VSD Overcurrent Limiting Setup
Requirement:
In certain scenarios, when pumping water (due to pressure changes in the system), it could cause motors to draw high currents when running at full speed until the pressure stabilises as expected. This typically happens in areas where pivot irrigation systems are located on steep up and downhill slopes, and also happens in scenarios where irrigation systems are being adjusted (changing or opening additional irrigation sections causing sudden drops in pressure etc.). Running at these high currents is obviously not desired and could damage the motors. Normal drive overcurrent protection features will assist in stopping the motor when the current is too high (for too long), but this will stop the motor and the drive will show an error. Ideally in these scenarios one would rather prefer the drive to slow the motor down (reduce the running frequency to a speed where the current returns to an acceptable value) while pressure builds up and then speeds up again.
Solution/s:
- VSDs offer a current limiting feature which can be adjusted. Adjust parameter A2-00 (Current Limit Level – default = 150%) to a lower value (~120%) which will limit the frequency of the VSD when the current rises too high (see U0-19 monitoring parameter to view impact/adjustment). Please note that this feature is not applicable for Solar Drives (please alternative Solar Drive solution HERE). Please see Current Limiting Explained section later on.
- For scenarios where a PID is used for constant water pressure setups, one could also consider setting the PID Initial Value parameter C0-16 (FA-21 for Solar Drives – set target Hz as % of the upper limit frequency) and the PID Initial Value Holding Time parameter C0-17 (FA-22 for Solar Drives) to limit the frequency for a period of time before speeding up (to give the pipe time to fill up and pressure to rise before speeding up). This typically only helps for scenarios where the high current issue occurs during initial start-up (not for scenarios where the current rises during full speed operation such as when changing or opening additional irrigation sections causing sudden drops in pressure or pivot systems operating on steep up and downhill slopes).
- For scenarios where the issue only occurs during initial start-up, one could also consider using the Simple PLC feature to control the start-up procedure. Please see the FAQ page regarding this for VSDs HERE and for Solar Drives HERE.
Current Limiting Explained:
Relevant Parameters:
- A2-00 (Current Limit Level – default = 150%): This value is a percentage of the set motor rated current (d0-02). This current value (when exceeded) is used as the starting point to start the overcurrent stall suppression action.
- A2-01 (Current Limit Selection – default = 1): To activate/deactivate the feature (0 = Invalid; 1 =Valid).
- A2-02 (Current Limit Gain – default = 20): This parameter is used to adjust the over current suppression capacity of the drive. The larger the value is, the greater the over current suppression capacity will be. In condition of no over current occurrence, should set A2-02 to a small value. For small inertia loads the value should be small, otherwise, the system dynamic response will be slow. For large inertia loads the value should be large, otherwise, the suppression result will be poor and over current fault may occur. If the current limit gain is set to 0, the over current stall function is disabled.
- A2-03 (Compensation Factor of speed multiplying current limit – default = 50%): Reduce the high-speed overcurrent stall action current, which is invalid when the compensation coefficient is 50%, and the action current in the weak magnetic zone corresponds to 100% of the recommended setting value of A2-00.
Explanation:
When the output current exceeds the value set in A2-00 (during acceleration, constant running, or deceleration), the current limit feature is enabled, and the output frequency will start to drop until the output current drops to below the current limit level – after that the output frequency will start to rise again attempting to reach the target frequency again.
The Current Limit Level Above Rated Frequency = (fs/fn) x k x LimitCur
- fs: Running Frequency
- fn: Rated Motor Frequency (d0-02)
- k: Compensation Factor of speed multiplying Current Limit (A2-03)
- LimitCur: Current Limit Level (A2-00)
