Problems in starting synchronous motor in a pump station 3

[Bronzeado]

Hi folks,

A 5.5 MW, 6.9 kV synchronous motor did not start when it is energized. Then we moved the rotor position and the motor starts. We have observed that the motor only runs if the rotor is in a particular position.
I wonder if anybody has faced a problem like that.

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro

 

[LionelHutz]

Can you further explain what not starting means? Doesn't rotate? Trips during starting? Something else?

 

[waross]

Have you checked the amortisseur winding for opens? Possibly multiple corroded connections?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Is it one of those motors with a rotating transformer exciter, a rectifier and a thyristor bridge that shorts the rotor winding during start? They can be tricky and cause some unexpected results if the setting has changed or if something in the thyristor gating circuit has come loose.

OT:
I had a case of sympathetic inrush in a number of pump stations a few months ago. Caused even harmonics which made a filter resonate (was not supposed to see any even harmonics) and "pumped up" the DC link in idling VFDs. Your articles were very helpful.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Bronzeado]

LionelHutz,
The motor does not rotate during starting.

waross,
The amortisseur (damping) winding is not open.

Skogsgurra,
The rotor winding is short circuited by a resistor during motor starting.

(Obs.: Could you, please, send the reports (if any) on your case of sympathetic inrush? I am happy in hearing that my papers wee helpful to you. Thank you!)

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro

 

[Bronzeado]

Dear LionelHutz, waross and Skogsgurra,

We are suspecting that it may be a magnetic locking between the stator teeth and the rotor teeth, knowing as a "cogging" phenomenon or magnetic locking of induction motor (mind that the synchronous motor atarts as a induction motor).

I wonder if you all could comments on that suspecting. Thank you.

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro

 

[edison123]

"The rotor winding is short circuited by a resistor during motor starting."

Is this motor with brushless excitation or with DC sliprings?



Muthu

http://www.edison.co.in

 

[waross]

Cogging is jumping from one slot position to the next rather than moving smoothly.
How are you starting?
DOL? VFD? Soft Start? Other method?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[panter]

Bronzeado,
Have you had some activity on the motor or the problem occurred during normal operation .Large current and moment are at the start so you should be brave and turn the rotor if motor will not start.Did you controlled inrush current and are they symmetrical .

 

[waross]

Starting from one position and not when the rotor is in other positions suggests hidden damage to the amortisseur winding.
Is the field fed by sliprings? Are there bad spots on the slip rings?
Is the air space clean? I had a motor that had magnetic dust accumulate in the air gap. It would spin free but would lock up when trying to start.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

More details like type of synchronous motor and starting method could help. How about some history. Is the motor new or has it been working? If working, was the motor recently serviced?

What is the field data and what size of resistor are you using?

Assuming the motor is otherwise healthy, cogging would be a design issue.

 

[Parchie]

Assuming the motor is otherwise healthy, cogging would be a design issue.

I agree. IIRC, this problem was already solved decades ago by using skewed slots in the rotor! Or am I missing a lot?

 

[Bronzeado]

edison123,

The motor is with DC sliprings.

waross,

We try to start directly and also with Soft Start.
The field is fed by DC sliprings. There is no damage to the amortisseur winding nor bad spots on the slip rings. The air space clean.
During the starting when the motor did not move, we just move the rotor to another position and it runs normally. ????????

LionelHutz,)4 Mar 16 14:01

The motor is brand new. The rotor has both squirel cage and field windings. We try to start directly and via Soft Start. Just now, I don not know the size of the resistor. We also change it trying to solve the problem.

Parchie,

"Quote (LionelHutz)
Assuming the motor is otherwise healthy, cogging would be a design issue.
I agree. IIRC, this problem was already solved decades ago by using skewed slots in the rotor!"

We also think that it is a design problem. Last week the manufacturer taked the motor to factory.
I will report the work result when to motor return to the pump station.

Thank you all for the "brain storm" in this subject.

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro

 

[LionelHutz]

WEG motor, right? From what I've seen at other sites, the amortisseur winding in a WEG synchronous motor is rather crappy.

 

[Skogsgurra]

Isn't it time to record what is happening during a start? Guessing games are great party activities. But the truth - like stator and rotor voltages and currents, plus rotor speed or position, are easy to record and add substance to the analysis. Any records available?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[panter]

Soft Starter is a strange decision for your motor and there will be highly demanding find solution in setting ramps for good start .
Good luck !

 

[waross]

Out of the box, but does this motor have Babbitt type bearings?
Too high an oil pressure may force the dry shaft against the top of the journal. A half of a turn would carry enough oil to the top of the journal to substantially reduce the stiction.
Years ago I was involved with multiple starts on strings of five DC generators with a combined output of 5750 HP.
The sets did not have turning gear and did not have oil pumps. They had to be manually turned enough revolutions for the oil rings to carry oil up to the shaft.
Dry Babbitt bearings are a drag, Really a drag. grin.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dArsonval]

"...synchronous motor did not start when it is energized. Then we moved the rotor position and the motor starts.
We have observed that the motor only runs if the rotor is in a particular position..."

The OP's reply to warcross was, "...The amortisseur (damping) winding is not open."
What tests were conducted to confirm the amortisseur winding is NOT open?

The statement declaring that when the rotor position is moved and then the motor starts... is a signature
indicator of an open bar circuit.

John

 

[edison123]

Any design that depends on the amortisseur winding for a synch motor start is indeed a bad one.

Years ago, we did a rotor rewind of a salient pole 750 RPM motor which had a three phase rotor winding in the pole faces for starting and separate DC fed field coils for running.

Right now, we are rewinding a 25 year old synchronous induction motor where the rotor, after starting as an induction motor, is fed DC near the synch speed by shorting two phases. The phase that carries the full DC current has wider slots and a wider slipring.

Both are very interesting constructions of ye old, which present day bean counters wouldn't allow, let alone understand.



Muthu

http://www.edison.co.in

 

[waross]

Hi Muthu: Is it feasible to convert a wound rotor motor to a synchronous motor? What conditions would the rotor have to meet?

Bill
--------------------
"Why not the best?"
Jimmy Carter