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VFD as static freq converter.

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itsmoked

itsmoked

Electrical
Feb 18, 2005
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I have a case where a yacht from Europe has 50Hz A/C equipment. It now permanently resides in the USA. The A/C consists of 4 units between 3 and 6hp 240V 50Hz. Replacing these compressors and opening the systems to do it would be spectacularly expensive, way up in the mid 5 figure realm.

Has anyone successfully used a VFD as a static converter set permanently at 50Hz to run 50Hz single phase motors? I'd see them as replacing the motor contactors. The VFD would be powered always and the contactor signal would merely close a relay that tells the VFD run forward at 50Hz.

Of course the VFD would need to be larger than normal since it only gets fed single phase.

Anyone tried this?





Keith Cress
kcress -
 
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The problem I see is using a single phase output. The drive will see that as a faulty motor.
I can give you a couple of transformer connections to develop single phase from three phase.
There will be equal current in each phase.
The capacity will be 2/3 of the three phase capacity.
The hitch is that with a resistive load, one phase will be at unity, one phase will be 50% lagging and one phase will be 50% leading.
(By the way, that is how you get full current in each phase but just 2/3 output.)

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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The other issue is that the VFD will not really be rated to handle the inrush current of the motor. Normally when a VFD is controlling a motor, it is in total control of the ramp rate etc. In your scenario, the VFD output transistors are already at full output potential when a contactor closes. The magnetic inrush of the motor windings is high enough to damage the transistors unless you SEVERELY over size the drive, like 5x the largest motor size, at which point it becomes impractical. In addition, most single phase motors will have starting caps which, as Bill pointed out, will look like a short circuit to the VFD output anyway and trip it off line.

There are single phase output VFDs out there, you could in theory put one on every single phase motor. In the US you can buy them from Bardac and Anacon, they are both brand labeling a product from the UK called Invertek, but Invertek only sells them here through these partners. But here again, starting caps are a problem for the drives, so they can only be used on PSC motors (because the caps are always in series with the windings, not switched out with a centrifugal switch). So that would be an option only IF your single phase motors are all PSC. Unlikely however. (They can also be used with Shaded Pole which can be used with simple dimmers anyway, but those have few uses beyond fans).


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
 
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Thinking out of the box, let's step back and take a wholistic view.
What do we have? 50 Hz AC units, 60 Hz power.
What do we want to do? Run the units and cool the yacht.
I seem to remember an old trick from the days when I dabbled in refrigeration.
We would come across a bad design or a valve replaced with the wrong valve. For some reason we would have a hermetic unit overloading.
Compressors have the heaviest load when the back pressure is highest.
On the big old ammonia plants, it may take quite awhile to pump down the system. It was common on the old six shooters to have a valve that would run all the cylinders, cut out two cylinders and run on four, or cut out four cylinders and run on two.
They would start on two cylinders. The operator would watch the ammeter or the crankcase pressure, and when the back pressure had dropped enough cut in two more cylinders.
When the motor load dropped to a safe level again he would cut in the last two cylinders.
What has this got to do with AC units? Lots. If you can get the units to start on 60 Hz, you can throttle the suction line and drop the crankcase pressure enough to drop the load on the motor to a safe level.
At one time there were suction regulator valves on the market for just this purpose.
I suggest to try it on one unit. If it can be made to work, it may be a cost effective solution.


Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Hi Bill!

I'm thinking by the time we've added fairly large VFDs and then ZIG-ZAG transformers four times we're probably going to have space problems.

I like the crankcase pressure regulating valve idea and understand it (in fact we had to put one on the last rail car problem) but this yacht is a long way from me and that method requires cracking open a system with reclaiming, vacuum time, recharging, testing monitoring and commissioning to make sure things weren't destroyed. By that point it might make more sense to swap out the compressor.

Not as simple as a simple freq change. Dang.

Jeff; The inrush.. What if the VFD were told to start at 0 and ramp to 50Hz in a second? I realize at some point the start cap is going to switch out but the motor is still there to help carry the disturbance.

Ahh the start cap.. High frequency modulation.. totally shorting that out.. so, pretty much no motion can occur and the VFD would protect itself. Dang. z = 1/2fC Dang.



Keith Cress
kcress -
 
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How about interposing a single phase transformer to block the modulation from the start cap. Then we could leave the VFD running and use the original contactor on the load side of the transformer to bring on the compressor?

The VFD has to be larger for feeding it single phase but that simultaneously makes the output drivers bigger too. Use say a 7-1/5Hp VFD on a 3hp compressor.

Keith Cress
kcress -
 
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Rotary frequency changer, or a UPS without a battery? Some of the larger commercial units are approved by the manufacturer for use as frequency changers.
 
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A standard 3-phase VFD just won't work on this application. If you have start capacitors, then a 1-phase VFD won't work either.

This would be a case where a MG set powering the systems would be the easiest and cheapest option. Probably not the most efficient option though.

Talking to industrial/commercial UPS manufacturers would likely be the best route towards a solid-state solution.

A TX between the VFD and the motor would remove the voltage PWM reaching the motor. But, I still doubt you could start the motor with a contactor without massively over sizing the VFD.
 
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Can't find my email... It's essentially in my tagline.
URL]



I think I have a beater VFD laying around and I'm going to thrash it with a 60Hz single phase motor and see what happens.

In this case the Yacht owner going to use his generator to bring the place down and will follow this using cheap temporary window unit to keep it comfortable after that. This modus operandi will continue until the Yacht transits to Australia for the next three years where it will be happy as-is.




Keith Cress
kcress -
 
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Hi Mike!

Advanced Power & Controls, LLC got back to me in minutes and stated that their static converters were specifically for lab use but they graciously directed me to two companies that had "marine versions".
Atlas Marine Systems
ASEA Power Systems
In both cases he warned me to hold onto my wallet with both hands.

Visicomm Industries didn't bother to reply at all.

Adaptive Power Systems, Inc requested, "I call them", I haven't had time.

Keith Cress
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