Induction And Synchronous Motor Control - Siemens 6SR41 series Product User Manual

The overall control strategy is similar to Open Loop Vector Control, except for the flux
regulator implementation as shown in Figure "Block Diagram of Vector Control Algorithms for
Induction and Synchronous Motor Control". For synchronous motors, the flux regulator
provides two current commands, one for the field exciter current, and another for the
magnetizing component of stator current.
With synchronous motor control, scanning the motor frequency to determine motor speed is
completely avoided. The control uses information from the rotor-induced speed voltages on
the stator to determine rotor speed. The drive begins (in the Magnetizing State) by giving a
field current command, which is equal to the no-load field current setting to the exciter. This
lasts for a time equal to the programmable flux ramp time that is entered through the menu
system.
After this period of time, the drive goes into the run state. In most cases, the regulator in the
field exciter is slow, and the drive applies magnetizing current (through the stator windings)
to assist the exciter in establishing rated flux on the motor. At the same time, the speed
regulator commands a torque-producing current to accelerate the motor to the demanded
speed. Once the field exciter establishes the required field current to maintain flux in the
motor, the magnetizing component of stator current reduces to zero. From this point onward,
the drive provides torque-producing current (for acceleration or deceleration) that is in-phase
with the drive output voltage. In other words, under steady state conditions, unity power
factor condition is automatically maintained at the drive output. The field current command is
provided to the field exciter with the use of an analog output signal. Other differences
between SMC and OLVC are summarized below.
● The motor no-load current parameter represents the Field no-load current value in SMC.
● With SMC, the flux loop gains are slightly lower than with OLVC.
● Spinning Load should always be enabled with SMC.
● The drive magnetizing current regulator uses only the proportional gain for the flux
Product User Manual
Operating Instructions, Version AE 12/2009, A5E01454341C
exciter.
Application and Operation
6.2 The Control Modes
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