Induction Motor and Batteryless Inverter

[mcorner]

I am still getting up to speed on some of the principles of induciton motors, so bear with me.  I could have many things wrong here.  The goal is to connect a batteryless inverter to an self-excited induction motor.   Hopefully, this can be done to avoid charging batteries for no reason (this is an on grid system).  The turbine will use a turgo runner.  (Yes, I have looked into connecting the induction motor right to the grid, but the regulations and equipment make this much more expensive/complicated/time consuming, so I want to use an inverter, and yes I know this is less efficient

As I understand things, if you connect an induction motor to the grid it will run at its nameplate speed.  Any torque you apply to it will produce current.  Right? You then want to size your runner's peak efficiency to this speed.

If you connect the induction motor to a transformer/rectifier/battery bank, this same principle holds.

However, what happens if you connect it to a rectifier and then to a batteryless inverter?  What speed does the motor run at?  Is it determined by the size of the capacitors?

Two betteryless inverters exist that I know of: SMA's windyboy, or the Power One Aurora wind interface and inverter.  I have been looking at the Power One inverter as it seems like it is better built, has a rectifier that works with the inverter and has a wider input range.

Much appreciation for any help...

-Mark

[Flux]

You need to excite your motor with capacitors and rectify the output to dc. You can then use a batteryless grid tie inverter and even the solar based Sunny Boy should do for this job as long as the dc volts you generate is within its operating range.

With rectification to dc you would be well advised to use a 3 phase motor, I really wouldn't go down the single phase route, I think it would be far from satisfactory.

With an approved inverter you should avoid all the hassle of trying to interconnect an induction generator, which in reality is far more logical, but politics counts for more than common sense.

You must be able to run your motor fairly near to its nominal motoring speed but you can go a bit either way by changing the capacitors.

Flux

[mcorner]

Yep, I had planned to use a three phase motor, the Aurora wind interface box takes three phase.

However, I guess I still don't understand how the RPM in the generator is set.  Do I need to fix the output voltage with a voltage clamp?

[vawtman]

Hi Mark

 The motor is a battery less inverter.The speed is determined by it's ratings.Not good for small wind in bad locations but great for hydro if you match the load.

 Mark

[mcorner]

I am not sure I understand that comment, but I was looking some more at the wind inverter datasheets.  The MPPT tracking feature seems like the right thing.

Below Vmin, it doesn't extract any power and the motor should continue to speed up until it reaches Vmin and then the inverter will start to extract power.  If the voltage continues to rise it will extract more power until it reaches Vmax.  After Vmax the voltage will continue to rise until the wind interface box turns on the diversion load.

Vmin and Vmax should be pretty close together to get a narrow range of operating RPMs.  It shouldn't go beyond Vmax unless the inverter disconnects during a grid outage.

The only thing that worries me is that the Aurora has a pretty high voltage for the diversion load (530V).  That seems too high to prevent over speeding.

[Ungrounded Lightning Rod]

As I understand things, if you connect an induction motor to the grid it will run at its nameplate speed.  Any torque you apply to it will produce current.  Right? You then want to size your runner's peak efficiency to this speed.

Close.

The field-rotation speed is the line Hz, time 60 (to get to RPMinute from RPSecond), divided by half the number of poles.  I.e.:

 60 Hz, 2 poles = 3600 RPM.

 60 Hz, 4 poles = 1800 RPM

 50 Hz, 2 poles = 3000 RPM

 50 Hz, 4 poles = 1500 RPM

and so on.

If the rotor were frictionless, not under load, and running in a vacuum, it would quickly end up being dragged around at the field rotation rate.

Now assuming it's spinning:  If you put a load on it, it starts to "slip" with respect to the field rotation rate.  This causes the field to move (slowly) through the rotor, which induces currents in the "squirrelcage" conductors, which induce a magnetic field in the rotor.  This magnetic field is a bit behind the rotating field and attracts it, causing the field to pull the rotor along harder.  The more the load, the more the slippage, the stronger the eddy currents, the stronger the induced field, and the harder the fields on the rotor and the stator pull against each other.

Energy from the line provides power to overcome the resistive losses in the squirrel cage and to tug the load along, in good old conservation-of-energy fashion:  Voltages induced in the stator windings by the lagging magnetized rotor cause the current from the line through the stator coils to increase, pulling power from the line.

The nameplate RPM is the field rotation RPM for the rated frequency minus the slippage under the rated load.  That's why you see numbers like "3550 RPM" rather than "3600 RPM".

Now if, instead of dragging the rotor slower, you push it faster, it similarly slips as it speeds up beyond the field RPM.  Again the rotor's field, and the resulting drag against the rotating field from the stator, increases with additional torque and excess RPM.  But this time the drag is pulling energy from whatever is providing the push and putting it into the line.  The field induced in the stator by the rotor's field pushes current back INTO the power lines, for a net gain (once the current you're pushing exceeds the in-phase current pulled by the stator windings to make up the resisitve losses of the current magnetizing the stator).  Meanwhile, there's some additional drag from generating the squirrel-cage currents.  Thus energy is again conserved.

This time the rotor RPM is HIGHER than the field rotation RPM by the amount of the slippage - and higher than the nameplate RPM by TWICE the slippage (once you're pushing hard enough that you're getting as much slippage in the generator mode as you'd have in the motor mode at rated load).

So if your nameplate RPM is, say, 1775 RPM your field rotation will be 1800 RPM and you'll want your turbine trying to push it at something like 1825 RPM with up to a bit more torque than the motor would have provided at full load.

[electronbaby]

I would use a PM alternator for the turbine. You could design it to meet the requirements of the Aurora (which is an awesome inverter in my book). However, you still need to program the MPPT algorithm for YOUR alternator. As I understand, they are programmed at the factory for the turbine they will be used with. Im pretty sure you might be able to get a hold of the software though.

Make sure you have a good voltage clamp, and a good furling mechanism, because when the grid hiccups, you get the "5 minutes till destruction" countdown, thanks to UL1741. :-)

[Flux]

Hope this turns up in the right place.

As others have said you must choose your motor pole number to match the turbine rotor ( or vice versa if you can change the rotor).

Ideally you ought to generate at the peak power point of your turbine rotor but you can shift things a bit with the capacitors. Off load your turbine will likely at least double its speed and the capacitor excitation will violently saturate your motor, it will load fairly heavily and prevent much of the speed increase, the volts will probably rise 50% at the very most.

Probably the ideal control would be a normal micro-hydro regulator that adds resistive load with rising voltage to hold effectively constant speed. In normal use you would set the regulator to divert nothing and the inverter would take it all. If you got a significant voltage rise the load regulator would clamp the voltage a bit above normal inverting volts and maintain the turbine speed.

During start up the load control would run the alternator into resistors at say 10% above your normal speed. After a short time the inverter would synchronise and load to the grid. If programmed correctly it would load more than the dump loader and the speed would drop to nominal and the heaters divert nothing.

If the inverter comes off line the load control would take over, diverting the power to heaters with only a small increase in speed and volts. There would be no danger to the turbine, alternator or the inverter.

Alternatively you can as you suggest use a voltage clamp set to limit turbine speed and it will also protect the inverter if set low enough.

Using the clamp will mean the turbine /alternator running fairly fast during synchronising with a significant internal loss in the alternator. This should be ok for short periods but if the inverter tripped due to loss of grid power you would need to shut things down fairly quickly to prevent over heating and voltage stress to the capacitors.

I don't know enough about programming the inverter to help you, a simple manual programme where you could just increase load until the turbine slows to nominal speed would be very simple ( not sure modern things cater for simple minds like mine).

Others have suggested a pm alternator, this would be my preference, but I have no idea of the power you have in mind. It would be more efficient and avoid expensive and sometimes not so reliable capacitors. The over speed control would have to be a bit more refined as you would not have the serious power dump during over speed in the alternator itself.

Simple in theory and shouldn't be difficult if you are good at electronics. Otherwise you will be at the mercy of someone to supply a controller to do what you want and not what they think you want.

Flux

[thefinis]

All been hashed out several times try a search for induction generator. Here is a good long thread on grid tied induction generator.

http://www.fieldlines.com/story/2006/7/2/8853/36057

Due to its small rpm range a direct tied induction generator is not well suited to most wind turbine designs. What Flux and others are suggesting is better suited for most turbines.

Finis

[mcorner]

Yes, I have read through that thread.  I definitely want to stick with an inverter based design due to regulations.

Are PMGs more efficient than induction motors?  I know they are more expensive.

What is the right process for matching a PMG for a turgo runner?  I know that Turgos matched with an induction motor need to be targeted to a particular peak power RPM, but in the case of a PMG am guessing this doesn't really matter.

For instance I have been chatting with Joe from http://h-hydro.com/ about Turgo runners.  For 45 feet of head and 300GPM the induction motor needed to run about 1000-1200 RPM (given a few more spoons, I think it could go as low as 900RPM).  If I go to a PMG, then I need something that will run at ~1000RPM and produce about 1.5kW.  I looked at: http://www.ginlong.com/, but those are better suited to low RPM wind power (~400RPM).  What is a good source for efficient PMGs?

[Flux]

There should be little difference in efficiency between a self excited induction generator and a PMA. The PMA is just so much more convenient in that it will work at any speed. You have little range of speed with an induction generator and below 1000 rpm they are not very good. Six pole is about as slow as I would go for good results.

If you know your optimum turgo runner speed then you just choose a PMA that will give sensible inverter input volts at that speed. If you are way out on the turbine speed you still have a chance that the PMA will be ok. You would have to change the induction motor if you missed the speed by much.

If you are looking for a commercial device then the PMA may not be a good choice, as you say they are expensive and not much choice unavailable. It is fairly easy to build your own. If the voltage is reasonable the Ginlong alternator should be fine, it will work ok at 1000 rpm or above and the high frequency is not a problem as you will be rectifying it for your inverter. You probably have to run them at that sort of sped to get the claimed rating anyway.

If you are reasonably sure of your turgo on load speed then the induction motor should be fine, the big nuisance is having to play with banks of capacitors to get it to excite at the right speed.

Flux

[mcorner]

I have read this a few times and I think it is starting to make sense

I have looked at PMAs and the choices are overall pretty poor.  For instance the Ginlong PMAs run up to 500V at realtively low 600 RPMs.  This makes them probably unsuitable for direct drive hydro (1200RPM).  I looked at the supercore PMAs (http://www.survivalunlimited.com/windpower/pmasc.htm) but if I draw 1500W I need to run the alternator at 2400RPM or more to get proper cooling...

So, the induction motor might be best, but as you say there are a few complications.  I understand what you are saying, but putting it into practice is another thing.

The Power-One Aurora Wind Interface and Inverter looks pretty good, but it has one shortcoming: it doesn't use the diversion load until it hits 530V.  I am assuming that if I let my induction motor run up to 530V bad things would happen, or as you say if it saturates the caps and only runs at 50% over voltage the diversion load would never turn on.  This would overheat the motor eventually.  I should check with Power-One to see if the diversion load voltage setting is programmable.  If it is, it looks like you could use their MPPT curve to basically function as a voltage controller and thus an RPM controller.  The higher the voltage in the generator goes, the more current the inverter will draw, thus increasing torque on the motor, slowing it down.  Right?

If the diversion load setting cannot be changed, ARE wind (http://www.abundantre.com/ARE_Wind_Turbines.htm) makes a voltage clamp and diversion controller that could be set a bit above the maximum voltage point of the wind interface MPPT.

This has gotten a bit expensive though  The ARE voltage clamp is probably close to $1000USD and I don't know what the Power-One interface/inverter box costs, but probably close to $3000.  Then the induction motor might be $1000.

At that point, I might just bite the bullet, get a proper induction motor grid-tie for $5000 and go through all of the "I-don't-use-UL1741" hassle with the electric company.

-Mark

[robl]

Unless I missed something, one of the tricky parts is getting is creating the 530V (DC?)needed to trigger the diverter circuit. One option there would be to use a 240 to 600V step up transformer. (Hammond sells them in lower wattages as control trnsformers). I use a 6 pole 3 phase 1200RPM delta wound induction motor and have been very happy with the results. Wayyy cheaper than a commercial PMA but a little finicky when it come to certain kinds of loads and a tendency to lose its magnetism when really loaded down. The latter can also be seen as a positive aspect since the generator shuts down when over-loaded.

OFF TOPIC ALERT: Has anyone on this  list have the hydro version of a Sunny Boy running off a hydro turbine in the same fashion as Mcorner is proposing for the Aurora setup?

Regards

Rob