5-8 Control Loops 590 Series DC Digital Drive
2. Enable field weakening control (FLD. WEAK ENABLE = ENABLE), with analogue tachogenerator, encoder or microtach speed feedback,
correctly installed and configured for extended speed operation.
3. Run the drive and slowly increase the speed demand so that the field is being weakened by the gain-limited PI controller. Change the MAX
VOLTS parameter down and up by 10% to generate field current transients.
4. Alternately increase the P gain (using the dc-gain parameter EMF GAIN) and reduce the integral time-constant (parameter EMF LEAD) until
the loop is correctly tuned (see the Current Loop "Tuning Hints" above).
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 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.
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.
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