Choosing right size of Emergency generator 2

[Bab AK]

Here we have 5 set of 55 KW inductive motors running DOL, I want to choose a diesel generator to feed all of them and provide their starting parameters, our derating factor here is about 20%. How much would be the right size in KW approximately?

 

[waross]

55 kW = 55kW/.9PF = 61 KVA
61 KVA x (3 + 4) = 427 KVA
This is for staggered staring. 3 X KVA of starting set plus KVA of running sets. (Plus any other base load.
You could probably get away with 400 KVA for a naturally aspirated set or for a prime power rated set.
The Cover Your Assets number would be 450 KVA.
Judgement call:
If the running motors are lightly loaded you may reduce the size accordingly.
There should be no reduction in the demand of the starting motor.
A naturally aspirated set accepts block loading better than a turbo'ed set.
A prime power set is de-rated 10% in comparison to a standby set. A 400 KVA prime power rated set is the same as a 440 KVA standby rated set.
If the original intent was to start all five motors simultaneously, try very hard to arrange staggered starting.
A simultaneous start of the five motors would want a 61 KVA x 5 x 3 = 915 KVA set (+ base load).
I generally size according to motor nameplate current and generator nameplate current rather than kW or KVA.
I've been burned a couple of times when someone else sized a set based on kW at 240 Volts for a 208 Volt system.
I have had customers order their sets without allowing me to vet the order. Not a good idea, but the customer's always right. (Until he's wrong. Then just keep your mouth shut and try to live with it. He's still the customer.)
Or sign in to the Caterpillar site or the Onan site for a generator sizing program.

Bill
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"Why not the best?"
Jimmy Carter

 

[Parchie]

Here we have 5 set of 55 KW inductive motors running DOL, I want to choose a diesel generator to feed all of them and provide their starting parameters, our derating factor here is about 20%. How much would be the right size in KW approximately?

How much voltage dip does your system allow? If these motors start at the same time, you may need larger genset (830kVA) or if they can be staggered you may need around 400 kVA!

 

[Bab AK]

About the allowed voltage drop it could be about 20% just for emergency purposes then the voltage could reduce until 80% of nominal when we apply our diesel generator as emergency power supply

 

[waross]

The question that is more important than voltage drop is the starting sequence of the motors.
Your running load with five motors running will be 305 KVA.
Sizing the set to start the five motors together will require about 925 KVA.
You may end up with a 1000 KVA set. Cat sizing may be larger.
Motors are seldom loaded up to exactly 100% of rated HP.
A 1000 KVA set may realistically be running most of the time at 25% or less of rated capacity.
That is not good.
You will have bad fuel economy.
You may have cylinder glazing.
You may have oil pumping or slobbering.
Worst case you may have blobs of burning oil blown out of the exhaust.
I have had a new set pump out all of its oil and shut down on low oil pressure the first time in service.
I had a supplier's engineer insist on a five day load test on a new set to set the rings to avoid glazing and oil pumping.
The set capacity was needed to start one large motor. Much of the time it ran at about 10% to 20% of rated capacity.
It is worth the effort and expense to arrange staggered starting of the motors.

Bill
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"Why not the best?"
Jimmy Carter

 

[Bab AK]

Waross, your mentioned points and hints are really useful and it is obvious that you have a long term experience in around diesel generators and emergency systems, thank you so much for your answers and I appreciate your care and time

 

[waross]

Thank you for your kind words.
By the way, the Cat software will allow you to specify a maximum allowable voltage drop when starting motors.
The default voltage drop setting in the software will result in a slightly larger set than my rule of thumb.
If you are working to a specification that includes a maximum allowable voltage drop then you should use one of the free (the last time that I checked) software packages to size your set.
I have many satisfied customers with sets sized to my rule of thumb.


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

 

[Bab AK]

Finally, I have found the way to considering motor start effect on sizing of my diesel generator calculations, but after that the important topic is the correct derating factorcalculations and inserting in in the right position of final conclusion formula.
Here is about 2560 m above sea as altitude
Humidity and corrosive weather could be neglected
Maximum annual temperature is about 40 Celsius degree
Standard frequency is 50 hertz
And also the base loud is 65 KW ( 50 KW lighting and 15 KW panels, PLC, office small loads )

 

[waross]

And also the base loud is 65 KW ( 50 KW lighting and 15 KW panels, PLC, office small loads )

You are safer to use KVA or Amperes to size a generator.
50 KW of lighting ballasts will be more than 50 KVA.
KVA is the generator end limit.
KW is the prime mover requirement.
Neither should be exceeded.

I have assumed the motor KVA as 61 KVA.
Please check the nameplate current rating an use that figure.

Set size example:
Lighting, 50 KW at an assumed PF of 75% = 67 KVA
Office panel, 15 KW at an assumed PF of 95% = 16 KVA
Four motors at 61 KVA = 244 KVA
One motor at 61 KVA, starting = 183 KVA
Recommended set size before derating for altitude and ambient temperature = 510 KVA
If the motors run lightly loaded you may be able to justify using a base number of 500 KVA for a standby rated set.
For a prime rated set, you may use a base number of 464 KVA. The motor starting duty will be well within the allowable overload for a prime rated set.
The difference between a 464 KVA prime rated set and a 510 KVA standby rated set may be a larger oil sump and/or an added oil cooler.
Often no difference.

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