Squirrel Cage conversion to Alternator

[alser]

Hi There,

I understand that you can retrofit the rotor of a 3 phase squirrel cage motor with a suitable magnetic material to act as the induction source within the alternator.

My questions are:

Has anyone done this successfully?

If so what rating for the motor has this been done to? I hope to build a 10 kw wind turbine or larger at 8 m/s

what speed of squirrel cage motor is best used where n= number of pairs of poles  IE a 2 pole 4 pole 6 pole motor, for a low RPM wind source?

What's the best motor for low speed conversion?

What speed does the prime mover have to rotate to give the nominal voltage IE if the motor is 240v delta or 415 star config what prime mover speed would have to be achieved to make the nominal supply voltage? IE if the motor is a 415v at 1500 rpm and 10 kw, would the prime mover have to rotate at 1500 rpm to develop the same supply voltage and power output?

What type of magnets should I use i.e. what are the best most cost effective magnets to use?

What quantity should I use IE if its a 1 pair pole motor

if i use say 36 magnets will this increase the output on a 1 pair pole motor stator will this increase the kW output at the same prime mover speed?

IE if I use the same rotor IE with 36 magnets on a 4 pair pole motor stator will this increase the kW output on the same prime mover speed?

Is it easier to build your own alternator taking into account the above? should I bite the bullet and build my own?

Thanks

Alan

[ghurd]

Look up Zubbly's work.
G-

[Flux]

So many questions and no way of answering without a real understanding of what you want to do. From the questions I have a strong suspicion that you are in a complete muddle about what is needed for wind power.

Yes you can replace the cage of a motor with magnets and produce power but beyond that you are venturing into unknown territory.

To use the original winding you need the same number of poles on the rotor as the stator is wound for but the speed you need to rotate it at to get nominal voltage is a guess. If you want nominal volts and frequency you will be very lucky if you get both right and even then it will depend on load.

If you try to get output at low speed you will have much less power than the original rating. If you are trying for a nominal voltage you will only get it at one speed so there is no possibility of taking load directly from it  or synchronising it directly with the mains.

Wind turbines work over a range of wind speed so you either charge batteries or use a grid tie inverter so the generated voltage is very nominal.

You can't increase the number of poles at random without rewinding the stator to suit so you either use the original winding with the same number of poles or just use the core punchings and rewind it to suit your chosen number of poles.

Th get 10kW at low speed with direct drive you will end up with a monster so you will probably need a transmission.

Can't give any real help without knowing what you intend to do.

Flux

[alser]

Thanks Flux,

Firstly I am not looking at a Direct connected device here! so no synchro to the mains! I am planning to create Dirty AC and turning this Dirty AC into a PWM DC then to AC or to DC (coupled with an AC supply) to produce a regulated voltage and frequency output.

I work for Schneider Electric here in Ireland its a long time since I've done generator theory !

I have access to group of excellent products which I wish to try this out on, one of which or more may work with the re polarised magnetic rotor that I wish to use.

These products are called

1 an active front end
2 an ATV61/71 drive configured in Constant voltage and frequency output
3 a UPS

The theory behind the active front end. is that it takes the regenerated kinetic energy on dynamic electrical loads which have high inertia and then in essence it turns the back emf from a stopping load into DC and then into a PWM AC, at constant Voltage and frequency.

The input voltage and frequency into the Active front end doesn't matter because it will give a constant voltage and frequency as long as the peak to peak voltage is maintained for some of the inputted voltage cycle, but I understand that now they have a new revision of firmware which also uses the input primary input voltage to generate the peak to peak voltage for the output also, although this is just hear say and I have yet to check this out.

The second alternative is to use an ATV61/71 of the correct KVA rating to the output of the turbine, turn the output voltage from the modified motor (IE the now alternator) into a 3 phase bridge and input this DC voltage to the DC bus of the ATV61/71. The minimum DC voltage is the peak to peak voltage from the rms supply, so if its single phase 230v output you require from the ATV 61/71 the DC bus voltage at the ATV 61/71 should be about 394vDC (that would be (230 * 1.717), similarly if its a 3 phase output one requires about 686vDC (400 *1.717).

Lastly I can use a correctly rated UPS to charge the Battery string at DC voltage levels and or use the proposed turbines input into the UPS to 1 charge the batteries or 2 supply the UPS.

Phew! sorry that was a lot to take in there!

So Flux and ghurd, my plan or perhaps dumb idea is to take a 10 KW squirrel cage motor, engineer the rotor,  with the best magnets available and then subject it to the prime mover of wind... and input the crappy voltage into either of the 3 above to get some free power!

I know I'm mad...... but I love a challenge!


My query really boils down to these questions even if its theory only:

if the n of the motor is 1500 rpm  and I put in the same amount of magnets in the rotor as there are coils in the stator, will I have to turn the rotor at 1500rpm to create the nominal face plate voltage?

would you say its the same for a n of 3000 rpm?

which would even in theory generate and produce more power for the same speed i.e. will a a3000 rpm motor produce more than a 1500 rpm modified alternator motor because it has more physical coils at low RPM?

If so how many magnet salient poles do I need per n of the motor

If I put X times the amount of magnets in the motor will I increase the power output per generation cycle or increase the voltage?

as far as I recall m (mue)=B * I * L where m= the permeability of the material B= the direction of the magnetic field force?  I= the current and L = the CSA

[Flux]

Basically the motor will run with the core nearing saturation so if you convert it to an alternator with permanent magnets then if you can get enough flux to be near saturation you will need to turn roughly at its nameplate speed to get nominal volts.  You will need a lot of magnet to get that state. In many cases you may need more than nominal speed to get the volts up to nameplate value.

It doesn't matter a lot whether you start with a 1500 rpm or 3000 rpm machine. The difference will be that the 3000rpm machine will be smaller than a 1500rpm one for the same output.

If you want more power out and are not confined to a fixed frequency then the faster you drive it the more you get out. If you have too many volts at the higher speed you can reconnect for a lower voltage and get more current to keep the power up.

As I said previously the size goes up very quickly for a given power if you drop the speed. You will be limited by the problems of holding the magnets in place if you start raising the speed and at 3000 rpm you will need some careful thought to get it to hold together.

Unless you can get magnets made specially you will have a serious challenge finding large magnets with suitable curvature for a 4 pole machine (1500rpm). The challenge will be worse for a 2 pole machine.

As you are not tied to frequency I think you will find it easier to use 6 poles or more but without rewinding you will struggle to find more than a 6 pole motor ( 1000rpm).

This is a big undertaking with lots of powerful and expensive magnet, make sure you consider the problems of handling a permanent magnet rotor of that size, if it attracts a large piece of steel there could be serious consequences and threading it is not to be undertaken lightly.

Flux

[oztules]

"If you want more power out and are not confined to a fixed frequency then the faster you drive it the more you get out. If you have too many volts at the higher speed you can reconnect for a lower voltage and get more current to keep the power up."

The second sentence of this statement by Flux is very relevant. You are building what is essentially a current limited alternator. As the rpm rises and the current rises so to will the ampere turns in the stator. In this configuration with fixed excitation, it will current limit. Speeding it up further without easing the current problem will result in no further output.....

So if it current limits at say 1500 rpm (from back MMF), driving it through a gearbox to 5000 rpm, will make no difference, unless you transform the AC or buck the DC in order to use the extra EMF in the coils due to the extra rpm.

If it hits current limit it will runaway with extra input. (from the wind)



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

[SparWeb]

You could look at my stuff.  I documented the work in enough detail so that someone could do the same if they wanted.  I can't say I'm interested in all of the electronic conversion you're talking about.  My goals are rather mundane compared to yours!

www.sparweb.ca    look at the "motor conversion" alternators.

I've done a lot of other stuff that isn't on my blog, so you're welcome to ask for more; I might have an answer!

[alser]

Wow !

Thanks a lot guys!

Theres is a lot to digest here... so I'll be studying all weekend!

I will be in touch soon to ask some more wonderful questions.

Thanks

Alan