Rockwell Automation 7000L PowerFlex Medium Voltage AC Drive (C Frame) - Classic Control - Manual (Page 309)

Chapter
5
7000 “C” Frame 7000L-UM300I-EN-P – June 2013
PowerFlex 7000 Functional Description
Introduction
The PowerFlex 7000 is an adjustable speed ac drive in which motor
speed control is achieved through control of the motor torque. The
motor speed is calculated or measured and the torque is adjusted as
required to make the speed equal to the speed command. The motor
and load determine the stator frequency and the drive synchronizes
itself to the motor. This is in contrast to the volts/hertz ac drive in
which the drive determines the stator frequency and does not attempt
to synchronize its output to the motor.
The method of control used in PF7000 is known as direct rotor flux
oriented vector control. The term rotor flux vector control indicates
that the position of the stator current vector is controlled relative to
the motor flux vector. Direct vector control means that the motor
flux is measured, in contrast to the indirect vector control in which
the motor flux is predicted. In both control methods, the stator
current (I
s
) is split into flux producing component (I
sd
) and an
orthogonal torque producing component (I
sq
) which are controlled
independently. The aim of vector control is to allow a complex ac
motor to be controlled as if it were a simple dc motor with
independent, decoupled field and armature currents. This allows the
motor torque to be changed quickly without affecting the flux. For
typical Medium Voltage motors the rotor time constant is in the
range of seconds and therefore the flux cannot be changed quickly.
The PowerFlex 7000 drive can be used with either induction
(asynchronous) or synchronous motors. Synchronous motor drives
are identical to induction motor drives except for the addition of a
current regulated field supply to the synchronous drive. The features
that are unique to a synchronous motor drive are confined to the flux
control function and the encoder option.