Sporadic VFD overcurrent faults
Application: “Grit Separator”: VFD running a 1.5hp motor which turns a gear drive. The gear drive rotates a torque tube with paddles that rotates in a conical shaped pit in the headworks of a wastewater treatment plant to separate grit from the influent flow passing through the chamber and keep the grit fluidized. The Grit Separator rotates continuously, however every two hours a grit pump pumps the collected fluidized grit to a grit classifier which separates the liquid and grit, then dumps the grit onto a conveyor belt. The grit pump and classifier run cycle last for 20 minutes. The system collects about one 5gal bucket worth of grit every two hours.
Loading of the motor due to the collected grit is minimal. (5 gal worth every two hours)
The VFD overcurrent shutdowns are random (have occurred both day or night) and sporadic (3-8 days between).
Feeders between the VFD and Motor are 12 awg, 3 phase with ground, approx. 65’ run, local disconnect at the motor with interlock to prevent from VFD running if disconnect is opened.
The feeders, motor and disconnect have been meggered and check out.
The electrician ran the VFD feeds in the same conduit and the grit pump and grit classifier. (Three, 3 phase feeders in the same conduit).
The motor typically runs at 1.2-1.3 amps and the VFD is set to trip at 2.3 amps. The VFD runs continuous 24/7 at 17.5Hz, so there are no accelerations or decelerations. The amperage remains steady while running (1.2-1.3 amps observed, see below). The motor also has overload protection, however the thermal switch has never tripped out.
The equipment vendor has replaced the VFD and motor, as opposed to legitimately troubleshooting it, and we are still getting the same overcurrent fault.
There is an existing system adjacent to ours;. same manufacture and set up, which has never experienced a VFD overcurrent fault.
The VFD is housed inside a control panel, so unless you have the control panel door open you cannot observe the VFD. I have set up my office inside the control room this past week, with the control panel door open and set the VFD display to show amperage and have been observing the system trying catch any abnormalities. So far I have not been able to catch it fault while the door has been open. The amperage remains steady even when the grit pump and classifier cycle their power on and off, as well as other equipment being fed out of the same MCC. I thought I might see instances where maybe the amperage would rise up on occasion, even if it did not reach the 2.3amp VFD overcurrent fault set point. Instead it has been steady as can be, so I am not seeing a condition developing even though it might not have been enough to trip the VFD. I have to believe condition must be limited to the actual trips and must occur quickly. With the control panel door closed (not being able to observe the amperage) I have seen the trip occur and was able to cycle power to the VFD, clearing the fault and restart the system back up quickly, expecting that it may again trip out, but it did not and continued to run for the next three day before tripping again.
The electrician insists even though he ran all three feeds in the same conduit this is not causing any issues because we are not observing any amperage spikes when the system cycles. I am not sure if combination of interaction between the three feeds and whether harmonics from the VFD feed and/or EMI might produce conditions for a perfect storm to cause a random current spike every 3-8 days or not.
I just not sure where to go from here. Any thoughts?
