Omron C200H-MC221 Operation Manual page 38

Motor Runaway
Runaway Due to
Disconnected Wiring
1, 2, 3...
24
Runaway can occur not only from reversed wiring of phases A and B of the feed-
back inputs, but also from reversed wiring of the speed control voltage and the
ground lines (crossed dotted lines at 2 in the figure above).
The servomotor runs out of control not only when the position loop is not correct-
ly formed but also when the position loop is not interrupted due to disconnected
wiring.
Amp
Error
counter
Direction
discrimination
circuit
1. Wire breakage while the servomotor is rotating:
While the servomotor is rotating, the speed control voltage is not 0 V be-
cause of the signal from the error counter. If the feedback line is broken, no
feedback signals will be given to the error counter and the speed control
voltage remains unchanged from the value existed before the line break-
age, causing motor runaway.
2. Wire breakage while the motor is stopped:
If the feedback line is broken while the servomotor is stopped and correct
feedback signals cannot be returned, the speed control voltage remains at
zero without changing. Therefore, the servomotor also remains stopped. In
fact, however, the motor may move in one direction without stopping.
This is caused by a discrepancy between the 0 V of the MC Unit's control voltage
and the 0 V of the servodriver's voltage input.
Speed control
voltage (0 V)
When the two 0 voltages do not match, an electric potential difference is gener-
ated, resulting in a false control voltage. This in turn causes the servomotor to
move in one direction without stopping.
In order to prevent this, repair the wiring or adjust the 0 V of either the MC Unit or
the servodriver so that the 0 V levels match.
Control voltage
Servodriver
Phase A feedback
Phase B feedback
Broken lines
Servodriver
Difference in electric
voltage input (0 V)
potential
Section 3-2
Servomotor
Encoder