Ultra clean mains voltage

[neazoi]

Hello, I have an idea which I would like to discuss with you.
I wanted to clean up the mains voltage of any waveform impurities, spikes etc.
One could use a transformer and PSU to convert to DC and then feed this to a sinewave inverter, but I have thought of another way.

Two motors from a microwave oven blowers (brushless moving magnet) will be connected back to back. One motor is used as a motor and the other as a power generator. The motor moves the power generator and produces an ultra clean sinewave, because the motor motion (and the flywheel effect drom the moving magnets weight) cannot respond to sudden mains spikes and noise.

I hope the motor can operate at 50 times/sec as it's label states 220v 50Hz.

My idea can be improved even further. One could connect the motor and the generator using a set of gears. Depended on the gear size, the output frequency of can alaso ghange. For example one could convert 50Hz to 60Hz. However the voltage output will also change and this can be compensated with external transformers.

How do you find my ideas, any comments are appreciated?
Do you know of any such commercial product?

 

[Dan Mills]

Thats an MG-set and was standard technology about 100 years ago....

You might want to look into the operating principle of a 'ferroresonant transformer' as well, these are effective for cleaning up critical mains power feeds and have no moving parts.

The modern way is a switched mode affair with a high voltage intermediate DC link.

Regards, Dan.

 

[neazoi]

Thats an MG-set and was standard technology about 100 years ago....

You might want to look into the operating principle of a 'ferroresonant transformer' as well, these are effective for cleaning up critical mains power feeds and have no moving parts.

The modern way is a switched mode affair with a high voltage intermediate DC link.

Regards, Dan.

An MG-set, thank you!
Reinventing the wheel...
However I am satisfied I have thought all the aspects for it's use before I read about it
Anyway, it would be interesting for a small set to be constructed for the bench, as a ultra low noise AC PSU. All the units I find on the net, are of huge power.
I have thought to replace the previously mentioned gears with pulleys, to minimize mechanical noise and avoid lubricant. Like the old Unimat DB-200 multiple pulley diameters could be combined into one metal piece to change the rotation of the motor or the generator.
Why on earth would someone wand other AC frequencies than the 50 and 60Hz is a mystery, but since I have the motors...
Thanks anyway for the useful info!

Have you got any idea if a motor that is designed for 50hz 220v, will ritate 50 times per second or not?
I mainly consider these brussledd motors from microwave oven fans.

 

[FvM]

I'm a bit surprized that your microwave has BLDC motors (= synchronous machine with permanent magnet excitation in general electrical machines terms). Most microwave fans I know are using simple (shaded pole) single phase asynchronous AC motors. But may be I'm not aware of home appliances state of the art.

A synchronous machine is rotating at fmains*60/N RPM, respectively fmains/N revolutions per second, N is an integer defining the number of pole pairs, e.g. 1, 2 or 4. Asynchronous motors are rotating slighly below synchronou speed, asynchronous generators sligthly above, described by slip versus torque curve.

 

[neazoi]

The motor is a Shaded Pole induction motor like this https://www.mpoweruk.com/images/shaded.gif , not a synchronous one. The label on it states 50Hz 220v.

So I should not expect 50 rotations per second I guess... What is your guess, should I expect even close to it?

 

[FvM]

So I should not expect 50 rotations per second I guess

5 to 10 percent below when loaded.

Operating an asynchronous machine as generator isn't so easy and requires special prerequisites. I haven't yet heard of an shadow-pole motor operated as generator, but it may be possible though. Shadow pole motors have very low efficiency (< 40 percent), so you get about no output power from the generator if it works at all.

 

[schmitt trigger]

To operate an induction motor (that is what a shaded pole motor is) as a generator, requires some sort of initial excitation.
There are many circuits that describe how to do this using the motor's magnetic remanence and external capacitors, but all of them are three phase motors (even though one only taps a pair of windings for the single phase output).

And to FvM's point, small induction motors, have terrible, terrible efficiencies.

Also, an induction generator cannot provide lagging power factor, which discounts many applications.

Having said this I have actually built your idea out of curiosity, and for the same reasons you state. It was an interesting project, but in addition to the pitfalls I mention above, on a generator for which you cannot independently control the excitation, via a separate field, the resulting waveform disturbances will be far larger than what you get from the powerline itself.

 

[neazoi]

the resulting waveform disturbances will be far larger than what you get from the powerline itself.

I am amazed about this fact.
Can you propose another type of motor that can eliminate these problems when used as a generator?

 

[schmitt trigger]

As I mentioned, you have to have field control. The field is wound on the rotor. An induction motor has a squirrel-cage rotor, not a wound one.

An electronic circuit (voltage regulator) senses the armature output voltage, and adjusts the field current accordingly.
This is the principle of all the AC synchronous generators you see on the market.

What is done with induction generators is to add or remove a parasitic load to the output, in an effort to regulate the voltage via brute force. There are many such projects on the web. Google induction generators.

 

[dick_freebird]

You might be better off with an isolation transformer
(which with big iron probably won't pass too much HF)
and some line capacitors on the output to soak up the
residual.

Nothing will get you to perfection. What suffices for
"good enough"?

 

[D.A.(Tony)Stewart]

Power Line quality is measured by "per unit" variations of voltage, frequency, transient noise, harmonic content.
This is greatly affected by line impedance. The grid has a much lower impedance than any small generator and especially much lower than a UPS.

UPS work well for most applications, but the flat top can induce some PSU failures, generate significantly more hum and harmonics in common mode situations such as floating laptop charger in audio systems. Special line filters and /or audio CM chokes may be needed here.

Since the noise in distribution is often generated by transient loads ( starting motors, arc welders etc), the power quality must be distributed carefully to prevent interference. As many non-linear loads can cause accumulate into excessive Neutral Current or Harmonic Current, special transformers such as high k or better yet, Harmonic Mitigating Transformers (HMT) which by means of 30 deg phase shift of parallel windings can cancel harmonic currents.

Line Impedance Stabilizer Networks (LISN) or large passive LC filters on 1 to 3 phase lines, can also be used to isolate noise from the grid in both directions.

Ferromagnetic Transformers are popular for applications demanding constant voltage but tolerant to transient noise as they tend to perform worse for noise immunity.

Power Factor Correction is used to reduce Current pulse noise and phase error upstream rather than improve power quality downstream.

 

[jiripolivka]

In all the above discussion I have not seen any mention of load impedance.
Taking about "microwave oven motors" as a generator is completely nonsense. This motor type cannot be used as a generator.
"Clean" AC voltage at 50 or 60 Hz is defined by harmonic distortion like any audio signal. Also harmonic content can be used. Instead of motor-generator pair ( AC mechanical generators often generate not-too-clean output; try a car alternator, for instance), try to design a good audio generator (wien-bridge or similar), then use a good audio amplifier to get the power your need.