sizing step-up transformer

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carelbrouwers

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I have to hook up a sprinkler pump motor
(139 Running Amps / 834 Starting Amps) 3 phase 460 Volt (90 kW)

The available voltage is 127/220V 60 Hz so I have to us a step-up transformer (NEMA type 2, enclosed, dry type transformer)
220V / 460V 60 Hz

My question:

What kind/size step-up transformer do I need?
 
carelbrouwers said:
I have to hook up a sprinkler pump motor
(139 Running Amps / 834 Starting Amps) 3 phase 460 Volt (90 kW)

The available voltage is 127/220V 60 Hz so I have to us a step-up transformer (NEMA type 2, enclosed, dry type transformer)
220V / 460V 60 Hz
What kind/size step-up transformer do I need?
Click to expand...


You'll need more motor data to get more accurate but, minimum transformer kva is 125% of running amps (144 kva) or next larger,150 kva.OCP is based on locked rotor amps.With 834 amps @ 480 starting, the 220 amps will be close to 1800 amps.Your going to need switch gear to handle that.
Rick
 
wasasparky said:
This almost makes it sound like a sizing requirement, is this just how you would size it?
Click to expand...

The requirement is from article 695.5(A).

(A) Size Where a transformer supplies an electric motor-driven fire pump, it shall be rated at a minimum of 125 percent of the sum of the fire pump motor(s) and pressure maintenance pump(s) motor loads, and 100 percent of the associated fire pump accessory equipment supplied by the transformer.
Rick
 
RUWired said:
The requirement is from article 695.5(A).
(
Click to expand...

Oops, didn't presume this to be a fire pump.
I saw sprinkler and got irrigation stuck in my head.
Other than this, I do not believe there are any sizing requirements for transformers?
 
Thats one heck of a sprinkler pump.

Typically, a motor connected across the line to a transformer could require a transformer to be 200% to 250% the KVA size of the motor. A quick motor starting alaysis can be done to determine voltage drop during starting to see if there will be a problem.
 
Nec 695.7

Nec 695.7

kingpb refers to NEC Article 695.7 which defines voltage drop criteria for fire pumps - 15% during startup and 5% at 115% of the motor fla.

I did a fire pump design in the past for a 75HP fire pump located in the middle of a warehouse. The Transient Motor Starting Analysis mandated a 300 kVA padmount transformer - driven by the 115% max voltage drop during startup.

The TMS study was provided to the Utility (who supplied the xfmr) and to the electrical inspector to show the resultant voltage drops.

JM
 
I think I worded my post wrong, because I was trying to suggest that being a "sprinkler pump" it was related to watering your landscape.:rolleyes:

Anyway, the 15% drop during start-up is consistent with motor design (see NEMA MG-1), and practical engineering design. The 300KVA transformer, depending on the transformer impedance is not all that surprising.

Was the driving force for sizing the 5% @ 115% FLA, or the 15% at rated FLA?
 
300 kVA xf for 75 HP pump

300 kVA xf for 75 HP pump

In response to a question as to what drove the sizing of a 300 kVA xfmr for a 75 HP fire pump - it was the inrush current on startup.

I had another experience where I did an electrical design for a pumpstation that had (2) 75 HP pumps which were across-the-line started, at a location about 125' from the Utility transformer. The Utility placed a 300 kVA, 480V service xfmr.
Upon starting of a pump, the breaker would trip.
I get a call from the A&E firm stating the problem, and as any EE consultant could attest, I was extremely paranoid that I undersized the feeder. So the first thing I did was a Voltage drop calculation for the feeder. It was fine.
Then I ran a Transient Motor Starting study and determined that the xfmr was undersized at 300 kVA, and needed to be 500 kVA minimum. The Utility swapped the unit out, and it fixed the problem.

So from these two cases I came up with a rule of thumb that's fairly conservative, but is a good place to start, as follows:
Size a xfmr that feeds a motor at the inrush current of the motor.
i.e. with a HP about equal to a kVA, a 75 HP motor draws 5-6 times fla at startup, size the transformer at (5* 75) + existing load, in this case another 75kVA = 450kVA.

There are other factors that play into this analysis, like Utility strength. But generally speaking, I think it's a safe place to start.


John M
 
... and one last comment on this topic

... and one last comment on this topic

I think "carelbrouwers" should consider a separate Utility service for the pump, because as RUWired pointed out, the 220V amperage will be over 1800 amps on startup!.. and that's just wrong...

John M
 
Wouldn't it be better and less costly to just add a VFD to ramp the motor up easy and avoid all this in rush current altogether?

Less stress for both the premise and utility wiring.
 
a response....8 months later.....

a response....8 months later.....

Bob,
I was looking back at some of my posts, and in the interest of completeness wanted to respond to your question:

Wouldn't it be better and less costly to just add a VFD to ramp the motor up easy and avoid all this in rush current altogether?

Less stress for both the premise and utility wiring.

I think the answer to that is YES!!!

But IMHO, the VFD has a 5-8 year life expectancy. It may last longer, but before long there will be no parts available, and it will have to be replaced with the latest and greatest available.

Whereas, with a good-old-tried-and-true across-the-line starter, it goes and goes and goes. I've seen fully-functioning 50 year-old NEMA 4 starters going strong - with replacement parts available at the local electric supply house.

Certainly if you're speed matching, the VFD is appropriate. But if all you want is that the motor start, a FVNR starter is the cheapest, most effective way to go. Granted the system has to be stiff enough to carry the starting requirement - but that's the Utilities call. My first choice is always FVNR, subject to the Utility approving the size.
JM
 
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There are other 'tried and true' soft starts available.

Autotransformer; expensive, HEAVY, adjustable taps, works with any motor, reliable. I've seen some still in service after 40 years. Uses standard contactors, (easily replaceable), easy to troubleshoot.

Wye-Delta; Mid-range cost, needs 6 wires from starter to motor, motor must be designed for this type (this size of motor is likely a 12 lead delta, so it'll work ok), reliable, uses standard contactors, (easily replaceable), easy to troubleshoot.

Part-winding; less expensive, very simple design, motor must be designed for this type, (12 lead delta will only work on low voltage connection), would work ok on centrifugal pump, but not much else due to very short start time, reliable, uses standard contactors, (easily replaceable), very easy to troubleshoot.

Given the choice, I'll nearly always opt for the old fashion mechanical starters over the new-fangle electronic ones. They're tougher, much easier to troubleshoot, no programming, and WAY more reliable.
 
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