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4 FAILSAFE OF THE SYSTEM WHICH USES THE DRIVE UNIT
When a fault occurs, the drive unit trips to output a fault signal. However, a fault output signal may not be output at a drive unit
fault occurrence when the detection circuit or output circuit fails, etc. Although Mitsubishi assures best quality products,
provide an interlock which uses drive unit status output signals to prevent accidents such as damage to machine when the
drive unit fails for some reason and at the same time consider the system configuration where failsafe from outside the drive
unit, without using the drive unit, is enabled even if the drive unit fails.
(1) Interlock method which uses the drive unit status output signals
By combining the drive unit status output signals to provide an interlock as shown below, a drive unit failure can be
detected.
(2) Backup method outside the drive unit
Even if the interlock is provided by the drive unit status signal, enough failsafe is not ensured depending on the failure
status of the drive unit itself. For example, when the drive unit CPU fails, even if the interlock is provided using the drive
unit fault signal, start signal and RUN signal, there is a case where a fault signal is not output and RUN signal is kept
output even if a drive unit fault occurs.
Provide a speed detector to detect the motor speed and current detector to detect the motor current and consider the
backup system such as checking up as below according to the level of importance of the system.
1)Start signal and actual operation check
Check the motor running and motor current while the start signal is input to the drive unit by comparing the start signal to
the drive unit and detected speed of the speed detector or detected current of the current detector. Note that the motor
current runs as the motor is running for the period until the motor stops since the drive unit starts decelerating even if the
start signal turns OFF. For the logic check, configure a sequence considering the drive unit deceleration time. In addition,
it is recommended to check the three-phase current when using the current detector.
2)Command speed and actual operation check
Check if there is no gap between the actual speed and commanded speed by comparing the drive unit speed command
and detected speed of the speed detector.
No. Interlock Method Check Method Used Signals Refer to Page
1)
Drive unit protective
function operation
Operation check of an alarm contact
Circuit error detection by negative logic
Fault output signal
(ALM signal)
Refer to Chapter 4 of
the Instruction
Manual (Applied).
2) Drive unit operating status Operation ready signal check
Operation ready signal
(RY signal)
Refer to Chapter 4 of
the Instruction
Manual (Applied).
3) Drive unit running status
Logic check of the start signal and
running signal
Start signal
(STF signal, STR signal)
Running signal (RUN signal)
Refer to Chapter 4 of
the Instruction
Manual (Applied).
4) Drive unit running status
Logic check of the start signal and output
current
Start signal
(STF signal, STR signal)
Output current detection signal
(Y12 signal)
Refer to Chapter 4 of
the Instruction
Manual (Applied).
Drive unit
Controller
System failure
To the alarm detection sensor
Sensor
(speed, temperature,
air volume, etc.)