Active power factor correction

[Palmitoxico]

Hi everyone,

My father has installed a solar powered water pump system in community nearby and he is experiencing some problems with the 48VDC->220VAC inverter he is using. The system consists of 4x 350W solar panels, 4x 12V 120Ah lead batteries connected in series, a 48VDC->220VAC inverter, a single-phase -> three-phase frequency inverter and a 3HP three-phase water pump (block diagram attached).

The 48VDC->220VAC inverter has a sinusoidal output and is rated for 3KVA continuos so in theory it would be enough for the 3HP (2237W) load. But as you may guessed, the frequency inverter has a pretty low power factor due to AC->DC rectification (~0.6) and it surpasses the power capability of the first inverter. I am trying to think a practical solution to this and these are what I got so far:

• Get rid of the 48VDC->220VAC inverter and replace it with a DC-DC boost converter;
• Modify some what the  48VDC->220VAC inverter to output a square wave;
• Design an external active power factor correction device for the frequency inverter.

1) would be the best engineering solution, but designing and building such DC-DC converter wouldn't be easy or cheap, 2) is risky and could reduce the efficiency of the first inverter (it has a 60Hz laminated steel core transformer that might not like high frequency harmonics). I'm gravitating towards the 3) solution, but I don't have any experience designing active pfc systems (let alone for a 2.3KW load), do anyone know a good literature about this?

If someone can think of a better solution do this I would appreciate.

Thanks!

[Siwastaja]

Boost 48V to the DC bus voltage (320VDC) of the VFD. 48->320 is such a high ratio that bog-standard boost won't be doable (or far from optimum), so you need a different topology such as center-tapped boost or flyback or half/full bridge transformer based, but the end result is still fewer components and less conversion losses than what you have now.

Thinking about it, maybe your sine wave inverter does have this boost stage already? You could bypass the output inverter stage and directly connect the 320V-ish DC bus of the sine wave inverter to the rectified DC bus of the VFD.

Any of this is far from effortless, of course.

[TheMG]

Thinking about it, maybe your sine wave inverter does have this boost stage already? You could bypass the output inverter stage and directly connect the 320V-ish DC bus of the sine wave inverter to the rectified DC bus of the VFD.

He did mention the inverter has a 60Hz transformer so it's likely not a SMPS type, the 60Hz transformer does the stepping up from 48V PWMd sine to 220V, thus no high voltage DC buss to hook onto internally. Doing such a thing would also likely bypass any kind of over current protection the inverter may have.

The best and easiest solution of course would be to replace the motor with a 48V DC equivalent. But I guess this is not feasible because you already have the 3-phase motor and motors that size aren't cheap.

[Palmitoxico]

Hi  Siwastaja,

Boost 48V to the DC bus voltage (320VDC) of the VFD. 48->320 is such a high ratio that bog-standard boost won't be doable (or far from optimum), so you need a different topology such as center-tapped boost or flyback or half/full bridge transformer based, but the end result is still fewer components and less conversion losses than what you have now.

Yes, I was thinking of using a coupled inductor boost converter so the PWM duty-cycle would be near 50%. A flyback topology would be a good bet too.

Thinking about it, maybe your sine wave inverter does have this boost stage already? You could bypass the output inverter stage and directly connect the 320V-ish DC bus of the sine wave inverter to the rectified DC bus of the VFD.

Any of this is far from effortless, of course.

Unfortunately, this inverter uses a huge 60Hz transformer to step-up the voltage (48VDC -> 34VAC 60Hz -> Transformer -> 220VAC 60Hz) so I can't take this route.

[Palmitoxico]

The best and easiest solution of course would be to replace the motor with a 48V DC equivalent. But I guess this is not feasible because you already have the 3-phase motor and motors that size aren't cheap.

Yes, unfortunately I don't think there are any off-the-shelf 3HP 48VDC motors were I live. Another concern is maintenance, DC motors require periodic replacement of its brushes, while induction motors are more robust in this regard.

[rvalente]

What kind of topology is this inverter using? How does it boost to 220V and them, does the AC switching?

I've read you're running the motor with a VFD drive, so you could tap the high voltage dc of the inverter and feed the VFD straigh, you may have a DC+ and DC- in the VFD or just feed it through the AC terminals, would be fine.

[Palmitoxico]

What kind of topology is this inverter using? How does it boost to 220V and them, does the AC switching?

I've read you're running the motor with a VFD drive, so you could tap the high voltage dc of the inverter and feed the VFD straigh, you may have a DC+ and DC- in the VFD or just feed it through the AC terminals, would be fine.

As I said previously, the 48VDC->220VAC inverter doesn't generate high-voltage DC, it converts to low voltage AC (PWM) and feeds directly to a big 'n old 60Hz transformer that steps-up the voltage to 220VAC.

[ConKbot]

Since your load is relatively fixed, an input inductor would do fine for improving power factor and making the input cirrent less spiky. Documentation for VFD line reactors should help you size one appropriately.  It may not be the most ideal solution, but it is a known off-the-shelf solution.

Given the equipment on hand, it may be the cheapest solution too.

[Siwastaja]

Adjusting the VFD settings to ramp up the pump more slowly, reducing maximum torque setting, and/or limiting maximum speed to a lower value, could let it run while still providing enough pumping action. Have you looked at this?

[bdunham7]

The 48VDC->220VAC inverter has a sinusoidal output and is rated for 3KVA continuos so in theory it would be enough for the 3HP (2237W) load.

I'm not sure what theory you are using, but you need to re-examine that statement in practice.  It looks to me like you are way underpowered.  Many inverters are fine with power factor issues, especially the low-frequency transformer type.  In  some cases they may have a kVA rating and a separate, lower watt rating--you might check that first--but in any case, your 3HP pump likely draws a lot more power than you think, not even considering startup transients.

You'd be best off measuring the actual power used by the pump under operating conditions, but you can get in the ballpark by taking into account the efficiency of the motor and its service factor.
I don't know your details, but a bog-standard pool pump motor (very common here in SoCal) might have an efficiency of 65% and a service factor of 1.5.  So take your 2237W x 1.5 / 0.65 and you have 5162W maximum power, assuming the motor is loaded that much and has those specs.  Your numbers will likely be different.

Your square wave mod idea may have some merit--but it would certainly be an experiment, and sometimes experiments end in smoke.  If you can use the motor drive inverter to slow down the motor, even from 60Hz to 50Hz, you may reduce the power requirement enough to allow the system to work.  Fluid pumps have a third-power exponential rise in power with speed--cut the speed in half and the mechanical power needed goes down 8-fold.  Going from 60Hz to 50Hz will lower the motor speed to 3000RPM (if it was 3600 to begin with) but save you 40% in power--in theory.

[rvalente]

Share the equipment  manuals, maybe there is a way

[Palmitoxico]

Adjusting the VFD settings to ramp up the pump more slowly, reducing maximum torque setting, and/or limiting maximum speed to a lower value, could let it run while still providing enough pumping action. Have you looked at this?

We already ramp the VFD (10s ramp) and have no problems at startup, then after ~3 minutes running the inverter shuts-down. Next week we will do some measurements (mosfets temperature, transformer temperature, more accurate power measures).  We configured the VFD to deliver a 54Hz output reducing the pump power and it increases the running time to ~15 minutes. It may be that the thermal protection is not well adjusted because my father said it seemed not too hot upon first inspection, but we need to measure it.

Since your load is relatively fixed, an input inductor would do fine for improving power factor and making the input cirrent less spiky. Documentation for VFD line reactors should help you size one appropriately.  It may not be the most ideal solution, but it is a known off-the-shelf solution.

Given the equipment on hand, it may be the cheapest solution too.

Humm, I will check that, maybe a passive PFC solution may be enough. I couldn't easily find a suppliers for such reactors, I may have to request some place to custom built it.

The 48VDC->220VAC inverter has a sinusoidal output and is rated for 3KVA continuos so in theory it would be enough for the 3HP (2237W) load.

I'm not sure what theory you are using, but you need to re-examine that statement in practice.  It looks to me like you are way underpowered.  Many inverters are fine with power factor issues, especially the low-frequency transformer type.  In  some cases they may have a kVA rating and a separate, lower watt rating--you might check that first--but in any case, your 3HP pump likely draws a lot more power than you think, not even considering startup transients.

You'd be best off measuring the actual power used by the pump under operating conditions, but you can get in the ballpark by taking into account the efficiency of the motor and its service factor.
I don't know your details, but a bog-standard pool pump motor (very common here in SoCal) might have an efficiency of 65% and a service factor of 1.5.  So take your 2237W x 1.5 / 0.65 and you have 5162W maximum power, assuming the motor is loaded that much and has those specs.  Your numbers will likely be different.

Your square wave mod idea may have some merit--but it would certainly be an experiment, and sometimes experiments end in smoke.  If you can use the motor drive inverter to slow down the motor, even from 60Hz to 50Hz, you may reduce the power requirement enough to allow the system to work.  Fluid pumps have a third-power exponential rise in power with speed--cut the speed in half and the mechanical power needed goes down 8-fold.  Going from 60Hz to 50Hz will lower the motor speed to 3000RPM (if it was 3600 to begin with) but save you 40% in power--in theory.

I'm pretty sure that the motor is 3HP rated and the efficiency would impact only the pumping capacity (3HP mechanical from the shaft, 2HP pumping power), but I would try to measure it more accurately with my oscilloscope and be very careful to isolate the ground and not blow up my probes (I don't have a proper power meter at hand).

[NiHaoMike]

I don't know your details, but a bog-standard pool pump motor (very common here in SoCal) might have an efficiency of 65% and a service factor of 1.5.

65% for the motor sounds really low (most motors of that size are in the 80s), but might be right for the motor+pump combo.

Would it be possible to replace the motor with a BLDC?

[Palmitoxico]

Share the equipment  manuals, maybe there is a way

We only have the documentation of the VFD  (WEG CFW500):
https://static.weg.net/medias/downloadcenter/h21/h6b/WEG-CFW500-programming-manual-v3.2x-10006739425-en.pdf
https://static.weg.net/medias/downloadcenter/hd7/h49/WEG-CFW500-user-manual-10001278006-en.pdf

The step up inverter is custom built, after we measure all parameters we will contact the manufacturer.

Would it be possible to replace the motor with a BLDC?

It would be relatively expensive to do it right now, I don't think we can easily source a powerful enough brushless motor here (we would probably need to import and pay insanely high taxes and fees).

[bdunham7]

I'm pretty sure that the motor is 3HP rated and the efficiency would impact only the pumping capacity (3HP mechanical from the shaft, 2HP pumping power), but I would try to measure it more accurately with my oscilloscope and be very careful to isolate the ground and not blow up my probes (I don't have a proper power meter at hand).

It doesn't work that way. A 3HP motor should produce up to 3HP (x the service factor if greater than 1) at the shaft.  The amount of electric power in watts required to power the motor at a given mechanical power level determines the efficiency.  So if your motor is loaded at 3HP and is drawing 3KW, it has an efficiency of 74.6% at that operating point.  The efficiency will vary widely according to load, input voltage, etc.  I think it is very unlikely that your motor would exceed 75% efficiency at full rated load, especially factoring in the losses in the VFD.  It is possible, and even likely given that the system works at all, that the load of the pump is less than 3HP so that your motor is not fully loaded.  It is still a marginal system as any change in pumping conditions could increase the load.

[bdunham7]

65% for the motor sounds really low (most motors of that size are in the 80s), but might be right for the motor+pump combo.

Would it be possible to replace the motor with a BLDC?

Not at full load, depending on the design and age.  Replacing pool pump motors with more efficient designs as they come out is a big business here in SoCal.  I have a two-speed one in the 80s, the one it replaced was 2HP and 65% at best.  Now they have nice 3HP variable-speed models at about 90%--probably just what the OP needs.  3HP nameplate, 92% efficiency and it still needs 3.2kW supply.

https://www.pentair.com/en/products/pool-spa-equipment/pool-pumps/intelliflo_vsf.html

[Palmitoxico]

I'm pretty sure that the motor is 3HP rated and the efficiency would impact only the pumping capacity (3HP mechanical from the shaft, 2HP pumping power), but I would try to measure it more accurately with my oscilloscope and be very careful to isolate the ground and not blow up my probes (I don't have a proper power meter at hand).

It doesn't work that way. A 3HP motor should produce up to 3HP (x the service factor if greater than 1) at the shaft.  The amount of electric power in watts required to power the motor at a given mechanical power level determines the efficiency.  So if your motor is loaded at 3HP and is drawing 3KW, it has an efficiency of 74.6% at that operating point.  The efficiency will vary widely according to load, input voltage, etc.  I think it is very unlikely that your motor would exceed 75% efficiency at full rated load, especially factoring in the losses in the VFD.  It is possible, and even likely given that the system works at all, that the load of the pump is less than 3HP so that your motor is not fully loaded.  It is still a marginal system as any change in pumping conditions could increase the load.

Humm, I don't know exactly the VFD efficiency, but I imagined that a three phase motor of such power would be > 85% efficient at nominal power. Of course the pump efficiency would be significantly lower (it is a 3 stage pump, it needs to pump water up to > 50m + fluid viscosity losses). That said, I don't believe it needs to operate full power, but I need to do more measurements to confirm.

[bdunham7]

Humm, I don't know exactly the VFD efficiency, but I imagined that a three phase motor of such power would be > 85% efficient at nominal power. Of course the pump efficiency would be significantly lower (it is a 3 stage pump, it needs to pump water up to > 50m + fluid viscosity losses). That said, I don't believe it needs to operate full power, but I need to do more measurements to confirm.

Is that 50m distance or height?  Does the motor have a data plate?

You really can't talk about pump efficiency in the same way you do motor efficiency.  The amount of recoverable work in any pumping system is usually pretty low unless you go to great lengths to reduce losses--an energy storage system, for example.  And many systems, like my pool, have 0% efficiency because there is no recoverable work done--the water stays in the pool--and all of the pumping energy is lost at one point or another.  And, as far as  your electrical system is concerned, the pump efficiency is irrelevant.  You could be driving a friction brake with it and the motor/VFD/inverter/batteries would neither know nor care.

[Palmitoxico]

Humm, I don't know exactly the VFD efficiency, but I imagined that a three phase motor of such power would be > 85% efficient at nominal power. Of course the pump efficiency would be significantly lower (it is a 3 stage pump, it needs to pump water up to > 50m + fluid viscosity losses). That said, I don't believe it needs to operate full power, but I need to do more measurements to confirm.

Is that 50m distance or height?  Does the motor have a data plate?

You really can't talk about pump efficiency in the same way you do motor efficiency.  The amount of recoverable work in any pumping system is usually pretty low unless you go to great lengths to reduce losses--an energy storage system, for example.  And many systems, like my pool, have 0% efficiency because there is no recoverable work done--the water stays in the pool--and all of the pumping energy is lost at one point or another.  And, as far as  your electrical system is concerned, the pump efficiency is irrelevant.  You could be driving a friction brake with it and the motor/VFD/inverter/batteries would neither know nor care.

Sorry, I wasn't clear, 50m height (~400m distance). Yes, the total efficiency doesn't concern the inverter provided that its output is 3KVA maximum. Let me search for a photo of the motor data plate.

[CoteRotie]

There are several topologies that could work, but a flyback is generally limited to maybe 200W max, and I wouldn't even go that high.  Anything in the KW range is full-bridge or PSFB territory, though you could conceivable do a tapped inductor boost at relatively low frequency, and several other topologies could potentially work.

[Palmitoxico]

Here is the motor plate (attached).

REND: Rendimento = Efficiency
FS: Fator de Serviço = Service Factor
FP: Fator de Potência = Power Factor

[bdunham7]

That helps a lot.  It is a brand new, very efficient motor, so going to a BLDC setup isn't going to help all that much.  It's interesting that they list 50Hz specs, although there doesn't seem to be any real design involved--almost any 60Hz 3-PH motor would behave similarly.  Note the 1.00 service factor at 50Hz. 

Does this motor come attached to the pump as a unit or are they two separate items that you have combined?  If the pump/motor is sold as a unit, does it have specs for flow and head pressure?

So I get 3288kVA and 2926W at full service factor load at 60Hz, not counting any VFD losses or tolerances in specs for the motor or the inverter--both of which could be rated optimistically.  Better than my earlier estimates, but still either cutting it close or just too much, depending on the loading.  The 0.6 PF of the VFD may not be helpful, but you're still at or near the limits.  The fact that reducing the VFD frequency to 54Hz extended the run time seems to back that up.

One other thought--you mentioned that the pump ran for 15 minutes at 54Hz.  If the input to the inverter is, say 3.2kW, that is a roughly 66A load on the batteries, more than C/2.  Have you checked to see how low the battery side of the inverter is going?   That 120Ah is likely the C/20 rate, at more than C/2 you are pushing the limits there too.

[rvalente]

Also, what kind of lead acid battery?

AFAIK there are 3 basic types of lead acid batteries: automotive, stationary and traction. As described, you're using 120Ah, so i believe they're automotive, right?

[NiHaoMike]

Also, what kind of lead acid battery?

AFAIK there are 3 basic types of lead acid batteries: automotive, stationary and traction. As described, you're using 120Ah, so i believe they're automotive, right?

Most likely deep cycle.

[rvalente]

The CFW500 has a bunch of nice features, had you ever checked the motor power on the VFD to see how much do you really need?

If yours have the plug-in card you can connect to the superdrive G2 software and monitor it.