590 Series DC Digital Drive Control Loops
5-7
Field Control
Set-Up Notes
The setting of the P + I gains for the current controller is done manually in much the same way as described in Chapter 4: “Current Loop - Manual
Tuning”, and one convenient way is to switch several times from "quench" to "standby" mode and observe the current response 0 à 50% for rise time
and overshoot.
Current Control
The field current loop can accept a demand directly from the plant and/or an outside field weakening loop and forms the error signal which is the
difference between demand and feedback. The error signal is fed into a P + I compensator which produces the output of the field loop, i.e. the field
firing angle signal.
The firing angle signal is translated into a certain time delay from the mains zero cross point (obtained via the same Phase-Lock-Loop as for the
armature) and this results into a firing command being issued to the field bridge every 1/2 of a mains cycle in steady-state.
Voltage Control
This offers the facility of an open-loop voltage control for motors which do not provide in the nameplate the field current rating. The field voltage is
controlled by the specified FLD. VOLTS RATIO which defaults to 90.0%. This is the maximum dc Volts that can be obtained for a given ac RMS
input in a single-phase rectifier, i.e. 370V dc for 415V ac supply. The specified ratio determines directly the firing angle at which the controller
operates and therefore the thermal effects on the field resistance as well as mains voltage variations are not compensated for. It is also worth noting
that in this mode the field overcurrent alarm is not active (since there is no current scaling) and therefore this mode is not recommended for use with
supplies much greater than the field voltage rating.
Field Weakening
Motor field weakening is used to extend the speed region of the motor above its base speed (the motor speed resulting at rated armature voltage, rated
armature current and rated field current), in a constant power mode of operation (motor torque reducing with increasing speed).
Note that the motor should be rated for field-weakened operation, in terms of rotational speed and reduced field current, before utilising this mode.
The drive includes a field weakening loop that, above base speed, can control the field current demand to the correct level required to maintain motor
back-EMF at a pre-defined level.
NOTE Field weakening is not possible when running with Armature Volts feedback. Although field weakening can be “Enabled” in this
instance, a software interlock clamps the field demand at 100% and will not allow the field weakening to reduce it.
When the back-EMF measurement is higher than the MAX VOLTS setting (default 100%) the excess voltage is presented to the field weakening gain-
limited PI controller as an error, and this controller reduces the field current demand accordingly.
The gain-limited controller is tuned as follows:
1. Ensure that the armature current, speed and field current loops are correctly tuned.
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