Rockwell Automation 1557 MEDIUM VOLTAGE AC DRIVE - Manual (Page 38)

5-20 DESCRIPTION OF OPERATION
1557-UM050F-EN-P – June 2013
Motor Model
The function of the motor model block is to calculate the motor current, voltage, flux, torque, power, and
slip frequency.
After the three phase to two phase conversion described in the machine converter feedback section, the
resulting stator current feedback signals (alpha and beta) are converted from analog to digital form. They
are then transformed from the stationary reference frame to the rotor flux reference frame that rotates at
stator frequency. This transformation produces two components of stator current which, ignoring
harmonics, are dc quantities in the steady state. The two components of stator current are the direct axis
current (Isd) in phase with the rotor flux, and the quadrature axis current (Isq) which is displaced 90
degrees from the rotor flux. The stator current magnitude is calculated by taking the square root of the
sum of the squares of the two components of stator current.
The two components of stator current are used along with parameters Rotor Time Constant and
Magnetizing Inductance to calculate the slip frequency and the rotor flux. Slip frequency is used in the
calculation of the motor speed. The rotor flux calculated from current is used in the low speed range on
drives fitted with a tachometer. The rotor flux determined from voltage and current that is described in the
machine converter feedback section is used in the high speed range and is not usable at very low
speeds. Without a tachometer, the flux feedback selection is permanently set to the flux feedback
derived from voltage and the flux feedback derived from current is not used. Therefore, there is no flux
feedback available below about 5 Hz and open loop control is employed as described in the speed control
and flux control sections.
The flux feedback is used along with the two components of stator current, the stator frequency, and
parameters Stator Resistance and Total Leakage Inductance to calculate the stator voltage. The motor
torque is calculated by multiplying the flux feedback by the q-axis current, which is then multiplied by the
speed to determine the motor power. Motor voltage, torque, and power are therefore all calculated rather
than measured quantities.