5-77
H12
Instantaneous Overcurrent Limiting (Mode selection)
H12 specifies whether the inverter invokes the current limit processing or enters the
overcurrent trip when its output current exceeds the instantaneous overcurrent limiting level.
Under the current limit processing, the inverter immediately turns OFF its output gate to
suppress the further current increase and continues to control the output frequency.
Data for H12
Function
0
Disable
An overcurrent trip occurs at the instantaneous overcurrent limiting
level.
1
Enable
The current limiting operation is effective.
If any problem occurs when the motor torque temporarily drops during current limiting
processing, it is necessary to cause an overcurrent trip (H12 = 0) and actuate a mechanical
brake at the same time.
The similar function is the current limiter specified by F43 and F44. The current
limiter (F43, F44) implements the current control by software, so an operation delay
occurs. When you have enabled the current limiter (F43, F44), also enable the
instantaneous overcurrent limiting with H12 to obtain a quick response current
limiting.
Depending on the load, extremely short acceleration time may activate the current
limiting to suppress the increase of the inverter output frequency, causing hunting
(undesirable oscillation of the system) or activating the inverter overvoltage trip
(alarm
0u
). When specifying the acceleration time, therefore, you need to take into
account machinery characteristics and moment of inertia of the load.
When the instantaneous overcurrent limiting is enabled, the motor output torque could
drop. For driving elevating machinery which could cause a serious problem with a drop of
the motor output torque, therefore, disable the instantaneous overcurrent limiting. Note
that disabling it will cause an overcurrent trip when a current exceeding the inverter
protection level flows, so secure the protective coordination using a mechanical brake.
An accident could occur.
