VFD; is power still power

[Shaneyj]

I'm working on a design with requirement of 1000 hp pumping capability.
The pump must operate at different speeds so we're using a drive.
The drive max output current is 1300A.
The drive manual is calling for 5 parallel 350s (185mm) from drive to motor.
But the pump will be limited to a fixed power output.
At 1000 hp, accounting for 125% continuous load, current calcs come to about 1050 amps. (600v, .9 pf, 95% eff)
I'm using type P cabling so ampacity per IEEE 45 OR 1480, 95 degree column (depending on which cable mfg cut sheet is being looked at both of these standards show up with associated current ratings). As an aside an explanation of how those ampacities are derived is appreciated.
The drive manual is providing for full 1300A output with its recommended cabling.
Am I missing something if I provide only for 1000 hp output besides limiting drive capability?



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[texie]

Look at 430.122. But note that in the 2017 and prior editions it is silent on the conductors from the drive to the motor. In the 2020 edition it is required to size them per 430.122(B).

 

[Shaneyj]

Look at 430.122. But note that in the 2017 and prior editions it is silent on the conductors from the drive to the motor. In the 2020 edition it is required to size them per 430.122(B).

Thanks sir. I'll read in the am.
Code aside, am I missing something from a fundamental perspective?

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[don_resqcapt19]

I'm working on a design with requirement of 1000 hp pumping capability.
The pump must operate at different speeds so we're using a drive.
The drive max output current is 1300A.
The drive manual is calling for 5 parallel 350s (185mm) from drive to motor.
But the pump will be limited to a fixed power output.
At 1000 hp, accounting for 125% continuous load, current calcs come to about 1050 amps. (600v, .9 pf, 95% eff)
I'm using type P cabling so ampacity per IEEE 45 OR 1480, 95 degree column (depending on which cable mfg cut sheet is being looked at both of these standards show up with associated current ratings). As an aside an explanation of how those ampacities are derived is appreciated.
The drive manual is providing for full 1300A output with its recommended cabling.
Am I missing something if I provide only for 1000 hp output besides limiting drive capability?



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Type P cable is covered in the 2020 code and you are required to use the ampacity tables in Article 310. Unless the terminals are rated for more than 75°C, you will be limited to the 75°C column.

 

[junkhound]

am I missing something from a fundamental perspective

Did not see you mention distances? If long lines:
Pay attention to impedances other than current carry capability (L&C). Voltage amplification on long lines due to standing wave reflection at VFD switching frequency,
any length: VFD frequency motor bearing currents, skin effect of conductors, etc.

 

[winnie]

Is you output 1000 Hp regardless of speed? If so then remember at lower speeds your voltage is lower but your current higher.

Jon

 

[retirede]

Is you output 1000 Hp regardless of speed? If so then remember at lower speeds your voltage is lower but your current higher.

Jon

Trying to think of a constant HP pump design…..I’m coming up empty.

 

[winnie]

Something that is limited to 1000 Hp because of external power available and has to pump different materials or against different heads.

I can't imagine any pump situation where reducing speed and increasing force go together if you are keeping things like the material and piping constant.

But if the mechanical loading is changing (say because you are filling a reservoir and the head is increasing, or you are pumping different materials with different viscosities, or you are switching pipes to feed different processes) and you want to maintain a fixed maximum power, then you might be operating at 1000 Hp and different speeds.

I admit that my comment comes more from my experience with traction loads than with pumps. For traction loads you often want to output all available power at different speeds.

-Jon

 

[Shaneyj]

Is you output 1000 Hp regardless of speed? If so then remember at lower speeds your voltage is lower but your current higher.

Jon

Great responses so far. Thank you.
I am following the rabbit hole of type P ampacity (337.80) - thanks @don_resqcapt19. Manual only states using cables rated minimum of 90 C, nothing about terminal temp ratings. I submitted an RFI to vendor this morning.
This is an oil well pump application. I have field data for engine output at different torques - but no gear info so we can't relate it back to hp input to pump accurately.
What I do have is pump torque/hp/pressure/rate charts where we can figure worst case scenarios.
Customer feedback is that operating at low speed high torque is a decimal fraction of their duty cycle, but it exists, so I will account for it.
@winnie and @retirede Pump hp is not constant, but is 85% of full capacity at a fixed rate 50% of the time - the remainder are variables in well profile which lead to both increased and decreased power input at varying rates.
Several variables and all moving targets.
All of this is great food for thought but much to digest and understand. Thanks for your input.

 

[tortuga]

I imagine your RFI wont come back with much unless your asking the drive and pump supplier, as they will be providing the terminations.
At 1000HP I imagine you'll be using a 4160/2400 V system, so your terminations will likely be 90C
See 2020 NEC 337.80, and 110.40 (all recent versions)

EDIT nevermind I saw you posted 600V so as Don said 75C terminations and that would be 110.14(C)

 

[don_resqcapt19]

Great responses so far. Thank you.
I am following the rabbit hole of type P ampacity (337.80)...

In general, for an NEC application, you never use any ampacities for conductors from the manufacturer's. You only use the ampacities found in the code book. Often manufacturer's will give you the 90°C, free air ampacity, typically the same as what is in the NEC for that application, but it is rare where we can actually use that ampacity for a real world installation. Even if you do have a free air installation, you have the issue with the 110.14 termination temperature ratings.

 

[GeorgeB]

Trying to think of a constant HP pump design…..I’m coming up empty.

While not really applicable to this thread, variable displacement pumps with variable displacement motors are used as constant power systems. Classic (and only personal experience) use is in winders where web tension needs to be controlled and diameters vary significantly. Winding (and/or unwinding) paper and steel strip are common applications.

 

[Shaneyj]

In general, for an NEC application, you never use any ampacities for conductors from the manufacturer's. You only use the ampacities found in the code book. Often manufacturer's will give you the 90°C, free air ampacity, typically the same as what is in the NEC for that application, but it is rare where we can actually use that ampacity for a real world installation. Even if you do have a free air installation, you have the issue with the 110.14 termination temperature ratings.

Thank you sir. Not an NEC application, and 90% of what I do is not beholden to NEC; I know its not a design guide, but I do adhere where possible which in most cases is achievable. This particular project has some constraints where it makes sense to deviate where acceptable - particularly in termination temp in order to get what I can out of these type P cables.
I have learned that this topic is covered by IEEE 2740-2020.
As far as I can tell at the moment, if my RFI comes back and termination temps are above 75C, then I can bump up ampacity of cables.

 

[retirede]

While not really applicable to this thread, variable displacement pumps with variable displacement motors are used as constant power systems. Classic (and only personal experience) use is in winders where web tension needs to be controlled and diameters vary significantly. Winding (and/or unwinding) paper and steel strip are common applications.

I figured there had to be some (unknown to me) application out there. Thanks for sharing.

 

[Jraef]

Look at 430.122. But note that in the 2017 and prior editions it is silent on the conductors from the drive to the motor. In the 2020 edition it is required to size them per 430.122(B).

430.122 only addresses the input. Rules on the motor leads are no different from any other application; sized per the HP tables in article 430. If your load output is less than the motor HP rating that does not matter.

But if you aren’t following the NEC, I guess that’s irrelevant.

Output current of a VFD is based on the torque required by the load. On a centrifugal pump you only need full torque at full load, which is full speed, so that’s your worst case scenario on current. At any lower speed the HP required by the pump drops at the cube of the speed change, so for example at 1/2 speed the pump only needs 1/8 the Hp it needed at full speed, so the current is much less as well.

 

[Shaneyj]

430.122 only addresses the input. Rules on the motor leads are no different from any other application; sized per the HP tables in article 430. If your load output is less than the motor HP rating that does not matter.

But if you aren’t following the NEC, I guess that’s irrelevant.

Output current of a VFD is based on the torque required by the load. On a centrifugal pump you only need full torque at full load, so that’s your worst case scenario on current. At any lower speed the HP required by the pump drops at the cube of the speed change, so for example at 1/2 speed the pump only needs 1/8 the Hp it needed at full speed, so the current is much less as well.

Thanks Jraef.
This motor does not have a hp rating, but rather a torque rating.
Its the pump (positive displacement) that is rated at 1000 hp.
Engineers from motor/drive supplier required several data in order to bless motor/drive package supplied.
I'd like to understand better current output of drive.
Is it the standard 3 phase current calculation dividing power (kW) by rt 3 x v x pf x eff ?
If it is, and speed is being varied; rpm dropping as a function of voltage being decreased (in a fixed ratio to Hz). Then for a fixed power output, as voltage goes down current goes up.
So highest current drawn would happen at lowest rpm available.
Is my understanding accurate?



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[Jraef]

It’s different with a drive because you are varying the frequency. In a Constant Torque application like a PD pump, the drive if maintaining the same V/Hz ratio that the motor was designed around, so torque capability remains the same at any speed. Current is what creates torque, so it follows very consistently. If your motor was designed to provide the necessary torque for the worst case on the pump curve, the current should remain constant regardless of speed.

 

[texie]

430.122 only addresses the input. Rules on the motor leads are no different from any other application; sized per the HP tables in article 430. If your load output is less than the motor HP rating that does not matter

Drive to motor conductor size is now covered under 430.122(B). Although it does not change what you are saying in most cases. My only point was that the code use to be silent on this but the 2020 NEC now does cover this in 430.122(B).

 

[tortuga]

Thank you sir. Not an NEC application, and 90% of what I do is not beholden to NEC

I am curious where people are working that the NEC does not apply?
And what codes are you under Canadian, Mexican, Saudi, Roman?
Your profile says your in Texas I thought they were under the 2020 NEC

I once met a traveling sparky whom had worked in Antarctica and I was like wow must be no codes there, nope they had one (cant remember what it was).

 

[Shaneyj]

I am curious where people are working that the NEC does not apply?
And what codes are you under Canadian, Mexican, Saudi, Roman?
Your profile says your in Texas I thought they were under the 2020 NEC

I once met a traveling sparky whom had worked in Antarctica and I was like wow must be no codes there, nope they had one (cant remember what it was).

Oil field. Mobile Equipment.
In most cases it is the wild west with little oversight. In fewer cases there is an adopted standard chosen by customer.
The American Petroleum Institute has adopted some IEEE standards.
IEEE has some standards which apply to which I adhere.
In all cases to date (going on 3 years in my design career) I have followed NEC minimum standards...
Maybe not this project but that remains to be seen until my RFI comes back.
I've done a couple overseas projects that required Atex and IECEx mandates.
Mostly the "code" to which I operate is design robust and don't do anything stupid.
Seriously though, besides customer checking the boxes on their CYB forms, there is no other type of oversight - not in the traditional sense of an inspector anyway.