't a low-current thing, it is just a type of motor control. High current vs low current is just how much current that's available and/or used, and applies to all forms of motor control.

Also, 3000w is not an RPM, it's a power level.

I understand the need of field weakening. Say a motor is to produce peak 6Kw and the rated rpm is 3000w (Mid drive). Instead of operating the motor in the rated conditions for peak torque and in the flux weakening region for higher speed, wouldn't it be advisable to build the motor a bit more robust, so that at the rated rpm the motor produces enough torque for top speeds and at for acceleration the peak motor torque is increased (Higher Current via FOC). The reason to do this is because the field weakening region is highly inefficient. I would like your thought on this. (P.S. the average speed is assumed to be 40-60 km/h)

 

[mxlemming]

I understand the need of field weakening. Say a motor is to produce peak 6Kw and the rated rpm is 3000w (Mid drive). Instead of operating the motor in the rated conditions for peak torque and in the flux weakening region for higher speed, wouldn't it be advisable to build the motor a bit more robust, so that at the rated rpm the motor produces enough torque for top speeds and at for acceleration the peak motor torque is increased (Higher Current via FOC). The reason to do this is because the field weakening region is highly inefficient. I would like your thought on this. (P.S. the average speed is assumed to be 40-60 km/h)

In addition to amberwolf comments, it's worth noting that some motors respond really really well to field weakening. You can get hugely higher speeds for not much current. Others... Not so much.

Also worth noting is that at high speeds there are eddie current losses in motors. With flux weakening, you can actually reduced the losses to Eddie currents, since it reduces the total field in the iron. At high speed for motors with high inductance and low resistance this can actually result in reduced losses. (Don't confuse this with mtpa which is a whole other topic)

Of course, your mileage may vary.

 

[Elios.pk]

Also worth noting is that at high speeds there are eddie current losses in motors. With flux weakening, you can actually reduced the losses to Eddie currents, since it reduces the total field in the iron. At high speed for motors with high inductance and low resistance this can actually result in reduced losses.

Would that mean that we could use Flux Weakening to increase range / thermal performance of an existing motor that can already reach our target top speed ?

 

[mxlemming]

Would that mean that we could use Flux Weakening to increase range / thermal performance of an existing motor that can already reach our target top speed ?

There are motors where this is true, but it isn't generally true. The gain is usually marginal.

 

[Elios.pk]

Ok and I have another question since you seem to be very knownledgable about Ebike motors.

I am trying to configure a direct drive hub motor and want to get the most performance out of it (torque, top speed, acceleration and efficiency). My motor specs would be a direct drive 205 45H motor.

What I am trying to reach is a top speed of around 80Kph (I guess around 800Rpm no load speed since I use a 27.5" wheel) and totally max out the torque value (more than 300Nm) while using 90A.

What I don't really understand is that every motor don't seem to be performing equally.

For example, Surron drive train have around 250Nm (At around 90A DC so almost 3Nm/A), a top speed of around 75Kph and a 0-50Kph in 4s at 6000W.
I haven't seen any conventional direct drive hub motor from know brands offering this kind of performance per watts from QS, MXUS, BAFANG, etc.

On the other hand, I have seen specs of an Electric motorcycle hub motor that could reach a torque per Amp ratio of around 4Nm/A while keeping a decently high no load rpm (700Rpm).



In contrast, there is the QS205 V3TI that can output only 181Nm at around 100A DC (1.8Nm/A...) and while making even less rpm (674rpm).



Am I missing the real picture ? For the motor I want to produce I want to max out the Torque, Max RPM and efficiency and don't understand which parameters takes play to reach a ratio of around 3 or 4Nm/A while keeping a decent top rpm. Is it the copper filling ? The magnet grade ? Number of pole pairs ?

 

[E-HP]

For example, Surron drive train have around 250Nm (At around 90A DC so almost 3Nm/A), a top speed of around 75Kph and a 0-50Kph in 4s at 6000W.
I haven't seen any conventional direct drive hub motor from know brands offering this kind of performance per watts from QS, MXUS, BAFANG, etc.

I just tested my 1500W Leaf direct drive the other day, since I replaced my tires with slicks (Hookworms). 0-30mph = 3.8 sec, so roughly the same. 0-40mph was 5.89 sec. Amps peaked at 92A, but voltage sagged so watts peaked at 7053W. It's just like with cars, if the power to weight ratio is roughly the same, so is the acceleration.

 

[Elios.pk]

0-30mph = 3.8 sec, so roughly the same. 0-40mph was 5.89 sec. Amps peaked at 92A, but voltage sagged so watts peaked at 7053W.

Really impressive to run a 35H motor at such power levels. How is the motor thermal performance ? Do you use statorade and hubsinks ?

Anyway my problem with the leaf is the torque. I think it only have a peak torque of around 100Nm.

Running it at 7000W I wonder if the motor isn't already saturated and you have extreme diminishing return in torque.

I used a regular 45H motor in 45A with a sabvoton and only reached a 0-30mph in 7s. I don't know if this time is reasonable given the power draw. My top speed is 50mph in 72v when crouched under the handlebar.

 

[E-HP]

Really impressive to run a 35H motor at such power levels. How is the motor thermal performance ? Do you use statorade and hubsinks ?

Anyway my problem with the leaf is the torque. I think it only have a peak torque of around 100Nm.

Running it at 7000W I wonder if the motor isn't already saturated and you have extreme diminishing return in torque.

I used a regular 45H motor in 45A with a sabvoton and only reached a 0-30mph in 7s. I don't know if this time is reasonable given the power draw. My top speed is 50mph in 72v when crouched under the handlebar.

I had the exact same 0-30 run time with knobbies but hit 120A off the line, which is why I wanted to test the slicks. Per the Grin simulator the motor is hitting 160NM peak off the line. Temps are fine with Statorade for the terrain I ride on. I’ve been doing temp testing for the last couple of years to get a baseline before adding heat sinks, when I get around to it.
The speed run is with the 3 speed switch set to medium (no flux weakening), but running in bypass throttle mode (which isn’t rideable, so only used for the testing). The testing is for fun, I like data, since I ride normally around 19mph most of the time.