Sizing Generator- Florida Nursery

[theplantman]

We have recently installed a new 20hp well that runs some critical watering areas on our farm. We want to have a standby generator installed, and in reaching out to generator/electrical companies, we are getting many different opinions on what we need. As a result, I have landed here to ask for advice. Here is what we have that we want to run, and the equipment that has been installed thus far:

Duke Energy 230 3-phrase power (I am told this is the highest we have available)
Pump Motor: Franklin 20hp pump model #236604 Franklin Motor Link (see three attaches pages on this motor)
Danfoss VLT AQUA Drive FC-202P18KT2ERH (see photo)
(18) Greenhouse Fans. They draw 1amp each, wired 110v.
(6) Very small irrigation timers.

The most complete quote, was a custom Diesel Generator made for this application. See attached quote.

We have gotten proposals from companies for 30kw, 40kw, 65kw with either Propane or Diesel. The issue is no consistency or agreement between electricians we have reached out to, so makes us worry about selecting a company/generator and it be to small and not work, or way overkill and burn up unneeded fuel. If there are any questions or further information I need to gather, let me know!

Thanks!

Alex the farmer

 

[waross]

Duke Energy 230 3-phrase power (I am told this is the highest we have available)
????
I thought that 230 3-phrase power died with the Dodo.
Do you have a niche open delta wild leg service?
Some guesses based on incomplete information:
Simple calculation based on 120/208 V service. 30 KVA.
Based on 230 V, wild leg service, 40 KVA.
Pump on DOL, 65 KVA.












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[TugboatEng]

In that size range, if you go diesel, I think you'll find that the 65kW unit burns less fuel at all power levels the smaller units are capable of producing.

Pricing is going to vary wildly.

 

[theplantman]

Yes we have 230 3-phrase power on our farm. We are way out in the country and thats the strongest power available out here. What other information do you need Waross?

There are only 2 transformers on the power pole, and the well man says we have "niche open delta wild leg" service. He called it "open delta" power.

 

[TugboatEng]

It has nothing to do with your problem it's just interesting to note that some voltages are not so common anymore and the difference will have little effect on your system. I have had boats in my fleet set up to run on 440, 450, 460, and 480 volts. When we plug into shore power I get 495V. Everything still works fine except the consumer grade UPS's sometimes had to be replaced with industrial units. I re-tapped transformers where I could. The 450v is a typical problem I run in to with marine electricians. If nominal is 110-115 then 112.5 is perfect! Also, DC wiring needs to be at least 10x larger than AC wiring of the same ampacity.

You'll be able to fudge the specific numbers with your voltage regulator. Give yourself some fudge room on the nameplate rating if you're going to be operating at a lower voltage.

 

[waross]

You'll be able to fudge the specific numbers with your voltage regulator. Give yourself some fudge room on the nameplate rating if you're going to be operating at a lower voltage.

BE VERY CAREFUL FUDGING.
More than once I have spec'ed a standby set very close for a known load.
I spec'ed but didn't order nor did I have a chance to vet the order before shipping.
Ordered was a specific KVA at 120/208.
Shipped was the spec'ed KVA but at 120/240 wild leg.
So now I have a set that is 208/240 = 87% of the intended load.
No problem right? Just send it back to the supplier and have it replaced with the set as ordered.
When the supplier is an ocean voyage away, that is not an option.
Not even discussed.
You live with what you got.
I had to come up with a creative automatic load shedding scheme when the standby kicked in and live with an unhappy customer.
Yes, I could have solved the problem with a transformer bank but cost was an issue.
I did negotiate a discount for both customers but that didn't completely cure the pain.
With as installed onboard sets and lots of shore power, tweaking is fine
I have often done that myself when I had a lot of marine work.
But, when sizing a new set, there is no room for tweaking.
Do it right and get it right.
If possible vet the order before it is shipped.

 

[LittleInch]

So what does your VFD run? that 20hp motor?

Do you have anything like a single line diagram or circuit diagram showing what is connected to what?

what is the usage / load profile? i.e. when running is the load fairly constant and mainly the pump or do you want the unit to run all the other bits and not the pump?

Generally generators like running at somewhere between 70 to 90% of rated load on a regular basis and don't like sudden large increases in power, but with a VFD you should be able to reduce this surge load. Propane and gas generators in particular don't like going from 20-30% to 80% in one jump. Diesels are a bit better, but will still struggle a bit.

So get the information together to let people take a better view of life. If the vendors thought your pump was just a DOL starter no wonder they went for 65kW...

 

[theplantman]

The VFD only runs the 20hp pump. The Pump, Fans, Irrigation switches are the only thing connected to this power pole....and will be the only thing run by the standby generator. I do not have a single line diagram. We have a power pole, with a service meter, and it goes from breaker, straight to this pumping station. All within 20ft of each other. Another breaker for fans. Another breaker for irrigation timers. Think simple farming deal here. But if a diagram is a must, say the words and I will draw one, or go shoot a video of the area.

I have been watching the VFD and it slowly cranks up the motor....and most of the time its between 12hp-15hp on the digital screen. During the day, the Fans are always running (18 amps of draw)

From the pump motor manufacture: 362 Locked Rotor Amps (Is this something to worry about? seems high)

What does "DOL Starter" mean? Waross also mentioned that.

From the pump motor manufacture: There are two types of generators available, External and internal regulated....most are externally regulated. They use an external voltage regulator that sense the output voltage. As the voltage dips, at motor start up, the regulator increases the output voltage of the generator.

Internally regulated (Self excited) generators have an extra winding in the generator stator. The extra winding sense the output current to automatically adjust the output voltage.
External Regulated: 75KVA
Internal Regulated: 37KVA

What should I be asking for at a custom generator manufacture to ensure we have an Internally Regulated Generator?

Thanks everyone!

 

[LittleInch]

DOL means Direct Online. Basically the motor starts within 1 to 2 seconds up to full speed and tends to draw about 6 times full load current for a few seconds. Fine if ypu are connected to a grid supply, but a big issue when connected to a generator when that 6 times current can result in an oversized generator.

The internal external thing I have no idea.

 

[Gr8blu]

As LittleInch pointed out - DOL means "Direct On Line". That means you close the breaker and the pump winding sees full voltage - and the unit begins to accelerate from zero speed. A couple of seconds later, it is at speed and humming along nicely. However - between closing the breaker and settling into a regular "load" rhythm, the pump draws a very high current (aka "locked rotor current").

Locked rotor current - for a machine that predates the new "premium" and "super premium" efficiency classifications required by the Department of Energy is around 6X normal full load current. For the newer, more efficient designs this is closer to 9X or 10X. This level of current is drawn from the supply, through any connections and breakers, and into the motor windings for almost the entire time it takes to accelerate to speed. It does drop somewhat (to about half of whatever it started at) as speed increases, until at running speed it settles into its "normal" running load state (e.g. 1X).

The OP starts and runs on a VFD - which means there is going to be some sort of current limit setting, which will keep the current draw down (typically in the 1X to 1.2X full load range) but extend the time for the pump to get to speed.

As to the "internal" vs "external" regulation discussion:
https://www.cat.com/en_US/by-indust...icles/White-papers/excitation-selections.html

The link has a reasonable explanation of the different excitation types.
The bottom line, in a nutshell: If it comes from the generator output, it's external. If it comes from another internally generated source, it's internal.
The difference being when heavy load is applied, the output voltage drops and that drop also applies to the voltage regulator power source - so it can't increase the field current as effectively (therefore slower to react). The internal power source isn't affected by the volt drop on the output so it can provide full current to the field (yielding somewhat quicker response).

 

[waross]

Generally generators like running at somewhere between 70 to 90% of rated load on a regular basis and don't like sudden large increases in power,

Like it or not, life is hard for a standby generator.
I have yet to find a customer willing to pay extra to make life easier for a standby generator.

 

[theplantman]

Thanks everyone. I am getting somewhere with this sizing deal!

So I need to find out the 'Current limit setting' inside the VFD. That sounds like it will reduce the 'Locked Rotor Amp' significantly.....thus reducing the size of the generator needed.

Gr8Blu- Thanks for the CAT paper. That really dials it down, and I have sent that to the custom generator manufacture. Will see what 'type' of generator system they were proposing for my job. Of course they dont Guarantee or size generators for jobs, but claim they will help customers.

 

[FacEngrPE]

Choice between diesel and propane may be driven by air permit considerations. If an air permit is required for a permanent backup generator, propane or Natural Gas have lower emissions, and are therefore simpler to permit.

The transformer arrangement for a high leg service with neutral ground (described above as your service) is called a scott T

Phaser diagram for service.

In-order to have your site system grounded correctly when running on generator, several choices:

• Have the generator wound special for the actual phasor diagram. Likely cost prohibitive.
• Place a transformer between your generator and your transfer switch that that converts the generator's 480 volt output to the high leg neutral arrangement.

Reference Hammond Power Solutions (this document confirms that for a high leg system some of the source windings must be sized for higher currents, which is applicable to a special wound generator, or for a transformer connecting a 480 volt generator to the 230 Volt High Leg Delta service. It likely also impacts the required generator KW rating, but not the prime mover requirement.).

Recommendation - hire an electrician, and make him responsible for sourcing all of the components needed.

Service voltage - discussed above
In the US the standard is ANSI C84.1
For your 230 Volt connection, the following could be expected :
Service Voltage (at meter socket) 252 to 234 volts.
Utilization voltage (at using equipment, under load) 252 to 216 volts.
The low end of utilization voltage usually indicates not accounting for voltage drop on long wire runs.

 

[waross]

The transformer arrangement for a high leg service with neutral ground (described above as your service) is called a scott T

Sorry. That is not a high leg vector diagram.
The Scott connection was originally developed to transform and convert between two phase systems and three phase systems.
About 50 years ago there was a flurry of Scott derived lighting transformers to transformer 480 VAC to true 120/208 VAC with just two transformers rather than three transformers.
Back in the day I saw quite a few "T" connected 25 KVA lighting transformers taking 480 VAC down to 120/208 VAC to feed industrial lighting panels.
There is no connection between Scott transformers and the classic 120/240 Volt high leg system.
Everyone familiar with the common 12 lead generators and 120/240 Volt high lead systems knows that a 12 lead generator may easily be connected for either 120/208 Volts or for 120/240 Volts, high leg.

It likely also impacts the required generator KW rating, but not the prime mover requirement.).

I would remove that site from your bookmarks.
The generator KW rating may not exceed the KVA rating.
The generator KW rating may not exceed the Prime Mover available KW.
Below these limits, the generator KW is equal to the Prime Mover available KW.
A caveat: For a wild leg system, the KVA must be considered for each phase independently.
One phase may be overloaded (KVA) even though the total KVA is less than the set rating.

 

[TugboatEng]

In my experience, if you want a durable reliable generator you need to buy something in a minimum displacement of ~4.5 liters. With modern engines this puts you at 65kW in a naturally aspirated package and 99kW in a turbocharged package.

The reason I suggest the larger engine is that the you get better injection systems, better pistons, better valves, better liners, etc when compared to the smaller units.

And with all of those better components, the larger engine will typically get better fuel economy than the smaller ones.

I operate a fleet of 99kW 4.5L sets with an average load of around 10-15kW. I don't load bank them and they tolerate the light loading just fine. We expect 35,000 hours between overhauls.