Variable Frequency Drives (VFDs) are widely used in industrial as well as marine industry automation. They are used to control the speed of the motor, optimize the energy consumption, and increase the efficiency of the system. However, the most confusing problem faced by engineers as well as technicians working with these drives is the overload trip of the VFD, despite the fact that the motor has been sized adequately.
The truth of the matter is, the motor has been sized adequately, but there are other factors at play. There are other factors affecting the operation of the VFD.
What Does “Overload Trip” Mean in a VFD?
An overload trip in Variable Frequency Drive can be described as a safety device whereby, once the Variable Frequency Drive overload trip is activated, the Variable Frequency Drive freezes whenever the motor continues to draw current beyond its maximum safe value. In fact, this indicates that the motor is being overworked and stressed, and this, in the long run, may result in the motor overheating and eventually breaking down. The Variable Frequency Drive frequently monitors some parameters, including current and time, and it makes use of a thermal model to calculate the motor’s temperature. The Variable Frequency Drive freezes if the motor continues to be overloaded for a long time.
Key Reasons Why VFDs Trip on Overload:
1. Incorrect Motor Data in VFD Settings:
If the VFD is not programmed with the correct motor nameplate information such as rated current, voltage, frequency, and RPM, it cannot compute the load conditions. The VFD may then mistakenly sense an overload even if the motor is running normally.
2. Acceleration Time Too Short:
If the acceleration time is too short, the motor is asked to start up too quickly. The high inrush current may exceed the VFD's allowable current and may cause it to mistakenly sense an overload condition. The high inrush current may also cause the motor to overheat and fail prematurely.
3. High Inertia Loads:
High inertia loads such as large fan motors and conveyor motors have high start and stop requirements. The high inertia of the motor may cause it to draw too much current during start-up and may cause the VFD to mistakenly sense an overload.
4. Mechanical Issues in the System:
The system might be affected by various issues like misalignment, worn out bearings, friction, or blockage of equipment which will result in the higher resistance of the system. As a result, the motor will have to provide more power hence consuming more current that might lead to the VFD tripping due to overload.
5. Cooling and Environmental Conditions:
The system might be placed in an inadequately ventilated, hot ambient, or dusty environment that can cause overheating of the motor or VFD. This is a straightforward reason for the overload trip as overload protection is generally temperature-based.
6. Incorrect Overload Protection Settings:
VFD's overload protection feature can be triggered at very low levels of setting, which can cause it to trip even when the machine is operating normally. This might result from the incorrect setting of the thermal or current limits.
Don't interpret overload trips of VFDs in industrial and marine automation only as instances of motor sizing problems. On the contrary, these events are useful signals to perform a system-wide assessment of the drive system, including VFD settings, motor parameters, load conditions, and environmental factors. Issues leading to frequent trips might occur even when the motor is properly sized, if for instance, the motor data aren't configured correctly or the acceleration time is too short, the load inertia is high, mechanical resistance exists, cooling is insufficient, and overload protection settings are not right. A properly configured Variable Frequency Drive together with good upkeep and precise parameter tuning leads to higher energy efficiency, reliable motor control, and lesser downtime. Initially, with a system perspective and a good understanding of the interactions between current, thermal behavior, and load dynamics, engineers can eliminate unnecessary overload trips and thus, achieve smooth and efficient operation of their automation systems.
