permanent magnet motor/generator - Smokstak

The standard answer is that if it is a DC motor, it can be used as a generator to generate DC current.

You may still get indication of AC power even if the source is DC using a VOM (volt-ohm-meter) or other instrument utilizing a d'Arsonval meter movement. The d'Arsonval meter movement is a DC current movement and to measure AC voltage or current requires the use of a full wave bridge rectifier in series with it.

Thus, your DC generator output will be able to move the meter even though the switch is on AC.

The reverse is NOT true. A DC meter movement will only "oscillate" on AC current without the bridge rectifier.

Also, the possibility exists that someone "opened" your PM motor/generator and "reversed" one or more of the magnets. You may be able to tell this by turning the shaft with your fingers "slow" and using the meter on DC current. Displacement of the meter pointer should be nearly all in a "positive" (or the same) direction.

Now accuracy is another matter. AC voltage (which varies continously) is generally expressed as RMS (root mean square) of the peak voltage. Our 120VAC current actually has a peak voltage of nearly 170VAC. But power wise and effectively, it is the equivalent of 120VDC, and 120VAC is the RMS reading convention that is in common use.

Mostly RMS voltage is what is shown on most d'Arsonval voltmeters although there are exceptions. Whether or not yours is an exception, there may be "compensation" built into the meter which is why you see a difference in the AC versus DC reading of the meter from the DC source.

Trust the DC reading if you're reading a known DC source.

Hope this helps.

Best regards, Joe I was just looking at one of those 90 volt DC permanent magnet servo motors the other day at a local surplus place. They wanted too much for it as they do for most of their other stuff.

Joe's comments on this subject are excellent, except for the possibility, or maybe the probability, of someone having moved the magnets around. I am not saying it can't be done, but the magnets are frequently epoxied or glued in place to the outside of the motor housing and prying on them could break the magnet material. It would be a difficult chore at best. Maybe on the larger motors of this type, the magnets could be held in place mechanically by screws or something similar making the job a little easier.

These motors make excellent generators once you get them up to speed. There is also a similar DC motor made for the old style computer tape reel to reel drives. These were designed as relativly slow speed high torque DC motors to pull the recording tape quickly back and forth through the recorder. The ones I have are rated at several amps at around 30 volts. You can spin one of these by hand and fully light a 12 volt brake light bulb with a twist of the wrist. Since the wrist can't make continous turns, this sort of limits the power generating capability by this method.

I wouldn't attempt to mount a windmill propeller directly to the DC motor shaft as it wasn't designed for the propeller loads that it would have to take. An outboard shaft and bearing assembly along with some sort of a chain, or belt drive to the motor would work. This would also allow for some sort of a braking assembly to be installed to protect the unit from stormy winds, or for maintenance purposes.

The permanent magnet motor used as a generator has no practical inherent voltage regulation capabilities, so you would have to use it to charge some sort of a battery bank for a practical source of power. Then you have to protect the battery bank from overcharging with some sort of a regulator. They make battery charging regulators for solar panel charging systems that could do the job as long as the voltage and current ratings are adhered to.

Otherwise, you could use an adjustable resistor bank to regulate the charging current. This would require frequent attention to maintain a maximum charging rate and still protect the batterys. You could also accept a less than optimun average power output with a lot less changing of the resistor settings if the resistor setting's remain appropriate for high wind conditions.

Since for a particular horsepower rating there is a rated current and voltage, this limits the power output without overloading the armateur windings and commutator brushes. You can charge a lower voltage battery bank, such as a 24 volt system with a 90 volt motor, as long as you do not exceed the rated current rating of the motor. Of course you will get a little less power generated this way than if you had, say a 64 volt battery bank, but accessories that can run off the lower voltages of 12 or 24 volys are more available. Otherwise you could use an inverter that uses 48 volts or higher to produce 120 VAC from the battery bank.

I am sorry Joseph, but I do get carried away sometimes on these sort of discussions as I find the subject very interesting. There pretty much isn't any way to control the output voltage of the motor used as a generator, except to vary the speed you spin the shaft. Actually there are DC to DC convertors that could be used to convert say the 90vdc to 12v for example but they are a bit expensive and have some quirks that would make them a fairly large hassle to use. The current that your motor will put out will vary with both the speed you turn it and what kind of a load u put on it. If you can set it up to have a constant load i.e. 3 car head lights or an radiant electric (no fans) heater then you can play with the gear/belt ratio (start with the gen/dc motor shaft turning relatively slow)between the engine and gen while setting up your display/system to get the voltage at the load in an acceptable range i.e. if the 3 12v car lamps you are using are connected in parallel they would need 12v and the gen would need to be putting out probably 35-40amps, but if you connected them in series (that is all in a string) then they would take 36v and about 12amps, the later setup would be be more suited to using the motor you want too, because in the first case the generator would be turning very slowly, in the 2nd case it would need to turn aprox 1/3 its rated RPM. I doubt your generator could output 12v at 30something amps. If you found an old 110v radiant heater you could use the output of the generator pretty much with out having to worry about any regulation. There is a gotcha though. DC motors are rated at a specific load or horse power rating, if the one u have is a 1 hp motor and is designed to run on 90vdc it will draw a certain amount of amps aprox 10 in this case, once u feed it the correct voltage and amperage it will spin at a specific RPM doing about 1 hp of work. Now lets take the motor, we set it up to be spun by a engine at the same RPM and don't put a load on it. We connect a voltmeter to our motor now generator and the voltage will be way higher than 90v probably nearly 140vdc because there isn't a load on it. Assuming the engine spinning the now generator can produce at least 2 1/2hp (It takes a 2 1/2hp gas engine to do the work a 1 hp electric motor can do) then we can add loads to the generator and when we add enough load say a w radiant heater which will be drawing aprox 10 amps then the voltage the generator is putting out will have dropped to about 90v. The voltage output is directly proportional to the load. The more amps you pull out of the generator by adding load the lower the voltage drops. If you set it up to run at 1/3 rated speed it is OK to draw the rated amperage out of it at any speed as long as the output voltage doesn't rise extra ordinarly high say 130volts. Another interesting load you could put on the motor made gen is old drill motors that don't have variable speed controls, they are built using what is called a universal motor and it doesn't really care too much whether the power feeding it is AC or DC. So if you hook up you gen and have a meter on it to make sure the voltage doesn't go over about 135v then the drill motor would work fine as long as it is indead the older ac/dc motor without variable speed. There is a definite danger in playing with the voltages your set up could put up. If you were spinning the gen at its rated RPM and accidentally shorted the wires the resulting arc could definitely cause damage by fire and literally exploding wires. If your motor is in the small fractional hp range the previous statement isn't as applicable. The DC motor is very efficent as a motor and generator and can put out a very large surge current if you give it a chance. If you're not familiar with wiring up stuff make sure you get a more experienced person to check it over. Nuff said fer now it's late. If u have more questions I'll be glad to offer what knowledge I can. The numbers I've used are ball park and somewhat pulled out of the eithers but I think are good enough as long as we aren't trying to get anything to run at 100% rated power where margins begin to stackup. One of the things I have wondered about when using a motor of any kind in windmill service is the ability of the motor bearings to be able to take the unusual strain of the propeller as compared to a simple "V" belt, or gear drive load on the shaft. It might seem advisable to use a seperate bearing to address the propeller loads. If the assembly is swinging back and forth because of changing wind direction, the centrifugal forces on the shaft can be considerable.

For an example, take a bicycle wheel and hold it by the shaft. Have someone start the wheel spinning and then carefully attempt to turn the wheel to either side. Use care, wear gloves and don't allow the fingers to get into the spinning spokes. You will immediately notice some strange effects as you attempt to turn the wheel. Now imagine the effects of a gusty wind rapidly whipping the windmills tail feather back and forth. There would be a lot of stress at the hub and bearing.

Electrically I don't have any real good simple answers. It would seem that some sort of a current limiting circuit would be required to protect the generator (motor) windings from over current if a battery bank of a significantly lower voltage rating than the generator was connected.

You would also have to use sme sort of a disconnect devce like a relay, or better still a diode to keep the battery bank from motoring the windmill during times of low wind speed.

I over heard some comments about a commercial wind generating station near where I live. The individual assumed that ecause the blades were still turning when there wasn't any wind that the utility must be supplying power to keep the generator motoring over. They forgot that the wind generating station is located on a hill top where the wind blows much of the time. Also even in a low wind speed condition the generator may still be producing smaller ammounts of power as long as the rated 30 RPM was being maintained.

I am working on a small wind generator using permanent magnet tape recorder drive motors. These are usually rated around 24 volts and are wound for slow speed operation. These motors came out of those large rack mounted reel to reel tape drives you see in the movies because the look impressive while running because the operation can be seen. I can spin one by hand at the hub and they will generate enough power at that speed to light a car stoplight bulb. Again, it is the bearings ability to handle the load that I wonder about.

coolio3,

Welcome to PF!

The three phase axial flux design you referenced was engineered by Hugh Pigott for use in HAWTs (Horizontal Axis Wind Turbines), and is a good choice for DIY construction with basic tools. There are lots of variations on the basic design, google: "Axial Flux Alternator" for lots of information.

A three phase alternator requires a bit more complicated rectifier than a standard single phase bridge rectifier, but that is covered on page 38 of the article you posted a link to.

The total cost of the alternator depends greatly on what you have on hand and what you can get from a salvage yard versus what you have to "buy new". I would guess one could be built for $300 to $500.

and would the electricity it produces be free?

"Free"? hrmmm, that depends a lot on how you define "Free", and what the source of mechanical energy is. I am going to assume you do not understand what an alternator or generator is and explain it clearly to prevent confusion later on:

An Alternator converts mechanical energy into electrical energy and you ALWAYS GET LESS electrical energy out of the alternator than you put into it in mechanical energy.

So, if you are planning to install an HAWT, the energy it produces will come from the wind (which is still "free" in most countries), so your only costs will be construction, maintenance and any associate land use costs. If your HAWT cost you $10,000 to build, has a peak capacity of 5kW and produces ON AVERAGE at your location 1kWh, then (assuming electricity is $0.15/kWh) you will get 1kWh * $0.15 * 24 * 365 = $/year of electricity. If maintenance costs $200/year averaged over a 10 year period, then in about 9 years you will get your first truly "Free" energy.

If your mechanical energy source is a river, your cost of construction could be considerably lower than an HAWT, and the output could be potentially much higher and hence your "return on investment" might be much sooner.

If you are thinking you can turn the alternator with an electric motor powered from the grid, you will INCREASE your power bill by at least 40%.

If you are planning on using an ICE (Internal Combustion Engine) to turn your alternator, the fuel will cost you at least twice as much as purchasing the same electricity from the grid.

At the end of the day, no matter how expensive you think electricity is, it is the cheapest energy available unless you have access to a river, live in a high wind area or have some other source of readily available mechanical energy.

Most RE (Renewable Energy) sources cost between $5 and $25 per capacity Watt to install, and very few put out more than 25% of their rated capacity, so for a 10 year return on investment, your investment could not exceed: $0.15 * 1W/ * 24 * 365 * 10 * DF = $13.14 * DF/Watt, where DF = "De-rating Factor". For instance a solar PV installation might have a rating of 5kW, but if this rating is only achieved 6 hours a day, then the daily de-rating factor would be 6/24, or 0.25. So, $13.14 * 0.25 = $3.29/W. If you then add in 50 days a year when there is cloud cover, you would get 315/365 = 0.863, so $3.29 * 0.863 = $2.84. Solar PV actually costs about $10 per installed watt, so an un-subsidized installation could take as long as 60 years to "break even". Solar PV has an expected useful life of 20 to 40 years.

Anyway, I have digressed. I worry when people mention "free energy". The electricity you get from the grid is the closest thing to "free" you will get until some major discovery reduces the cost of grid supplied electricity.

Fish coolio3,

Welcome to PF!

The three phase axial flux design you referenced was engineered by Hugh Pigott for use in HAWTs (Horizontal Axis Wind Turbines), and is a good choice for DIY construction with basic tools. There are lots of variations on the basic design, google: "Axial Flux Alternator" for lots of information.

A three phase alternator requires a bit more complicated rectifier than a standard single phase bridge rectifier, but that is covered on page 38 of the article you posted a link to.

The total cost of the alternator depends greatly on what you have on hand and what you can get from a salvage yard versus what you have to "buy new". I would guess one could be built for $300 to $500.

and would the electricity it produces be free?

"Free"? hrmmm, that depends a lot on how you define "Free", and what the source of mechanical energy is. I am going to assume you do not understand what an alternator or generator is and explain it clearly to prevent confusion later on:

An Alternator converts mechanical energy into electrical energy and you ALWAYS GET LESS electrical energy out of the alternator than you put into it in mechanical energy.

So, if you are planning to install an HAWT, the energy it produces will come from the wind (which is still "free" in most countries), so your only costs will be construction, maintenance and any associate land use costs. If your HAWT cost you $10,000 to build, has a peak capacity of 5kW and produces ON AVERAGE at your location 1kWh, then (assuming electricity is $0.15/kWh) you will get 1kWh * $0.15 * 24 * 365 = $/year of electricity. If maintenance costs $200/year averaged over a 10 year period, then in about 9 years you will get your first truly "Free" energy.

If your mechanical energy source is a river, your cost of construction could be considerably lower than an HAWT, and the output could be potentially much higher and hence your "return on investment" might be much sooner.

If you are thinking you can turn the alternator with an electric motor powered from the grid, you will INCREASE your power bill by at least 40%.

If you are planning on using an ICE (Internal Combustion Engine) to turn your alternator, the fuel will cost you at least twice as much as purchasing the same electricity from the grid.

At the end of the day, no matter how expensive you think electricity is, it is the cheapest energy available unless you have access to a river, live in a high wind area or have some other source of readily available mechanical energy.

Most RE (Renewable Energy) sources cost between $5 and $25 per capacity Watt to install, and very few put out more than 25% of their rated capacity, so for a 10 year return on investment, your investment could not exceed: $0.15 * 1W/ * 24 * 365 * 10 * DF = $13.14 * DF/Watt, where DF = "De-rating Factor". For instance a solar PV installation might have a rating of 5kW, but if this rating is only achieved 6 hours a day, then the daily de-rating factor would be 6/24, or 0.25. So, $13.14 * 0.25 = $3.29/W. If you then add in 50 days a year when there is cloud cover, you would get 315/365 = 0.863, so $3.29 * 0.863 = $2.84. Solar PV actually costs about $10 per installed watt, so an un-subsidized installation could take as long as 60 years to "break even". Solar PV has an expected useful life of 20 to 40 years.

Anyway, I have digressed. I worry when people mention "free energy". The electricity you get from the grid is the closest thing to "free" you will get until some major discovery reduces the cost of grid supplied electricity.

Fish