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relationship between rotor flux, Ke, Kt

bin gao1
Intellectual 270 points

Dear Sir,

From user manual(spruhj1f, Page 396), the rotor flux (which is also equal to the back-EMF constant (Ke) in SI units). Also,

Mtr(s)=3/4*P*rotor flux; ??? since it is the transfer function between q-axis current and motor torque is: Is that Kt, that is Kt=3/4*p*rotor flux, since rotor flux is Ke in SI units, it means Kt = 3/4*P*Ke ??? Is my understanding right ?

However, from another equation, I get

Kt=Ke/104.7 where Ke unit is V/krpm, Kt unit is nm/A

It seems there two Kt is not equal ???

Thank you

  • 0
    TI__Expert 4760 points
    Hello bin gao1,
    If you are using a PMSM, and Ke is in units of peak volts line-to-neutral per radian per second, and you are using the amplitude invariant form of the Clarke Transform, then yes...Ke will be numerically equal to the rotor flux in webers. Using these same assumptions, the motor torque (in nm) will also equal Iq * 0.75 * P* Ke.
    In your other expression for Kt, I suspect they are using different units for voltage. It is very common to instead represent Ke using voltage in units of peak volts line-to-line, or voltage RMS line-to-neutral. You must also convert mechanical RPM correctly into electrical radians per second. If not, then that could be another reason for the discrepancy.
    Regards,Dave