Converting 2 pole induction motor to generator, no. of poles?

[damob]

Hello all,
i am doing a college project where i am converting a 1.5kw induction motor into a generator, i have seen all the posts on here about putting permanent magnets in the rotor, my idea is to turn down the rotor, use two magnet poles, one north, one south, i will use lots of circular magnets.
Am i correct with just two poles? also the rotor is dia. 80mm, what size neodymium magnets should i use?
I understand that the 2 pole motor is not ideal but i am just hoping for 200 watts at about 300rpm?

[ghurd]

Just to be clear, the motor is 2-pole?
The magnet poles must match the motor's winding.
A 50Hz 2 pole motor name plate says about 2950RPM.
A 60Hz 2 pole motor name plate says about 3550RPM.

To me, 200W at 300RPM sounds a bit optimistic with a 1.5KW 2-pole motor.
G-

[damob]

Just to be clear, the motor is 2-pole?
The magnet poles must match the motor's winding.
A 50Hz 2 pole motor name plate says about 2950RPM.
A 60Hz 2 pole motor name plate says about 3550RPM.

To me, 200W at 300RPM sounds a bit optimistic with a 1.5KW 2-pole motor.
G-

The motor is 2 pole, it has a stator with 24 slots

[SparWeb]

Hi damob,

Ghurd was trying to get this cleared up but I think it's still not confirmed.
A motor of any speed or any number of poles can have any number of stator slots, so you can't make a conclusion from that.
Your motor should have a data plate, and there should be a rated speed on that.
What is that speed?
That is the only fact required for confirming that the motor is 2-pole, 4 pole or anything else.

[damob]

Hi damob,

Ghurd was trying to get this cleared up but I think it's still not confirmed.
A motor of any speed or any number of poles can have any number of stator slots, so you can't make a conclusion from that.
Your motor should have a data plate, and there should be a rated speed on that.
What is that speed?
That is the only fact required for confirming that the motor is 2-pole, 4 pole or anything else.

This is the motor spec:

Specifications
  Height    212mm 
  Length    284.5mm 
  Motor Rating    2HP 
  Output Speed    2870rpm 
  Output Torque    370Ncm 
  Power Rating    1.5kW 
  Supply Voltage    220 → 690Vac 

Thanks
  Type    Three Phase 
  Voltage    415Vac 

[Flux]

This seems to be a 2 pole 50Hz machine. Yes you are correct that it is 2 pole. The voltage is somewhat confusing and I suspect it was intended for a Vf drive.

Most likely it is a nominal 415v 50Hz machine. I seriously doubt that you have links for the other voltages, if it has 3 leads it will be 415v . If it has 6 leads then it will be 230/415v star/ delta.

You probably won't have many connection options and it may not suit 12v very well, if you want 24v it may be better.

Your 300 rpm seems very slow, if it's for a VAWT then good luck ( you'll need it). If it's for a HAWT then I suspect you ought to be aiming for your 300W at a higher speed, in which case you have a good chance of making it.

Flux

[damob]

Yes it has 6 leads for star or delta, would i be right in using 1 large south pole and 1 large south pole made up of smaller disk magnets?  It will be used with a vawt, thanks for the info!

[Flux]

One of the difficulties with 2 pole conversions is the problem of finding magnets with a suitable curvature. you will be forced to approximate with round or rectangular magnets and to get even a half decent approximation you will need lots of small magnets, otherwise you have large air gaps on the pole surface.

It's a pain trying to fix lots of magnets close together to make a large pole as they all try to repel and will flip given half a chance, probably one of the best ways is to use round magnets in holes in a non magnetic sleeve, that way you have lots of area for the glue to hold. If you stick rectangular magnets on you will have to do one row at a time and let the glue set unless you devise some form of magnetic shunt to divert the flux while the glue sets.

You need the maximum possible flux but with small individual magnets I doubt that you will get near saturating the core so the more magnet the better. I would aim for half the rotor as a N pole and the other as a S pole. Normally using more than 2/3 of the pole arc as magnet is not worth the effort but in this case it May still give you more out.

Flux

[opo]

300W @ 300rpm its a bit optimistic. But don't be discouraged by it. Keep going!. I also recommend putting as many magnets as physically possible, measure carefully how much is to be removed from the rotor because gap between magnets an laminations  is an important parameter.

I did a 1hp 2 pole conversion and I'm very happy playing with it learning the many aspects of wind generation. I have messed with it @ 12,  24 and 36V. This one seems to like 24-36 V better than 12v.

Cheers,

Octavio

[SparWeb]

I hope this doesn't sound too discouraging.  I just don't want you to invest too much time and energy into a project that goes in the wrong direction.
I'm very satisfied with my HAWT turbine using motor conversions - but it's important to point out that the one time I used a 2-pole motor the results were far from ideal.
I can just imagine it in a VAWT being even less impressive.

Some numbers (free of charge)

Say your target is 200 Watts output in a 10 meter/second wind.  The generator producing 200W will require at least 2X the input power because you should only expect 50% efficiency from a motor conversion when turning at a good clip.  Also set a (optimistic guess) target of Cp=0.3 meaning 30% of available wind energy captured by the turbine rotor.  That means that your VAWT blades must capture at least 1500 Watts, give or take.  To do that, it must sweep enough area.  Using sea level air density and the 10 m/sec wind the swept area of the VAWT must be around 30 square feet (about 3 square meters).  You'll also have to keep in mind that VAWT's operate at rather low TSR, say TSR=1 although this is difficult to narrow down because so many designs have many different "happy" speeds.  Knowing that TSR, the wind speed, and an approximate speed of rotation (300 RPM) then I'd say that the ideal radius has to be 1/2 meter, maybe less.  That leaves you needing your VAWT to be about 6 meters tall.  It's not impossible, but it's challenging to say the least.

Just look at the trouble Mariah Windspire has been having - with dimensions roughly the same.

[opo]

300W @ 300rpm its a bit optimistic. But don't be discouraged by it. Keep going!. I also recommend putting as many magnets as physically possible, measure carefully how much is to be removed from the rotor because gap between magnets an laminations  is an important parameter.

I did a 1hp 2 pole conversion and I'm very happy playing with it learning the many aspects of wind generation. I have messed with it @ 12,  24 and 36V. This one seems to like 24-36 V better than 12v.

Cheers,

Octavio

I was not paying full attention. I was thinking ,in my previous post, in a HAWT set up instead of your intended VAWT. A 2 pole motor conversion will require 200-300RPM just to reach cut-in @ 12V, so a HAWT is probably a better setup for the motor you have. Even in a HAWT the two pole motor is far from the ideal set up, a 4 pole motor will be better for example.

Cheers,

Octavio

[SparWeb]

I was called away from the computer, but I have a few more thoughts to add:
I was making several assumptions in my last post, so don't take it as written in stone.  All I wanted to convey was that the VAWT's aren't a good match for motor conversions.  Like you, I started out with VAWT's.  I wasn't so much disappointed by my experience, as I learned a lot, but decided after a couple of years that I would find a HAWT more practical and satisfying.  Which was true for me.  Especially since I turned to motor conversions instead of the axial flux alternators.  
Your college project may have some fixed goals or some flexible ones - I'd suggest looking for a way to find a better match for what you have on hand and what will provide results that you can use in your project reports.  It is possible to construct an axial-flux alternator better suited to a VAWT, for example.

[damob]

Thats some good info.!
 IT needs to be vawt as it is a another student designing the blades,
is 4 or 6 pole a beyyer option?

[ghurd]

The more poles the better.
G-

[Flux]

Yes that's about right. Low speed alternators are short and fat, high speed ones are long and thin this rule applies for large turbo alternators and hydro generators as well as for motors.

Low speed cores are large diameter so that you can get many poles in, this is normally a requirement to obtain the desired frequency which doesn't worry you, but the 6 pole cores have the larger diameter and shorter teeth that you need to make a decent low speed machine.

Unless you are prepared to rewind then you will be far better off with a 4 or 6 pole machine.

Flux

[damob]

ok, so does a 4 pole motor have more turns than a 2 pole overall?
What size motor should i look for in a 4 pole?
Thanks

[Flux]

That's far too complicated to answer but in reality the number of turns total per phase doesn't change a lot with the number of poles, the big advantage for you is the larger rotor diameter and the chance of using far more magnet material effectively.

Within reason the larger the machine the better your chance of obtaining your power at low speed but something in the 2 to 3hp region should be a good starting point. Larger machines have lower resistance which will help you in addition to having more magnet space in the rotor. If it is a VAWT then weight may not be a big issue. Just don't go crazy, if you chose a 30hp machine you would probably not supply the core loss.

Flux

[damob]

I am starting to get it now, i just thought it would be fairly easy get 200w from that size motor.
Would this motor work ok?
http://www.inverterdrive.com/group/Motors-AC/ac-Motor-2HP-4Pole-4-pole-Marelli-MAA90L4-B35/default.aspx
Thanks,
Damien

[Flux]

That may be a better starting point. What you need to remember is that you are only working at 1/5 of the design speed so if it is rated at 6A at 230v you are now looking at something like 6A at 50V. if you are intending to work at less than 50v then you have even more derating.

For wind duty you can seriously overload things during gusts so the thermal rating will almost certainly be adequate as long as you can force your power out of the thing in the first place. You will probably be looking at cut in of half your full load speed to get the output with the internal resistance so possibly it may be ok at 24v.

It's a bit of a long shot predicting things as a pmg from the original spec as an induction motor so there has to be a fair bit of trail and error. Before the days of neo this sort of rating was out of the question.

Flux

[SparWeb]

It's not really what I'd suggest.  My approach has always been to find old motors that have been retired for some cosmetic or regulatory reason or other, and convert them.  Buying a new one is paying money for something not suited to your purpose, and then you must invest work to make it more suitable - though only partly so.  I still think you should be wary of using a big motor-conversion in a VAWT system because there are some drawbacks that a VAWT is not the best at overcoming.
If you're going to spend some money, then try to locate a DC motor, either on e-bay or some local motor repair/overhaul/sales shop.  Explain what you're after and what range of speeds you are trying to accommodate.  Most people you deal with in that manner won't know how to help you, but they should know the equipment they sell and point you in the right direction.
For example, finding a 1 HP DC motor whose nominal speed of rotation is 1200 RPM and design voltage is 100 Volts will work as the generator of a 12-volt system, charging a battery up to 14-15 volts when turning at 150-180 RPM.  Some variation of the same ratio could be just as suitable.  This would save you a lot of trouble with all the fabrication work required to convert an AC motor.

[Flux]

This is very much my thoughts but for those in education things sometimes work differently. Scrounging from the scrap yard may not be as acceptable as buying new.

The big snag with dc machines is that low speed ones are not common. The old wound field machines are not suitable and there is not a lot of market for conventional dc motors that are intended to cover the range that induction machines with VF drives can deal with cheaper.

There may be some available but finding them is not easy and if it's your project and you need to get it done now it may change things.

I won't make further comment about what goes on in education sometimes but we can't blame students for that, they are given a task and have to deal with it. Far better his colleague made blades for a hawt and he did the motor conversion to suit but I won't open that can of worms.

Flux

[damob]

Thanks for all the advice so far, i managed to get a 2hp 4 pole motor, however i am not sure if i am calculating the emf correctly, could someone advise me, these are my calculations so far:
emf per phase = N x change in (flux x area)/ time
N= number of turns = about 250 turns
flux = 0.5T
area = about 25 10mm dia. magnets per pole x 4 poles = 0.0314m^2
Time- 300 rpm = 5 revolutions per second, which is 1 revolution every 0.2 sec.
Working this out i am getting 19.625V
Could it be this low? The rotor is dia. 100mm by 100mm long
Thanks,
Damien

[SparWeb]

Damien,

I'd have to get out my pocket calculator to be sure, but you may be under-estimating the EMF, unless it's for just one split phase at a time.
My previous motor-conversions have delivered at least 15 volts for 100 RPM, and in my most recent case over 30.  At 300 RPM that would be 45-90 volts EMF.
I measure open-circuit voltage after rectifying to DC, so working backward I think that range would be closer to 25-50 per split phase (off the top of my head).
There is some size difference between your 2HP motor and my 3HP motors, which would make up the difference between your calc's and my measurements, as long as you are talking about each separate phase.

Flux,

Your point is well taken.  At times I can step back and realize that I can't second-guess it, and stick to the question at hand, and at other times I can't help but to question the goal.  Knowing when not to and when it's invited...

[damob]

That is the phase voltage, if i multiply it by 1.73 and 3 amps i get about 110 watts of power at 300 rpm, is this all i can expect?
Thanks

[ghurd]

Some coil configurations can be a factor of about 2.1 instead of 1.7.
A converted 3-ph will sometimes show that.

I do not know where you came up with any estimate of amps.
If there is a good solid way to estimate it before the other factors, I do not know what it is.
G-

[damob]

If it is wired in star configuration will that not give me the current per phase of about 3 amps?
Thanks

[ghurd]

Where did 3A come from?
G-

[damob]

In star it is 230v/ 3.45 amps?

[ghurd]

OK...
Where did 230v/ 3.45 amps come from?
G-

[damob]

Fron the motor spec., it can be wired star 230v/5.97amp or Delta 480v 3.45 amp, this is from the motor tag.?

[SparWeb]

The data on the motor's website spec is:  "Full Load Current - 6.2A at 230V, or 3.6A at 400V"  ...close...

Anyway you can't judge much by these figures.  They are strongly dependent upon the motor acting like a motor - no longer the case.
I've never made the calculations work very well at predicting performance myself (goodness knows I have tried).
IMHO using calculations to show that your motor, when converted, will do what you expect is a stretch.
Even more difficult to forecast is the efficiency of the machine when you're done.  If you can get that 300 Watts with 50% efficiency, then you're doing well.
Kind of why I'd rather steer you toward an axial-flux generator, but it's been pointed out to me once already that it's not my project to decide.

On the other hand, you will have many electrical options if you choose that motor or something like it.  It will also be pretty rugged and be easier to mate with the turbine your partner eventually comes up with.  Why don't you take a look at my project and see what happens when giving one of these motor-conversions a load test:  www.sparweb.ca   The "Baldor" conversion is the most detailed, but there's similar info on the "Toshiba" conversion which was a much larger 7.5HP motor to begin with.

[Flux]

Predicting the output is way more complicated than you are imagining.

There are various ways of predicting the voltage from the flux per pole and the number of turns per phase, the big snag is actually knowing the flux per pole. The flux density in the air gap with well chosen magnets should be above 0.5T, you can get that sort of figure very easily in air gap machines so why bother introducing iron and all its associated problems for such a low figure. I suspect you ought to be looking at something over0.85T to get the power you want at very low speed from a small motor.

I won't give you the equation I use for emf because it goes back to basics and involves flux per pole and frequency. You will find simpler versions based on gap flux, speed, number of poles and turns per phase.

The line volts will be about 1.7 times phase volts and the dc mean of a 3 phase rectifier is about 1.4 times this.

That sorts out voltage and it can be sorted fairly accurately if you know the gap flux.

Current is entirely different and is not really related to anything on the machine name plate, that is only really a guide to its long term current handling without over heating and even that is not very relevent as the original fan cooling fails at low speed.

The factor that determines the current you get is the effective voltage driving the current through its internal resistance. Basically open circuit volts less battery volts divided by effective winding resistance. You will have to measure the winding resistance and you can only guess what the effective resistance will be ( in general it is somewhat higher than the resistance between a pair of terminals).

You can get volts at any speed with any size motor by altering the number of turns but power involves the interaction between volts and current and you will find that achieving the required current becomes the factor that dictates the size of motor you need.

Most of what you need is here somewhere but it is all scattered in various places among good and bad advice and it will take some finding.

Flux

[damob]

I have been on the look out for magnets to use in the conversion, i used 0.5 T as the magnets below give a performance of this at each face, are there better magnets with higher flux density available?
Also if i measure the resistance in a single phase, can i then calculate the current using I=V/R ?
Thanks


F674 - 10mm dia x 10mm thick N42 Neodymium Magnets (Pack Of 10) (A8/Y) 

Pack size

A pack of 10 magnets

Geometry

10mm dia x 10mm thick neodymium magnet.
The NORTH pole is on one circular face and the SOUTH is on the opposite face.
The NORTH and SOUTH poles are 10mm apart.

Plating

These magnets are plated with 3 layers of protective coatings - Nickel + Copper + Nickel (Ni-Cu-Ni)

Performance

These magnets will have approximately 5,200 Gauss on each face.
Each magnet can support a steel weight of up to 2.1 Kgs