Power factor

[patpappas]

Waste Water Treatment Plant, 480 volt three phase service, using a rustrak data recorder the PF hangs around .46, when the blower motors start it climbs up to .75 and as two or more start it goes to .86. There are no capacitive loads on the system however the PF jumps back and forth from lead to lag. Using a small hand held harmonalyzer the low PF comes from the same MSB (800 amp 480v 3ph) as the blower motors but the PF is down while they rest and improves as they come on line, there are six small (20 amp 480v 3ph) fan motors running almost constantly but I fail to see these do anything so drastic to the PF. The load is surprisingly well balanced, voltage is constant, no signs of any loose connections, checked for the posibility of single phasing motors. I'm stumped, anyone have any ideas?

 

[Smart $]

Are you certain there are no correction cap's involved? Just your statement about jumping between lead and lag makes me wonder :-?

 

[brian john]

Durn SMART you are smart GOOD guess/idea.....then maybe his CT polairty.

 

[Smart $]

...GOOD guess/idea.....then maybe his CT polairty.

Perhaps the increase in PF is indicative of initial lead angle decreasing. That is, perhaps the system has unswitched main PF correction caps. With minimal inductive loading the PF is 'heavily' leading. As inductive loads are added, the leading PF diminishes and the current angle approaches unity—i.e. PF = 1.00. When the system is loaded to the maximum, the PF crosses over into lagging.

Just a guess.

 

[patpappas]

There are no caps anywhere in the building, the average PF from the utility bill is .78 which we pay penality for. We satreted this project wanting to istall capacitor bank for correction then this came up.

 

[rdelauter]

I have to wonder if there are drives the have could be changing the PF when the motors are starting with their own PF correction installed? Yes No?

 

[drbond24]

There are no caps anywhere in the building, the average PF from the utility bill is .78 which we pay penality for. We satreted this project wanting to istall capacitor bank for correction then this came up.

Does your utility bill specify if that 0.78 is leading or lagging? The only way the OP makes sense is if your PF was leading to begin with. Only then would the motors actually be correcting it.

 

[winnie]

Are there any synchronous motors in the facility? Synchronous motors can act as capacitive loads, and in fact synchronous motors without output shafts are sometimes used in place of capacitors.

Note also that a low power factor can be caused by harmonics; if this is the case than a 'leading' or 'lagging' indication might be meaningless.

-Jon

 

[patpappas]

To the best of my knowledge there are no synchronous motors in the building, all I have seen are induction, the Utility does not specify lead or lag in the statement I seen. I do know there are no drives on any motors. The mcc that seems to be the culprit (as per the harmonalyzer) has six small 3 PH 20 amp air exhaust fans.