Using VFD as a phase convertor to run 3 phase pumps 9

[Wedoca]

Hi people ,

I recently have seen a Culter Hammer VFD been used as a phase convertor get gain 3 phase at an area where only single phase is avaliable. now ....the problem is that when every we run the pump the output frequency would only go up to around 50 hz.. and it gets worst when I try to run both pumps (its a dual pump station) at the same time (Frequency drops to around 45 hz) ..I have checked the VFD setting and its all correct .. I have consult with the VFD Manufactor tech. and he said its because the utility is not supplying enough power to this local station......can someone tell me if thats true ?? or is there any other possible explaination??? is there a solution???

 

[Wedoca]

guys here is the latest ...

the DC bus voltage on the VFD drop from 336 to 287
as I turn on the pump, input voltage drops about 5 volts from 248 to 244.

base on that can I conclude that the cause of all these madness is because the utility incalbable of supplying power to the site?? and possiblely need a bigger transformer ?

 

[jraef]

How fast are you trying to start the pump? It's possible that you are attempting to accelerate as if it were starting DOL, which may indeed be too much for the supply. But by having the VFD, you may also be able to stretch out the acceleration time enough to reduce the strain on the supply system. Usually, it is possible to start pumps with no more than 100% FLA using a VFD, as long as there is no mechanical issue with long slow acceleration (which can sometimes be a problem with self-lubricated pump designs).


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[Wedoca]

I tried with 3 second ramp up time and 7 seconds ...and its makes no difference

 

[itsmoked]

Tell us more about your setup.

Motor size?

Measured line voltage to the VFD?

VFD model?

Pump type?

Pumping what?

Expected approximate pressure desired?

Estimated head?

What is the power source? (Like a utility pole transformer)

How long is the wire run from the breaker to the VFD?

What size wire is it?


Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

No, it appears there is not enough DC buss capacitance.

A 4V drop in the line voltage should not cause a problem. Your line voltage is also high side to begin with allowing a higher voltage drop before it becomes an issue.

 

[Wedoca]

Motor Size: 10HP FLA 25A

Line Voltage: Phase to Phase: 248.6 volts
L1 to Ground: 123.3 volts
L2 to Ground: 122.6 volts

VFD model: Culter Hammer SVX9000 25HP

Pump model: submiseable

Pumping Media: Sewage (station is tested using water)

Estimated head: 108ft

The wire run from the breaker to the VFD: 3~5' Max its all inside of a panel

power source: see attached picture

Wire size: unknown

 

[Wedoca]

power source: utility pole transformer

 

[jraef]

No attached picture.



"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[markcraig]

What about the cable from the VFD to the Pumps? Is it VFD grade cable or is standard submersible cable being used for the run to the pumps?
Not sure exactly how these effects work, in fact no idea at all, but I have seen 100KW supply fans with VFD's not reach speed due to problems with cable from the VFD to the motor. We replaced the cable and the fans worked. Should have taken more interest in the theory but it was in the middle of a big project and not my part of it.

 

[Wedoca]

sorry about the picture

 

[jraef]

Wow, someone needs to trim that plant!

Appears to be a 15kVA 1 phase transformer. If so, it's probably marginally sized for ONE 10HP 3 phase load when you factor in throughput efficiencies of the VFD conversion, let alone two! Did the PoCo authorize this much load on that transformer?

Most likely your VD situation is happening too fast for your meter to pick up, but is triggering your VFDs Current Limit / Stall Prevention feature and automatically limiting output frequency.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[burnt2x]

Service transformer is too small I believe. Since you have said you have two submesible pumps, that's too much for the 15 kVA supply. Ask for a bigger transformer from power company.

 

[DickDV]

Let's see if I've got this straight. You've got a 230V 25hp VFD on a 10hp 230V motor on a pump. The power supply is 240V single phase and runs from 244-248V over load changes.

We surely can't fault the drive sizing. It is more than twice the size of the motor.

We can't fault the power supply. At motor maximum speed it holds up to 244V.

So, it seems to me that the system is either running up against a current limit setting somewhere or the drive is simply not programmed properly.

I'd give improper programming about a 90% lead in this case.

Has the correct motor data been entered into the drive?

What is the current limit setting in the software?

When the motor hits the speed limit, what is the drive output current at that point?

 

[Wedoca]

Sorry Guys ,, its actually a 20HP drive my mistake ..all motor data has been input in the VFD and but the current limit is set at the FLA which is 25A ... the drive never reach the speed limit the limit is speed set at 60hz 3480 RPM

 

[burnt2x]

Wedoca measured the DC bus link dropped from 336 to 287V (see post 15 Jul 09 11:18). I believe the supply voltage at that 287VDC instant couldn't be higher than 200VAC. Am I correct to say that the measurement done by Wedoca are off by some percent?

 

[jraef]

I think that's a distinct possibility, I'm surprised that none of us picked up on it earlier. PLS for you.

Wedoca,
It's entirely possible that your voltage readings are skewed by the harmonics on the line as a result of the VFD.

Be that as it may, the fact that you have the current limit set for 25A on a VFD capable of 61A, it's entirely possible that the VFD is artificially limiting frequency to maintain the current limit setting. It may also be related to the following:

620 Load drooping
Setting a proper value to the Load Drooping parameter enables the load drooping.
This function causes the speed to decrease as the load torque increases. At rapid load
changes this gives a smoother torque response, when a part of the required power is
taken from the system inertia. This operation causes a constant, torque-dependent
steady-state error in speed response.

So check to see what parameter 620 is set at.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[ozmosis]

I think your current limit is set too low.
For the SVX9000 to operate on a 1ph 240V supply at such high current ratings you will have high instability due to the ripple on the DC link. I have seen this with other 'brands'(vacon) same as the SVX9000 where the current limit PI gains are all wrong and the current limit is not set up for a 1Ph supply.
It is a common issue for drives that simply disable the input phase loss detection, oversize the drive (as you have done) but then do not further work on commissioning the drive correctly to cater for the instabilities working on a 1ph supply.
I have no idea what the settings should be but suggest you maybe leave the default motor settings in the drive and adjust the current limit up from a point where it stops going into current limit. Not too scientific but unless Eaton provide you with actual settings running on a 1ph supply, it may be all you get.
Make sure you have motor protection (e.g. thermistor) if doing this to prevent motor burnout.

 

[ScottyUK]

If the smoothing caps are desperately undersized - which they probably are because a 3-phase rectifier has comparatively little ripple at 300Hz compared to large amount of ripple at 100Hz for a 1-phase rectifier - then an average DC bus voltage of 287V could quite possibly be a correct reading from a 240V incoming supply. The DC would be very 'ripply' to say the least. Lionel already hinted at this in a previous post.


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If we learn from our mistakes I'm getting a great education!

 

[burnt2x]

ScottyUK,
I cannot reconcile 287VDC coming out of the DC link supplied with a 240VAC, 1-phase!
Unless a DC boost is employed in this SVX9000 unit or better still voltage doublers!
A full-wave rectifier (resulting from non-supply of the 3rd leg) will roughly produce:

V[sub]average[/sub] = V[sub]DC[/sub] = 2 X sqrt(2) X V[sub]RMS-INPUT[/sub]/PI.

My hand calculator tells me I can only have around 216VDC. Or did i miss a lot here?

 

[ScottyUK]

Well, if you had a massive DC link capacitor you would find the DC link was running about 340V with a 240V single phase input.

If you have a bit of DC link capacitance but not enough to smooth it properly then you will strike somewhere in the middle ground between having no capacitance (your equation) and having a very large capacitor which would charge to the peak of the rectified sine at about 340V, so the 287V and the fact that the voltage collapses rapidly under load both support the idea that the DC link capacitor is inadequate.

You didn't miss a lot, but maybe a little bit.


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If we learn from our mistakes I'm getting a great education!