Starting 6 kV , 1.35 MW Syncronous Motor 6

[Masbibe]

Lansford,

You are right, but:

- Another thing that is problem with starting like this is voltage drops that occur on other devices on the same 35 kV /6 kV trafo station.

- and one more important thing is that I am working in company that would like to solve this problem and also to earn some money. But profit will be very low if we solve this problem by only increasing settings on protection.

Thanks.

 

[edison123]

Milovan - I see what you mean (BTW, I think you have 2 dc motors connected with the synch motor wrongly).

What you need is a two pole isolator-cum-changeover switch or contactor wherein in one position, you connect the external dc drive supply to the generator armature (to run it as a motor) and in the other position, you connect the DC motors to the generator armature. All of them must be rated for 1 KV and I suggest to make all changeovers at zero or low armature voltage to reduce arc flash.

Am I boring you too much ?



Muthu

http://www.edison.co.in

 

[Masbibe]

Edison123,

No , No (I am think on boring).

Now its OK about three position switch disconnector, but with 1000 kW rating (not because of drive, because of motors).

This thing with changeover on low voltage I agree with you especailly because current is DC.

And what about pony motor. You prefere more this solution with dc generator?

Thanks, bye.

 

[edison123]

Are the DC motors hard-wired directly across the generator terminals without an intervening breaker ? If yes, how do you protect the motors from faults ?

Muthu

http://www.edison.co.in

 

[Masbibe]

"Are the DC motors hard-wired directly across the generator terminals without an intervening breaker ? If yes, how do you protect the motors from faults ? "

Edison123,

As I now connection is direct. This is some old Russian solution and they have some current measurement in DC circuit (because of speed regulation on dc motors) and I assume that they use this measurement also for protection. So if they want to switch off dc motors they simply reduce exitation on DC generator to zero.
But I am not 100% sure on this.

Sorry if my english is bad.

 

[Masbibe]

Edison,

One more thing about protection from fault. If you have some short circuit fault in DC circuit, I now that decreasing exitation on dc generator is too slow.
But than this cause also increasing current in 6 kV AC side and you have circuit breaker here.

 

[Masbibe]

Edison123,

I looked now some schems that I have and it seem to be that there is no CBs in DC circuit.

 

[edison123]

It's possible they have wired to trip the generator field in case of any fault. My suggestion would be to introduce a two pole contactors/isolators for both the motor (100 KW rating) and the external dc drive supply (100 kw ? rating) and have an interlock mechanism to switch only one of them at any given time.

As for pony motor, does your synch motor or even one of the generators have a shaft extension in the opposite end to fit a pony motor ? Then you need a VFD to power up this motor and a VFD will be a costlier option to buy and maintain than a dc drive ? And then there is the additional cost of purchase, erection and maintenance of the pony motor.

Muthu

http://www.edison.co.in

 

[Masbibe]

Edison123,

Yes there is possibility to install pony motor on shaft. I agree that this will be a little cheaper solution (but realy a LITTLE cheaper because we can buy 55 kW motor for 3200 euros and this contactors or isolators are not cheap).

 

[edison123]

Then it's time to work out the comparative costing. Good luck with your project.

Muthu

http://www.edison.co.in

 

[Masbibe]

Edison123 thanks.

 

[edison123]

correction

"My suggestion would be to introduce a two pole contactors/isolators for both the motor (1000 KW rating) and the .... "

Muthu

http://www.edison.co.in

 

[Masbibe]

Edison123,

I notice that this was accidentally mistake.

 

[edison123]

Yes, as my age goes up, my intelligence seems to go down.

Thanks for an interesting discussion. Come back here when you solve it and update the story.



Muthu

http://www.edison.co.in

 

[Masbibe]

Edison123,

OK I will.

 

[waross]

In regards to the bearing issue;
If you have babbit bearings the weight of the rotors will squeeze the oil out of the bearings when the machine is sitting. Typically the machine is turned slowly until the oil rings carry sufficient oil into the bearings from the oil sump.
The torque required to turn the rotors when dry is much greater than the torque required to turn them when the oil is flowing. Possibly more than ten times the torque is required to start the rotors turning than is required when they are running free.
There are several options;
DOL starting will create issues with both extended starting times and bearing wear.
If the issue is bearings, you may use a small pony motor to rotate the rotors slowly until the bearings are properly oiled and the turning torque drops. Then start DOL.
In regards to weight, mass inertia;
Our sets weighed 80 tons to your 50 tonnes. BUT you don't accelerate the whole mass. You only accelerate the rotors which is a small part of the total mass.

NO breakers;
This is a common Ward Leonard scheme. Our sets used loops of two generators and two motors in series for each loop. Protection is by generator field reduction and stopping the main synchronous motor. The possibility of expensive mechanical damage to the machine is greater than the possible electrical damage if a drive loop goes open with an elevated load on the machine.

Using a DC motor for a pony motor;
Use full field. Start with zero voltage on the armature and ramp the voltage/current up. Your current will follow ohms law. R will be the resistance of the armature. E will be the applied voltage minus back EMF.

An anecdote to illustrate motoring DC generators.
Many years ago I was serving as a replacement instructor at a vocational school.
In our shop there was an old DC generator that had been donated to the school. As I remember it was about 250 kW or 300 kW. In class we had been discussing DC machines and went out to look at the old generator. I decided to see if we could motor the old generator. We had power supplies available that were designed for electronic experiments. The output was about 3 amps. We used the largest supply to energize the field and connected a variable supply to the armature. We set the current at about 2 amps and the generator started to turn very slowly. As the rotor turned the current dropped. As the current dropped, we set the voltage a little higher. The old machine eventually reached a speed where the floor was starting to bounce.
I was about to terminate the experiment and let the achine coast down when one of my students tripped over the extension cord to the power supplies. The inductive kick let the smoke out of both power supplies.

Suggestion, It may take much less energy than you expect to spin unloaded rotors.

Suggestion, Lift the brushes on a couple of motors to break the loop. Connect a welding machine to the generator armature (with the field applied) and monitor the current and voltage to determine the capacity you will need for the permanent drive.
With full field applied to the generator the speed will be proportional to the applied voltage.
Before going further you should determine what type of bearings are installed on the generators.

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

 

[Masbibe]

Waross,

Thank you very much for this detailed desctiption. Also that ancdote is very nice.

I dont believe that there is babbit bearing but I will check that if I could, because this machine is not in my country and my company only gives offer for this job.

About full field. I know its better to have full field because of smaller armature current. But then I have problem to find DC drive with 900 VDC output (we commonly use Siemens or ABB drives and they dont have drives for such high voltage).

Because you have so much experience I would like to hear what is your opinion on using serial connected two DC motors on one DC drive.

Thanks.

 

[edison123]

Milovan

I meant to ask you this. How a 1.35 MW synch motor is able to feed 3 MW DC load ? The power balance doesn't compute.

Muthu

http://www.edison.co.in

 

[waross]

May I answer that, Muthu? The loading on the various machine functions is such that the generators are almost never loaded to the maximum simultaneously. That is, if the digging motors are loaded to the maximum, you won't be swinging. If this is a dragline type of excavator, the drag and hoist winches oppose each other. When the hoist is winding in, the the drag is paying out with very little load back to the sync motor.
The machine I was on had 5800 Hp total of motors on each 3000 Hp sync motor. There were four such sets.

I will think for awhile before answering the question Re two DC motors on one drive.

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

 

[Masbibe]

Edison123,

Generators are working at same time but laod is very variety with time and maximum load on 3 generators are not in the same moments so 1.35 MW is enough for this 3 genrators.