FAILSAFE SYSTEM WHICH USES THE INVERTER
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FAILSAFE SYSTEM WHICH USES THE INVERTER
When a fault is detected by the protective function, the protective function activates and output a Fault (ALM) signal. However, a fault signal may not be output at
an inverter's fault occurrence when the detection circuit or output circuit fails, etc. Although Mitsubishi Electric assures the best quality products, provide an
interlock which uses inverter status output signals to prevent accidents such as damage to the machine when the inverter fails for some reason. Also, at the same
time consider the system configuration where a failsafe from outside the inverter, without using the inverter, is enabled even if the inverter fails.
Interlock method which uses the inverter status output signals
By combining the inverter output signals to provide an interlock as shown below, an inverter failure can be detected.
Backup method outside the inverter
Even if the interlock is provided by the inverter status signal, enough failsafe is not ensured depending on the failure status of the inverter itself. For example, if an
inverter CPU fails in a system interlocked with the inverter's fault, start, and RUN signals, no fault signal will be output and the RUN signal will be kept ON
because the inverter CPU is down.
Provide a speed detector to detect the motor speed and current detector to detect the motor current and consider the backup system such as performing a check
as below according to the level of importance of the system.
• Start signal and actual operation check
Check the motor running and motor current while the start signal is input to the inverter by comparing the start signal to the inverter and detected speed of the
speed detector or detected current of the current detector. Note that the current is flowing through the motor while the motor coasts to stop, even after the
inverter's start signal is turned OFF. For the logic check, configure a sequence considering the inverter's deceleration time. In addition, it is recommended to check
the three-phase current when using the current detector.
• Command speed and actual operation check
Check for a gap between the actual speed and commanded speed by comparing the inverter's speed command and the speed detected by the speed detector.
Interlock method
Check method
Used signals
Refer to
Inverter protective function
operation
Operation check of an alarm contact.
Circuit error detection by negative logic.
Fault (ALM) signal
Chapter 5 of the FR-A800
Instruction Manual (Detailed)
Inverter operating status
Operation ready signal check.
Operation ready (RY) signal
Chapter 5 of the FR-A800
Instruction Manual (Detailed)
Inverter running status
Logic check of the start signal and running
signal.
Start signal (STF signal, STR signal)
Inverter running (RUN) signal
Chapter 5 of the FR-A800
Instruction Manual (Detailed)
Inverter running status
Logic check of the start signal and output
current.
Start signal (STF signal, STR signal)
Output current detection (Y12) signal
Chapter 5 of the FR-A800
Instruction Manual (Detailed)
Inverter
Controller
System failure
To the alarm detection sensor
Sensor
(speed, temperature,
air volume, etc.)
