# DC-AC inverter feeding VFD



## micromind (Aug 11, 2007)

I'm replacing the electronic linear actuators at 5 gates with hydraulic cylinders. Obviously, I need a pump at each one. 

In order to keep from completely wrecking the gates, I need to start slowly and stop slowly so I'm using a VFD to power the pumps. This also gives me the ability to control the top speed of each gate. 

4 of the locations have single phase utility power, one of them has solar panels, 2 - 8D batteries and a basic 5KVA inverter. All locations are either 208 or 240. The VFDs will be single phase input and I'm guessing the motors will be around 1/2HP. Because of the duty cycle, I'll need to upsize the VFDs a bit. 

Since the inverter produces a 'modified sine wave', meaning it's stair-stepped rather than a curved line, my question is will a VFD accept this type of power? 

I have a feeling it will but it might be rough on the input diodes and the soft-start circuit might not like it. Might need to upsize the VFD even more.......

Has anyone connected VFD to a distorted waveform? 

A line reactor or a basic transformer might smooth it out to some degree but maybe not enough.


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## mitch65 (Mar 26, 2015)

I'd use DC motors. DC hydraulic pumps are easy to find, use the AC to charge batteries, use straight DC on the solar one. DC to run the forward/reverse valve.......no need for drives or use dc drives.


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## RePhase277 (Feb 5, 2008)

The basic circuitry of a VFD is a rectifier feeding an inverter. That circuit won't care a lick about the modified sine wave.... BUT the intelligence inside the controller might see it as a fault.

And why not DC pumps?


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## JRaef (Mar 23, 2009)

We have "modified sine wave" UPS inverters powering VFDs, but the VFDs are extremely over sized for the load, as in 3x and the inverter is something like 5-6x the size of the VFD. I don't know if all that de-rating is just for the VFD issues or because other things are being powered by the UPS, I didn't design the system. 

What I do know is that the modified sine wave doesn't like the harmonic rich current draw of the VFD, and the VFD DC Bus capacitors run extra hot because of the added ripple caused by the way the diodes draw current from that modified size wave. Basically, they aren't really compatible, but you can make it work if you throw enough money at it. A reactor ahead of the VFD would help, but it's difficult to quantify how much. If anything, I would either try to find a Sine Wave inverter for your PV system, which has the filter built-in, or get a Sine Wave Filter instead of a plane line reactor ahead of the VFD.

Or, get electric linear actuators...


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## micromind (Aug 11, 2007)

I thought of a DC system but because of the trouble they have had with the solar one, they don't want DC. Yes....dumb....but it is what it is......


Since this installation is under an unventilated shelter, the caps would suffer a lot. 

Maybe I'll see if they'll accept one DC system and the rest AC. 

They're looking for reliability more than anything else so I don't want to take very many chances.......lol.


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## JRaef (Mar 23, 2009)

micromind,
At that HP range, some small VFDs can accept AC or DC input from the same unit. So from the end user standpoint, the drives would be exactly the same, the only difference would be where the two wires land; L1 and L2 for AC single phase, or DC+, DC- for a DC input. To make this possible, the VFD must have a pre-charge circuit on the down-stream side of the DC input. 

I'm sure more than one are available, but the one VFD I know for sure has this is an A-B PowerFlex 520 series, like a 523 (assuming you don't need Ethernet).


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## frenchelectrican (Mar 15, 2007)

JRaef said:


> micromind,
> At that HP range, some small VFDs can accept AC or DC input from the same unit. So from the end user standpoint, the drives would be exactly the same, the only difference would be where the two wires land; L1 and L2 for AC single phase, or DC+, DC- for a DC input. To make this possible, the VFD must have a pre-charge circuit on the down-stream side of the DC input.
> 
> I'm sure more than one are available, but the one VFD I know for sure has this is an A-B PowerFlex 520 series, like a 523 (assuming you don't need Ethernet).


JRaef .,,

I have ran into the 520 series few time and yes they can take full DC input but the question on this part which I do not know what voltage did Mircomind have see on that location .,, 

But the one I did work on it can take 72 VDC but kinda little hard on the VSD but it will be nice if it was on 120 VDC then it will be fine on that voltage level. 

so just wait until Mircomind reply to this part then we can able go from there.

unless he mention anything under 48 vDC he may have to get combo converter and VSD hook up together., so I dont know what his gameplan is yet.


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## micromind (Aug 11, 2007)

The DC at the solar gate is 24 volts. It is then inverted to 240AC. 

I haven't actually scoped the output of the inverter yet but it looks like it would be the type to output a modified sine wave. 

I suppose I could rectify the 240AC and feed the VFDs DC bus with it but rectifying a non-sine wave could prove difficult. Maybe a choke/capacitor smoothing circuit would work better than just capacitors. 

One nice thing is the current is fairly low.


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## frenchelectrican (Mar 15, 2007)

Ahh small package .,, but with modifed sinewave it will be little hard on VSD but if change to true sinewave then it will work just fine.,,

the other thing is battery .,, how big the battery bank and how often do they ran the gate ?


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## telsa (May 22, 2015)

What has Faraday, Volta, Edison, Tesla wrought ?


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## bill39 (Sep 4, 2009)

I may be missing something about what you're trying to do but if you are using hydraulic cylinders why not use valves & flow controls on each of them and an across-the-line starter for the pump?


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## JRaef (Mar 23, 2009)

micromind said:


> The DC at the solar gate is 24 volts. It is then inverted to 240AC.
> 
> I haven't actually scoped the output of the inverter yet but it looks like it would be the type to output a modified sine wave.
> 
> ...


It's the rectification that's difficult here, so since a VFD is already doing that, adding another rectifier just adds another component to the system. If you are going to do that, I'd just add a DC boost converter to your 24VDC to take it up to 330VDC for feeding the drive, forget that inverter altogether. These guys will have (or make one) for you, or if you are feeling ambitious, you can find plenty of circuit designs on the web to roll your own, they aren't that difficult. When people make and sell solar powered well pump drives, that's what they are doing; using a DC boost converter or something like it to step the solar array/battery DC voltage to what the drive needs for an AC motor. In fact that's what is inside of your inverter now, it's just that it also includes that modified sine wave output that, in this case, you don't really want. 

In fact, if that inverter already exists, it might be worth checking to see if you can just access it's DC bus directly, because if it has a 240VAC output, it must have that 330VDC bus in there somewhere.


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## JRaef (Mar 23, 2009)

Re: rolling your own.
An easy way to do this instead of trying to manufacture an SMPS based boost converter is to use a simple "voltage multiplier" circuit that is a series of capacitors and diodes in a specific arrangement. When you buy a VFD that takes a 120V input and runs a 230V 3 phase motor, there is one of these ("voltage doubler") on the front end of that VFD.

Here's a tutorial on it, I've used this in the past. For yours, you would need 14 stages to boost your 24VDC up to where the VFD can use it to make 230VAC. The trickiest part is finding a source for reliable components that would survive your environment. 
http://www.electronics-tutorials.ws/blog/voltage-multiplier-circuit.html


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## micromind (Aug 11, 2007)

My first thought was to use hydraulic controls for the soft-start and soft-stop of the gate but the VFD was less $$$ and even better, less complicated. Plus, with the VFD, I need only one position switch for up and one for down. With the hydraulic controls, I'd need 2 each or timers. 

Plus, if it blows through a position switch, I can program the VFD for lower than the actual motor current so it'll trip instead of deadheading until something bad happens......

I'm now thinking about seeing if I can get a 'true sine wave' inverter for the solar gate. Considering the small size, it's likely I can find one that's oversize enough to handle the VFD reliably. 

But again, I don't know how these would work with a VFD. 

The idea of buying or even better, building a DC-DC converter looks pretty good too.


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## frenchelectrican (Mar 15, 2007)

Mircomind.,,

How big the motor are we talking about for this set up ?? half horsepower or what ?? 

I may have some idea but before I get my feet wet I need to know a rough estamate of motor size so either myself or Jaref one of us will come up some better idea with this set up..


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## tates1882 (Sep 3, 2010)

micromind said:


> I'm replacing the electronic linear actuators at 5 gates with hydraulic cylinders. Obviously, I need a pump at each one.
> 
> In order to keep from completely wrecking the gates, I need to start slowly and stop slowly so I'm using a VFD to power the pumps. This also gives me the ability to control the top speed of each gate.
> 
> ...


skip the dc motors. siemens has a setup, a vfd that will run directly from the solar panels. Give me a few and ill post up the info.


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## micromind (Aug 11, 2007)

Considering the size and weight of the gate, I'm guessing 1/2HP. Maybe even smaller.


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## tates1882 (Sep 3, 2010)

Ill get the rest of the details tomorrow but Sinamic V20 Works directly on DC supply applied at its DC link. It uses mppt to avoid ol trip on a cloudy day or low battery situation.

On a side note one of my customers is based in Dayton, NV, he needed a test station built at his shop consisting of a vfd, start/stop station and a twist lock. He had three different electrical contractors stop by and none of them could figure it out. I offered to swing by but a 18 hr round trip was to expensive.


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## sparkiez (Aug 1, 2015)

The vfd may be cheaper, but i would just hardwire this bad boy and use a pair of limit switches and a second valve. 4/3 way with spring return. Center would be normal travel speed while the limit switches activate hydraulic cushions on up and down. This way they have something reliable that isn't quite as susceptible to the heat.

Another way would be to use a meter out circuit on the return line that only has flow when the limit switches shift the valve to slow it down upon raising or lowering.


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## micromind (Aug 11, 2007)

tates1882 said:


> Ill get the rest of the details tomorrow but Sinamic V20 Works directly on DC supply applied at its DC link. It uses mppt to avoid ol trip on a cloudy day or low battery situation.
> 
> On a side note one of my customers is based in Dayton, NV, he needed a test station built at his shop consisting of a vfd, start/stop station and a twist lock. He had three different electrical contractors stop by and none of them could figure it out. I offered to swing by but a 18 hr round trip was to expensive.


Dayton is about 40 minutes from my house........in fact, I'll be passing through tomorrow on my way to Carson City. 

That test station sounds incredibly simply but it doesn't surprise me than 3 contractors couldn't figure it out. There are VERY few guys around here who understand VFDs and controls, the vast majority of guys here do buildings (some are rather huge.....) and connect power to factory built equipment.


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