Low P.F.

[surineman]

Hi there,

I have a large amout of mixers in my plant with low P.F.

(0.51 @ 100% load ).
There are 50 of them each 20 HP (600 V) and 36.0 A FLA. (High current)
With this P.F. I probably have to double size of transformer.

Or

Is there any way around this? what about capacitor banks?
I need some reference and feedback about them.

Thanks in advance.

Regards,

 

[wareagle]

Capacitors will corect the pf and reduce the required transformer size. The pf on these motors seem very low. Is there any thing special about the motor. Warning: If the motors have variable frequency drives you need to be very careful when adding capacitors. Drives and Caps do not like each other. Using table 430.150 in the NEC a 208 volt 20 hp is rated at 59 amps and at 480 27 amps three phase.

 

[jraef]

You have something very out of the ordinary here. 20HP at 600V should be around 20 FLA. Even if it were a 1.3 service factor motor, that FLA is still too high. On top of that, if the motor were that loaded, the PF would be higher, not lower. It sounds almost as if you have a 60HP or 75HP motor that is running seriously UNloaded. Don't start adding things to solve a problem that is as yet unidentified. Post ALL of the motor nameplate info. and maybe we can help. Also, post a description of how you determined that power factor. Is it calculated or are you reading it from a PF meter?

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[digitrex]

Motors with very low PF must be of low speed type. I have 6.6kV 1.2MW motors which are of 10poles type and the nameplate PF are 0.57. The frame sizes are much bigger compared to 2 or 4 poles type.

Capacitors as suggested by wareagle is a good idea, but need to check the harmonics.

On the other hand, if those 50 small low PF motors constitue a small percentage of the total plant load, and they don't incur penalty on your utility bills, may be you can just increase the transformer size because I would prefer one equipment less to be maintained in a plant.

 

[keewee]

Surinenam, your figures of 20 HP motors , running at 36A FLA, and a PF of 0.5 doesn't sound right. What kind of motors are they ? Why do you have to double the size of the transformer ? Are those motors an addition ?

All the replies in this thread are valuable here. I think Digitrex raises the most salient point that if you are meeting the PoCo PF requirement at the supply terminals, then no correction is needed.

 

[surineman]

Everyone,

the motor is a Flygt mixer model 4670, here are some information they provide on their website:


Rated Power:14.9kW 20.0 hp
voltage:600 v
Speed (r/min):440
Rated Current: 36.0 A
No load Current: 28.0 A
Locked Rotor Current: 105.0 A
Starting kVA: 109.2
EEMAC CODE : F
Efficiency @ 100% load: 77.5
Efficiency @ 75% load: 76.0
Efficiency @ 50% load: 71.0
Power Factor @ 100% load: 51.0
Power Factor @ 75% load: 43.0
Power Factor @ 50% load: 34.0
Power Factor @ start: 36.0

It would be 50 of them in the plant, at least 25 of them running together.

These are the major loads, although I have a 320A main breaker dewatering unit with some VFDs inside, and a 200 A service for Admin. building and workshop.

Regards,