Rockwell Automation 7000A PowerFlex Medium Voltage Drive (A-Frame) - Classic Control - Manual (Page 297)

Functional Description 5-29
7000 “A” Frame 7000A-UM150F-EN-P – June 2013
The slip frequency required to provide the desired flux and torque is
calculated by the motor model. The slip frequency is integrated to
get the slip angle and added to the measured rotor angle to obtain the
flux angle. Indirect control can be used at any speed, but its weakness
is that the calculated slip is sensitive to errors in the motor parameters.
Errors in slip frequency increase the coupling between flux and
torque which adversely affects the stability of the flux control. Since
large motors generally have lower magnetizing current and lower
slip than small motors, they are more sensitive to parameter errors
(i.e. a small error in slip produces a large error in torque and flux).
Because of its salient pole construction, the position of the rotor flux
in a synchronous machine is not arbitrary but is determined by the
physical position of the rotor. A synchronous machine therefore
requires an absolute position encoder instead of an incremental
encoder for indirect vector control. The encoder must also be aligned
with the direct axis of the rotor. To avoid having to physically align
the encoder, an offset angle specified by parameter Encoder Offset
(644) is added to the encoder output to compensate for the difference
between the encoder zero and the direct axis of the rotor. Parameter
Enc Direction (643) is provided to reverse the encoder rotation in
software if it does not match the rotation of the motor. There is no
parameter to specify the encoder resolution; it is inferred from the
number of motor poles.
Synchronous Transfer
Synchronous transfer is an optional feature of the PowerFlex 7000
drive which allows either a single or multi-motors to be transferred
between the drive and a fixed frequency supply in either direction
without stopping and with a very short interruption of power.
Compared to a simple non-synchronous transfer in which power to
the motor is interrupted for a significant length of time, the transient
drop in motor speed is much less with synchronous transfer.
In order to perform a synchronous transfer, a drive output contactor
and a bypass contactor are required as shown in Figure 5.11. The
name bypass indicates that the function of this contactor is to connect
the motor directly to the fixed frequency supply, bypassing the drive.
An additional Voltage Feedback Board (VFB) is used to measure the
bypass voltage on the line side of the bypass contactor. These inputs
are brought in through the machine side Signal Conditioning Board
(SCBM) and used in synchronizing the motor voltage directly to the
bypass voltage resulting in a reliable synchronous transfer. In addition
the measurement of bypass voltage allows certain protection features
to be built in. The synchronous transfer is automatically aborted if
the drive detects an over-voltage or under-voltage or reverse sequence
in the bypass voltage.