Controlling an AC Load with DC Voltage

relay of three position switcher.

How much help do you need - an analog transducer with a series resistance will fill the bill. Try any of the over the counter control companies and specify the load and range of control. You should be able to drop the voltage in a linear mode from 28 to 2 vdc and range the output to 12 vdc with some simple circuitry. Siemens, Automation Direct, etc should have the gear you need.

I need help finding the gadget or someone to build it for me - have not had luck finding such a gadget. But then, I have not talked to a control company. Recommendations?

The AC heat load starts at zero and ranges up to 15amps at 60Hz 115vac. The control must do nothing below 28.6vdc and must apply the AC load in a linear fashion as the DC control voltage increases from 28.6 to 29.6-30vdc.

If necessary, the entire load can be applied as soon as the DC control voltage exceeds 28.6vdc, but I would like the device to be solid state for fast switching and reliability??? It would probably do a lot of cycling on a windy day.

I know what I need this device to do, but finding/building/designing such a device is way beyond my meager electric talents. I can wire the thing in, and that's about it. This is a dump-load for my wind generator.

If I have to go to a DC dump-load, I'm gonna need about 8 times the heating coils, compared to an AC load. The morningstar dumpload controller is intended to apply a DC load above the set point, but an AC load would be a lot easier. With the right device, I could remove the morningstar controller from the circuit, right? When the battery bus voltage reaches the set point, it begins applying the AC load.

What you need is a DC controlled transformer [variable inductance] ,what this is is a transformer with 3 windings your AC input primary an AC output secondary and a DC control winding.

By varying the dc through the DC winding you will alter the impedance of the transformer which will vary the AC output voltage.

The windings will have to be calculated for current and voltage etc but I dont have time at the moment to go in to further detail, no doubt somebody with a bit more time can elaborate.

Though the DC voltage range is a bit limited you may have to use a power amplifier to give a good range.

This is called a Saturable Reactor, a device frequently used as a dimmer for theater lights before high-power thyristors became readily available and inexpensive. Since saturable reactors tend to be large and heavy, they have mostly been supplanted by controllers that use SCR's and Triacs.

I gather you're using a diversion load controller. Why not connect to a series of water heater elements? Why are you focused on using AC? Heater elements are cheap and they'll work with AC or DC.

The OP has a 24V battery bank. Can you get heater elements for 24V? Obviously he can use 110V elements but he is only going to dump 1/16th of the rated power.

Chas

Yes. My suppliers have 12/24/48VDC elements for water and air. The H2O heaters are rated to 45 amps and the air heaters up to 40 amps. Check AEE if you want to source them yourself.

Good point, give or take a few percent!

All the nominal 110 V, 115 V, 117 V AC systems are now all really pretty standardized at 120 V, so 1/25 of the power, except these semi-incandescent loads have rising resistance with temp. As I recall, the resistance of a cold light bulb is very low relative to the full power volts / amps, like 1/2 to 1/10, which explains some of the thermally induced stress of turning them on and off, so exact numbers require experimentation: try it and see.

Of course, the standard 12V DC vehicle battery is really 12.6 V DC chemical, is often charged at 13.8 to 14.4 V, and will sustain the higher voltage for some time after charging stopped, perhaps due to the temporary hyper-concentration of excited ions near the poles. Not much is exactly as labeled in this world. So the 24 V may be 25.2 V, or 28.8 V is dumping while charging at max. I usually charge at 14 (28), being wary of boiling off the water. Often, some sort of stepped fold-back charging plan is best, charing a N amps until you get to M volts, then at M volts until charging falls to P amps, then fall back to maintenance at Q volts. Now, with natural system power inputs, the availablility becomes another variable, but that sort of plan is a good limit to maximize charging without buying a lot of distilled water.

Maybe you can dump the extra power in a dehumidifier and use the water on the batteries!

All you need is a device called an SSR (Solid State Relay), and maybe with an analog DC control input. You are asking for a very narrow bandwidth of control there, but it could be done.

However, you have one issue somewhat wrong. Amps are what the load TAKES, not what the device gives. The device can control the output Voltage, but the Amps that are drawn will depend on the load. You can also do what is called "Current Limiting" meaning that you can ramp the voltage and when the load starts to draw more current than you want it to, the voltage ramping ceases at that point so that the load is unable to draw more current. But before you go off on this, examine you goals first..

On an AC heater element, ramping the voltage to control the Wattage output of the heaters is very common. However, do you want an infinitely adjustable output? Or are you simply looking for a ramp function, what is called a "soft start"? Soft starting is a typical feature used to avoid high inrush currents on certain types of heater elements that have a high hot-to-cold difference in resistance. For example, if the material has a very low resistance when cold, turning it on means a big spike in current for a few seconds until it heats up and the resistance increases. This is called a "positive coefficient of resistance" or "Positive Temperature Coefficient" (PTC) and is common in most metalic heater materials such as NiChrome. So if you get an SSR with a 30VDC input signal and a soft start ramp function, it will avoid that current surge as the element heats up and you no longer have to deal with soaking the ramp yourself.

I'm not sure why this is complicated. I set-up dump loads for hydro-electric all the time and 1.8kVA is not much with which to deal. A couple of Trace (oops xantrex) C-40 controllers and few heater elements.... The C-40's allow a gradient of output based on your voltage above your bulk battery settings, eg. 0-40 ADC. I'm assuming some things about your system here, so correct me if I'm wrong. Or be more complete in your system description.

Try a 'Saturable Reactor' or sometimes called a 'Transductor' or sometimes called a 'Magnetic Amplifier'. 1950/60 technology may be hard find anyone making them now.

This Man need a Omniterm TWT Universal Input Two Wire Transmitter. We used to convert 0 - 94.88 Volts to 4-20mA. You can clamp your low and high input limits to any value between 0 to 250 Vdc. It is programable and auto scaling. you can program it such that 28.6 Vdc must give 4mA and 30 Vdc will give you 20mA. Then on the AC zone use the Solid state relay that can be controlled with an input signal of 4-20mA from the omniterm TWT Universal Input Two Wire Transmitter Model C2406A.

Please Google - omniterm TWT Universal Input Two Wire Transmitter Model C2406A for data sheet.

Sophisi:

Your setup may work for me, depending upon cost. Could you advise cost for the C2406A and for the appropriate SSR (model)? and/or where should I purchase such devices?

Thanks

Please try googling - RS components. But not to sure if they sale intarnationaly.

A DC voltage comparator - op amp with high impedence inputs, adjustable comparator range, proportional output set to control the AC load with a triac or something similar. You could switch a solid state relay but that seems like overkill and kinda hard on the relay. If you can't find something at Allied Electronics call FSI Systems in Farmington NY. They should be able to design and breadboard this in a day if they have the parts on hand.

This is a simple requirement set which any digital phase angle controller should e able to accomplish. You can create an offset on an analog input with a simple voltage divider, or some controllers will let you set the input offset digitally.

If you need only one or two, I'll gladly help find what you need.

If you need any kind of quantitiy, let me kow - I can customize firmware on one of my controllers for you.

Rgds,

Chris

www.clicsys.com

chris@clicsys.com

At the moment, I need only one controller, and I sure do need help finding such a device - please advise.

Wildshot,

I agree with many here that this is not all that difficult a task. My preference would be using a little analog circuitry (op-amps etc.) for control and then triggering a solid state relay, SSR. But as you've noticed here this is just one of many sound approaches. Are you sure that you require linear control for just a 1.4 volt change? Noise sensitivity might make that tricky.

If your handle included any location information some of us might help you to build the circuit or direct you to a local contact. You might want somebody's "hands on" help because unanticipated complications normally occur the first time any design gets made.

Last I checked, long ago, the cheap control for 15 A 120 V AC was a triac. As I recall, you can turn them on gradually with DC -- current turns them on in 3 of 4 sectors of load and current polarity. So, I visualize a resistor, an zener diode and a triac as minimal parts. The design questions are:

1. Where is the DC coming from?
2. Do you need isolation or can the controller and dc control source float on the AC line (additional cost and complexity for isolation)?
3. Does the load mind chopped AC voltage/current (most heaters are dumb resistors, and OK)?
4. Is there anything nearby that would dislike RF noise (cost of suppression)?
5. Does the control have to be linear, or just monotonically or mostly ascending (comparison and feedback circuits add cost)?
   

If enough of the above complicate it, then a packaged item like SSR might save a lot of mucking about.

I put the answers behind your questions:

1. Where is the DC coming from? system voltage
2. Do you need isolation or can the controller and dc control source float on the AC line (additional cost and complexity for isolation)? don't think I need isolation
   
3. Does the load mind chopped AC voltage/current (most heaters are dumb resistors, and OK)? the load is a bathroom heater with w fan (AC motor)
   
4. Is there anything nearby that would dislike RF noise (cost of suppression)? no
5. Does the control have to be linear, or just monotonically or mostly ascending (comparison and feedback circuits add cost)? mostly ascending

What will it take to do this?

RE: 1, is the system grounded to a power system or for safety, that the control voltage generator can float?

RE: 2, just don't touch anything while it is running.

RE: 3. Most motors do not like chopped AC, but if the motor could be separated and turned on full when the heat rises, then the rest of the heater could eat chopped AC. Can we isolate the fan motor? It's another minor design to make it run only when the heaters get hot. The system now eats line power to make generated power go away!

PS: Have you considered a more intermittent load, where you turn it on full when charging hits an upper limit, and turn it off when batteries show a low threshold voltage? Then, the load device is working with the intended voltage and waveform, just not on all the time.

This article is nice in more precisely describing the triggering of triacs by either dc or ac currents, where quadrant I-III is an AC trigger and II-III is a dc trigger:

http://www.littelfuse.com/data/en/Application_Notes/AN1002.pdf

Commonly found circuits show simple AC triggering, but do not consider DC triggering:

http://www.aaroncake.net/circuits/dimmer.asp

You can get one for $.69 + S&H!

http://www.newark.com/jsp/search/browse.jsp;jsessionid=NAUBRLKGISEROCXDUZ0G13Q?N=1001819+511+296208+290236+290245+290250+290208&_requestid=412489

Radio Shack does not seem to stock them, but you can find the supporting components there.

RE: 1, the system is grounded, and I don't know what float means.

RE: 2, roger that.

RE: 3, the fan motor can be isolated, I am sure I can deal with that.

I am looking to buy a solution, I am not the kind of guy who can build some electronic whizbang thingy and expect it to work. All you guys, give me a quote and a delivery date for your device.

I have got a 90amp ssr coming and will experiment with that. The morningstar device will send a 28 volt signal to the ssr and turn on the AC, that is what I expect, but it will not be proportional right? all or nothing? this may work for the moment.

Wildshot,

I'm fairly certain you will not get a quote here. You've yet to even identify where you are. You probably have some local zoning codes that must be followed. Even if you're in the lawless back-lands of Libya, local conditions will demand engineering requirements that have not been discussed here. As your OP implies, you are looking for a non-standard configuration. Having trained eyes on the scene can be invaluable.

We will still gladly provide you with ideas and sometimes circuit schematics if you choose to tinker up a system for this project. As things go awry, we will certainly assist in righting the course if you get baffled.

To answer one of your questions though, floating of an electrical system means that there is no designed conductive path to earth. The electrical system of an automobile is floating for the design does not have a planned electrical connection to earth. (Tires often do not even conduct off static charges.) This has critical personnel and equipment safety implications. An incidental connection of one and only one lead to earth will not be detected as a possible fault and cannot electrocute an individual. But this can produce a false sense of safety, for despite a car's electrical system being isolated removing a functioning spark plug wire can still produce a substantial shock.

RE: 1. Electrical circuits are about the current running around the circuit, and voltages relative to each other in the circuit. Their relative voltage to earth=ground=the-case is only a safety and insulation concern, and they actually have no relative voltage if they are not connected to "ground" at some node in the circuit network.

So, if you have a floating circuit and want to control it from an existing system that is grounded, you can attach it, and it will float at whatever voltage the existing system gives it. If you have two grounded circuits, you have to run interference, like with a comparison amplifier, to move the voltage between nodes a1 and a2 of the existig circuit to nodes b1 and b2 of the grounded second circuit; amplifier, please make input node b2 the voltage ( a2 - a1 + b1 ). Floating is cheaper.

High voltage (shock capable) circuits are never copmpletely safe to touch even if they are grounded, but some have major portions that run at isolated low voltage. This is a plus for safety and ease of diagnosis, but may add to the cost. There are nice devices like transformers and optical links that isolate circuits from each other.

The 120 V AC line has one side that is 'neutral', which is a safety distinction from 'grounded'. The power company transformer usually has a 240 V AC output to your house, with a center tap that is grounded to set the relative voltage of everything as low as possible, else leakage from the really high voltage on the other side of that transformer would stress the insulation downstream, not to mention the poeple that touched anything and devices. Neutral and ground are one copper bar node inside your main breaker panel, usually with a wire out to a real ground rod.

Since neutral is a current-carrying line, it might blow open out, so they run a ground wire along with it, just as big, that carries no current except to blow fuses and breakers and to electrocute people. If your house has a blown neutral, half the lights get too bright and half get dim, as the neutral voltage floats around. If this happens, turn off the main breaker, not the bright lights!

Neutral somenhow relates to the wide prong on your 'polarized' newer two prong plugs, although it just boggles my mind to think that safety authorities endorse a device that trusts outlets to be wired so neutral is always on the silver screw!

If you are generating power for your home, you might want it to have a grounded neutral, so it is no less safe than the normal systems!

RE: Proportional: When AC power is controlled by electronic devices, they almost always switch on and off to achieve this. They go from ~0 Amps at 120 V to 15 A at ~0 V over the device, so they only consume power during the transition, and for the small ~0 on and off Volt X Amp products. So you get three choices of output:

• On and off for multiple sine waves, usually for seconds, like a manual switch,
• On somewhere in the middle of every half-sine wave and off at the end, giving a lower average voltage and 60 HZ with lots of odd multiple harmonics,
• On and off all over the place, possibly reversing the power leads using additional switches, to achieve the net effect of a different voltage at a different frequency (and lots of energy at many frequencies).

Controllers that hang in the middle, or pretend to, can generate 60 HZ lower voltage, but most motors want lower frequecy when they get lower voltage, perhaps 1/4 speed at 1/2 voltage, so proportional, in the fine, is not good for much but low harmonics and RFI (Radio Frequency Interference).

Proportional in the gross in a longer time interval average. How long can be a complex question, but an important one for the design.

To All:

I am astounded by the response, many thanks to all.

I will need some time to digest all of your suggestions, it would seem I am on the right track with your help.

A bit more info:

My solar controller drops the photovolaic load at 29.6 volts. The wind power continues into the batteries, so as to avoid free-wheeling destruction (because their is no dump load yet), and so my goal is to put a load on the wind genny AND use ALL of the available power to heat instead of letting it go to waste. For various reason, I prefer to heat air rather than water. For wiring purposes, an AC load would be ideal.

The morningstar dumpload controller appears to be working as planned: I hooked up a 24vdc fan to the dumpload circuit and sure enough, when the system DCv reaches 28.6 the fan comes on. Then the solar controller shuts off the PV juice, and the voltage drops down, then the PV comes on and this cycleing continues. I will probably change the dumpload controller to activate at 28vdc not 28.6.

The reason I want to control the AC heat load in a more or less linear fashion is so that as the DCv increases, the AC heat load increases, as a way to use all the power and to stay beneath the solar controller 29.6v cutoff.

If this is not making sense, it may be because I don't know what I don't know. Or something like that. Many thanks and have a good weekend!

Hello

You'll want to set your diversion load controller slightly higher than your bulk voltage setting on your batteries or your batteries will never reach full bulk charge. You'll also need to shut down your diversion load or adjust the settings on the controller if you want to equalize your batteries. Both AAE and Alternative Power and Machine sell resistive, air heating diversion loads.

BTW, I have heard that sometimes you get a better charge with an ac-dc mix, depending on the battery, naturally. Apparently pulling a bit out before pushing some more in moves the chemicals around in the cell for a flatter plating, or something like that. Maybe someone more a chemical engineer can comment.

I suspect isolation and RFI supression are both requirements here, as this is not an isolated, insulated and sealed installation (yet).

Taking the most elegant of energy forms, electricity, and using it to make the most lowly, hot air, is a bit of a shame, but maybe later you can make hydrogen and oxygen by electrolysis, use some power to compress them, and not have batteries.