Rockwell Automation 1557 MEDIUM VOLTAGE AC DRIVE - Manual (Page 27)

DESCRIPTION OF OPERATION 5-9
1557-UM050F-EN-P –June 2013
Speed Control
The function of the speed control block is to determine the torque current command. The inputs are the
speed reference from the speed ramp, and the stator frequency and slip frequency from the motor model.
Either tach feedback is used, or the slip frequency is subtracted from the stator frequency to determine
the motor speed, which is then made negative if the motor is rotating in reverse. The result is filtered by
an adjustable second order low pass filter to obtain the drive speed feedback.
The speed feedback is subtracted from the speed reference to determine the speed error. Parameter
Speed Reference Step, which is used to demonstrate the step response of the speed loop, is added to
the speed error to form the input to the speed regulator. The response of the speed regulator is
determined by parameters Speed Regulator Bandwidth and Total Inertia. The rate of change of the
speed regulator output is limited to the value specified by parameter Torque Rate Limit. The output of the
speed regulator is clamped to the value specified by parameter Torque Limit Motoring for positive values
and to the value specified by parameter Torque Limit Braking for negative values. The clamped output of
the speed regulator is the torque reference. This is divided by the flux reference to determine Isq
Command, which is the torque component of the stator current command.
Without a tachometer, reliable speed feedback is not available at stator frequencies below about 5 Hz.
The speed regulator is therefore disabled at low speed and the torque reference is determined by open
loop programming. The torque reference changes linearly from the value specified by parameter Torque
Command 0 at zero speed to the value specified by parameter Torque Command 1 at Ramp Speed 1,
and remains constant above this speed if the drive is still in start mode. When tach feedback is enabled
the open loop torque programming is not used.
An estimate of the torque producing current supplied by the motor filter capacitor is calculated from Isq
Command, stator frequency, and parameters Total Leakage Inductance and Filter Capacitor. The
resulting value is called Icq Command and is subtracted from Isq Command to determine Iy Command,
which is the torque component of the dc link current command.