Hi.
What is the input ? What is the circuitry inside ? Does it rectify the AC mains and then creates another mains outputting a different frequency AC mains ?

If rectifies the inputted voltage first; could it be modified to accept DC as input and create AC at a certain frequency output ? Sort of selectable/variable frequency inverter ? Do they exist single phase output ?

 

Please post the link to this item. It should have more information.
I think some are to fix the 50hz vs 60hz problem.
I think some are used to get variable speed out of an AC motor. I think we used one on the lathe.

I have something like this for testing power supplies. It is very old and about 100 pounds.
My machine outputs frequencies from 10hz to 500hz. I use that to test 50/60hz and some time to make certain my power supply will work at 400hz.
My machine outputs any voltage from very low to very high. Normally I use it from testing 50hz at low line and 60hz at high line. Those ate two test point are commonly used. Some time I use it to see where the break point is on a supply.

 

Thanks, Ron. The image above is just an image; irrelevant, not looking for that one, ignore, as am trying just to learn what are my choices to a VFD that will run from 120VAC 60Hz single phase mains and deliver 120VAC 60Hz to a single phase compressor motor of a plain 0.75KW air conditioner.

 

Yes, the typical VFD has a 3ph rectifier followed by a large capacitor bank, These can often be used on 1phase up to around 5hp.
The internal electronics typically uses BJT devices to output the 3ph.
Also they usually come with an internal isolsted PLC for the LV control circuitry.

 

Thanks, Ron. The image above is just an image; irrelevant, not looking for that one, ignore, as am trying just to learn what are my choices to a VFD that will run from 120VAC 60Hz single phase mains and deliver 120VAC 60Hz to a single phase compressor motor of a plain 0.75KW air conditioner.

I'm a bit confused (what's new?). You want to take single-phase 120 VAC/60 Hz and deliver single-phase 120 VAC/60 Hz.

Why would you need a VFD at all in this application?

 

I forgot to add, the use of 1ph output VFDs fell out of favour due to dropping out of run under load or low RPM.

 

Thanks, MHR.

Yes, WBahn, it appears confusing.
As the 'front end' first stage is rectification from mains; DC can be fed instead of the intended 120VAC input for the VFD.
And that is what I have: Solar 120VDC source capable of 10 Amperes that I want to run a 5000 btu 120VAC airconditioner with, if possible. I have a 120V battery to parallel too.

120VDC--->VFD--->120VAC/60Hz

I think that VFDs can raise voltage a bit too to fill the 160VDC rail gap from the shy 120VDC source.

 

Thanks, MHR.

Yes, WBahn, it appears confusing.
As the 'front end' first stage is rectification from mains; DC can be fed instead of the intended 120VAC input for the VFD.
And that is what I have: Solar 120VDC source capable of 10 Amperes that I want to run a 5000 btu 120VAC airconditioner with, if possible. I have a 120V battery to parallel too.

120VDC--->VFD--->120VAC/60Hz

I think that VFDs can raise voltage a bit too to fill the 160VDC rail gap from the shy 120VDC source.

Hi,

I am guessing that you are thinking of making your own DC to AC converter to run the air conditioner when the power goes out.
I am also guessing that you looked at converters that already do this and they don't like motor loads like an air conditioner would have.

VFD's will normally take an AC input, single phase, or three phase, and output an AC voltage with a way to vary the frequency.
The AC input is rectified and depending on the filtering used, it can be capacitors or an inductor and capacitors. Inductors are used for high power stuff because the peak AC currents in capacitor input filters is very high and inductors help a lot to reduce that. Most likely a swinging choke type inductor that lowers its inductance as the input current goes up.

In theory, you would apply a DC voltage to the internal DC buss. The DC buss comes from the rectified AC. The voltage depends on the filtering so you should measure that first.
This is pure theory though, which means it assumes that every other detail is taken care of. For example, it would also depend on the battery(s) series resistance which means the voltage comes down with load, especially heavy load. The batteries also have to be properly specified for the current demand. The range of battery voltages would also have to be considered. If you have to have a battery bank of 250v to supply 120vdc that won't work as there would be some limit on the upper voltage.

Maybe you can look around for a converter that is already built for this. It's quite a risk to try to modify a device for operating loads like this. If something blows out you can be out a lot of money.

 

In the early days it was with PWM. Then Toshiba came along and made one that could have maximum torque @ 0 rpm. Yep, I used quite a few of them in three phase 480VAC for typically 15Hp or less variable speed-controlled processes. I'll not even attempt to explain just how they did it as it is pretty well explained here: Variable-frequency drive - Wikipedia

There is no quick easy answer, but it has to do with quadrature phase control and some hefty industrial solid-state components. I could fit one into a standard Motor Control Center bucket instead of the typical motor control starter. The Toshiba drive is actually smaller!

 

Thanks, MHR.

Yes, WBahn, it appears confusing.
As the 'front end' first stage is rectification from mains; DC can be fed instead of the intended 120VAC input for the VFD.
And that is what I have: Solar 120VDC source capable of 10 Amperes that I want to run a 5000 btu 120VAC airconditioner with, if possible. I have a 120V battery to parallel too.

120VDC--->VFD--->120VAC/60Hz

I think that VFDs can raise voltage a bit too to fill the 160VDC rail gap from the shy 120VDC source.

You just need an inverter.

 

You just need an inverter.

Do you have an example ?

 

I am guessing that you are thinking of making your own DC to AC converter to run the air conditioner when the power goes out.
I am also guessing that you looked at converters that already do this and they don't like motor loads like an air conditioner would have.

To run the air conditioner when under full sun and perhaps a few hours at night time from battery. There is no utility power to go out at that rural site.
I have not found a converter that does this from 120VDC, other than 'off grid string inverters'

 

To run the air conditioner when under full sun and perhaps a few hours at night time from battery. There is no utility power to go out at that rural site.
I have not found a converter that does this from 120VDC, other than 'off grid string inverters'

Hello,

Oh ok, then maybe you could measure the DC buss voltage and current under various loads and go from there in deciding it batteries can handle it.
You'll also have to look carefully at surge current because air conditioners can draw high surge currents for short periods of time. The VFD has to be able to support that current without faulting out.

Now the type of air conditioner makes a difference too. The older units are terrible at startup currents that could be 6 times higher than the normal run current. That means that a 4 amp AC unit could draw 24 amps for a short time. The modern ones use, ironically, a converter that regulates the motor speed and soft start mechanisms so the motor speed ramps up gradually, thus drawing a much less startup current. Probably max 1.5 to 2 times the normal run current.

You should look into this as that will solve the startup problem. The inrush current is the biggest problem when it comes to using some sort of converter/inverter with an air conditioner.

You might also have to be careful when connecting batteries to the DC buss. The capacitors will draw a huge surge current even before the converter is turned on. If you use an inductor between the batteries and caps the inrush current would be minimized, but then if you disconnect the batteries the voltage spike from the inductor would have to be dealt with.

Are you forced to use a battery bank that has 120vdc nominal voltage?

 

You just need an inverter.

Hi,

It looks like he needs 120vdc input and the typical inverter/converter does not handle air conditioners very well. I am wondering if it would work better with a modern air conditioner that would have an internal converter that regulates motor speed and has soft start to reduce inrush current.
The inrush current is what prevents a lot of inverters from working with air conditioners.

 

Do you have an example ?

https://www.victronenergy.com/inverters/phoenix-inverter-12v-24v-48v-800va-3kva
I know it's nominally 48V DC, but it's just an example of one that I'm familiar with. You'll find plenty with different DC input voltages.

 

The input can be anything, it could be a parameter set inside the controller, it could be potentiometer, it could be digital signals, it could be PROFINET, PROFIBUS....

 

https://www.victronenergy.com/inverters/phoenix-inverter-12v-24v-48v-800va-3kva

Hi,

It looks like he needs 120vdc input and the typical inverter/converter does not handle air conditioners very well. I am wondering if it would work better with a modern air conditioner that would have an internal converter that regulates motor speed and has soft start to reduce inrush current.
The inrush current is what prevents a lot of inverters from working with air conditioners.

A modern air-conditioner will have a VFD to drive its compressor. The AC input will be rectified (and probably boosted by a power-factor controller) to give a DC voltage (usually about 400V) to run the VFD.
The right battery voltage connected to the right part of the circuit would mean neither VFD nor inverter would be required.
The PFC is simply a boost converter, so in theory would work with a DC input.
However, there may be other parts of the circuit that do run off AC (smaller fan, perhaps, condensate pumps etc.)

 

Thanks, Ian. 48V does not work as example. Not applicable, and not 'plenty of higher input voltages'.

The right battery voltage connected to the right part of the circuit would mean neither VFD nor inverter would be required.

I do not understand that. Change your perspective : Make a small 120VAC 5000BTU airconditioner work from 120VDC. That is the whole point.

 

The input can be anything, it could be a parameter set inside the controller, it could be potentiometer, it could be digital signals, it could be PROFINET, PROFIBUS....

The input to what can be anything ? What controller ? Potentiometer? Really ? Digital signals ? What are those ? Where/how does networking play any role here ?

 

I'm a bit confused (what's new?). You want to take single-phase 120 VAC/60 Hz and deliver single-phase 120 VAC/60 Hz.

Why would you need a VFD at all in this application?

as the name suggest VFD/VSD is mainly used to adjust speed. for example it can fix the problems where mains frequency is different. but that is not the only use as they typically have a lot more options since controlled digitally. one can easily set current limits, acceleration, deceleration, etc. just about every variant will have an option for closed loop control, following another axis etc. they can be tuned to load, sometimes more than one which allows axis to operate smoothly in both directions even under asymmetric load (like vertical). interface can be discrete I/O)digital and analog) or fieldbus. they can monitor number of things and react to problems (over and under voltage, over current, overtemperature, change in load etc.)