Siemens sync motor. Starting current 1

[Skogsgurra]

Hi,

Long time no type.

Anyone that knows anything about starting a Siemens Synchronous motor on a weak mains. Motor data: 11,5 kV 9 MW, P.F.-controller sets excitation current. We experience starting times longer than 20 seconds, which trips the circuit breaker. Adding more Mvar does not seem to help. Voltage drops from a low 11,3 kV to a low-low 10,4 kV.

We are waiting for "better" voltage. Anything that can be done else?

 

[DougMSOE]

My first question is,
Did the motor start sucessfully be for and your starting troubles are recent?

Long starting times could certainly be caused by low voltage but may also be caused by the partial loss of starting torque.

 

[Skogsgurra]

Doug,

It started in 12,5 seconds when we had local power available. The voltage was 11,8 kV then, so it seems to be the voltage that is a problem. What I was thinking is if anyone has specific knowledge about this motor type (squirrel cage rotor for starting AND DC excitation for running). There is also another problem with this motor; it goes out of sync now and then - for no appearent reason. The motor P.F. is slightly inductive and we think that more excitation (on the capacitive side) can make it less senitive to torque variations. The guys that run the motor seem to be very reluctant to generate any reactive power - I think it is due to some funny economic thinking that I cannot understand, but that's the way it is.

Does anyone have numbers on this? What P.F. would give the best shock-torque "unsensitivity"?

 

[GusD]

hello Skogsgurra.

Your low voltage Power Supply is the cause of your troubles.Your field is probably running weak and motor pulls out of sync.A more leading PF may help with voltage sags.The choices you have are few,at least until your get better voltage.If you don't think your motor's squirrel cage is not at risk from the long starts,than the timing of trips and out of synk conditions can be adjusted to facilitate a start.
I believe that having to subject this motor to multiple starts, (because it has tripped due to Sync pullout or Main Breaker trips) ,can do much greater harm than set your controlling devices in a more generous manner.After all,you are already stressing this drive by repeated starts.If the control settings allows you more successful starts ,that will be a plus.Something gained ,nothing lost.

GusD

 

[jbartos]

Suggestion: Has the motor starting been simulated by any software for its otimal starting?

 

[Skogsgurra]

GusD

This is exactly what we would like to try (running at a higher P.F. to get a better "grip" and not lose sync as easily as we do now). But the process people are reluctant, probably because they do not understand what we are talking about. I will make them think in the right direction by quoting your advice. Thanks!

 

[swgrmfg]

Three questions: 1) Do you have any other synchronous machines on this bus?, 2) What type of rotor is this machine?, 3) How many MVAR's have you added that didn't make a difference?

GH

 

[Skogsgurra]

All is well that ends well

I just got words that there is a major mechanical problem with the heat pump attached to the 9 MW synchronous motor that started this thread.

When the mechanics inspected the system (after two weeks of saying that the problem was entirely electric and that the pump and auxilliary equipment were in mint condition) they found a one-wave valve (a "hydraulic diode&quot where half of it was missing. So much for "mint condition" and mechanic expertise. The valve is now being replaced and the local generation capacity will be available when the repair is finished.

So, there was no electric problem at all. But the excercise taught me something about synchonous motors. And ruined my Ascension holiday.

 

[jbartos]

Cheer up!

 

[GusD]

Hi skogsgurra

Thanks for letting us know the reason for your problems.
We all should take notice and do the same whenever we have the answers to our problems.Few as they may be.Corrections I mean!

Thanks again


GusD

 

[Skogsgurra]

Thanks all!

I guess that you had a hard time finding out what a "one-wave" valve is. It is not. Should read "one-way" valve.

Skogsgurra

 

[juanito]

Sir:

With regards to your synchronous motor, I have several brief comments. As you noted, the motor starts like an induciton motor, which allows it to be modelled as such during starting. I have employed this method with similar Siemens synchronous motors. With regards to the operation of the motor in a leading power factor be certain of the specifications for this. The motors that I evaluated were limited not only to quantity, but also for a limited time. This was associated with a thermal limit on the motor for such operation.

 

[Shortstub]

What are load parameters? Would "valve" failure resulted in starting the motor "under" load? What is starting time of uncoupled motor?

 

[Shortstub]

Skogsgurra,
This is in no-way a put-down of your knowledge of english, but a "one-way" valve is commonly called a check-valve.

 

[Skogsgurra]

Thanks, Shortstub.

I searched several Swedish/English dictionaries - even a technical one - but could not find the right word. I have seen the word "check-valve" several times, but never understood what it is. Thought that it might be a valve that you open to check something, level perhaps.

FYI: it is called a "backventil" in Swedish. If you ever need that piece of knowledge.

 

[jbartos]

Suggestion: Visit

http://www.redvalve.com/products/tideflex/Applications1.html

for check valves. It prevents backflow, which is close to Swedish "backventil."

 

[Shortstub]

Back to the original query.

Then, the motor could have been started under load! Even partial load could have substantial impact on starting time which agrees with your observation.

PF correction capacitors will not provide substantial improvement! The reason is they are usually sized to offset no-load reactance, not starting reactance.

Based on your data, i.e., a starting time of 12,5 seconds @ 11,8 kV, then the starting time would have been about 15 seconds @ 10,4 kV. Therefore the additional time was caused by "partial" load condition!

The concern, then, is the starting time with a 10% lower terminal voltage injurious to the motor?

Also, you haven't answered my question about a speed increaser (gear box) between the motor and pump! This has an impact on the total inertia factor, GD2t, consisting of the motor factor, GD2m, + the pump factor, GD2p. If the pump speed is greater than the motor's speed then GD2p factor must increased by multiplying it by square of the ratio of pump to motor speed, (Np/Nm)^2!