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Generators & Motors General Discussion Antique Generators, Light Plants and Old Electric Motors: Questions and answers about restoring and showing old power generation systems.

Generators & Motors General Discussion

permanent magnet motor/generator


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  #1  
Old 09-22-2001, 11:16 PM
Joseph Turrisi
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Default permanent magnet motor/generator

I have a permanent magnet 90 volt dc motor that i plan to used as a generator on a wind mill. My question is when used as a generator will this motor produce AC or DC voltage. I hooked it up to my meter and on the AC setting it produce about 30 volts while i spun it by hand and on the DC setting it produce about 20 volts. Does anyone have any ideas.
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  #2  
Old 09-25-2001, 08:39 AM
Joe
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Default Re: permanent magnet motor/generator

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
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  #3  
Old 09-26-2001, 04:37 PM
Russ Hughes
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Default Re: permanent magnet motor/generator

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.
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  #4  
Old 09-26-2001, 07:23 PM
Joseph Turrisi
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Default Re: permanent magnet motor/generator

Russ don't worry about getting carried away i found what you had to say very interresting. I have been trying to figure out how to control the voltage but have not nailed it down yet. If you think of any thing else please let me know also i would like to thank you and joe for your ideas.
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  #5  
Old 10-02-2001, 03:28 AM
Jack Daniel
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Default Re: permanent magnet motor/generator

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 1000w 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.
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  #6  
Old 10-02-2001, 03:42 AM
Jack Daniel
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Default Re: permanent magnet motor/generator

I just reread your question and can answer the part about having both DC and AC outputs on your motor when spun by hand. The AC signal is a result of the communtator switching coils in and out of the circut as the coils go thro the permanant magnet fields. There really isn't a useable amount of AC because as you spin the generator faster the DC output voltage increases and the AC stays the same. There is a AC reading because your meter is seeing the DC as a series of rapid pulses instead of a smooth voltage level. If you want to make a 12v wind generator you might want to consider a alternator off a small car with a speed increaser using pulleys and belt. You wouldn't have to worry about overspeed conditions which will cook batteries if you use a 90v motor as a generator and it gets driven too fast.
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  #7  
Old 10-03-2001, 03:48 PM
Russ Hughes
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Default Re: permanent magnet motor/generator

I was thinking about the maximum output you could expect from one of these motors turned generator. With a voltage rating of 90 volts, the rated current would be only a few amps. If the unit was to be used for charging a 12 volt battery bank, the current rating would still be the same based on the wire sizes used in the motor. The operating speed of the unit when used in a 12 volt installation, would have to be slow enough so that it didn't try to exceed the maximum current rating of the wires.

If the unit had been designed as a 1 hp motor at 12 volts, the wire sizes would have been larger to handle the required current. I picked up a brand new unused 12 volt DC permanent magnet 3/4 H.P. motor and the current ratings are around 65 some odd amps. (I am too lazy to go out and look for the exact value.)

So it would seem that to get the most efficiency out of this particular unit, that it should be used at, or close to its rated voltage, probably charging a battery bank at that voltage. Then some sort of an inverter could be used to get to the voltage you need for whatever load you may have.
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  #8  
Old 10-10-2001, 05:39 PM
russ hanks
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Default Re: permanent magnet motor/generator

90v. DC PM generator should make a good wind machine. Normally you want one that can deliver full output at 300 RPM or less. What RPM is the motor rated for? An 1800 RPM 90v. motor would give you about 15vdc @ 300 RPM. If that is what you want it for, you can probably use a wind generator controller. Of course, you would need the right size prop.

As far as a controller goes, there are a number of regulators used for alternative energy. Some of these are pulse-width-modulated types that work from about 12v-90vdc input. I should be able to dig up some web pages if you are interested.

Russ Hanks
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  #9  
Old 10-11-2001, 08:32 PM
Joseph Turrisi
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Default Re: permanent magnet motor/generator

Russ the motor is rated at 1725 rpm I was able to turn the motor at 550 rpm and it produce about 30 volts. I would appreciate any info that you could provide on wind generator controlers.
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  #10  
Old 10-09-2003, 01:38 AM
Citizen Zero
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Default Re: permanent magnet motor/generator

I recently got a hold of a 100VDC permanent magnet motor out of an old tredmill. It currently has a large flywheel/weight attached that I am going to have to figure out how to remove. I hooked it up to a voltage meter and get 4-5V when spinning it by hand. Does anyone have any idea what rpm these things are optimal at? It is rated at 1/2 HP @ 100VDC. I had not really been out purposely looking for a motor for a windmill, I ust happened into it by accident and thought, hmmm windmill. Any help and/or pointers would be highly appreciated.

c0
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  #11  
Old 10-09-2003, 01:43 PM
Russ Hughes
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Default Re: permanent magnet motor/generator

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.
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  #12  
Old 10-09-2003, 02:06 PM
Matt Fretwell
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Default Re: permanent magnet motor/generator

Hello,

This website has some info regarding the basics on this.

http://www.otherpower.com/

Matt
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  #13  
Old 10-09-2003, 02:10 PM
Russ Hughes
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Default Re: permanent magnet motor/generator

I have to respectfully disagree. An AC voltmeter of the analog type (which has a pointer) is a DC meter that has a rectifier connected inthe circuit to change the AC to DC. I don't know about a digital meter in this regard.

In any case, an analog meter that is set to read AC will still read the DC component of a source that is supplying the power although the calibration, or reading will not necessairly be correct.

While it is true that the switching operation of the commutator will produce a certain amount of hash with an AC component in the output, it should be insignificant as you say. If you wanted to get an estimate of the amount, or AC voltage of the commutator hash, you could connect a non-polarzed capacitor in series with the analog meter which is set on AC. The capacitor would, after charging up, block the DC component and pass the AC component, which the meter could then read without be affected by the larger DC component.

If the capacitor was old and leaky, some of the DC component would be passed, so a new non-polarized capacitor in good condition is required. Also, the capacitor, if too large of a value (micro farads 'mf'), it will take a long time to charge up to the DC value through the resistance of the meter. If the capacitor is to small in value, it may not pass the AC component very well while still blocking the DC component.

On the other hand, if the commutator and brushes are not sparking abnormally under usual operating conditions, I just wouldn't worry about the value of the commutator hash.
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  #14  
Old 10-09-2003, 03:50 PM
Howard Yoshida
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Default Re: permanent magnet motor/generator

Aloha Russ,

"Again, it is the bearings ability to handle the load that I wonder about". Russ, you could get a larger bearing that would be appropriate for the job and make a mount and shaft to fit the new bearing. Then couple the new shaft to your tape drive motor but then this would require some machining. I think this will handle the heavier loads applied to the small motor shaft.

Mahalo, Howard
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  #15  
Old 10-09-2003, 08:52 PM
Mike Shearer
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Default Re: permanent magnet motor/generator

IMHO D.C. motors used for power generation are better suited for for D.C and items that have low voltage inputs and/or automotive devices, (voltage regulation issue aside) and A.C. motors would be better suited for home power generation applications.

Any A.C induction motor ($10 at the junk yard) can be employed to generate household compatible power and would cost less for the same voltamp size. and will generate almost the motor rating requirements in output power.

This site explains how it is done by turning the motor just above the RPM rating and adding some A.C. capacity across the output. I found this very interesting and much easier then using a D.C. motor to do an A.C. job.

http://www.qsl.net/ns8o/Induction_Generator.html

--mike--




Induction Motor Power Generator
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  #16  
Old 10-12-2003, 03:05 PM
Russ Hughes
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Default Re: permanent magnet motor/generator

Howard, that's pretty much what I had in mind also. I suspect that a large pillow block type of bearing installed next to the propeller would do the job with the other end of the shaft being attached to the motor/generator shaft several inches away. What is amazing about these tape drive motors is the slow RPM required to get a reasonable amount of power out of them.

An automotive alternator usually requires several thousand RPM to produce a reasonable amount of its rated power. The system, be it belts, chains, or gears used to achieve this speed increase from a much more slowly turning propeller would soak up some of the available energy.

A direct drive generator that could produce its rated output directly at the propeller RPM would be simpler and more efficient. In any case, what can be defined as a reasonable amount of power depends on the application and the equipment available. These tape drive motors are only good for a few amperes at 12 to 24 volts and if more than that is required, then something else would obviously be required.

In any case, it is an interesting subject to contemplate. Maybe I should go and finish the small wind generator project I started last winter when it was a little too uncomfortable to want to go do outside things.

...
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  #17  
Old 10-12-2003, 06:56 PM
Joseph Turrisi
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Default Re: permanent magnet motor/generator

You should goto www.fieldlines.com as homemade windmillls is one of the main things covered on that site.
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