9-2
Control Loops
590+ Series DC Digital Converter
Bridge Changeover Delay
The bridge changeover "dead-time", i.e. time interval of zero current, is programmable from 1 to
1500 (via Reserved Menu) with a default value of 1.
For values from 1 to 6:
The delay can be set at multiples of 1/6 mains period, i.e. max. 6 x 3.33 = 20ms at 50Hz. This is
relevant for use with large power converters where it is advisable to allow more time for
snubber currents to subside before reversal is enabled. It is also relevant for motors with very
large armature inductance where zero current detection is more sensitive and therefore a "factor
of safety" in the bridge changeover delay is advisable.
For values from 7 to 1500:
The delay corresponds to 7 x 1.33
P
s up to 1500 x 1.33
P
s = 2ms maximum.
Manual Tuning
Note:
This procedure is rarely used or required, if possible use Autotune.
You may need to perform a manual tuning as Autotune does have two limitations:
1. It requires the field to be switched off and therefore the shaft will need clamping when
autotuning a permanent-magnet motor or very rarely with a wound-field motor of relatively
high permanent magnetism.
2. Part 1 of Autotune determines the discontinuous to continuous boundary level, i.e. the
average value at which the armature current becomes "just" continuous. This is achieved by
automatically disabling the field and advancing the firing angle at small steps until the slope
of the current "envelope" changes substantially indicating continuous region of operation.
Part 2 of Autotune applies a step change in the current demand within the continuous region
as determined by Part 1. When the current feedback approaches the final settling value
within 1 to 2 steps, the autotune function terminates and returns the "FIELD ENABLE" to its
previous state. The P & I gains and the value of discontinuous boundary current should then
be saved.
If the value of boundary current (Part 1) is very high (larger than 150% or so), then the
Autotune Part 2 step change will be in the region above 200% which might result in
overcurrent trip. In this case it is advisable to set the I gain to a large enough value (typically
10) to give fast response throughout the discontinuous region, a low value for the P gain
(typically 1, not important since there is no effective armature time constant in the
discontinuous region to compensate for) and finally eliminate the adaptive mode by setting
"Discontinuous" to zero. At the same time though, one must disable the Missing Pulse alarm;
this is activated when the load current is above the "Discontinuous" level and in this case it
would give erroneous trips if left enabled. In order to disable this alarm the special "super-
password" reserved for SSD Drives personnel needs to be entered. Next in the "Reserved"
menu, which will then appear as a submenu of "SYSTEM", a parameter called SYS
HEALTH INHIB should be set to the hexadecimal value 0x0002.
The above suggestion assumes that the current limit will prevent the motor from operating in the
continuous region, i.e. above 150% in the example above. If this is not the case, as for example
when the current limit is set at 200%, then a manual tuning will be necessary.
Set the DISCONTINUOUS parameter to the correct value by disabling or disconnecting the
field, set the current limit to zero and start the drive. Gradually increase the current limit
observing the current feedback waveform (see Diagnostics below) on an oscilloscope beam.
When the pulses "just come together", with no zero interval between them, read the value of
current limit (or indeed current demand) and set the DISCONTINUOUS parameter to this value.
If this value is very high (above the current limit), then it should be set to zero and follow the
suggestion in
2
above. In this case the drive will not perform any adaption in the discontinuous
region, so some loss in performance may be noticed in the current loop response.