What load must be available for motor while starting in Y/D? 2

[nifrabar]

Hi!
On board a ship we are having a diesel-generator set which supplies 44 [E] kW.
I do have a basic [E] load of about 35kW.
Q:
How do I calculate if I can start a compressor using Y/D starter in this configuration?

TIA
-Bart

 

[nifrabar]

Dear jraef,

Thanks for your comprehensive information which real makes sense.
I think the Korndorfer starter may be a good solution in this case.
I agree with others that still some 'factors' may disturb the proper work of the mains o/b the vessel.

Would there be some more info available for Davidbeach solution:
The closed transition version of the S-D starter
Maybe a scheme somewhere what tells more than a hubdred words?

TIA and thanks for your time so far.

-Bart

 

[waross]

I assume that the crane will be used mainly when the vessel is along side the wharf.
It may be possible to turn off selected equipment to allow enough capacity to start the crane. Any heating loads are a good starting place.
The KVA of the motor start will probably be at a very low power factor. The power factor of the base load will probably be at a much higher power factor. Due to the different phase angles of the starting current and the base load current the total current and the total KVA will be less than the sum of the parts. This is to your advantage but also complicates the calculations as to whether the set will start.
Take a look at the base loads and see what may be safely turned off for a few seconds to allow the motor to start.
I have found that the Cat programs are quite conservative. I ran the numbers of a few marginal installations that I had been stuck with that involved motor starting. (Residential with large A/C units) All of the installations were working adequately but the Cat software said that all would fail.
The Cat software is good for initial sizing but when you are stuck with a large motor to start on an existing set, it is discouraging.
Based on direct experience starting motors on existing gen-sets with existing base loads, your installation is a little worse than marginal. There are a few factors such as the ratio of the motor size to the base load and the phase angle difference between motor starting and the base load that work in your favor. If the gen-set is prime rated rather than standby rated that is a big factor in your favor.
At the end of the day, the installation is still marginal. I have found that load curtailing is the most dependable and cheapest solution. This may be manual with suitable instructions, semi automatic or fully automatic.
In residential applications I have had good luck with semi-automatic schemes. The selected loads are tripped off automatically and manually restarted.
This is a case where you must make several assumptions in order to calculate the possibility of starting the motor. Then you go out in the field and see which assumptions were erroneous if the motor doesn't start.
You may consider a trip to the vessel and some testing to determine what conditions are.
I don't think that a generator will be happy with either a VFD or a soft start under marginal conditions. Both of these devices have input current waveforms that are challenging to the AVRs of generators. We are back to assumptions as to whether there is sufficient reserve capacity for these devices to function properly.

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

 

[LionelHutz]

I missed the part about it being hydraulic before. That means it's likely a piston pump. You will likely have to use an unloader valve to reduce the pump load enough to get it to start use a Y-Delta starter.

The comments about the second current surge during transition were not fully explained. There are actually 2 different causes of current surges during the transition.

The first is caused by closing the power onto the motor when the motor has residual back EMF and is also out of phase with the line power. In this case, you can see a few cycle current that could be 10 to 20 times the rated current. The closed transition starter helps with this.

The second is caused by the motor speeed when the transition occurs. Most typical motors have a high current draw until the motor reaches around 90% to 95% speed and then the current quickly drops off. The motor current will jump back to the full-voltage levels after the transition so the transition needs to be performed after the motor has reached about 95% speed. Otherwise, the motor current can easily reach 400% plus levels after the transition until the motor finishes accelerating to full speed. The motor needs to have a small enough load it accelerates close to synchronous speed before transitioning to avoid this issue.

 

[electrolitic]

It is not a technical answer.
Is just a remembering of an old engineer:Weak back up system,
motor starting problems,transition peaks, etc and a wound rotor motor getting a starting kVA around 1,5 times running kVA.
Simply the easiest type of motor starting for a weak gen set.
Old memories...