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???

 

[jraef]

A couple issues.
ANY VFD is inherently capable of "converting" 1 phase power to 3 phase. they rectify the AC to DC, then invert the DC back into pseudo AC. So to the VFD, the AC source power is just a "raw material" and it doesn't matter if it's 1 phase or 3 phase. But in converting from a 1 phase source, the amount of current it draws is going to be 1.732 times the amount of current that the motor consumes in 3 phase. In addition, the added ripple in the DC from converting 1 phase is higher and needs more capacitance to smooth it out. For those reasons, the VFD that you chose when you do this must be twice the size of the AC FLA of the motor. Some people either are unaware of this or think it's OK to ignore it because they think that if you have a VFD, it's because you WANT to run slower and therefore ill not need all of the power.

Issue #2 is that VFD manufacturers play some games in the sizing of VFDs for pump loads as well, stating that a drive that is rated for one size motor is capable of running a larger motor if it is a centrifugal pump. While not inherently incorrect, that rule does NOT apply if you are using a 1 phase supply, yet many VFD sellers are again either unaware or uncaring.

tying that together for your situation is the likelihood that you have one or the other of the above working against you, possibly even both, leaving you with a VFD that is incapable of supplying enough power to your motor if running full speed. How that translates to your not being able to get past 50Hz with one pump or 45Hz with 2 is that most VFDs can be programmed to respond to an overload by automatically reducing the speed in an attempt to keep the output current below the drive's capacity. In other words your speed is dropping because the drive it trying to keep from frying itself. the drive tech is probably incorrectly assigning that to the supply when what he means is that someone sold you a VFD that is too small for what you want to do, which he may he loath to tell you.

The only way I can think of that an inadequate supply could cause this is if the full loading of the pump or pumps at full speed is causing a line voltage drop, and that is in effect increasing the current that the motor(s) will draw, so the VFD responds in the same way. If that's the case, only your utility can help by installing a larger service transformer, something that is not going to come cheaply. You may need to re-think your capacity.


"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

 

[sibeen]

A gold star for jraef. A succint and helpful answer.

 

[LionelHutz]

The ripple voltage on the DC buss increases as the VFD load increases. I'd guess what is happening is a combination of line voltage dip and ripple voltage creates a lower effective DC (or average) buss voltage.

The VFD will be programmed to ouput the maximum AC voltage possible by taking the DC buss voltage and dividing by sqrt(2). So, with the effective DC buss voltage lower, the maximum AC output voltage is also lower. Since the drive follows a V/Hz curve, the lower maximum AC output voltage corresponds to a lower maximum frequency.

The end effect is that you need more capacitors in you VFD, obtained by buying a larger VFD and/or you need to increase the input voltage.

If the incoming voltage is 208VAC then don't try to set the motor voltage to 240VAC. You can't get out more than you put in. If the drive has DC buss terminals then you could build up an additional DC capacitor bank and add it to the drive.

 

[DickDV]

Golly, jraef, that answer was a work of art. It belongs in the archives. Well done!

 

[sibeen]

If the incoming voltage is 208VAC then don't try to set the motor voltage to 240VAC. You can't get out more than you put in.

I'm going to throw in a question here. I worked in the power electonics industry for about 20 years and yet never worked on a drive.

I thought the drive industry had mostly shifted over to active (force commutated) rectifiers. Is this the case or has some sales engineer been pissing in my pocket? The question was inspired by the answer by LionelHutz. If using an active rectifier, then you're basically using a boost circuit on the front end and the DC link voltage is then limitied by the front end inductors and the switching frequency.

If some have shifted to force commutated, are these rectifiers designed to operate in four quadrant mode?

 

[ScottyUK]

I think most of the commodity drives are still dumb six-pulse diode rectifiers unless you specify otherwise.


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

 

[jraef]

I thought the drive industry had mostly shifted over to active (force commutated) rectifiers. Is this the case or has some sales engineer been pissing in my pocket? The question was inspired by the answer by LionelHutz. If using an active rectifier, then you're basically using a boost circuit on the front end and the DC link voltage is then limitied by the front end inductors and the switching frequency.

If some have shifted to force commutated, are these rectifiers designed to operate in four quadrant mode?

Probably deserving of a new thread (hijacking is strongly discouraged in this forum), but we'll let this slide this one time.

Certainly there are a few VFD manufacturers who OFFER an active front-end inverter option, but they are essentially twice as expensive as a passive 6 pulse inverter, so they are by no means "mostly shifted over"; the market couldn't stand the price increase! So the salesman who told you that most likely had an agenda; i.e. convince you that you were buying the latest and greatest over priced drive because that's what he wanted to sell you. I know of no mfr that is offering that exclusively. Now certainly, if you have a high harmonic mitigation issue or a severe braking (4 quadrant) application, then it may be a BETTER choice than other options, but again, there is no universal movement in that direction.


"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

 

[itsmoked]

And "active (force commutated) rectifiers" does not infer "boost" of any kind.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

The active PFC types do boost the DC link slightly above the peak of the AC waveform, but force-commutated (thyristor) types won't: is terminology is getting mixed up?


----------------------------------

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

 

[sibeen]

is terminology is getting mixed up?

Could be, Scotty. I'm way down under and we may use different terminology here

When I talk force commutated I mean the actve IGBT front end style. In this topology the DC needs to be boosted, otherwise it will just operate as a 6 pulse diode bridge.

 

[jraef]

Yeah, maybe I mixed them a little too much, but does anyone really use force-commutated thyristor front ends any longer? I just ASS-u-me-d...

Keith,
A true active front-end drive can boost the output voltage to about 20% above line voltage, or in other words, continue at full output when the line drops to 80% of normal.


"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

 

[itsmoked]

Hmmm interesting. So they use the IGBTs as voltage doublers? Anyone got a schematic lying around?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

This App Note from Vicor explains the principles - basically a boost converter is running continuously from the rectified unsmoothed line input.

A word of warning about active PFC circuits - our IT dept had an expensive power supply front end blow up when trying to follow a heavily distorted voltage waveform with a multiple zero crossing. The distorted waveform is another story but the active PFC certainly did not like it.


----------------------------------

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

 

[sibeen]

Hopefully the pdf I've just done up helps. It is a fairly simplistic expalnation but hopefully it gets the idea across.

It's one of those things that given 5 minutes and a whiteboard it's fairly easy to explain

 

[Wedoca]

I have checked the pump and it is 10.1HP, and the VFD is 20HP. so I would assume that the VFD is sized properly, and that would elimated the possibly of low DC link voltage caused by not having enough capacitance to smooth out the DC ripple. so the next possibly would be because the utility transfomrer is too small ...but how do I verify that???

 

[itsmoked]

Observe the input voltage to the VFD. Does it drop a lot?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Wedoca]

I don't know have to check....

 

[Wedoca]

what percentage of line voltage drop is acceptable and whats not ?

 

[itsmoked]

5% is fine. More, may be fine, but should be assessed as that can relate to supply hardware heating up, (or not).

Keith Cress
kcress -

http://www.flaminsystems.com