AC servo motor burning out 1 phase (motor side) but ok VFD side

[Blikzoggy]

Hi all,

First post, so not sure how this will work out…

I’ve currently got a 3 phase AC servo motor (Lenze brand) at work that seems to be overloading 1 phase (U phase of motor)…
This motor is VFD driven by again a Lenze brand 9300.

The load for the motor is a coupling driven pressure roller, with another identical motor/vfd/load setup above for identical application and use when needed.

As far as I’m aware these motors have always been fitted and always driven by the same VFDs…

During inspection some time ago after a catastrophic motor failure I found all phase terminals meggered to earth and terminal box burnt to hell. So decided to replace for the same identical motor…

Fast forward 6 months or so and on routine maintenance found the U phase of the replaced motor terminal was starting to become brittle and discoloured, tested the motor windings and all balanced at 1.4 ohms and meggered fine between eachother and to earth.

After a thorough inspection of the VFD side terminals, no burning or discolouration was showing and all connections tight.

After checking parameters on the Drive, all fitted the spec of the motor and as explained above, have never been touched or adjusted.

During running conditions checked VFD output to motor and all ampages showed roughly balanced (as expected with a balanced motor) reading were U- 8.6A V- 8.5A W- 8.75A.

Checked mechanical load and pressure roller bearings were free to turn by hand…

Getting abit stuck now with paths to go next, and don’t want another motor burn out.

Any advice would be greatly appreciated.

Thanks in advance.

 

[James]

I would be looking carefully at the cable ends, crimp lugs and motor terminal studs as it looks like under load there may be a higher than normal resistance on one or more terminations.

 

[Blikzoggy]

I would be looking carefully at the cable ends, crimp lugs and motor terminal studs as it looks like under load there may be a higher than normal resistance on one or more terminations.

I would be looking carefully at the cable ends, crimp lugs and motor terminal studs as it looks like under load there may be a higher than normal resistance on one or more terminations.

Hi James,

At the inverter end the multi strand is ferruled and torqued in the terminal and at the motor end they are the “wago type” push fitting again ferruled but just sat in the push fit.

I just can’t grasp as there’s an identical motor, with the same push fit above with the same load etc, with no overheating. And also how it’s just the one phase?

Thanks for the reply James