Hi everyone,
I am using a vesc 100/250 with an internally geared QS138v3 on 22S
Thus far I have the phase current at 320a and it is plenty of power. No issues.
I am wondering if I can get more RPM through the use of field weakening. There is no question that it will work but the question is if it will kill the vesc.
I am unsure about the inner working of field weakening however I was under the impression that it may increase the BEMF voltage at peak to above what the mosfets in the vesc are rated to and cause a melt down.
Because I am so close to the voltage limit anyway i'd rather not try it unless someone who knows more than me can give me a guide.
Is there a way I can know if field weakening will kill the mosfets before I try it out?
The obvious solution is to change the gear ratio. I have more sprockets on order
Any help appreciated
Thanks,
Reuben
I've been pondering this same question and wrote the following notes to myself. (I think the reasoning is correct.)
Flux weakening can be implemented as part of the FOC algorithm. It allows a motor to drive a load at speeds (RPM) otherwise unobtainable.
As a motor's speed increases, its back-EMF also increases at a linear rate.
Assuming the load will allow it, a motor reaches its maximum speed when its back-EMF equals its supply voltage.
Flux weakening decreases the motor's back-EMF by injecting negative current in the D-axis and thereby allows it to attain a higher speed with the same supply voltage.
The danger of flux weakening is that if control is lost for any reason and the motor is running above its “base” speed, the motor will act as a generator and a higher than normal back-EMF will be seen by the controller. If the controller's DC link capacitors and power semiconductors are not rated to withstand this higher voltage, they can be damaged.
The limitation of flux weakening is a speed/torque trade-off. There is no free lunch, maximum power will be constant and any increase in speed will come at the cost of reduced torque.
Hi konrad,
Thanks for the reply. Somehow I missed it.
If my understanding is correct then when the controller is working correctly, FOC is safe because (somehow) the mosfets are engaged in a sequence that prevents the BEMF voltage from exceeding the FET voltage.
If the rotor position is lost or another fault takes place then it could cause a failure as the motors BEMF is higher than the FET voltage rating.
I understand there is no free lunch however a bit more top speed with reduced acceleration (under field weakening) sounds like a great tradeoff. The thing that concerns me the most is the possibility of failure if the VESC loses rotor position.