10P-1000HP - Shaft Hunting 1

[macmckim]

Good Day

Have issue with a 1000HP, 10P (713 RPM) 4kv / 60HZ, TEAAC compressor motor - VFD powered - Variable torque application.

Motor is direct coupled (flexible coupling) to VRU ring compressor and is VFD power from 660 - 740 RPM (fully loaded)
The motor shaft starts hunting - movement is over 6.5mm. Hopefully you will be able to see attachment (short video)
The compressor shaft does not have any movement, only the motor shaft/coupling is hunting.
Shaft hunting starts at 660 RPM - does not happen below this speed.

When motor/coupling are disengaged from compressor (no load) - motor is tested at operating speeds, the motor shaft does not move
Vibration levels, bearing RTD temps, full load current are all in acceptable range.
There are 3 compressor motors - two operate correctly and third is hunting.

Any idea's

thanks
Mac

 

[dArsonval]

An electric motor reacts to its power supply, and to its load.

If the motor runs fine decoupled, it is not a motor issue.

It reads as though the refinery crew may pat their back when they get lucky, and
point fingers when, when, when, the luck does not exist.

Gotta find the "sweet spot" for the installation.
Unfortunately, finding that spot takes less talk and a little more work.

John

 

[edison123]

Simple check. Run the motor decoupled. Check if the rotor oscillation is still there. If no oscillation, scribe the magnetic center near the DE bearing housing on the outboard. Stop the motor, bring the rotor to the magnetic center and measure the gap between the couplings. If the gap is more than 1 to 2 mm, move the stator/motor towards the compressor by (gap-1) mm, realign and couple.

If the rotor oscillates in decoupled state, then the issue is with the motor. Improper assembly, skewed bearings, fan axial thrust etc.

Muthu

http://www.edison.co.in

 

[Marke]

I witnessed a problem like this a number of years ago and the result was a destroyed fan unit.
The problem was speed instability in the VFD which was set in Torque mode meaning a sensorless flux vector mode.
Changed the setup to V/Hz (OK for Fans) and the problem went away.
If the speed loop PID is out of adjustment, then you will get a speed instability like this. - too much loop gain.
Try reducing the Proportional gain in the speed loop PID if available, or run in V/Hz mode.

Best regards,

Mark Empson
Advanced Motor Control Ltd