.8 pf

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72.5kv

72.5kv

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How did .8 pf on large power generators come about?

Is it becasue the average motor has a pf of .8
 
72.5kv said:
How did .8 pf on large power generators come about?

Is it becasue the average motor has a pf of .8
Click to expand...
PF is determined by the load. If your load is mainly motors,the PF can be 0.80 To correct this, you need to add the correct number of capacitors.
Edit: correct 0 to 0.80
 
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72.5kv said:
How did .8 pf on large power generators come about?

Is it because the average motor has a pf of .8
Click to expand...
When I worked for a major utility (1973) in a power station, I was told that the ratings are at 0.8 because that is the typical load. By changing the exciter (field current), we could modify an individual generator's PF. More current, and lower lagging PF, was more stable. I was told stories of dropping rotor current and having the synchronous generator drop; high current and magnetic field strength was considered safer and better operation.
 
i understand that the load is what set up pf.
eg 1000Kw genset with a pf. of .8

S= 1250KVA

S=1000-j750K

is it correct that the .8pf value indicate how much reactive power this machine can deliver without damaging itself
 
72.5kv said:
is it correct that the .8pf value indicate how much reactive power this machine can deliver without damaging itself
Click to expand...

That is only true if the machine is putting out rated kilowatts (real power). At lower kilowatts the machine can supply more reactive power (kilovars) based on its reactive capability curve. At lower KW loads, it may be possible to supply 120% or more of rated KVAR. Operating power factor will be less than 0.8, maybe even as low as 0.2 or 0.1.
 
bob said:
PF is determined by the load. If your load is mainly motors,the PF can be 0. To correct this, you need to add the correct number of capacitors.
Click to expand...


I thought most motors ran in the 0.8 PF range. How would the PF ever approach 0? The only way I could see this happening would be if the motors were completely unloaded. Then they might look like a reactive load, but the total draw from the source would be so low it wouldn't really matter.

STeve
 
steve66 said:
I thought most motors ran in the 0.8 PF range. How would the PF ever approach 0? The only way I could see this happening would be if the motors were completely unloaded. Then they might look like a reactive load, but the total draw from the source would be so low it wouldn't really matter.

STeve
Click to expand...
Thats a typo. It sould have been 0.80. My error.
 
I'm a journeyman, not an engineer so if I'm wrong, please correct.

My take on the .8 PF has to do with the economics of wire size vs. engine size. KVA has to do with wire size, overcurrent protection, etc. (amps) KW has to do with how hard the prime mover has to turn the shaft. (horsepower).

Here's an example; let's say we have a generator that's 1000kva and a PF of 1.0. That would be 1000 kw as well. Let's say the prime mover is a diesel engine. In my experience, this engine would be around 1400 hp. Now let's say it is powering a plant with a bunch of motors and transformers, all lightly loaded. Let's say the PF is .5. If the plant is 480v 3ph, and the current is 1200 amps, that's about 1000KVA. The generator stator winding and plant wiring are loaded to the gills. But the KW is only about 500. Our 1400 hp diesel is running at less than half of it's capacity. Alot of wasted iron.

This is an extreme example, and it wouldn't make much difference on a 50KVA unit, but what about a 2000Kva one? In my experience, most 3ph services over a few hundred amps will run .75 to .85PF whether it's a manufacturing plant, high-rise, hospital, or whatever. Why have such a huge engine if it's never run hard? Diesels generally don't like to just plug along, they like to work. Hence the realistic .8PF.
 
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