Starting 6 kV , 1.35 MW Syncronous Motor 6

[Masbibe]

In one excavator there is 6 kV synchronous motor starting direct on line. Motor is starting with small load but with high inertia. Motor is running 3 1MW DC generators (Vard - Leonhard group). Starting time is almost 1 minute. Because of direct on line start starting current is too high and circuit breaker that supply this excavator trips. We must to solve this problem but with low budget. Idea is to use small AC motor with frequency converter to run this Vard Leonhard group up to synchronous speed and then to close motor circuit breaker but with no exitation on motor. After this we will increase exitation on motor up to nominal value.
I am interesting what is your oppinion on this and which starting current should i expect with starting like this.
Also another question is how much should be nominal power of this small motor.
Sorry for my bad english.

 

[ScottyUK]

Difficult to say for sure, but as an example a pony motor arrangement used with a four pole 3.3kV 2000HP induction motor was about 90kW if I remember correctly. If this is a two pole motor then I expect the power required would be higher.

Your English is quite clear, so no need to apologise. In England we know the motor-generator system as "Ward Leonard" sets which might help if you are ever looking for information on them.


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If we learn from our mistakes I'm getting a great education!

 

[Masbibe]

ScottyUK,

Motor is 6 pole , 50 Hz , 1000 rpm. Problem is that this pony motor should run this synchronous motor and 3 more DC generator 1 MW , 1000 rpm which are mechanically coupled on same shaft.
I have some information that on the same configuration somebody used 55 kW pony motor but this information is not 100% sure.
But what concern me more is how much will be current peak after I close main breaker of synchronous motor.
Anyway thanks for suggestion.

 

[TommyOO7]

Milovan, I would rather concentrate on Circuit Breaker trip settings. Your synchronous motor (SM) already has asynchronous motor (AM) inside it, much larger size then you are considering for your pony motor. SM can not start on its own, it must be brought to near nominal speed by other means. For that purpose, there is a cage added on the rotor, so SM can start as AM and then switch to SM mode. Control logic is a problem not the motor.

 

[waross]

What type of bearings do your generators have. If you have babbit bearings with oil rings you may find that turning the set by hand for several minutes to bring up oil may reduce your starting times and avoid tripping.
Years ago, we started every shift by manually rolling 3000Hp synchronous motors on an excavator. It took a minimum crew of four but went better with six men. After test and commissioning the machine ran 24/7.
You may be able to use the synchronous motor with a VFD instead of a pony motor. Some of the folk here have experience with this technique. Be patient.

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

 

[edison123]

Can't you not use one of these DC generators as a motor for start up ?

Muthu

http://www.edison.co.in

 

[djs]

Wouldn't a reduced voltage starter be less expensive than another motor and VFD?

 

[LionelHutz]

You are just accelerating the rotors of 3 DC generators. It is just an inertia load and even then shouldn't be a huge load for the motor. It doesn't seem right that this takes 50 seconds.

You would have to figure out how much HP is required just to spin everything at 1000rpm. Then you could size this pony motor.

The current peak when you close the main breaker would be the same as you see now, it would just be a much shorter time before the running current levels are reached. However, you could apply the field to the motor first and then synchronize the motor to the line power before closing the breaker to reduce the inrush.

If there was a bearing issue like Bill described, I would use a low voltage VFD connected to the motor before starting. You could run the motor at full torque at 3.2Hz using a 380V VFD by setting the proper V/Hz in the VFD. This would just require a VFD system that can handle the motor FLA which isn't that much.

 

[edison123]

The best bang for the bucks will be by running one of those DC generators through a DC drive. Best starting torque - smooth and with minimum current & fuss. Ramp down and disconnect the dc drive once the system is running up to speed with the synch motor.

Muthu

http://www.edison.co.in

 

[Masbibe]

"Your synchronous motor (SM) already has asynchronous motor (AM) inside it, much larger size then you are considering for your pony motor."


Tommy007 I cannot agree with you. This thing about cage added on the rotor is correct and SM can start direct online becacause of this cage. But current with starting like this is about 5-6 nominal current of SM. So this mean that in starting you have about 7.5 MVA and this is much more higher then 50-100 kW of pony motor.

 

[Masbibe]

Warros,

This with bearings is very interesting but we dont have so much people on excavator. There is only one man who is working on excavator per shift. But with more effective bearings we can reduce power of motor and VFD. Thaks for suggestion.
I dont believe that any solution on 6kV side can be cheaper than pony motor and VFD on 0.4KV.

 

[Masbibe]

djs,

I dont believe that any solution on 6kV side can be cheaper than pony motor and VFD on 0.4KV.

 

[Masbibe]

LionelHutz,

It is time like this believe me. This is hugh inertia because weight of this 4 machine is about 50t and also you have some friction losses.

About this that you wrote "The current peak when you close the main breaker would be the same" I thought like you just I am not 100% sure. If this is correct than we must see which protection is tripping circuit breaker. If this is some overload then this solution with pony motor is OK and if protection that trip CB is some kind of short circuit protection than we are in the problem because breaker will trip again wherever speed is synchornous or zero when you close CB.

And this what you wrote about firs exitation and syncronization motor on network before closing CB is best techincal solution and I was thinking abuout this just I am not sure how much money this will cost. But I agree with starting like this current will be very low.

Thanks.

 

[Masbibe]

Edison123,

I agree with you about using DC converter on one of DC generators. I was thinking about this also.
Only few things to explain.

Nominal ratings of DC generators are 900VDC, 1000kW, 1000rpm. We have 400 VAC available. OK we can reduce exitation on this DC generator and run this machines up to synchronous speed of SM like this. So when you rectify 400 VAC you have about 540 VDC so this mean that flux should be about 60% of nominal and stator current about 40-50% of nominal.

For this solution I need 2 extra more contactors for switching generator between drive and 2 motors (which this generator supply). This contactors should be for 900 VDC (and this is not standard) and one should be for 1000kW and another for about 70-80kW.

So when you calculate all of this price is more or less same. But this version of solution is still active.

But I am afraid what will be with starting current of SM when I close CB.

Thanks.

 

[edison123]

Milovan

Yes, you could achieve the final speed with field weakening of the dc motor (generator). Or a step-up 400/700 V trafo feeding the DC drive.

The set-up I see is

For the field circuit - Use the existing one.

For the armature - A parallel feed via 400/700 V AC; 520/900 V, (100 KW ?) DC drive with a 1000 V rated motorized isolator (instead of a contactor)at the drive output.

If you want a smooth synch of your motor with minimum inrush, you could also run your synch motor as a generator with external excitation and then synch with a synchroscope & synch check relay. But this again is a cost driver with associated maintenance/operation issues.

Muthu

http://www.edison.co.in

 

[Masbibe]

Edison123,

I prefere to do this without step up transformer (because DC drive for 900VDC is also not standard).

This motorized isolator then should have 3 pairs of contacts to connect: drive, generators and motors and should be for 1000 kW. Is that what you mean.

Do you maybe now how much cost some synch reley that I can use for synchronization.

Thanks.

 

[edison123]

Milovan

The problem with field weakening is it also weakens the dc motor torque and this could result in acceleration issues for you. Hence, it is better to resort to field weakening only as a last option.

I agree 700 V DC seems to be limit in the DC drives (at least per Allen Bradley). Still it is better than 520 V DC in terms of better acceleration with a stronger field.

My idea is to run a parallel DC supply to only one of the DC generators from the DC drive with a 1 KV rated motorized isolator. I’m rating this isolator at 1 KV since there will be a 900 V voltage backfeed from the DC generator when it is running normally.

The whole system will have a rating of about 100 KW (?) - you need to do the math for accelerating the 50 ton mass from 0 to 1000 RPM.


Muthu

http://www.edison.co.in

 

[Masbibe]

Edison123,

I draw and upload simple scheme for you. I cant use only one isolator to isolate only drive (when speed reach sync speed), because when I supply DC generator from drive I will supply DC motors also if I use only one isolator.

 

[lansford]

All of these discussions are good but the first order of business is to determine if it is a problem for the motor due to long acceleration time.
If it is OK thermally and OK for the drive bearings, then adjust the CB not to trip on acceleration.
JIM

 

[Masbibe]

Sorry,

When I draw this I make one mistake. On sycnhronous motor of course there is no DC motors. Correct scheme is below.