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How to rectify ac to run a dc motor?

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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I think there's a very good chance that one or more of the following are true:

1) the designed voltage is *much* lower than 130VDC
2) you are killing the motor
3) you are wiring the motor incorrectly.
4) you are placing yourself at risk of a deadly electric shock.
 

eKretz

Apr 8, 2013
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That should be fairly easy to check by measuring resistance between commutator segments to see if any are vastly different.
 

duke37

Jan 9, 2011
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The part of the commutator that is shown in the photo looks good. If there is a fault in the comm, then I would expect to see some signs of arcing between segments and I would not expect it to run nicely at low voltages.

You have not commented on the brushes in the photo. It looks as if they are wrapped with copper wire. A bad connection here will doubtless cause excessive heating.

Starting a DC motor such as this direct on line will take an enormous current untill it is up to speed.
 

poor mystic

Apr 8, 2011
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:)
Rich brown burning smells have never meant much good that I heard of. (Sorry, I know the smoke was white but I guarantee the smell was brown. And that the smoke was toxic.)
Just watch out, that motor could do anything, but work I think, and should be tested using vast caution, and an isolating transformer.
 

boomerdog

May 24, 2013
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Thanks for all the replies, but I think I' m in over my head here. I have a few projects that a slow speed motor would be useful for, maybe just not this motor. I'll seek local help if I want to use this motor in the future. i'm going to heed your cautions/warnings, the risk/reward isn't worth it to me. Thank all of you for your patience with me!
 

eKretz

Apr 8, 2013
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The brushes look fine to me. I'm pretty sure what you're describing as copper wire on the brushes is the brush springs. The comm does look a little odd to me though, black and appears to be some smearing between segments. I can't see clearly if there's a lot of wear on the comm or not. But I agree the motor shouldn't run well at low voltage if there's a problem.
 

Young

Feb 6, 2013
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A rectifier can never i repeat never rectify a voltage completely there will still be some ripples in the voltage,the voltage stil behaves like an ac you need a filter capacitor and that too must be calculated,if you dont believe me check the waveform using an oscilloscope,if the voltage output is low or higher i will recomend you use a step up or step down transformer before connecting it to the rectifier and capacitor afterwards you can use other components to modify the voltage and current to suite the motor and if you dont have a transformer or cant get the actual value you can modify a transformer using the transformer calculation formular..
 
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eKretz

Apr 8, 2013
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Ripples shouldn't affect a brushed motor though, should they? It would basically just act as a pulsed DC signal like PWM, just with an irregular waveform, wouldn't it? Here is a waveform from a rectified 24VDC motor that I checked with my scope, and it runs just fine:

image-5.jpg~original
 

Young

Feb 6, 2013
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This is different from his,the voltage wil stil have little oscillations between 24v and maybe 0v you may not note the effect because the voltage is not to high his voltage is very high compared to yours,atleast let him try it and see..
 

duke37

Jan 9, 2011
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Young may be right. The problem is not voltage, it is current.

If the voltage is higher than the back EMF then the current will be high, this can be smoothed with the addition of a parallel capacitor or series inductance.
Small motors are often run on rough supplies but big motors with low inductance will be more fussy.

The scope picture shows the voltage but not the current. The small ripples will be due to the commutator passing the brushes.
What does 10kSiemens/second mean?

EKretz. Could you put a low value resistor in series with the motor to measure the current, then put a load on the motor to push up the mean current?
 

eKretz

Apr 8, 2013
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It's 10,000 samples per second, the sampling rate setting I was using at that time. (DSO). I can measure with a resistor, but I think I have no way to load the motor; it's a blower motor and if I remember correctly it's enclosed completely. I will take a peek later today.
 

eKretz

Apr 8, 2013
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OK, with a 1 ohm resistor shunt, I'm getting a 350mv drop across the resistor unloaded. If I drag my finger on the motor fan, I'm getting a reduction to ~340mv drop. I then removed the shunt and measured with an ammeter. The shunt method showed me around 350mA current and the ammeter showed 260mA. The waveforms didn't really change much when I loaded the fan, just dropped the voltage down, retaining pretty much the same waveforms. The current barely increased while loaded when running in series with the ammeter. The load I put on by dragging my finger pretty much cut the RPM in half.
 

duke37

Jan 9, 2011
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eKretz

The idea of the shunt was to look at the current waveform. Current will only pass when the voltage supply is above the back emf. This will occur for only a short time. Thus your average of 300mA may give many times that peak.

If the loading is increased the current will have risen to get the lower voltage.

Duke
 

eKretz

Apr 8, 2013
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I see, that makes sense, but I expected the waveform to change I guess. It really looked identical. Would it be more obvious then if I were to record a shorter time period and zoom in? I used the signal from 2 channels on opposite sides of the load resistor and Ch 2-Ch 1 to show the differential. As I'm sure you can tell I am still fairly new to oscilloscopes but I like doing experiments and trials to learn more.
 

RITZEE

Apr 4, 2017
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I just registered to reply.. Any one saw the RPM of the motor.. Its very small 3 digits.. Looks like a low rpm motor..
But then its a very old post.. Never mind
 
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