Why does Kt vary with torque?
Why does Kt vary with torque?
(OP)
I'm using a 24V 3 phase BLDC servo motor with trapezoidal commutation to provide force feedback to a steering wheel. Torque should be greatest when the angle between the steering wheel centerline and the machine centerline is maximum (cranking the wheel for a hard turn). Since torque and current are proportional, shouldn't the torque constant not vary unless it's due to coil heating or current limiting? I'm seeing the Kt at max. when the wheel is first being turned.





RE: Why does Kt vary with torque?
Torque = Kt x Current
It should be (approximately) constant vs. angle.
As long as you are controlling the current, the only reason torque will change with temperature is that Neo RE magnents lose flux with increaing temperature (it recovers when they cool).
Is your Trap commutation actually working?
RE: Why does Kt vary with torque?
RE: Why does Kt vary with torque?
RE: Why does Kt vary with torque?
Second, torque constant may have tolerance +/-10% from Data Sheet.
Third, pay attention how motor manufacturer define torque constant - for trap or sine commutation?
And finally why don't use standard torque sensor directly? - see Futek for example.
RE: Why does Kt vary with torque?
RE: Why does Kt vary with torque?
1. The PWM signal modulates voltage -- the current is only an indirect result of the modulated voltage. Are your values for torque/current based on actual current measurements, or are you assuming the current is proportional to the modulated voltage?
2. A lot of motors can be driven into magnetic saturation at high current levels. A few years ago, I ran into a brushless DC motor whose Kt at 90% of rated (instantaneous) current limit was about 20% of the Kt at low current levels. This caused all sorts of problems in the application, because the users were expecting a constant Kt.
Curt Wilson
Delta Tau Data Systems