138 MOTOR DRIVE ALARMS
A continuous comparison is made by the controller of the speed feedback and armature voltage feedback. If
the difference is greater than the value set by 8.1.2 MO TOR DRIVE ALARMS / Speed feedback mismatch
tolerance PIN 172, the alarm is operated. If armature voltage feedback is selected, then the speed feedback
alarm is automatically suppressed.
If 103)FLD WE A K EN ABLE is enabled, then the controller suspends the speed-to-volts comparison in the
field-weakening region w here the volts are clamped to a maximum value. Instead, when in the field-
weakening region it checks w hether the speed feedback is below 10% of full speed. If so, the alarm will
operate. This means that it is not practical to start field weakening belo w 10% of full speed i.e. 10 : 1 range.
The automatic s witch to A VF feature allows continued running, although at the lower accuracy level of
Armature Voltage feedback. The A V F remains the source of feedback until the next STOP / ST ART
sequence. The original feedback source is then restored and the alarm reset to allo w auto A VF protection
once again. It may be necessary to reduce the 172)SPEED TRIP T OL to about 15% if a smooth transfer to
auto A V F is required. However, if the threshold is too low then an unwarranted transfer may occur during
speed transients.
There is a flag on hidden PIN 703 which warns of a speed mismatch after the normal delay time. This flag is
reset by a STOP command. It is suggested that the flag is configured to a digital output to provide a w arning
that the auto A VF has occurred.
The speed feedback mismatch alarm is normally triggered by failure of the feedback mechanism in one of the
following w ays:-
1) Disconnection of wiring.
2) Fai lure of the tachogenerator or encoder.
3) Fai lure of the tachogenerator or encoder mechanical coupling.
Note. Alarm delay time: 0.4 secs to TRIP, 0.2 secs to automatic A V F switch.
WARNING. The protection afforded in fi eld weakening mode is limited to total feedback loss only. This is
because the speed / AVF relationship is not maintained in field weakening mode. If a partial loss of feedback
occurs the motor will run to excessive speed. When the field has been completely weakened and is at its
minimum level, the armature overvoltage trip will come into operation. This may only occur at a dangerous
speed. It is therefore recommended that a mechanica l device be utilised to protect against this possibility.
Correct setting of 110)MIN FIELD CURRENT should ensure that the overvolts TRIP occurs just above the
maximum operating speed.
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