Standby Field - Parker 590+ DRV Product Manual

It is the over-voltage of the back-EMF that provides the error which drives this controller to weaken the field, and the over-voltage occurring during
ramp through base speed is dependent on the rate of ramp. If the over-voltage is excessive, then it can be reduced most simply by switching to the
ADVANCED field weakening mode. In the STANDARD mode, provide advanced notice of the overvoltage, in order to reduce it, by employing the
lag/lead filter applied to the back-emf measurement. This filter is disabled by default through its equal lag and lead time-constant settings, increase the
lead time-constant to provide advance notice of weakening to the controller.
Notes on field weakening controller usage:
1. The use of the back-EMF filter should be limited to 3:1 ratio of lag to lead time-constants, and the field current loop and field
weakening controller may need to be de-tuned, in order to maintain overall loop stability.
2. The gain-limit of the PI controller may also be adjusted in order to optimise the balance between transient and static back-EMF over-
voltage. Lower lag time-constants and lower dc-gains result in more static back-EMF over-voltage, but allow for more back-EMF
filter advance during speed ramps. The ratio of lag to lead time-constants should be typically maintained above 10 to avoid significant
dc over-voltage on the back-EMF.
ADVANCED Mode
The ADVANCED mode of the field weakener offers the following advantages over the STANDARD mode.
1. A feedforward control is applied in addition to the gain-capped PI controller. This term, which compares the actual speed feedback to the
calculated base speed, estimates the required field weakening. The use of this control term significantly reduces the overvoltage on transition
through base speed, prior to the application of any lead-lag compensation. Transitions through base speed can be more rapid without
overvoltage as a result. In addition, false weakening of the field is eliminated for speed transients just below base speed, if the lead-lag back-
emf filter is left disabled.
2. The back-emf control loop is gain compensated for reducing field level. Motor back-emf is related directly to the motor speed, and to the
motor flux level. As a result the transfer gain from field current to back-emf is directly related to motor speed. An adaptive gain element is
included in the ADVANCED mode that increases back-emf loop gain below full speed, and this allows improved control performance at the
field weakening boundary whilst maintaining stability at full speed.
3. The speed control loop is gain compensated for reducing field level. Motor torque is related directly to both armature current and motor
flux level. An adaptive gain element is included in the ADVANCED mode that increases speed loop gain below full field, and this maintains
speed control performance into the field weakened operating region. Note that the lag-lead back-emf filter can still be applied in the
ADVANCED mode to further improve voltage control during rapid excursions into field weakening.

Standby Field

When the armature current gets quenched, a timer starts timing-out and after a certain delay (FLD. QUENCH DELAY) it will either quench the field
totally (FLD. QUENCH MODE = QUENCH) or will reduce it to 50% of the current or voltage setpoint (FIELD QUECH MODE = STANDBY). This
applies to both current and voltage modes.
DC590+ DRV Series DC Digital Drive
5-7
Control Loops