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.