fanyu622
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- Location
- montreal, quebec, canada
Power factor correction capacitor sizing for a wye-delta motor connection
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Julius Right
Senior Member
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- Electrical Engineer Power Station Physical Design Retired
In my opinion, the problem is a little bit more complicated. See[for instance]:
https://www.scribd.com/document/108805884/IEEE-White-Paper-Capacitor-Application-Issues
https://www.scribd.com/document/108805884/IEEE-White-Paper-Capacitor-Application-Issues
kwired
Electron manager
- Location
- NE Nebraska
I assume you are saying motor is only loaded to around 58% of it's rating, current wise.
Keep in mind power factor usually varies as load varies, and if this motor is normally always operating at this current level then you very likely should base your correction values according to what the normal power factor values are, not what the full load rated values are.
First of all you will have a different PF triangle for the 58% current then you will for 100% current. Though power factor is likely lower at lower then nameplate load, the load (watts) is also lower which together those two will change the actual running KVAR as well from what it would be at full load rating.
I think his ? is does he size the cap for the Y starting i (FLA/sqrt 3 or 0.577 x FLA) or for the delta running 100% FLA
???
I would size for FLA and check against starting amps (keep pf <0.95 lag)
adjust kvar within those limits
Although corrected pf should be similar at both points
starting kva is lower but so is pf
running kva will be larger but with improved pf
so correction (kvar delta) for a given kvar should be similar at both points
may sketch it out later
???
I would size for FLA and check against starting amps (keep pf <0.95 lag)
adjust kvar within those limits
Although corrected pf should be similar at both points
starting kva is lower but so is pf
running kva will be larger but with improved pf
so correction (kvar delta) for a given kvar should be similar at both points
may sketch it out later
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Wye-Delta connections are usually used for soft starting the motor. The PF correction would normally be sized and applied to the running connection. If the starting time is severally long PF correction might be employed, but it is pretty much a moving target as the motor gets up to normal speed.
Starting capacitance is something usually associated with single phase motors and 'add-a-phase' systems.
Starting capacitance is something usually associated with single phase motors and 'add-a-phase' systems.
kwired
Electron manager
- Location
- NE Nebraska
I had thought of it being a question of sizing during starting.
Most of the time PF correction is done to minimize penalties from POCO. The duration of any non optimal PF during starting likely doesn't have much impact on any assessed penalties to be worth trying to correct in most instances, especially for a wye -delta starting method that is likely only in the wye connection for just a second at the most during starting. Once you get past that initial surge of energizing current you need full voltage to have enough torque for accelerating.
I think the 58% comes from 1/sqrt(3), the ratio of the wye to delta voltage.
Based on my experience, the motor reactive component doesn't change much with load. That's why a single, fixed value PFCC is usually what gets fitted.
kwired
Electron manager
- Location
- NE Nebraska
OK, so with a wye start delta run, is it worth while to even consider what the power factor may be during the very brief wye start period, especially if power factor averaged over a longer time period is what you pay penalties for and is only reason you are correcting PF in the first place?
Other special circumstances, may be different, but POCO penalties is likely the main reason most correction happens when the source is the the POCO.
I agree.The brief wye start can be pretty much disregarded in my opinion.
Most likely the starting condition can be ignored
but depends on a few things
Start under load?
duration
starts per hour
etc
but it costs nothing but a few minutes to see the impact of a cap sized for the run mode on the start mode
alot depends on the specific motor curves
but depends on a few things
Start under load?
duration
starts per hour
etc
but it costs nothing but a few minutes to see the impact of a cap sized for the run mode on the start mode
alot depends on the specific motor curves
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
The purpose of PFC caps is to correct the power factor of the motor when running. It does not matter where they are in the circuit as long as they are in parallel to the motor. They are sized to raise the PF to .95 or better by adding VARs to compensate for the inductive properties of the motor. Using the .58 value of the contactor in the sizing of the caps simply results in the caps being under sized by 42%. Might as well not bother putting them in at all because if the capacitance is insufficient to raise the PF to .95 or better, you will still suffer the penalty from the utility.
Given the potentially dangerous torque spikes that can result in using Y-Delta starting, I would never arrange for caps to be on line until after it has transitioned to Delta. For that reason, I always add a 4th contactor feeding the caps, controlled by the aux contacts so that the caps do not come on line until the Wye contactor opens. So you put a NC aux from the Wye contactor in series with a NO six from the main contactor.
Given the potentially dangerous torque spikes that can result in using Y-Delta starting, I would never arrange for caps to be on line until after it has transitioned to Delta. For that reason, I always add a 4th contactor feeding the caps, controlled by the aux contacts so that the caps do not come on line until the Wye contactor opens. So you put a NC aux from the Wye contactor in series with a NO six from the main contactor.
I agree with that. We always fitted a contactor for the PFC.
There is another reason too. If you don't disconnect the PFC, the motor can self excite. In fact, a good many years ago, we looked into doing that to get an inexpensive generator using a submersible motor/pumpset. We happened to have a 30kW submersible motor as part of a test rig and we got it up and running and producing significant output.
fanyu622
Member
- Location
- montreal, quebec, canada
Thanks a lot for the answer.
I sized the capacitor simply based on the motor KW. 58% was not in the calculation.
C=VAR/(6*pi*f*V2), and in delta connection, Vp=VL --> current is irrelevant, size of the capacitor that delivers the needed VAR will not change in this case whether being put on the main line, or the delta.
Last edited:
How did you arrive at the VAr and why 6 rather than 2?
what C (or VAr) did you arrive at?
fanyu622
Member
- Location
- montreal, quebec, canada
To solve for C using your equation you must know the VAr
how did you determine that value?
what is it? 50, 100, etc ???
what is?
voltage
HP
FLA
non-corrected or motor pf
???
fanyu622
Member
- Location
- montreal, quebec, canada
I calculated the VAR of the capacitor using ---> (VAR=KW x multiplier Kk from table)
The relation between VAR and C was just proving that I don't have to multiply the above equation with 58% to get the VAR, if the capacitors are placed on the Delta line as shown on the first post.
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
FanYu...
Are your Caps configured in Delta or Wye?
Regards, Phil Corso
Are your Caps configured in Delta or Wye?
Regards, Phil Corso
Um......check the diagram provided by the OP.
fanyu622
Member
- Location
- montreal, quebec, canada
The caps are configured in Delta.
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