800hp auto synchronous motor fault 1

[jezz1]

new boy here,hope someone could help with a problem thats got our team stumped!
motor runs a ball mill,motor runs up fine thru first and second stage resistances but nine times out of ten hunts badly with large current swings after shorting of rotor resistances and changing to exciter control
any ideas would be appreceated.

 

[edison123]

Looks like the motor (during induction motor start phase) has not yet reached close to synch speed at the time of synchronizing with dc excitation. If possible, increase the time of induction motor start a little more to see if you can synchronize smoothly. Also, larger the dc field current during synchronization, better is the pulling into step.

 

[jbartos]

Suggestion: In addition to the previous posting, the motor shaft load may cause larger slip in the starting asynchronous mode of operation. There may also be a problem with amortisseur winding that is used in the asynchronous mode of operation. More systematic troubleshooting by a synchronous machine specialist is recommended.

 

[jezz1]

many thanks eddison.
I measured run up times of the outher drive [witch is fine]
and this made it to 690 rpm [60rpm short of 750 rated]
before it changed to dc control;this machine seems to stay in it's accelleration stage for longer than the problem drive,i cannot turn it off for four days as it is critical plant.
the rotor resistors are liquid and appear to have been mismaintained so this could prevent motor reaching a
high enough speed prior to going to dc if extending run up time does not improve matters.

 

[jraef]

Am I reading something wrong here or is this a synchronous motor with a wound rotor and liquid rheostat on the amoritisuer windings? If so it is the first one I have ever heard of.

"Venditori de oleum-vipera non vigere excordis populi"

 

[edison123]

jraef,

There are two types synchronous motors that I have rewound.

First one was the synchronous induction motor, where a normal slipring motor, at minimum slip, pulled into step by applying dc to the star connected rotor winding and shorting two sliprings. The dc current was I in one phase and I/2 in the other phases of the rotor winding.

The second one was more intesresting salient pole type synchronous induction motor where a separate normal 3 - phase start winding is located in the slots cut into pole faces of the salient poles and is connected through its own sliprings to the LRS during start up.

 

[Dopler]

There are one more type, It is like the first one but instead of induction motor It is a wound rotor, It has direct load coupling, for starting like wound rotor It has LRS and when it is shorted, DC is applied in I one phase and I/2 other phases.
Difference is First and second starts with out load, synchronize and then you couple the load (pneumatic or magnetic coupling type).
Third starts with load (common used in mining an cement industry)

 

[jraef]

I learned something new today. Thanks

"Venditori de oleum-vipera non vigere excordis populi"

 

[jbartos]

Comment: The starting resistor values might be checked for their proper values or settings.

 

[jezz1]

the motor in question is as dopler shows lrs then two rings shorted with dc to pull in,on load start
rotor and exciter being tested on thursday as i have had tacho on shaft at start up and it only got to 600 rpm befor dc applied.
changed timers and got to 700rpm,big improvement but still fails to synch 3 or 4 times out of ten

 

[edison123]

Increase induction start time so that speed is about 730 to 740 rpm. Make sure your dc exciter voltage is up to the rated before synch. When you synch at low speeds, not only the motor has to be pull through a large angle for synch but also your dc exciter output voltage would not have reached the rated level at low speeds (assuming your dc exciter is coupled directly to / mounted on the motor shaft). As I mentioned before, strong dc current would help in quick and positive synch.

 

[jezz1]

increased timers but found that motor speed would not go above 700rpm until changing to dc
when the rotor is being tested i should be able to compare
the resistors to makers spec
by the way when the drive was new would the lrs have allowed the motor to get that close to synchronous speed?

 

[edison123]

Looks like some portion of your rotor external resistance is not being fully cut out and is still included in the rotor circuit. This would account for such a large slip and low full speed of 700 RPM (for induction motor operation, I would expect at least 735 RPM).

If possible, use a separate three phase contactor in parallel with the rotor external resistance, which will short out all the three phases of the rotor winding. After the last contactor in your rotor external resistance is closed, close this shorting contactor and see if the speed goes above 700 RPM. This additional shorting contactor should be opened again prior to application of dc to the sliprings. Bit of co-ordination reqd here. Good luck

 

[aolalde]

jezz1:

I am suspicious of the rotor starting induction winding condition. Make winding resistance, insulation resistance, PI and surge Test the three phases. If your winding is not good and balanced the motor will not accelerate close to 750 rpm even if the resistors and final short circuit of the rotor are in good shape and/or you increase the allowed acceleration time .

 

[Dopler]

Jezz1, have you checked the LRS density?, if It isn't normal, It indicates changes in resistance, and that is the pull in torque to synchronize.

 

[jezz1]

many thanks to all for your input,i now have a list of
things to check when the specialist motor guy comes on thursday.
still waiting for original lrs spec however an operatve says aguy from the starter makers drained refilled the lrs
need to speak with him soon.

 

[jbartos]

Comment: The rotor field increase should be made carefully and with fine grading, since the protective devices of the synchronous motor stator might trip if the synchronous motor starts with very high field excitation.

 

[jezz1]

had motor guy in on thursday,the two rotor windings checked
ok as did stator.
motor has been starting ok since I increased start up timer
but still only reaches 690 rpm before pulling in so iam now going to really look closer at lrs as I assume if rotor
ok lrs limit final speed befor synch.
could being too slow at synch demand to much current from
exciter at change over