Simple and cheap grid tie system, no need for inverter...

[The Crazy Noob]

Hi,



I've been thinking and searching for a cheap and easy to make grid tie system without fancy sinus wave inverters and all the bells and whistles.





The most simple way to get a cheap grid tie system would be to use an induction motor, used as a generator and power it with a windmill/waterwheel/... . The disadvantages are that the speed needs to be constant in order to get the right voltage/frequency.



A bit more sophisticated way would be to store the power you get form a windmill/hydro/solar/... in a small battery bank and connect this bank to a DC motor which in turn drives an induction motor used as a generator that is connected to the grid. Between the battery and the DC motor and between the induction generator and the grid, there will need to be control circuits; these can be simple or more sophisticated to allow for extra protection in case of a grid failure or other functions.

Because a picture says more than a thousand words, I've made a crude diagram:

One would choose the DC motor so that it has enough power to turn the generator and is capable of turning as fast (or faster) as the induction generator.

You then connect the motor to the generator and increase the voltage to the motor. When the generator reaches the desired voltage (a bit more then 120V/230V), you write down the voltage that was given to the motor. This is the voltage that you will need for your control circuit.

The control circuit would be made up in a way that when the battery voltage is for example 12.5V; the DC motor is turned on (with the right voltage from the previous step) and the generator gets connected to the grid (with a relais for example). When the battery voltage goes down to 12V, the system is shut down and waits untill the batteries are charged again. This way, you can make shure your batteries last long scince you can adjust the voltage to what they discharge (no real 'deep cycles' => long battery life) and you don't need a big battery bank scince you don't store the electricity for further use but just use the batteries as a buffer to dump the power on the grid.

You can also remove the batteries if you want it real cheap and set the control circuit to turn on when the voltage of you turbine/solar panel/... reach the voltage that needs to be given to the DC motor.





I can get my hands on a free induction motor (around 200W) to test those things with. If annyone has already built something similar please let me know. Oh, and would annyone know a good way to make the control cirquit (sheme/diagram...)?







The Crazy Noob.

[asheets]

see http://www.utterpower.com/playing.htm

[Experimental]

Hello Crazy,

    What you are speaking of, has been used for MANY years in most aircraft and these "inverters " are readily available from surplus suppliers -- usually in 24 volt..

    The big problem is , they are very inefficent, useing very large ammounts of DC power, to produce small amounts of AC power, where the electronic inverter units are quite efficent !!

    Many motor/generators (inverters) are still used to produce ,"high cycle" power, where the speed of motors and gyros they control is criticle..

    Your idea is good, but the efficency is going to be very poor, espically if you are not driving a true generator..

    I,m not trying to throw cold water on your idea, just thinking, you might save a lot of work, if you look at surplus sources, for that motor/generator..

   Happy building, Bill H....

[Nando]

Your idea is OK and there are several wind mills doing that.

BUT you need a controller and accept the efficiency around 50 to 60 %

The system : generator ( 3 phase) rectified.

A double duty PWM controller, or a Ballast controller & Variable frequency motor controller.

A VDC motor or a variable speed AC motor.

An induction motor.

The DC motor capable to start the AC induction motor with low input voltage

The PWM controller sets the DC motor to certain RPM as defined by the Induction motor and the GRID frequency and when excess power is generated a secondary Ballast controller enters in action to help limit down the wind mill RPM to maintain the Power Factor of the Power been injected into the GRID.

If a Variable frequency motor, then the controller will vary the frequency of the Variable Frequency Motor to attain the RPM + Slip for good PF to the GRID by the Induction Motor, also the secondary Ballast controller is needed when excess power is generated.

There are other variations which may include GRID power to the induction motor to start the rotation of the DC motor & Induction motor set up to reduce the starting currents.

A micro controller is REQUIRED because the steps are continuous and several points are needed to be covered for effective system operation.

These are the basics -- more detail info too long to type.

There are other systems, specially one that can use an Induction motor directly with a bit more sophisticated system controller with possible better power output if a Pitch controlled Wind Mill is used. about 80 % efficient.

Nando

[jimjjnn]

Most if not all those milspec aircraft generators generators were also 400 Hz too.

In my opinion, I think I would prefer an inverter that synchronizes to the grid.

[nothing to lose]

It would work many ways, but not everyone has a power meter than will turn in reverse when you make more than you use from the grid.

A basic system I tested, but don't use and NOT advizing to be used either.

Start an AC motor with Grid power. That motor is connected to a gas engine with a belt and proper pully ratio. AC grid fed motor is the Starter for the gas engine, when the gas engine starts it has been set to run at the correct speed which over speeds the AC motor about 10% or so through the pully/belt. As long as the Gas engine runs and the grid has power you are feeding power to the grid. If grid looses power the Motor SHOULD stop making power also, that could depend what motor and how it is set up. You do of course want the sytem shut down if grid goes down!!!

That was a basics. Sure I burned more gas than I made power, but when testing that I had nasty old varnish gas I needed to dump anyway and also testing something else too.

Well just change from Gas engine to a DC motor and maybe connect as a dumpload. If you have no use for heat why make it, use the dumpload to feed the grid?

I have not done so, but it would seem fairly simple to use some switches to turn on a grid fed AC motor and when a set RPM is reached flip on power to the DC motor which is connected at the correct ratio to over speed the AC motor thus feed the grid with it.

If this were a dumpload system then who cares if only 50% if you would have otherwise been making heat you had no use for as a dumpload. Making heat may be 99% but if you don't want it then it is 0%

I see alot about wanting to be grid tied without batteries and if that's what a person wants then go for it. I would never invest tons of money and still be without power WHEN the grid goes down, so for me I would use it as a dumpload system. Always on batteries, never with out power, and the high demand loads like a well pump would eat grid power and the dumpload hopefully replace alot of that grid power that was used durring the month.

[vawtman]

Dont we need expensive permits and specialized equipment just to get started?Thats what I found when looking into it.

[commanda]

Bureaucracy.

In Australia, you've got no hope of grid tie unless it's a commercial system installed by licensed tradesmen.

Before they killed the solar rebates, that also had to be a commercial system installed by specially licensed RE tradesmen.

Amanda

[Experimental]

Thats quite true Jim,

    But those you speak of were used for instrument and radar equipment, where motor speed was critical..

The inverters I,m speaking of, were used for lighting , galley, and bathroom plugs for passenger use -- they being 60 cycle 120 volt !!

  Good day, Bill H.......

[nothing to lose]

Yes you do in most places I think. It can vary where you are. Also perhaps in how you do it also.

Some places may be very simple easy and cheap, while other places may not be worthwhile because of all the problems involved.

[terry5732]

Before you get too excited, remember you need to match the timing of the frequency not just voltage and count of frequency. You can have 120 V 60 Hz  that is a dead short to the grid power.

[The Crazy Noob]

Yes, but there is a simple way to do this: connect the induction motor to the grid (so it starts turning and is in sync with the grid) and a few seconds later, start the DC motor. This shouldn't be too difficult to implement in the control circuit.

The main reason why I would like this system is becouse it would be cheap and easy to make (given that the control circuit stays small).
Yes, the overall efficiency will be rather low (like 80-60% when done right) but it would be a fun experiment. I'm not yet into the big power things and i don't have much room for a big windmill, nor the money for solar, nor a place for hydro; so a small windmill is all i can make. The power from this won't be much to use off-grid (maybe a pumping system to use rainwater for toilets/...?), so i feel better using the grid as a "battery" and just pushing the power down there. Becouse of the samll amount of energy produced, i don't think it will be harmfull for the powermeter since it won't turn backworths (power consumed>power produced) except maybe at night. So i hope i can try this without getting into paperwork and discussing with authorities/electricity suppliers/...

[Vince]

Wasn't somebody on the board working on a home-made grid tied inverter project ? Did anything ever become of it ?

BTW, if the primary point of a grid tied inverter is to stop sending power onto the grid if there is a power failure, so that there is no danger to repair workers, what about using a regular inverter with just a circuit that would disconnect if the grid went down ? Sure, you'd have to determine when the grid power returned and flip a switch to reconnect the system, but wouldn't it be a lot cheaper by eliminating the fully automatic circuitry ?

[The Crazy Noob]

The (expensive) grid tie machines also check the frequency and adjust it so that it is syncronised. A regular inverter doesn't do this and so you shouldn't connect it directly to the grid.

I would however like to see a DIY grid tie inverter (and plans!  :-) )

[oztules]

I have pondered this for some time now, and have decided that I will be doing that which you proposed initially, ie small batt bank, dc motor and induction motor to drive into the grid.

This is simple and robust. I wont be using induction motor as a backup when the power goes off, the ups does this fine already. ( the power goes off over here regularly )

The intention is not to drive the meter backwards, but merely to supplement the usage and so reduce the inflow from the mains.

I propose this for the relay system:(36v 1.7hp dc motor driving 5hp ac induction motor 240v)

Simple window comparator for the trigger device. 41.3v - 34v... on - off

when 41.3v reached.. turn on 3 phase 240v relay to switch the motor on (use 2 poles for active and nuetral) and the third pole in series with the coil of the 36v dc relay for the DC motor.

When AC motor engages it drives the dc motor up to speed, a sample of the dc motors leads will provide 36vdc (acts as dc generator) in series with pole three of ac relay, energises the DC relay to start dc drive to the system.

When Batt= 34v then open ac relay, ac induction gennie turns off, as does pole three of ac relay disengaging the dc motor relay.

A pwm to drive the dc motor at a constant rate completes the picture. The window comparator circuit will be driven off a 12v transformer, so that when power fails, system shuts down (comparator dies, so does the energy for the ac relay.) Choose PF capacitors that won't allow self generation without the mains present.

Hopefully this will be able to supply an intermittant 300-500 watts. Not enough to shape the world or drive the meter backwards, but enough to impact the energy bills by a bit, (but really to have an excuse to build my wind mill.)

efficiency probably 60-70%, but  the power is cheap. Got the motors, built the pwm, have some relays,just have to finish building the house so she will let me play....and then of course the mill.

 your  thoughts welcome.

..........oztules

[The Crazy Noob]

I like the idea, could you draw a scheme/diagram? Does annyone have information about DIY pwm-units?

[oztules]

Hi Crazy Noob, I will see if I can upload a diagram of the system when I scribble one out, uploading pics is the problem 900 bytes/sec is terribly slow, but will try.

The pwm units I have are ones that I built. They are over complicated and incorporate forward/reverse, soft start, potwiper loss, variable acceleration and fet junction current detecting for the overcurrent feedback..etc. I think I have a simpler version from earlier on using large darlingtons and a wire shunt for current detection, I'll see what I can find . (still living out of shipping containers....can't find a lot of stuff yet).

If you can locate old floor scrubbing machines (battery) they may have the motors(dc 1-2 hp 36v) you want as well as the pwm unit for the traction drive. Clarke, tennant, multiclean, all have these things on board 32"machines will ahve 2 big dc motors for the brush heads (approx 200rpm), and a third for the traction....with a controller. Just need a prop.

............oztules

[oztules]

sorry for the delay This should do the trick. the green box is the comparator, and could be made from a led light battery indicator..tap off from the leds corrosponding to the voltage stop/start you want. Will need to put in a buffer /latch (mosfet and small relay) to drive the three pole ac relay. Don't forget the back emf diode across the relay coil to stop the spikes at turn off back into the fet..

Once comparator sees correct voltage turn on 2 pole buffer relay.( pole 1 turns on ac relay, pole two latches the relay on until low volt led/buffer turns off, then releases the 3 pole as well.)

Buffer relay turns on ac three pole relay. 2 poles used for the active and nuetral, turns on ac motor . Third pole in series with + from dc motor and the dc relay. When dc motor is driven up to speed by the ac motor, the dc motor generates power to drive the dc relay on, and so then overspeeds the ac motor until batt gets low. buffer relay will drop out, 3pole will drop out,this breaks the circuit to the dc relay, all stops.

If ac drops out, ac motor stops generating naturally (no exciter current), and the 3pole ac relay drops out. dc relay circuit drops out, all stop. When power comes back, all starts up again.

PWM is a good option, but not needed if close match of batt voltage and motor is achievable without it. shorter/tighter generator cycles will be the only difference.

take great care if you are not completly confident, get someone who is

...........oztules

[The Crazy Noob]

sorry for the late-late reply

I've tested the thing with a 12V motor from a fan to cool a car-radiator connected to a 12V/15A power supply and a 100W induction motor (1 phase, it has a capacitor)

When I just turned on the AC motor, it drew about 0.5A (230V~ input). I put a kWh meter in between and it turned at a certain speed. (the DC motor was outputting about 8V)

When I applied current to the DC motor (it drew around 8A (wich is close to the 100W from the AC motor)) I could see the kWh meter turn slower (less power used) but the amps that were flowing between the grid and the induction motor rose to around 0.7A or so.

Does annyone have an explanation why the amp-meter showed higher power being used and the kWh meter showed that less power was being used? Does it have something to do with the voltage being out of phase so that the meters can't detect it the same? (something with vectors and those things?)

[Nando]

A bit late on this.

When the DC motor drives the motor at a RPM higher than the RPM+Slip the induction motor starts delivering amps to the grid, instead of taking it.

230 X0.7 = 161 watts - UNLESS THE motor was being turned backwards by the DC motor and presented a load to the induction motor.

The experiment needs to first make sure how the rotation for the AC and the DC motors is in the same direction for the system in this case to work.

To truly know if the power is delivery a detector is needed to discern the current direction.

Also, when you applied voltage to the DC motor it started as a load and not as a motor driven the AC motor.

To check the system run the DC motor driving the Induction motor ( dsiconnect the capacitor) and when the RPM is higher than the RPM+slip, then connect the AC induction motor to the GRID POWER You should see a current, write it down, then open the loop of the DC motor and read the current. - unhappily you do not have a directional current detector, unless you get an older house current meter to show the direction of the flowing current.

Nando

[oztules]

Hi Crazynoob,

I was hoping someone would clarify this for you, but thus far no-one has. So i'll try..... then I'll be corrected and we'll have it...there will be a lot of should's and maybe's.....

It could be true that the kwh meter is seeing only the watts actually used (in phase volts x in phase amps) The induction motor will have a power factor not unity, and so more current is drawn from the grid than is used in the motor.... yes a vector thinggy.

But which way did it go... in or out of the system.

Nando describes a way to do this..... but I'm not good enough to quite understand it. So hopefully he will put us right if I'm wrong.

I figure that a simple way to check it is like this,

If you put a 1 ohm (1R) resistor in series with the ac motor, you can measure the ac voltage on both ends of it with respect to the grid.

ie when running the motor @ 0.5A, you should drop 1/2 a volt over the resistor. On the grid side it should be nominal 230v on the motor side it should be 229.5 ( or 1/2 volt less than the grid side of the resistor)

This should clearly show which side of the Resistor is the load, and which one is the driving emf.

If you start the dc motor now, this situation should reverse.  ie the motor side should be 230.5v and the grid side should be 230v

You may need a pwm for your dc motor to keep within the slip range

The possible sting!

If you drive the ac motor too fast then it will probably start to work against the grid, and current will flow the other way again..I think... I have not tried this yet so hopefully someone can correct this if it is a poor assumption.

As you get further from the synchronous speed (plus slip), you will start to work against the mains instead of adding to it.(I 'm guessing thats why they use a computer to turn on and off asynchronous gennies on windmills. They turn on only in a narrow band of rpm otherwise they probably get too much slippage and the rotor will begin to look like a low impedance load..... just my guess)

Caution this is conjecture on my part crazynoob,and hopefully someone will correct or verify these comments. But in the absence of any other theories, ....

Sorry I could not be more definate on this.............oztules

[The Crazy Noob]

hmm,

I will try that trick with the resistor if only I wouldn't have been so stupid to ruine the connection piece that i used to put the two motors together ... must make another one.

I do have to mention that the DC motor ran at maybe twice the speed of the AC motor but the AC motor kept the speed down of the DC motor to about only a slight bit higher than the normal RPM of the AC motor (the RPM + positive slip).

I thought that it didn't matter how fast the DC motor ran, because the induction motor would keep the speed down. So if this isn't true, it might explain the strange measurement.

BTW: how do you get a directional current meter when you have AC? won't it just swing back and forth 50times/sec like a DC meter would do when AC is connected to it?

It might take a while until i get back to you guys ... I need to explain something, I'm 16years old and I'm having exams right now :-D. Allready interested in alternative energy and I will probably devote my studies to it. (studying science at the moment)

My dad studied for electrotechnician (a long time ago) so he understands basic electricity and schematics.

My dad can pick up a big, three phase induction motor (don't know the wattage, but the physical dimentions are about 40cm * 15cm diameter). I would think that feeding back to the grid is more easy with a three phase induction motor than with a one-phase motor that has a capacitor/... on it to do the phase-shift.

I'll probably convert the big indcution motor to a generator for a wind turbine.

[Tritium]

I can't even begin to count the ways this device is completely illegal to connect to the US grid.

By the way you posted your advertisement to a 4 year old thread.

Thurmond

[JoeD.]

Before you get too excited, remember you need to match the timing of the frequency not just voltage and count of frequency. You can have 120 V 60 Hz  that is a dead short to the grid power.

  New guy here.  I read all this about grid tie and I wonder the same thing. The power you are trying to force back out to the grid has to be in phase with that coming in or you let the magic smoke out of all your equipment.

  Even the big power plants do the same thing. Everyone has to be in sync before you close the switch.

  How you accomplish this with a grid-tie inverter, I am curious.  Of course, maybe there is a big difference in electricity coming from a spinning alternator versus a solid state inverter driven with smooth DC power.

  I've read something about how they sync big 3 phase alternators back in the day.  Something about using 3 light bulbs, one bulb connected between a leg from each alternator. When the machines are out of sync with each other, the bulbs lite. When the phases are sync'ed, the bulb would be dark. I know there is more to it than that, but that is the gist of it.

 
 
 

[Phil Timmons]

  How you accomplish this with a grid-tie inverter, I am curious.  Of course, maybe there is a big difference in electricity coming from a spinning alternator versus a solid state inverter driven with smooth DC power.
 

If you are using a UL listed inverter, the inverter senses the Grid connected power and creates a synchronized signal to control the inverter before connection.  That is the easy part. 

The tricky part is sensing the loss of the grid power and automatically disconnecting from it if the grid power is lost.

The UL listed inverter does that part by slightly shifting the its output frequency up and down from the steady state beat of the 60 Hertz grid.  If the grid is still connecting and functioning, the inverter will not be able to shift the frequency, and its sensors will not detect any frequency shift and it stays connected.  If the inverter can manage to shift the output frequency, it figures the grid is down and disconnects itself.