Small generator sizing

[schwarz]

Hi All,
We often have to size Diesel genset for telecom sites. My questions are A) how much continuous(8-12 hours) KW load (0.9PF) & no A/C can be put on a 1 PHASE 10KVA DG set. B)If we have SMPS load of 7.5KW (0.9PF) and A/C load 2KW (0.6pf) what should be min. DG set capacity in KVA &KW? Regards

 

[rbulsara]

The manufacurer of the genset will tell you if the unit is rated for continuous duty or not. Most gen sets are only stand-by rated. Nameplages usually inidcate this on larger units. Not sure of the smaller units.

 

[itsmoked]

Figuring the load on a generator changes from add this and that to something else when motors are in the picture because they require large starting currents.

Would the 7.5kW load be present when the 2kW A/C starts or would the A/C always be started first?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

7.5 KW @ .9PF = 8.33 KVA SMPS
2KW @ .6PF = 3.33 KVA A/C Running.
4KW Starting allowance for A/C
13.5 KW, Minimum.
Recomended rated PF 0.8, for 16.875 KVA
I assume random starting until proven otherwise.

Sets may be ordered as Standby, Prime power or Continous.
Your application would be prime power.
respectfully

 

[itsmoked]

Thanks Bill.
What's the difference between 'Prime power' and 'Continuous'?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Hi Keith.
I was unsuccesful looking for the definitive definition, but:
Prime power, supplying a variable load as the first or only source of power.
Continous power, running at a fixed, high power level such as a co-generation station that typically runs flat out. Load variations are dealt with by a different part of the system.
The diesel engine is typically rated at 100% of the KW rating for standby.
The diesel engine is often rated at 125% of the KW rating of a continous or prime set. This facilitates running the full, typical, 15,000 hours between overhauls. The engine can get old and tired and still carry the load.
respectfully

 

[TheBlacksmith]

Don't take this the wrong way, but have the diesel engine and genset manufacturers fired all their sales people? When I was in that business we had all kinds of charts, pocket whizwheels and computer programs to analyze the customers need and recommend a solution. Then the buyer just picked the lowest price, best sounding name, guy that picked up the most expensive restuarant tab, whatever. Why is everybody trying to do it themselves?

 

[waross]

Hi TheBlacksmith
That's a good question and I hope I have a good answer.
I do it myself after being asked to by 8 or 10 customers who were suffering with grossly undersized standby sets.
The originals were the lowest price.
Originals, 14 KW.
Replacements, ranged from 24 KW to 40 KW.
All customers are now happy with their sets now.
I have two other instances of undersized generators where we curtail loading when on standby.
1st. Rated at 240 volts when it should have been 208 volts.
Undesized by a factor of 208/240 or 87%.
2nd. Rated at 3 phase when 1 phase was ordered. Undersized by 2/3 or %67.
respectfully

 

[giuseppe]

Hi Theblacksmith,

unfortunately, the approach you stated appears to be the typical approach that possibly leads to disasters. When a seller is leading the choice of a generator, he will invariably try to sell the most expensive stock, regardless to the efficiency and reliability required, as we electrical may have to acknowledge at a later stage.
In Europe we are habit to do our homework producing a Data Sheets on generators and then ask the Manufacturer to compile only tiny parts of it, just to complete the typology of the generator they try to sell and convince us that it is what we are looking for.
If you don't know what type and size of generator you are looking for, it is always advisable to consult an expert, this being, of course, not the seller. Of course, it could be the Technical Office of the Manufacturer, sometime. If you want a nice restaurant then, better to pay for it with the savings made in choosing the right machinery...
But you're right anyway: the old fashion sale persons are not around anymore...unfortunately!
Sincerely.

 

[rheikaus]

I don't know about you guys, but in NYC you cannot sneeze without stamped drawings from our buddies in the consulting engineering firms. My job working for the manufacturer is to educate the CE's on the hardware so they can spec the right stuff. Good arrangement, as the engineers get paid for the job and I make sure the client gets the right equipment -- and has someone other than me to blame if not .

 

[schwarz]

Hi All,
my query is not fully answered.The GENSET is meant to be standby duty, typical 8 hours a day. We normally program A/C to start late or early.
Gensets we have specs like say for 15KVA GENSET- 15KVA continuous and 16.5KVA standby max.rating at 0.8 pf. This model has 19bhp @1500rpm engine.
warros- how do do we get this figure '13.5 KW, Minimum.' as 7.5KW and 4KW adds up to 11.5KW.
pls also explain -"2nd. Rated at 3 phase when 1 phase was ordered. Undersized by 2/3 or %67"
Idea is to just right size & right cost GENSET.Often we add up factors and in actual running GENSET is at 30%load which I understand does not help.

 

[davidbeach]

If you enter your projected loads into the gen-set manufacturer's sizing software what size does the software say you need?

 

[schwarz]

davidbeach: Our suppliers do not have such software.

 

[waross]

Hi schwarz;

my query is not fully answered.The GENSET is meant to be standby duty, typical 8 hours a day. We normally program A/C to start late or early.
Gensets we have specs like say for 15KVA GENSET- 15KVA continuous and 16.5KVA standby max.rating at 0.8 pf. This model has 19bhp @1500rpm engine.

A typical wording on a continous (or prime) rated set may be
"10% overload allowed for 1 hour out of 10 hours"
Standby sets are expected to run far fewer hours, so the 16.5 KVA is allowed at any time.
Apparently not all manufacturers make a distinction between "Continuous Power" and "Prime Power".
Some manufacturers use the same set with different ratings for standby and prime (or continuous). Some manufacturers add bigger oil sumps and possibly other options (oil coolers, larger fans, larger radiators) for continuous power or prime power sets.

warros- how do do we get this figure '13.5 KW, Minimum.' as 7.5KW and 4KW adds up to 11.5KW.

I added 2 KW for running the A/C, and added an additional 4 KW for the starting demand of the A/C unit.
I use a factor of three for starting an A/C unit.
For more than one A/C I add the KWs of the A/Cs and then add 2 X the largest unit for a starting allowance. I have had good luck with these factors for a number of installations of A/Cs on generators.
I appologise for not being more clear in my original post.

pls also explain -"2nd. Rated at 3 phase when 1 phase was ordered. Undersized by 2/3 or %67"

The customer required a 40 KVA set at 120/240 volts. Single phase alternators are not generally manufactured in this size. Typically a three phase set rated at 60 KVA will be used. The alternator will be reconnected for single phase (the double delta connection is the normal connection). The set will be re-rated to 40 KVA. The engine is now oversized, so the allowable power factor increases to unity.
The salesman knew that 3 phase sets were used for single phase but he didn't know that they must be re-rated. He sold a three phase, 40 KVA set that was rerated to 27 KVA.
I called the salesman and he was ademant that the set was properly sized. I pointed out that the main breaker was too small to allow a 40 KVA load. He referred me to the technical department with an appology for sending the wrong breaker.
The tech department explained the de-rating to me very politely. I asked for an explanation in writing. I took the explanation to my customer who went back to the managment of the vendor.
The set had been shipped from the US to Central America, and exchanging it was not a reasonable option. The vendor was reputable and gave a considerable discount to an already low price. The customer was happy to accept the offer and work with me on a curtailing scheme.
NOTE; schwarz;
From the specs, your set appears to be a single phase alternator. This is common at 15 KVA. Above about 20 KVA, reconnected three phase sets become common. This is more in answer to the suggestion to ask the salesman.
respectfully