Generator sizing

Posted by suniltdpa on February 23, 2007, 3:50 am

Hi,
I need to find a relaion between load power(non linear load) and the
rating of the load(emergency generator). I saw in different cases they
will consider it as 3 times the load rating. Anybody help me for
deriving a mathematical relation.
Regards,
Sunil

Posted by Vaughn Simon on February 23, 2007, 6:10 am

     Without bothering to look it up, I believe Real Power (in watts) = VA X
Power Factor.

     That 3X thing is just advertising.  For example: It is normal for
manufacturers of cheap UPS units to assume a .5 Power Factor.  That way that can
advertise a 500-watt UPS as a 1000 VA UPS.

  Your next question should be "How do I know what my power factor is?"

Vaughn

Posted by Norman Webb on February 28, 2007, 7:29 pm

Without bothering to look it up, I believe Real Power (in watts) = VA X
Power Factor. ???????

I thought a power factor of less than 1 gave a real power greater than VA.
Real Power = VA / PF

ie for VA = 200 and PF = 0.5 real power = 200/0.5 = 400

So if you are running a motor you would have to allow for the power factor
correction.

However STARTUP CURRENT is the biggy with motors.
I have a 70W bar fridge and it makes my 500W (750W peak) petrol generator
grunt every time it starts.

Regards
Norm (unashamed top poster )

Vaughn Simon wrote in message ...

that can

Posted by John on February 28, 2007, 8:19 pm

You should have looked it up.

PF = RP / AP

where

PF - power factor (0 - 1)
RP - real power (watts)
AP - apparent power (volt amps)

Posted by Norman Webb on March 1, 2007, 5:42 pm

John wrote in message ...

Thanks. Glad someone is watching out for bullshit.

I confused real power with apparent power. I knew the PF was 1 or less and
why the power companies get pissed if you are running low PF equipment.
Regards
Norm