Rockwell Automation 1557 MEDIUM VOLTAGE AC DRIVE - Manual (Page 19)

DESCRIPTION OF OPERATION 5-1
1557-UM050F-EN-P –June 2013
Introduction
The Bulletin 1557 is an adjustable speed ac drive in which motor speed control is achieved through
control of the motor torque. The motor speed is measured and the torque is adjusted as required to make
the measured speed equal to the speed command. The stator frequency is determined by the motor and
load and the drive synchronizes itself to the motor. This is in contrast to the more common 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 is known as direct vector control. The term 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 more common indirect vector control in which
the motor flux is predicted. In both cases, the stator current is resolved into flux producing and torque
producing components which are controlled independently. The intent is to achieve performance similar
to a dc machine, in which the torque can be changed quickly without affecting the flux. The flux cannot
be changed quickly because of the rotor time constant, which is in the range of seconds for large
machines.
The following description of operation is organized in the same functional blocks as the control block
diagram (Figure 5.1). Most of the functions are the same as those found in any adjustable speed ac or dc
drive. The features which are unique to a direct vector controlled drive are confined to the motor model
and flux control functions.
The Bulletin 1557 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 drive are
confined to the flux control function and the encoder option.