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Using VFD for pumps with no neutral, and taking neutral point from main source

Hesham1216

Plastic
Joined
Sep 25, 2023
Hi Guys,

I have submersible pumps that is mainly work on 60Hz in my country. But it will be exported to another country that has 50Hz supply.

Pumps are single phase. (220 V)

The power supply for the location at which it will be exported is 3 phase (5 wires: L1, L2, L3, N, E) (3 live, neutral and earth). (380 V)

I thought to use VFD to change the frequency, but unfortunately, I could not find any VFD with single phase out. So the idea is to use a VFD with 3 phase input, 3 phase output and use one of the phases to get 220V from the VFD.

Now the VFD doesn't have a neutral, so my question, can I use main supply neutral for the load? so the pumps will be connected to one phase of the VFD and neutral of the main.

Is it possible? is it ok for the VFD?

Pumps are working 8.3 Amps @ 220V, I am buying a VFD around 7.5 KW (not sure if I mis calculated)

I am a mechanical engineer not electrical so excuse my ignorance :D

Any help would be appreciated much.

Thank you guys..

(Note: the budget is also very tight so I can't buy more equipment other than the VFD :(()
 
it isn't going to work. A single phase pump is likely to have a motor run capacitor which is frequency dependent to generate the correct torque and phase shift for the motor. If you source the supply from a VFD then this gives out a complex waveform (its why the motor makes a whistling noise) and this may well damage the capacitor, the motor or both. Also, never connect any load between a VFD output and a neutral. The VFD rectifies the incoming mains (1 or 3ph) and you'll only ever get DC out of one phase to neutral. Not great for something requiring AC. I suggest you get the correct pump for the country.
 
The other way is to run the pumps at 50Hz instead of 60Hz. Generally speaking, the flow will be 5/6 as much and the maximum pressure will be 25/36 as much. If this is acceptable, it's the simplest solution. The voltage should be reduced, but most motors have enough capacity to cope with the 0-20% over voltage caused by running at 50Hz instead of 60Hz.
 
We made VFDs for a special application running an oil burner that had a run capacitor. Worked fine, but we did have sine wave output, not the "quasi-sine" that still exists with some inverters. Most have the same sine wave output that ours did.

There are many single phase VFDs (ours was single phase) however they tend to be at power levels 1.5 HP and lower. Invertek in the UK should have a good selection.

Far from not working, most single phase VFDs are intended for motors with a single start/run capacitor. Most will not work well with standard type induction motors.
 
Single phase input/single phase output VFD should be fine here. You connect it to one incoming phase and to neutral. The input of the VFD will usually be marked something like "L1/L L2/N"
 
The voltage is fine - 380V line to line is 220V line to neutral, and there's a neutral present.

The issue is frequency mismatch, which a transformer won't fix.

Running the pump at 50Hz instead of 60Hz theoretically requires stepping the voltage down to about 185V, which is probably going to mean a non-standard (PITA) transformer. Alternatively, a single phase VFD to raise the frequency.

Three-phase submerged pumps aren't available? A 480V 60Hz motor is natively a 400V 50Hz motor - the V/Hz ratio is the same, you just lose 18% power.
 
in 35 years of repairing and installing VFD's, I've never seen one with single phase output. VFDs work by synthesizing and altering the frequency (and voltage FWIW) of the rotating field to a 3-phase squirrel-cage motor, whether the VFD itself is supplied with single or 3-phase mains. Can anyone give me a link to a VFD (variable frequency drive) that is aimed at 1-ph capacitor run motors?
 
in 35 years of repairing and installing VFD's, I've never seen one with single phase output. VFDs work by synthesizing and altering the frequency (and voltage FWIW) of the rotating field to a 3-phase squirrel-cage motor, whether the VFD itself is supplied with single or 3-phase mains. Can anyone give me a link to a VFD (variable frequency drive) that is aimed at 1-ph capacitor run motors?

Well, THAT was easy:

 
That's a new one on me! But they're a new product I believe, reading the blurb. I've never seen one. Invertek (Optidrive) are a Welsh company and are easy to obtain in the UK. Most single phase speed control in the UK is traditionally done using a phase-angle controller similar to that on a ceiling fan. Thanks for the link!
 
They have made single phase drives for at least 12 or 15 years, as I recall. Not a new product.

I used to work with the US distributor from time to time. They had the single phase about that long ago, so by now it is a mature product.
 
We made VFDs for a special application running an oil burner that had a run capacitor. Worked fine, but we did have sine wave output, not the "quasi-sine" that still exists with some inverters. Most have the same sine wave output that ours did.

There are many single phase VFDs (ours was single phase) however they tend to be at power levels 1.5 HP and lower. Invertek in the UK should have a good selection.

Far from not working, most single phase VFDs are intended for motors with a single start/run capacitor. Most will not work well with standard type induction motors.

I contacted invertek, they have sales representatives in the middle east (UAE) and waiting for a quotation.

Will update when I come into a conclusion.

Again, thanks for your suggestions and ideas, it was really helpful.
 
They have made single phase drives for at least 12 or 15 years, as I recall. Not a new product.

I used to work with the US distributor from time to time. They had the single phase about that long ago, so by now it is a mature product.

Since you are from USA, what do you think of:



Since invertek seem doesn't have more than 1.5 HP VFD

The pumps are 2 HP and 1.5 HP and to be in the safe side I want to purchase a 3 HP and 2 HP.

Also from your experience, do you think this will work? with pumps with capacitor?
I am also wondering, the running current as you may know is different than the starting current, the starting current usually much higher right? will the VFD handle it?
 
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It depends on the type of motor and the characteristic of the load. They work best on motors that do not have a centrifugal switch - so, shaded pole or permanent split capacitor (PSC). Motors intended for very high starting torques like cap start/run (dual cap) usually don't do too well.

Are you sure your pump supplier can't manage a 50Hz variant? They're usually made in the same factory, just with slightly different motor-impeller combinations.
 
It depends on the type of motor and the characteristic of the load. They work best on motors that do not have a centrifugal switch - so, shaded pole or permanent split capacitor (PSC). Motors intended for very high starting torques like cap start/run (dual cap) usually don't do too well.

Are you sure your pump supplier can't manage a 50Hz variant? They're usually made in the same factory, just with slightly different motor-impeller combinations.

The supplier can of course, and I already contacted them they can ship directly to the other country, but the problem is the cost and also the time. It will take longer and the customer... well you know customers....
 
Since you are from USA, what do you think of:



Since invertek seem doesn't have more than 1.5 HP VFD

The pumps are 2 HP and 1.5 HP and to be in the safe side I want to purchase a 3 HP and 2 HP.

Also from your experience, do you think this will work? with pumps with capacitor?
I am also wondering, the running current as you may know is different than the starting current, the starting current usually much higher right? will the VFD handle it?
Don't know much about that company. However, looking at the pics, and reading the manual, it seems that they may be re-branded chinese VFDs.

There are different types of motor with capacitor. One type does not disconnect the capacitor, which is used for both starting and running ("PSC" type). Single phase VFDs normally work with those.

The other type includes standard capacitor start, and resistance start motors. These have different characteristics, and do draw large start surges, which the VFD must be designed for.

The diagrams suggest that the ATO units are made to use both types without the phase-shifting capacitors. I have long wondered why nobody does that, but have not seen such a thing in a manual until now. The VFDs seem to be three phase units with the required programming difference.

Given the probable source, I am rather suspicious of those units. The idea is sound, their implementation is what I am concerned about.

The more usual single phase VFD starts the motor at full voltage, and therefore is generally only made for the "PSC" and shaded pole motor types.
 








 
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