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I bought this DC motor

PostPosted: Mon Nov 17, 2008 6:36 pm
by apicella
Hi all.
I bought this DC motor and would like some help regarding if I should build a windmill or turbine.
I live in Orange Grove Johannesburg,I am not clued up on electrical stuff at all but I do want to have a 12volt system to run some lights in the house and maybe a small convector.The voltage of the motor is 72volts.
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Re: I bought this DC motor

PostPosted: Tue Nov 18, 2008 8:46 am
by windgat
Hi apicella, welcome!
First thing is to do some testing of output voltage vs rpm. On a 12V system, you would need to find an rpm which gives you more than 12V. That is an open circuit (OC) test.
Next you could add a load. Best way to do this is to get a flat(ish) 12V car battery (perhaps somewhere between 11.5V and 12V), then hook up the output of the motor to the battery via an amp meter (in series) which can measure a few amps. Don't use the 200mA setting on the meter - you will probably blow a fuse.
Ideally you should also have a voltmeter on the battery at the same time. Then crank up the motor (for a short while) using a drill or something, and measure three things simultaneously: RPM, volts, and amps. Then plot the volts x amps against rpm on a graph. That will give you watts vs rpm, which will be the basis of further design. Oh, and for the next step you will need to know the wind speed for which you will optimise the system (unless you have complex adjustable blades you need to choose a wind speed at which things will work the best.
Looking forward to hearing more! And feel free to ask if anything above is not clear.

PS: Is a convector like an extraction fan?

Re: I bought this DC motor

PostPosted: Tue Nov 18, 2008 1:40 pm
by apicella
Forgive me for been thick.
You say dont use 200ma.
On my electronic meter i have
DCA 1:200u 2:2000u 3:20m 4: 200m which one do I use to see the amps.

Re: I bought this DC motor

PostPosted: Tue Nov 18, 2008 1:52 pm
by windgat
Hopefully you will be making more than 200mA, so you need a meter that can handle 10A or so. These normally have a different connection plug for handling large currents. If you use a 200mA setting, turn the motor very slowly at first, because at about 250mA you will blow the fuse inside the meter.

Re: I bought this DC motor

PostPosted: Tue Nov 18, 2008 3:21 pm
by apicella
Got it, I ran it on a lathe so I know the RPM'S
325 RPM
1.55 Amps
13.74 Volts

125 RPM
2.40 Amps
11.5 Volts

Hope this is good news.
Leon

Re: I bought this DC motor

PostPosted: Tue Nov 18, 2008 3:31 pm
by windgat
So far so good. Its a bit puzzling for two reasons: I assume you had a beefy, flat 12V battery attached? So why did you get 13.74V? :?:
Perhaps you are using a tiny battery? Try use 100Amp hour or so, which will be pretty unaffected by an amp or two for a few seconds.

At lower RPMs you are getting 28W, and at high RPM 21W, which is opposite to expected. It may have to do with the different voltages.

Ideally you want to test at about the same voltage, at different RPM. That's why a flat battery is a good load, because it tends to keep the voltage within a range.

I suggest you do a range of readings if possible at about 10 different RPMs, and plot the results (volts times amps) on a graph. Its also useful to do that for open circuit (in which case you just measure the voltage) so that you can see the 'cut in' speed, which is the RPM at which you get around 13V.

Re: I bought this DC motor

PostPosted: Tue Nov 18, 2008 4:33 pm
by apicella
The battery I used had 12.5 volts an old car battery will look for a bike battery tomorrow and try again.
Thanks

Re: I bought this DC motor

PostPosted: Tue Nov 18, 2008 4:49 pm
by windgat
The car battery sounds good - usually bigger than a bike battery. I think the problem was that it was nearly fully charged, so couldn't absorb much current. I suggest you run a load off it, like a powerful 12V lamp (e.g. 50W), until the voltage settles just under 12V after letting it stand for 20 mins.

Re: I bought this DC motor

PostPosted: Wed Nov 19, 2008 8:17 am
by apicella
Ok I'm thinking ahead of time here but maybe I should build a vertical axis wind mill and I say this for two reasons the area were I stay and also any wind direction will spin it.
My idea is 1.5M heigh and radius a 27inch bicycle wheel with 110mm drain pipe as the veins.
Just a though for now, any suggestions.
Leon.

Re: I bought this DC motor

PostPosted: Wed Nov 19, 2008 4:11 pm
by apicella
Ok hope this is ok now.
Battery 11.65volts
Rpm Amps Volts
55= 1.78= 6.70
125 = 0.70 = 7.60
200 = 0.40= 9.50
300= 0.30= 12.00
325= 1.90= 12.80

the 325 rpm gives a big jump in amps from 300

Leon :roll: :roll: :roll: :roll:

Re: I bought this DC motor

PostPosted: Wed Nov 19, 2008 5:15 pm
by windgat
Hi Leon. I am still confused, sorry :?
If there battery is at 11.65, how do you get a reading of 6.7V at 55rpm?
For the open circuit test, the voltage is usually quite linear with respect to rpm.
So maybe I wasn't clear before (or maybe I don't follow your data). And I just realised that since you are using a DC motor, it will try and turn as soon as you connect the battery to it (in test 2 below). So either only connect the battery once its turning, or use a diode to block current flowing into (instead of out of) the motor.

What I meant was:

Test 1 (open circuit):
Connect a voltmeter across the motor
Measure voltage and rpm across a range of rpm, at least up to where you get 14V or so.

Test 2 (battery as load):
Connect the motor to the (flat) battery, with an ampmeter in series with one of the leads
I.e. motor negative to battery negative; motor positive to ampmeter connection 1, ampmeter connection 2 to battery positive
Connect a voltmeter across the battery (or the motor, should get same result)
i.e. voltmeter negative to battery negative, voltmeter positive to battery positive
Measure voltage, amps and rpm across a range of rpm.

For test 2, you should get 0 amps at low rpm, then at some point (cut in speed) you will start to get amps. For this test, all the voltages should be close to 11.65V (or whatever volts the battery is at).

Finally, plot V against RPM from test one, and then seperately plot V x A (ie watts) against rpm for test 2.

Re: I bought this DC motor

PostPosted: Wed Nov 19, 2008 5:32 pm
by apicella
Okay will do so tomorrow.
I hope I understand now as for a diod I'll have to go out and buy one but what do i Buy.
I have sent you a gift via email.
Perhaps you can sent me a diagram via email as to what I should do.
Leon
PS what do you think of a vertical axis windmill.

Re: I bought this DC motor

PostPosted: Wed Nov 19, 2008 5:35 pm
by windgat
If you get a bridge rectifier that can handle 30 amps that will be plenty. Should be around R30 or less. Get two, then you can use them if you make your own alternator (AC generator) one day.

Re: I bought this DC motor

PostPosted: Mon Nov 24, 2008 1:13 pm
by apicella
windgat wrote:If you get a bridge rectifier that can handle 30 amps that will be plenty. Should be around R30 or less. Get two, then you can use them if you make your own alternator (AC generator) one day.

I bought a Bridge recti on the week end now can you explain how I should run the tests, remember I have a DC pertinent magnet motor.
Thanks

Re: I bought this DC motor

PostPosted: Mon Nov 24, 2008 5:21 pm
by windgat
Ok, here is a diagram I made for you. Please let me know any questions.
MotorTest.png

Re: I bought this DC motor

PostPosted: Mon Nov 24, 2008 8:35 pm
by apicella
My plan is to build a vertical axis turbine but mount it on the top of a pole and not my roof.
This is because of my position and think that an all direction wind turbine should work best.
If the DC motor is no good them I'm going to try to build one of your generators with magnets and all.
This model according to the plans is 3ft diameter and 4ft tall.
http://www.windstuffnow.com/main/lenz2_turbine.htr

Re: I bought this DC motor

PostPosted: Tue Nov 25, 2008 9:41 am
by windgat
Sounds good, look forward to hearing more. Remember that a horizontal axis turbine is also an 'all direction' turbine, in that it rotates according to the wind direction. The vertical access design is less affected by constantly changing wind direction, but is less efficient in a constant wind due to the fact that the blades are moving into the wind about half the time.

Re: I bought this DC motor

PostPosted: Tue Nov 25, 2008 3:01 pm
by apicella
Ok Test One went off well but test two I dont know but I followed your diagram but once I connected the bridge Rec I got nothing out of the motor.Maybe I'm doing something wrong here.

Re: I bought this DC motor

PostPosted: Tue Nov 25, 2008 3:07 pm
by windgat
One of the tabs is different to the other three - that's the positive one. Try leaving the battery out at first to check you have everything correct. Then you should see some voltage when turning the motor. With the battery connected you will only get something when the RPM is high enough to generate more voltage than the battery.

Re: I bought this DC motor

PostPosted: Tue Nov 25, 2008 4:04 pm
by apicella
Does it matter which direction I spin the motor.

Re: I bought this DC motor

PostPosted: Tue Nov 25, 2008 4:23 pm
by windgat
I guess it would. To confirm, attach the voltmeter, spin by hand, and watch which way gives a -'ve voltage.

Re: I bought this DC motor

PostPosted: Wed Nov 26, 2008 8:36 am
by apicella
Did some research on the net last night and they say to use a Schott Diode in line so that the battery does not try to run the motor BUT they don't show how to connect.
Do you have an idea how to connect it.
It is a Schottky Diode 25A-45v Diode.

Re: I bought this DC motor

PostPosted: Wed Nov 26, 2008 9:16 am
by windgat
You can use this, but it is almost exactly the same effect as the bridge rectifier which I showed in the diagram above how to connect. As I recall the two outside pins (or just one of them for testing) go to the positive of the motor, and the centre pin to the positive of the battery. If you get it the wrong way around, the motor will spin (or try and spin if the voltage is too low).
Disadvantage of the Shottky is that you would need a heatsink in practice. I suggest just go with the bridge rectifier as initially planned. The voltage drop is slightly more but (if you look at the 'Electronics' page) I found it to be insignificant.

Re: I bought this DC motor

PostPosted: Wed Nov 26, 2008 10:17 am
by apicella
Ok I used the Bridge and found after the tests it was warm.
I hope that the RPM'S are correct.
Test 1
RPM
200-5.4 volts
250--8.10
360-13.35
400-20.00

Test 2 Battery at 12.10volts
Rpm-Amps-Volts
360-0.17-12.33
400-2.42-12-46
700-2.63-12.50
900-5.90-12.72


I hope that this is now correct as I am getting a little dis harted as the motor cost me R200 and hope I did not waste cash

Re: I bought this DC motor

PostPosted: Wed Nov 26, 2008 2:45 pm
by windgat
Ok! Looks right the right data. Here are the graphs of your results, plotted against RPM.
MotorGraph.png
Graph of motor output
MotorGraph.png (4.93 KB) Viewed 24224 times

You notice the line is not smooth. That might be inaccuracies in measuring - if it was me I would do a few more tests and see if the same results are obtained from the same RPM. From these graphs you can already tell two things:
1. You will need to get the motor to turn at at least 350 rpm on a 12V system
2. To get more than 50 watts or so, you would need to get the motor to turn at 1000 rpm or more, which means gearing would be needed.