In case anybody still reading has misunderstood anything: I have no intention or belief that I’m inventing anything here. No overunity discoveries are planned, nor will any be fabricated for the sake of argument. I’m just learning stuff and taking notes as I go. I go back to the books and I can sort-of see how some authors (Gary Johnson for example) have definitely done work like this and understood this better than me, but their didn’t offer enough detail for me to appreciate what really happens. Not as much as just doing it myself. Now that I have, I am going back to the books and realizing “that’s what they meant when they wrote that”.
Here are some detailed results from the tests. I’m collecting current at various points in the circuit, and in each phase to make sure they’re balanced.
For example, with 420 uF of capacitors in Delta, across the leads of the motor in series-Y, I see this:
Starting with the generator running Open circuit:
251 RPM
10% slip (belts and lathe drive motor)
12.6 Hz
142V line voltage
11.3 V / Hz
9.5A line current (2337 VARs)
5.9A capacitor current (1451 VARs)
18.8 Newton-meter torque
497 Watts mechanical power
Same setup, now loaded by just 60 ohms:
221 RPM
16% slip
11.1 Hz frequency
90V line voltage
8.1 V / Hz
4.5A line current (857 VARs)
2.9A capacitor current (447 VARs)
5.5A Load current (281 VARs)
242 Watts on wattmeter
86% load factor
22.3 Newton-meter torque
517 Watts mechanical power
47% mechanical efficiency
And that’s just one of the test runs. I’ve repeated this for various speeds for each capacitor value, and varied the capacitors many times. It’s a big spreadsheet…
One general observation: the V/Hz looks good when lightly loaded. The basic motor is dual voltage, and having it in series-Y makes it 460V at 60Hz. If you consider the rated speed actually has some slip, and the base frequency actually extrapolates to 480 V, then the standard V/Hz for this motor is either 7.7 or 8.0 depending on how it should be defined. Either way, the tests where I have the induction motor loaded, it is maintaining a loaded voltage that corresponds to this ratio rather well. Except in some cases where the speed isn’t high enough to maintain the field and the voltage is about to drop out anyway. It is as important to me to use different speeds on the lathe to investigate the BAD speed ranges as well as the good ones, to know what’s going on in both scenarios.
Some general comments:
Ahhhhhh residual.
Something to try.
Disconnect the load and caps before stopping rotation.
Let the generator coast to a stop with caps and load connected.
While residual is needed in induction generators, it may cause excess in rush current in transformers.
We have had a couple of discussions at an alternate site as to techniques to kill residual in transformers.
The various techniques to kill residual are based on reducing the applied AC voltage so that as the EMF drops to the point where it interacts with the magnetic hysteresis the residual will be left at zero.
You may be a victim of a similar, unintended consequence.
A thought:
What voltage and frequency are you seeing across the caps?
You are looking for VARs.
Volts times Amps(reactive).
A higher voltage on the caps may incur the magic of multiplication.
If you have much less than 480 Volts across the caps, you may be able to use transformers to boost your VARs.
Success will depend on frequency and V/Hz ratios.
Yours
Bill
A comment on slip:
When we talk about slip in an induction motor, we are looking at the difference between the synchronous speed and the actual speed. eg 1800 RPM vs 1760 RPM = 40 RPM slip.
When the 40 RPM is resolved into frequency, it gives the frequency of the rotor circuit.
In your case the slip of interest will be the actual RPM of the induction generator vs the frequency of the generated voltage.
In a motor the slip is closely proportional to the load, up to about 200% to 250% of full load.
In a motor the slip frequency is constant from zero RPM to at least 200% of rated speed.
That is, while the slip frequency varies with loading, the slip frequency for a given load remains constant across a wide speed range.
The real current for a given slip remains closely constant.
The slip frequency of your IG rotor may be of interest.
I’m not currently set up to measure the difference between the lathe motor’s slip and the slipping of the belts in the lathe gearbox. Maybe there is something interesting in there, if I just tried to measure it.
Actually, it should be more useful to measure the frequency in the generator itself.
So far I have been inferring it from the speed of the lathe but Bill you’ve just opened my eyes to the OTHER SLIP going on in this system.
I’ve been missing it all along. I can re-run a few tests to see if I can add some useful measurements.
About re-running tests: I’ve tried this a few times, such as when I got out my wattmeter. Running under seemingly identical conditions did not have exactly identical results. Close… It’s as if the generator is susceptible to temperature in my garage, I think.
I just realized that I never circled back to write a conclusion. I wrapped up the tests in April. I learned a lot (my primary goal). To anyone who has been thinking of using an induction motor in a wind turbine instead of a permanent magnet generator (PMG), here is what I learned:
• The project could be cheaper; especially if a used induction motor can be found (they are sometimes given away for free).
• The project won’t require special tooling like a lathe to modify a motor, nor a mould to cast an epoxy stator.
• You will save the cost of permanent magnets, but the capacitors may cost almost as much.
• Low wind speed performance won’t be good unless you use a lot of capacitors.
• You should test your system to make sure it’s stable at full power.
• You should design protection against the induction generator going off-line in strong winds. Have a braking or furling system that can immediately shed the power coming to the rotor.
• Equip the bank of capacitors with a way to loose heat.
• Don’t expect spectacular efficiency.
I suppose that doesn’t make it sound easy. Challenging may be a better word. PM generators are more common in DIY projects, for good reasons. Larger industrial turbines can use induction generators because large-scale projects have more regulatory constraints, and need more control and safety systems anyway, therefore the complexities that I’ve noted above are par for the course for Vestas and Siemens.
