Can a stand by generator 264 kW supply a 75 kW compressor + some loads?

[SA07]

Hi

Can you please confirm if we can connect an air compressor 75 kW (star-delta) to our stand by generator (330 kVA, 264 kW, 400 Vac) ? In case there is a power cut, the generator will operate and supply the compressor + other loads.

The other loads may vary and is now around 52 kW.

Any idea up to what unused kW on the generator should be available to be able to supply the compressor? Thanks

 

[LittleInch]

All depends on your start current.

Just try it?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[wayne440]

Also depending on the type of "other loads", you may want to avoid starting all of them simultaneously.

 

[TugboatEng]

Typically the generator should be nameplate rated at double the largest motor load for a direct on line start. This doesn't mean it needs to have double the available capacity. Your system should have minimal trouble starting the compressor even while carrying the 52kW load.

 

[waross]

I have installed a lot of generators that were challenged by motor starting.
These were residential standby sets with a lot of air conditioning loads.
I many of the sets had already been purchased before I became involved.
Once the set was installed, I would work with the owner to determine how many of his A/C sets could be operated at once.
Part of the job was adding contactors for each automatic start A/C units. When the power failed, the contactor dropped out and had to be reset manually before the A/C could be started.
Based on many actual field tests over a number of years, I have developed the following guidelines.
The spare ampacacity of the set must be three times the rated current of the motor to limit the voltage dip to an acceptable level.
I use nameplate amperage rather than kW. Motor starting demand is at a low power factor so KVA is more of an issue than kW.
So, rather than using 264 KVA, we can use 330 KVA for our calculations.
My numbers show that you will be just a little overloaded, but star delta starting should bring you into the safe zone.

Caveats:
1. The voltage dip will be more than normal, but was always acceptable to the customer, and did not cause any problems.
2. I had about a dozen satisfied customers when I came into the possession of the Caterpillar Generating sizing software.
I ran the numbers for my past installs and in every instance, the Cat s/w said that I needed a larger set. Remember, Cat is in the business of selling gen sets and the bigger the better. (Wet stacking notwithstanding.)
3. If the motor is the only load on the generator, you may be able to start at an ampacity ratio of 2 1/2:1, (250%) but I do not recommend using this ratio. If the voltage dip is enough to cause a contactor to drop out, the contactor will be destroyed in minutes.
It will "Machine Gun". The set slows down and the contactor drops out. The now unloaded set speeds up and the voltage recovers and the contactor drops out. Rinse and repeat.
In the event that winter fuel drops the output of the motor by 10% you will be dangerously close to Machine Gunning.

Notes:
1. If the engine is somewhat oversized for the generator, (Not uncommon) motor starting will be better.
2. A Permanent Magnet Generator (PMG) supplying the excitation helps motor starting.
3. An unloaded Naturally Aspirated (NA) set will be better that a Turbo Aspirated set (TA) starting a motor with no other load on the generator. If there is enough base load to spool up the turbo, the NA set has less advantage.
4. Poor fuel may be an issue.

Bottom line:
If the 330 KVA generator is ordered, on site or you have a special reason for using it, I would go with it.
If a set has not been ordered yet, I would consider going one size larger.
But it is almost too close to call.
After a close consideration, including an evaluation of the loads, the odds are good that I may go with your chosen set.)
The odds are excellent that your planned installation will give good service.
If you do find issues, all is not lost. Come back and I will go into mitigation issues.
Come back and tell us how the installation performs.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[SA07]

Thanks for your replies.
All the four compressors are already on site as well as the std by generator.
We are relocating all 4 compressors and plan to supply one compressor from the st by generator in case of a power cut.

 

[TugboatEng]

If you are concerned, a VFD can allow you to start a compressor with zero excess capacity. I recently downsized a fleet of generator sets running 150 kva gens to 99 kva. The application has a 10-20kW base load and 75 horsepower winch. The gens were sized for DOL start of the winch. The winch had vector control and required no excess capacity. I took some of the money I saved downsizing from a 9 liter diesel engine to a 4.5 liter and invested it in PMG excitation and a better voltage regulator. The smaller units perform better with less voltage drop.

 

[edison123]

Starting KVA is more critical than running KW for sizing a DG. A 75 KW cage motor with star-delta start will take about 250 to 270 KVA during starting. So a sequential start with the 75 KW compressor starting first and then the next smaller load starting should be ok with your 330 KVA DG but overall running cost will be higher since your engine KW is twice the actual running KW required.

Agree with tugboat. Given the cheap cheap VFD's now a days, it doesn't make sense any more to increase capital and running expenses by sizing DG's based solely on motor starting KVA.

Muthu

http://www.edison.co.in

 

[LittleInch]

The key thing is probably how good your engine is at picking up the load and how sensitive your switchboard is to under voltage and under frequency.

The settings for a grid supply might be much tighter than possible for a generator supply. Ask the question.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[FacEngrPE]

If your compressors are capable of starting "unloaded" enabling the feature will significantly reduce the load torque the motor needs to handle during the starting process. The impact on starting kVA demand is that the peak is the same, but the starting time is significantly reduced.

 

[Maximillianrichards]

To determine if you can connect the air compressor to the standby generator, you'll need to calculate the total power requirement of all connected loads, including the compressor.

For a 75 kW air compressor and 52 kW of other loads, the total power requirement is 127 kW. With a 264 kW generator, you have enough capacity to supply this load.

However, it's important to note that starting an air compressor typically requires a higher power draw than its rated power, so you'll need to ensure that the generator has enough capacity to handle the compressor's starting surge. This surge can be up to 6-8 times the compressor's rated power, so you may need to oversize the generator to accommodate it.

As for the unused kW on the generator that should be available to supply the compressor, it's generally recommended to have a reserve capacity of at least 20% above the total load. In your case, that would mean having at least 152 kW of unused capacity on the generator. However, this reserve capacity may need to be increased if the compressor has a particularly high starting surge.

 

[crshears]

Hello Max, and welcome to the foray, um, I mean fora . . .

Your input seems to completely ignore the submissions of others; what might be your thoughts on what the other posters have had to say?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[wroggent]

crshears, prior to seeing your post here I felt inclined to read all of Max's posts and after having done so I'm pretty confident they're coming from some AI.

 

[crshears]

Well! Ain't I dumb for falling for that! I took the poster at face value, never contemplating it might not have a face at all . . .

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[LionelHutz]

That could be. They're rather canned answers, and have been wrong too.