Power Generator Sizing

Some generators voltage regulators are negatively affected by soft starters and VFDs. I would suggest contacting the generator manufacturer concerning these issues.

MS, tks for youe reply,

Hien

Ms, it is me again for the question.

As I referred to ABB soft starter handbook, it is implied that normally the sort starter's stating current is around 3 to 4 times FLC. Since the motors are rather big so could you explain it again?

Nevertheless, I referred to some other manual provided by CAT - generator supplier. They recommended one calculation method based on NEMA std and it is so complicated . To do do this, I must gather many information such as SkVA/hp, RLC, Motor code... But these things are just applied within the US and we are not. In fact, do we need to follow this procedure? I took my looks over some suppliers like CAT, Siemens... they also recommended the same procedure. To be concluded, I found that whatever the method they are using, the original is to based on the starting capacity of the motor.

I have been struggling on how to grasp one good and simple method of generator sizing. I dont want to design by myself and I want to follow some standardized procedures but it seems impossible.

Hien

MS, I suggest to close the topic here since your answer said it all. It is very useful and I think I have got what I have been looking for.

Hope you will be always fine. Please contribute more for this forum. Thank you,

Hien

I have provided answers to some Hien's questions by email. For further reference, I pasted them here so that other engineers can be benefited in future:

1. At this case, we can start the biggest cooling pump first, then we go for the rest pumps. The generator capacity would be much reduced. Right? '

Yes, it will reduce to at least 50 KW to 100 KW

2. What does "voltage dip" mean? And 10, 20 or 30% are our requirement? It means if I am the customer, I will request the supplier for this?

Voltage dip is the momentary voltage drop within the generator during first few cycles of the motor starting. It is due to the synchronous reactance of the generator. The higher the generator capacity, the lower the reactance value, the lower is the voltage dip. See this link: http://www.re-volt.us/pdf/CPS_info_sheet_38_Voltage_Dip.pdf. This curve is almost same as NEMA 32.18.2 (definition of voltage dip).

3. Is that Ok with 30% voltage dip? Does it affect to the controller e.g the contactor coils will be unable to hold closing? In the case this generator will have to provide power for the lighting system, will 30% voltage dip be accepted?

Unless the system has very voltage sensitive loads (sensitive relay, contactor), 30% voltage dip is ok.

4. Is voltage dip different from voltage drop and how it is different? Normally, we need the voltage drop during motor starting to be kept within 10%. I don't understand why they have three kinds of voltage dip?

Yes. As already mentioned, the voltage dip is the momentary (few cycles) voltage drop within the generator, voltage drop is the within the cable and transformers.

5 . If some educational books say that we just need to gather all of starting current of the motors. Then choose the generator whose running current is equal of higher than these total starting currents. Is that not clear and incorrect?

This is correct if all of motors are started same time. However, it is even not 100% correct.

6. I have contacted some local suppliers and they obviously don't understand about the sKVA that the generator can provide during motor starting.

These are so technical terms; generally local suppliers don't have this level of knowledge. They generally use some short-cut calculation.

7. If all these things have been go through, do I need to double check by recalculate the "voltage drop" in the cable during motor starting. We need it within 10%.

Yes, it is correct. You can go up to 20% for total voltage drop during starting (for NEMA motor). After starting is over, it should be not more than 5%.

See more information in these sites:

http://ecmweb.com/training/electrical_basics/electric_sizing_gensets_large

http://www.buyerzone.com/facilities/generators/rbic-power-quality-08.html

http://www.myemergencygenerator.com/site/1324593/page/555882

http://www.gregorypoole.com/products/electricpowergenerator/Documents/LEBE5294-02%20Electric%20Power%20Application%20Sizing.pdf

http://www.dieselduck.ca/machine/03%20electricity/alternator.htm

http://www.cashmanequipment.com/UserFiles/Uploaded/cms/motor-start-concerns.pdf

http://www.gillettegenerators.com/sizing/sizing03.html

http://forums.mikeholt.com/showthread.php?p=1161144

http://www.buyerzone.com/facilities/generators/rbic-power-quality-01.html

http://www.kwietpower.com/_docs/motor_starting/sizing_inst.pdf

http://www.kohlerpower.com/common/pdfs/83474_GensetMotorStarting_Final.pdf

- MS

Nice work.

if we using VVVFD or VFD what will be the capacity of generator for 250 KW motor