View Full Version : Main motor running in reverse

10-07-2012, 02:44 AM
Hi Guy's
I have a problem here and wondering if anyone has had s similar experiance.

I have a soft starter, starting a 3.3kVa, 1270 HP motor. Whenever we initiate start, the motor starts in the wrong direction and the soft starter trips. After 3 or 4 (sometimes 2) restarts, it starts in the correct rotation and stays running.

The electrical team here seams convinced that the problem is with the motor but I've never come accross a motor that changes direction unless the phases are swapped, therefore it must be a soft starter issue...

Has anyone come accross a problem like this.?

The starter is a

AuCom - MVS series (I've never sean one before)



10-07-2012, 05:23 AM
definitly a soft starter problem, possible starting on two phases full voltage and third phase confused,so wrong direction more than likely a faulty what ever they are called, electronic contact. The initial fault would be unbalanced phases so will trip after partial ramp up time out. Fortunately you don't have a parallel line contactor that would apply correct phase direction, that would kill your motor. 3.3Kv is scary stuff.

10-07-2012, 05:01 PM
Thanks Magoo...
I recon your right on the money.... Definately a faulty starter.
The client is itching to get to production so they're accepting the fact that ,, yea.. it does a couple of trips due to reverse rotation but ,, eventualy it fixes itself and they're on their way..... Lucky for them that it does trip...
I've never seen a screw compressor run in reverse at 2950 RPM.... I wonder what happens..??
Anyone know.?..

Cheers mate.. Thanks for your reply


13-07-2012, 06:10 AM
Hi Peter,
fortunately screws start fully unloaded, but gases and wrong compression can play havic on thrusts, apart from making weird noises with oil injection etc... You need to convince client that drive unit needs to be sorted out, could be as simple as a clean and re-fit of the electronic contact on the dodgy phase. The Aucom starter is fairly user freindly, here they are supported by Danfoss Drives. There limitations are the fault displays, basically the stop/fault and little else info. Short of a tech analysis.

14-07-2012, 05:32 AM
Hi Magoo.
At this stage all is working well. We had several starts yesturday and only once did it bump backwards.
I wont have a chance in convincing them as they are behind schedule and under a lot of pressure to start producing LPG..
Even with correct rotation and compressor at minimum, we couldn't get it to take off. By the time the motor ramped up to nearly full speed it tripped on current overload. I installed a 2" manual by-pass from oil sep to suction and now it get's up running no problems..
So.. Thats it for me here... I'm heading back to Sydney....
Thanks for your feed back mate. Appreciate it


Hey.!.. What did the kiwi say to the statue.??

15-07-2012, 02:11 AM
Hi Petrock.
interesting that the manual bypass resolves problem, so effectively no load at startup will get motor started.
definitly a drive issue, ramp times probably. Are you sure the control system is not rapidly loading compressor after initial start, during ramp stage.
Had similar issues with a drive, even with minimal ramp times, same intermitent overcurrent faults. Sorted by falsifying the motor supply voltage set up inputs. Set up with motor supply voltage at 480 vac instead of 420vac which in turn fired proportionally higher voltage to motor through ramp stage.Lower amps, no more faults.
Still does not explain the wrong rotation at start, I still think that one of the "magic "contact thingos needs cleaning, they have a conductivity paste stuff between each bit, any resistance will give a low voltage at critical time. Another problem could be one current transformer coil is dodgy inputting weird readings to drive.
Put pressure on the drive maker and installer to identify the real problem.

and what did the Kiwi say to the statue, dare I ask.

16-07-2012, 12:27 AM
Hi Magoo,
Yes. Definately stay's unloaded. I temporarily programed a force by-pass to keep the unload solenoid energised, to make sure.
When starting a new screw you allways run it at minimum for at least 30 minutes. They've tried different ramp times and I'm not sure what else but to no avail. Definately a glitch in the drive somewhere... There's no stopping them now though... They're in production..

The Kiwi said...... " Hey.!!.... Statue Bro.!?"...

Cheers mate..

16-07-2012, 04:37 PM
In the words of Charlie Brown Good Grief

21-07-2012, 12:29 PM
very intersting issue you have there, I can't comment on the refrigeration side of things but I have had some experience with 3 phase induction motors. I will agree with the guys above in that it is very unlikely to be a motor issue, typically the motor will do what you tell it or do nothing.... If it's a typical squirrel cage motor then it is virtually impossible for it to run backwards, as the statoris generating a rotating field in the desired direction, if the field is going in one direction and the rotor is going in the other direction without being mechanically driven by the pump then you have yourself a perpetual motion machine! If the motor was 2 phasing ie dirty contactor terminal it will usually just sit there and growl it probably wouldnt spin up at all, it won't travel in the opposite direction though, well not at any appreciable speed anyway.

Could the variable speed drive be accepting an unreliable direction input from the local controller / PLC?
Usually equipment built for HV especially vsd's of this size aren't overly cheap and nasty so I should hope it's a configutation issue rather than an internal fault as an internal fault of this magnitude is a very big cause for concern.

good luck!