How long to idle 750 hp motor for cooling 4

[bacon4life]

How long should a 750 hp motor run at idle after unloading it to cool down the motor? This motor powers a water pump via a magnetic drive. The pump currently runs at idle for 30 minutes before shutting down. I was told the idle period was to cool down the pump, though I do not specifically know if the person was referring to the electric motor, the vertical lift water turbine, or the magnetic drive.

This long idle time appears to dramatically lower in overall power factor of the installation during months with low run times. Sometimes the installation has a power factor below 0.5, so the pump appears to be working for just a few minutes before idling for 30 minutes.

I remember thread mentioning that forcing a 2x start per hour motor to always run for at least 30 minutes might be much simpler control circuit than actually controlling the starts per hour. Before pursuing power factor correction, just want to check if there are any controls adjustments that should be made first.

 

[edison123]

In non-VFD environment, use of PFC is an universal practice with strong theoretical base to fix low PF's. In the absence of any motor voltage control during 'idling time', PFC is the only way to go. Questioning that time tested practice is just contrarian bullshit.

Muthu

http://www.edison.co.in

 

[LionelHutz]

Questioning if power factor capacitors should be used is not contrarian bullshit.

1st is determining if it's possible to stop running running a 750hp motor at idle for a large portion of 30 minutes every time some water must be pumped.

Only after it's determined that the answer to the above is there is no better way to pump that water should the power factor be investigated further. It is necessary to determine if adding power factor correction will actually provide a pay back. Every bill I have looked at uses a moving window peak kVA and kW demand recorder. The window is typically 15 minutes, but I've seen this vary. The demand charges are based on 90% of the kVA or the kW, whichever is higher. With this type of billing, if the motor runs loaded long enough just once in a billing period then there will be no demand savings possible by correcting the poor idle power factor.

I've seen bills where demand charges were being paid and the average running power factor of the site was crap yet capacitor payback was still in the 5 year range. The average operating power factor of a site doesn't tell you much about the demand charges.

 

[waross]

The bills that I have looked at all had separate charges for demand and power factor.
If you have to run a 750 HP motor you have to pay the demand for a 750 HP motor.
The good news is that once you run it continually for 15 or 20 minutes, your demand for the month is established, whether you run 15 minutes a month or 24/7.
The power factor payback in most plants is less than a year.
However, rain water pumping is a special case where there may be long periods of inactivity.
In such cases, the payback for PF correction can easily run into years.
It was easy back in the day when PF was calculated on the monthly KVARHrs and the KWHrs.
You could leave a small capacitor bank on 24/7and put KVARHrs "in the bank" to get your monthly average PF down.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

There isn't a utility around here where "banking" kVAR with a small capacitor would be of any use.

 

[waross]

Putting KVARHrs "In the Bank."
Many years ago, KARHrs were measured with a standard KWHr meter with a 90 degree phase shift device shifting the voltage.
Leading KVARHrs would run the meter backwards.
Later, ratchets were incorporated so that the meters were insensitive to leading VARs.
However, PF penalties started at 90% lagging. Running at 100% PF during idle hours put a buffer "In the bank" to be used to improve the overall average PF.
I have heard unverified reports that some utilities are recording all KVARHrs, leading and lagging and basing the penalties on that value.
That effectively blows the old "Art of PF Correction" out of the water.
Either correct each individual machine to 100% or install a Power Factor Control system.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Every bill I have looked at uses a moving window peak kVA and kW demand recorder. The window is typically 15 minutes, but I've seen this vary.

Agreed. I have seen specs for thermal demand meters with a 30 minute response but I have never seen one in the field.

The demand charges are based on 90% of the kVA or the kW, whichever is higher.

Back in the day, the demand was based entirely on KVA.
But with regard to the new tariff, the Utilities give nothing away.
Included with the shift to demand charges of 90% of the kVA or the kW, whichever is higher, would be an increase in the rate of about 10%.
The result?
If your PF is below 90% no real change in the charges.
If you have corrected your PF to above 90% you will be indirectly penalized for your good power factor.
Would a utility do that?
Are you following this thread Keith?
Do you think that Pacific Gas and Extortion would stoop to such a trick? grin
In so many fields of endeavor, the subtle tricks used by the better operators to gain a little advantage are no longer possible due to increased sophistication of the equipment, electronic and mechanical.
Case in point, automatic transmissions are becoming so common in over the highway trucks that it is getting more and more difficult to find a younger truck driver who can shift a 10 speed, 13 speed, 15 speed or 18 speed automatic transmission.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Automatic or manual? If the can’t shift an automatic there’s a real problem.

 

[LionelHutz]

Many years ago and back in the day it might have helped, but I still have never seen a utility bill where "banking" kVAR with a small capacitor would be of any use.

 

[waross]

Good point David.
I am contrasting the conventional stick shift and clutch with an automatic transmission.
Getting too far off topic. I am deleting the balance of this post.
--------------------
"Why not the best?"
Jimmy Carter

 

[bacon4life]

The eddy current drives description at

https://www.dynamatic.com/drives/dynamatic-water-wastewater-drives/

matches up quite well with the customer's description of their system. The Dynamatic drives can ramp the flows rate to avoid hydraulic transients. The Dynamatic drives do not have any sort of cooldown requirement, so still need to figure out where the cooldown practice came from.

 

[bacon4life]

I am surprised that any of you have experienced multiple utilities calculated power factor charges the same way. I checked the power factor charges at 12 utilities per the links below, and found almost that many unique formulas. Lumping the calculations in to broad categories:

-Five utilities used some sort of average power factor​

-Four utilities used a cost per kvarh charge​

-One utility used a peak calculation​

-Two utilities did not publish a clear methodology within the rate tariff documents that I could find​

Average power factor:

https://www.aepohio.com/account/bills/rates/AEPOhioRatesTariffsOH.aspx

https://www.myavista.com/about-us/our-rates-and-tariffs

https://ec.europa.eu/eurostat/docum...2012.pdf/692ab75f-c69a-4574-9eae-84469504bc8c

https://www.mytpu.org/wp-content/uploads/All-Schedules-2020_Emergency-Rate-Delay.pdf

https://www.snopud.com/Site/Content/Documents/rates/electricrates_010720.pdf

Cost per kvar:

https://www.ladwp.com/ladwp/faces/l..._afrWindowMode=0&_adf.ctrl-state=1bifaysbt0_4

https://www.seattle.gov/light/Rates/docs/2020/Schedule MDC Jan 1 2020.pdf

https://www.pse.com/pages/rates/electric-tariffs-and-rules#sort=@fdocumentdate85487 descending

https://www.sciremc.com/sites/sciremc/files/Rate Schedule IP - revised 10-1-15.pdf

kvar during peak kW:

https://www.myavista.com/about-us/our-rates-and-tariffs

Unspecified method:

https://app.bchydro.com/accounts-billing/rates-energy-use/electricity-rates/business-rates.html

https://www.portlandgeneral.com/our-company/regulatory-documents/tariff

 

[waross]

The Dynamatic drives do not have any sort of cooldown requirement, so still need to figure out where the cooldown practice came from.

With a limit of two starts per hour, the motor will always be able to start after a thirty minute cooldown.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FacEngrPE]

A 750HP motor is large enough that on starting a considerable amount of energy is dumped into the rotor as heat. When the starts per hour limits are not observed - for several years. The result can be broken rotor bars, which result in hard to diagnose vibrations.

It is unlikely that your magnetic coupling is the week link on the starts per hour limit. Any reasonable selection would allow continuous operation unloaded, and under reasonable operating conditions.

There are scenarios where magnetic coupling drives are more cost effective than inverter drives - both have first costs, and operating losses, so when built the system you have now may have been the best life cycle selection. If built today, the economic selections might be different, but not likely enough to consider early replacement.

I would consider adjusting the magnetic coupling to operate the pump at a slower speed, and see if the run time increases enough to make this a worthwhile operating mode. You need to verify the selection for high slip operation to be sure the coupling will not overheat with continuous operation with the desired output speed. The impact on utility costs depends on your tariff, and all of your other loads.

My utility charges energy by kW and demand by kVA with a ratchet, so pf is not directly an issue. Once you set demand, you are stuck with it until the ratchet lets it go.

 

[bacon4life]

Bill,

PG&E uses the monthly average calculation, as explained in this whitepaper and their tariff. The PG&E documents do not mention whether PG&E detents the meter to avoid the meter spinning backwards for negative reactive power.

The PG&E whitepaper contrasts PG&E's calculations to SDG&E and SCE.
-SDG&E use the max kw and the max kvar per rule 1 but I haven't yet figured out how much much they charge for power factor below 0.90.
-SCE appears to have removed all power factor charges from their most recent tariff.