Motor Seizing on Startup

[Nexen]

Hello all,

In my facility we have multiple 3HP 460V motors that are being controlled by VFDs which drive conveyor belts. The drives are randomly faulting for a motor overload when commanded to start by the PLC. My team have been unable to find any mechanical or electrical issues with the systems. When the issue occurs we can hear the motor humming, with zero rotation of the belt/motor and the VFD is in a current limit mode. Normally the motors are running at 50 to 75% of their rated current.

In the fault data captured by the drive we can see that the drive is outputting a negative output frequency of about -3Hz (commanded frequency is +57Hz). Using the drive software we can see that the output frequency during the acceleration ramp will randomly drop below zero before ramping to the commanded frequency. Sometimes it gets stuck in the negative values and produces the symptoms described above until it overload faults. The manufacturer of the VFD recommended an autotune and that seems to have resolved the issue.

Could anyone possibly explain what is going on with the motor or VFD? The manufacturer wasn't much help for an explanation.

 

[robertmee]

What brand/model drive? Do you have flying start disabled? Do the conveyors have backstops on the gearboxes or electric brakes?

 

[Nexen]

What brand/model drive? Do you have flying start disabled? Do the conveyors have backstops on the gearboxes or electric brakes?

Schneider Electric ATV340. I will have to check on flying start when I'm back in the office. There are no backstops or brakes on the gearboxes.