size diesel motor and generator 1

[dcset]

Trying to start 4 motor 330kw and two motors of165 kw, on direct start Mechanical output 330 kw Volts 4160 V current nominal 68 A pf 0,72 eficiency 0,9 start current 243 A starting torque 0,94 pf on starting 0,1. the other motor dates are mechanical output 165 kw, volts 4160 V, currento full load 33 A, pf 0,72,eficiency 0,94, current on start 128 A, pf on star 0,16, i´m thinking on using a generator 4000 kva, pf 0,7, 4160 V eficiency 0,95, with a boost system to mantain excitation.
When i got all the motor except one started, i got nominal mechanical power from motors 1320 kw, electrical power kw demanded for the motors 1400,7 kw, and pf 0,72,
then we try to start one of the 330 kw motors on direct start, and i get 1663,350 mechanical power on starting, and 1663,35 kw electrical demanding on the start moment from the motor, this means diesel motor demands of 1750 kw mechanical on shaft, with a power demanded from generator of 3696 kva, and pf 0,45., i just wonder if generator is enough and if a motor of about 2177 kw mechanical will be enough.
Cause customers says me that as the motors are 6 poles and ten poles electrical motors, we have to uprate the unit, cause the generator is four poles, i couldn´t understand this point, for me doesn´t matter what is the internal poles of the motor, we are talking about power mechanical and electrical, i know i have two questions, first what do you think about my calculations, and second, what do you think about the question about relation between poles on generator and motor.
thanks a lot for your time

 

[rbulsara]

You need to accommodate both kW demand and kVA demand. Staring methods had a great influence on generator sizing.

See faq237-766

The generator frequency need to match the nameplate frequency of the motor. It has nothing to do with the internal poles. You should consider soft start or reduced voltage starting.

Involve some more experienced engineer in the analysis. While generator may be able to start a motor, resulting voltage sag could be problematic for the starter and other loads on the bus.

Rafiq Bulsara

http://www.srengineersct.com

 

[dcset]

of course, generator is 60 hz and motors also, but i wonder about what this guys are talking about.... cause when i don´t understand never think about others fault.

 

[rbulsara]

You have to ask them for their explanation. As for validity of your calcs, read the FAQ and draw your own conclusions. Allow for efficiencies and step load responses of the engines.

There may be other factors in the project and on site, that may be necessary to consider before arriving at a final design. Having a alternator bigger than engine rating would be a custom unit.

Seeking some professional advice, would be beneficial. Free advice is worth just that.

Rafiq Bulsara

http://www.srengineersct.com

 

[dcset]

Ok, seems to be, that in any way, nobody hear nothing about that relation between number of poles of motor and generator, for me seems to be no sense, but i´m not a expert.
And about my calculations, if the generator can give 300% of the nominal kva during 10 seconds, we could think no big dip on voltage, cause when we start the lst motor we would be on 48% of the rates power of generator,
if our voltage didn´t droop more than in example 20% do you think we could get some problem for the last motor.
Cause they said that with this values, we couldn´t even start more than 4 of the motors.....(330 kw)

 

[rbulsara]

You need to do a proper analysis. There is no point in making assumptions.

Rafiq Bulsara

http://www.srengineersct.com

 

[dcset]

is there any tool on internet for doing it?
thanks

 

[catserveng]

CAT, Cummins, Generac, and Kohler all have very good sizing softwares that work well MOST of the time.

A google search of "Generator Sizing Calculators" yielded a number of links.

 

[waross]

Sum of all motors in kW or KVA, plus an additional 200% of the largest motor.
Oversize the Alernator??
Better to oversize the prime mover.
See:
DETERMINE THE SIZE OF GENERATOR
thread237-265425

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

 

[dcset]

Better to oversize the prime mover- waross said
Why you want to oversize prime mover, on starting the kw demand as the pf is low is not high, in the numbers i put from the test banch of the motors, they are water pumps, and i wonder if i could have same issue with this values of starting currents, and pf, and about motor..... but i don´t understand why to oversize to much the prime mover, based on your experience, does this motors could be started with the proposed generator with a pmg.... and i repeat, maybe you waross knows more about this, does any relation between the poles of the motors and the poles of the alternator?
thanks a lot

 

[waross]

Forget the ratios between real power and reactive power and remember that when starting a large motor on a gen-set, the alternator may be electrically overloaded (KVA) and the prime mover may be mechanically overloaded (kW).
The AVR will respond to keep the voltage up and the alternator will withstand the temporary overload quite well.
The prime mover does not respond to either block loading or overloading as well. If the kW overload slows the prime mover the UFRO will kick in and a Voltage dip is unavoidable.
An oversized diesel will have a much longer life. Prime power sets may be oversized so at the end of their life, when the set badly needs a major overhaul, it can still drive the full electrical load.
10% oversized is standard on prime power sets. I have seen sets with the diesel engine spec'ed at 125% of the alternator rating. Or think of it as a 500 KVA set rated at a power factor of 100%.

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

 

[dcset]

thanks, but i would like, just now i´m asking, if somebody could to make a quick calculation on my numbers and see if i could start the pumps or no... comparing with
a normal diesel motor and pmg generator or boost generator, cause i oversized generator for the run load, about 49% of rates values, and motor about 17% of rates kw on run condition.... maybe i select a generator too big, but i want to avoid the dip on the voltage....
thanks to all, you´re really gently.

 

[waross]

By the rule of thumb you would need 2300kW/2875KVA for a mixed load and 2145kW/2681KVA for motors only.
The Cat sizing software calls for 2000kW/2500KVA. Experience and judgment tell us that when sizing a group of large motors such as this the set may be a little less than "Rule of Thumb". This philosophy is reflected in the Cat Sizing software.
BUT, the allowed frequency and voltage drops are both 30%, with a calculated frequency drop of 5% and a calculated voltage drop of 21%.
I find myself corrected on some of my statements. Cat is over sizing alternators for some loads.
If we want to go to a 10% voltage dip, Cat recommends the alternator be increased to 2650 KVA.
Go on the Cat website and download the sizing software or ask your vendor to provide you with sizing software, do the sizing or both. With your own software you may try various scenarios and possibly make some judgment decssions.
As an example, you may arbitrarily decide that you want the voltage drop when starting motors to be 10% or less. You try the calculations based on a 15% voltage drop. You may find that one motor causes a 10.8% voltage drop on starting and all the other motors cause less than 10%. The soft ware has selected a larger set to bring the 10.8% voltage drop below 10%. You may be able to justify a less expensive set by allowing one motor to slightly exceed the arbitrary 10% voltage drop limit.
Sign on to Cat and download the software.

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

 

[dcset]

thanks again waross, your reply is very nice, do you know exactly or a a link to the catterpiller web where i could see this calculation program, but it´s seen that i use a big generator cause mine is 4000 kva, and the motor is no so big... i need maybe 100kw more on power to achieve the rule of thumb, ok, again thanks a lot, cause i was really worried about cause the customer said me a lot of crazy stories about other people that install motor in applications like that and they fail on starting, and then this people started to talk about the diference between the generator poles and motor poles, and for me i really don´t understand then, but now its more clear, and if i could cheek this calculation program is ok, i try to look on caterpiller web but it´s too huge for checking
thanks

 

[dpc]

There's this new-fangled thing called "Google". You should check it out sometime:

http://www.cat.com/cda/layout?f=111183&m=37508&x=7

David Castor

http://www.cvoes.com

 

[dcset]

hi, thanks dpc, i tried on caterpillar web and i found a lot info about scavengers, etc, that i thought maybe someone on the forum know the right adress, thanks to everybody for help and i´m going to try this software

 

[BrentTSL]

dcset,

Be careful when you size your required load using CAT or any software (it is only as good as the info you put into it)

Make sure you put the correct starting sequences into the software. i.e. 1st load step = 2x330kW etc. 2nd load step etc.

Also the max ambient operating temperature will have a bearing on the final analysis

macfarlane generators has a quick and simple tool if you want a ball park (without having to download software)

good luck