Author Topic: Induction motor Controller.  (Read 479 times)

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Offline DJ

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Induction motor Controller.
« on: April 02, 2017, 03:50:31 AM »

Wondering if anyone has a relatively easy method for Controlling 3 Phase motors used as Generators.

I have thought of Using an arduino or Chinese Control board to PWM or switch Capacitors in and out but I can't seem to find a Voltage Divider for AC to start with.  Thought of PWM'ing a Dump load to hold a stable load on the IMAG but doesn't seem to be any straightforward way of doing that.

Everything my feeble brain comes up with has a problem with non existent components..... at least as far as I can find.
Seems in this day and age it should not be a hard task to so something like this with an arduino or even hooking up some Chinese pre built control board but it's certainly escaping me.

Anyone come up with a solution for this? Given the amount of people into DIY power and the low cost of 3 Phase motors of decent output, it should be something that would be of value to a lot of people if there were a cheap and easy way to do this with a set of instructions they could follow for something like an arduino.

Offline lighthunter

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Re: Induction motor Controller.
« Reply #1 on: April 02, 2017, 06:15:36 AM »
Hi DJ,  :)

Problem is, your 3 phase motor wont ever generate power until it is powered up first from another source, then while maintaining shaft motion and a load, it will work as a generator.

The answer to your question has to do with what you want to do with it.

Whatever the case, for it to work as a generator once it has been started, you have to sustain the induced magnetic field in the rotor otherwise it will stop and have to be restarted. You maintain field by keeping current flowing in stator and shaft motion of course.
 
Are you thinking wind generator? or perhaps a genset? Both are possible but not easy.
If you just need power they grid tie nicely as is. Plug it in and turn shaft faster than its already turning and you are generating power into the feed line.

If you want control, now thats another thing.

If I wanted it for a wind generator I would buy (2) 5hp 3phase vfds off ebay for $120 each Everything is already done for you. All you need to do is provide the frequency setpoint based on actual windspeed (jump table and anemometer or relays).  Tie the DC buss + and - together on the two drives and put transformer and rectifier on the output of second drive and you can charge battery. You have to connect power to input of first drive single phase may be enough for most, this will rotate turbine at the frequency setpoint, as the wind turns shaft faster the DC buss voltage will rise and you need to monitor this and adjust frequency setpoint on second drive to regulate it. You could substitute the second drive with a pwm 3phase load or even 3 240v light dimmers. These loads would connect directly to the motor/gen leads.

Then you need a good brake that operates on its own automatically. Not really a project that can be tossed together in a weekend unless youve done it before.

All fun stuff but a bit hairy until the controls are built correctly.



LH

Offline Pete

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Re: Induction motor Controller.
« Reply #2 on: April 02, 2017, 04:41:19 PM »
Hi DJ, like lighthunter says it depends on what you want to do with it. I have seen a few small micro hydro turbines that use three phase induction generators. They basically hang a capacitor across one winding to act as an exciter winding. The outputs are then connected to a three phase rectifier and regulator to charge batteries.
That is the simplest way I have seen, controlling the output.
Lighthunters ideas of using a Variable Frequency Drive on the output will work if you want an AC output.
So tell us more of what you actually want to do with the generator.

Offline DJ

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Re: Induction motor Controller.
« Reply #3 on: April 02, 2017, 11:11:40 PM »

People never put enough info on these questions and now I'm Guilty of it myself!  :0)

I have played with the setup extensively using a diesel engine to spin up the the AC motor and make power.
I have done it straight grid tied, set it up with caps as 3 Individual legs and pulled power off each, set it up in a C2C config to get single phase power and  also done that and added a rectifier to power a solar inverter. For running fixed loads, the things are great and can do that all day long.

When using the induction motors as a standalone generator, the problem is always switched loads.  They are fine to run some fixed resistive loads like a light or heater but once you try to say power a home where the fridge kicks in and out and the TV and other appliances are switched on and off which vary the loads, it's basically unusable.

Either the field collapses because there is not enough capacitance for the load or the thing over shoots when a load is removed.
You basically have to balance the RPM of the engine to the load if the capacitance is fixed but there is very narrow limits on that before you really need to add more capacitance before the frequency is off the planet.

What I am looking for is a way to keep the output at a steady level.
It would seem to me that having a set load on the motor and a Dump load that was PWM'd to keep the output to the required electrical load  stable.
My limited knowledge of Arduino suggest to me that someone that was more expert in their application should be able to come up with something to do this with relative ease.

The other alternative may be switched in extra capacitance. I thought of varying the capacitance through PWM but seems that's not a feasible idea. 
I was also wondering about all the Chinese made boards that are so cheap and if they could not be fashioned together to do the same thing. There are so many out there with very puzzling descriptions that often bear no resemblance to their true function that maybe something is out there that does the job, it's just labeled as "Windscreen Wiping temperature controlled pressure check balancer" and therefore a board I am not aware of it's suitability for this job.

What I want is to be able to run the 3 Phase motor like a regular generator that will tolerate varying loads and something like an AVR for Induction generators.

Offline lighthunter

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Re: Induction motor Controller.
« Reply #4 on: April 04, 2017, 10:44:13 PM »
Hi DJ, I was hoping an expert would respond to your post as i havent played with this stuff as much as you have. I hope my post doesnt discourage you because you have very optimistic ideas :).

So here is my 2 cents, achieving what you desire is a little bit like designing a car with rear steering to travel at high road speeds. I believe it can be done as you obviously do, yet the control element is on the wrong end. Ive found keeping grid tie inverters happy can be challenging enough. Yet I think if you designed an arduino loop or similar to do variable loading to maintain constant current, it should work. .I'm guessing the constant current setting would need to be larger than the largest anticipated load. If the control loop was fast enough, the ac current would always stay the same no matter what you loaded it to. Kinda inefficient tho unless your pwm dump load is a usable power.

If you can get it to work, there may be a market for it!
LH

Offline DJ

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Re: Induction motor Controller.
« Reply #5 on: April 04, 2017, 11:55:07 PM »

I agree, "Optimistic" being the key word!   ;D

Got to say with the level of skill and creative electronic engineering people posses these days, I thought someone would have tackled this now and come up with a cheap and cheerful solution.

You are definitely right in what you say. Making power with these things is a cakewalk. Controlling it is a very different matter.

I found using the solar inverter had severe limitations as well. The inverter kept looking for the sweet spot and varying the load.
An imag is really only good for about half the motors rating which was not what I have seen written and threw me off for a while wondering why I couldn't get a lot more efficiency. I came to the 50% Conclusion about the same time I read something new on the concept which said the same thing.
Trouble was I was using an inverter that had about 50% of the load capability and my motor and therefore was trying to run it at the top end of its output. Once you get there, the load on the driving motor seems to go exponential.

I was trying to drive a 5HP motor with a 12 HP diesel engine and had the diesel smoking it was working so hard just to try and make 3Kw the inverter was trying to pull.  When running lights etc at 1800W, the engine was barely turning over and I have used a grunty 3 HP diesel to make the same power with no trouble. The 2 KW Solar Inverter I had just flat out didn't seem compatible with the power supply and would kick out almost as quick as it put load on the IMAG.

I was pretty hopeful of this setup. Bought some 3 phase rectifiers to make the setup easier than using individual single phase units ( I also bought a heap of) and thought I could get it to load constantly but couldn't manage that. The inverters have far greater tolerance in input than the IMAG has for load.
It seems to me that the best and most practical application for the IMAGS is to back feed the mains supply to spin your meter backwards or run lights or a heating element.   This works really well either by Direct connection, 2 Legs connected for single phase or 1 leg and neutral, a C2C setup to use all 3 legs and bring them back to single phase or all 3 legs for 3 phase.

 I found that unlike what everyone says, Running the load imbalanced seemed to not make a bit of difference to the motor.  I tested the thing well loaded up on both 2 legs and a single leg and Neutral and saw no frequency issues and certainly no overheating as I have read much about.  I figure that is really illogical anyway. The motor I have has a LOT of thermal heat sinking capacity and a very strong fan. By only using 2 legs you are under running the thing and leaving a lot of provision for heatsinking the load that the motor is carrying which in this case would be a lot less than the thing as designed for in normal full load operation.
Pulling all the power I could get off 2 legs barely made the motor warm although it was easy to tell where the 2 winding's were that were supplying the power.

The other thing I have done with these motors is run them off single phase and use the 3rd leg to supply power to a 3 phase load also running off a single phase output.  All you need to do to run a 3 phase motor ( at reduced output) is spin the thing to start it ( or use caps to phase shift).
I wrapped a rope around the pulley of the motor and spun it up lawnmower or outboard style then hit the switch connecting it to the single phase output.
They run perfectly.  The 3rd free leg is connected to the 3phase load / motor that is also wired to the single phase load and it can be switched on as normal.  Fun to play with and very simple.

Rotary converters are a bit wasteful I think for practical use. All you need to do to run a 3 phase motor off single phase is wire in come caps and they will work perfectly well.  Setting up a rotary is fun to impress friends and the electrically curious though.

So much you can do with these Cheap and plentiful motors but it seems setting them up as a stand alone generator is rather a challenge!   

Offline oztules

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Re: Induction motor Controller.
« Reply #6 on: April 05, 2017, 04:47:51 AM »
Maybe read this for some ideas that work for hydro units.


http://www.who.int/management/InductionGenerators%20forSmallHydroSchemes.pdf


.........oztules
Flinders Island...... Australia

Offline DJ

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Re: Induction motor Controller.
« Reply #7 on: April 05, 2017, 07:17:16 AM »

Yeah I saw that paper and apart from it being a bit over my head, there is no schematic on how to build such a controller which I think would be also beyond my abilities.

Just amazed no one has come up with something using an Arduino or cobbling together some off the shelf Chinese boards.

Offline welshman

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Re: Induction motor Controller.
« Reply #8 on: April 05, 2017, 09:09:45 AM »
even on my latest hyundai generator, the rpm is still manually set at the fuel pump. no electronic controlling of the rpm at all. they tend to be just set up so that the rpm is 1500 at middle of it's load rating, 10kva in my case. - i think thats why the engines are so large in cc.

Offline welshman

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Re: Induction motor Controller.
« Reply #9 on: April 05, 2017, 09:19:31 AM »
maybe go the route of dectecting the the current then pwm a transitor of the dummy load proportionate to the detected current. math should be easy, circuit should be easy. but then you will be driving the motor 100% load all the time. is that what you want? what kind of dummy load are you thinking? that would be the pinnacle here. depending on what kind of dummy load you would have would determine what kind of circuit to build.


Offline rossw

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Re: Induction motor Controller.
« Reply #10 on: April 05, 2017, 05:49:43 PM »
For modest power levels, one of the el-cheapo chinese inverter kits like I got for my centrifuge, looks like a good start.

Use the motor to drive the generator, the generator full-wave rectified into the HV DC input to the inverter, and leave the inverter to wory about output volts and frequency, entirely.

Then use the DC voltage on the input in a throttle feedback loop. Simple window detector - if the DC is below the lower window limit, crank up the RPM (higher duty-cycle on the PWM for the actuator). If the voltage is above the upper window, reduce RPM. Now, your engine should be running at something "close to" the ideal RPM for the current load, and your load will see a nice, clean, stable voltage and frequency.

(It's what I plan to do to one of my generators here, when I get time)

Offline oztules

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Re: Induction motor Controller.
« Reply #11 on: April 05, 2017, 07:23:46 PM »
Perhaps we go one step further, and use a lf inverter ( like the PJ or the 8010's we build)...... and use the induction motor as the mini grid tied asynchronous generator... it will drive the big loads, and charge the batteries back through the H bridge when the load is lower..... and use the rpm that Ross alluded too.... best of all worlds.

Should be reasonably efficient charger in that configuration.


............oztules
Flinders Island...... Australia

Offline lighthunter

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Re: Induction motor Controller.
« Reply #12 on: April 05, 2017, 09:51:27 PM »
Thats clever, simple effective solution. Nothing to build really other than the throttle control that watches current and voltage. Great idea!
LH