# motor as generator question

Discussion in 'General Electronics Chat' started by count_volta, Aug 14, 2009.

1. ### count_volta Thread Starter Active Member

Feb 4, 2009
435
24
I was just wondering, since a DC motor has a commutator, if I rotate it, will it produce a rectified AC current? It should look something like the output of a diode whetstone bridge rectifier right?

To produce an AC current I need an AC motor without the commutator right?

2. ### ELECTRONERD Senior Member

May 26, 2009
1,146
16
If you were to take a DC motor and spin the shaft, than DC would come out of the positive and negative wires. Rectified AC just looks like the negative portion is chopped off, but if you were to add a capacitor it would smooth the ripple out thus producing DC. Although if you spinned the shaft on an AC motor (invented by Nikola Tesla), you would produce AC on the output. You could put a wheatstone bridge rectifier bridge on the output to have the negative portion cut off; but if you didn't add that it would just be a pure AC sine wave.

I built a DC motor a couple years back and it had a commutator. I'm not sure if AC motors have them or not; but you could check on http://www.wikipedia.com/ and see.

Rubi Adams likes this.
3. ### count_volta Thread Starter Active Member

Feb 4, 2009
435
24
Thanks, thats what I thought.

Well the wheatstone bridge is used to fully rectify a signal. The output from a diode bridge looks like, ΩΩΩΩΩΩ while the output of a single diode looks like Ω__Ω__Ω__Ω

Well I think the output of a DC motor used as a generator is also ΩΩΩΩΩΩ because of the commutator. It doesn't cut off the signal on the negative part, but it always keeps the signal positive. Am I right?

Man I seriously need an oscilloscope. All my experiments involve AC signals lately.

Also do those hand crank generator radios that they sell in stores have AC generators or DC generators? I want to get one just for the generator.

4. ### ELECTRONERD Senior Member

May 26, 2009
1,146
16
I don't think that's necessarily true. Because when I put a single diode on AC it chops off the lower portion. So I think that if the peaks were closer together it would be determined by frequency. So, the peaks will stay the same if you have a single diode or a wheatstone bridge. I think your right about the commutator, it will produce AC but the time it takes to make each contact it is such a negligible amount that it is considered DC. It is important to note that time and frequency are related. 1/f = T and f =1/T where f = frequency and T = time. If I were to put the cathode of the diode to ground (which is what usually happens) it will cut the negative portion off; but if I reverse it and put the cathode on the positive side it will chop off the positive portion of AC. You should get an oscilloscope; it is very helpful. Hang on a minute and I'll get you some pictures for a demonstration.

As for those hand cranks, it would be a DC generator. You see that hook the shaft of the motor up to a gearbox which will make the shaft spin faster, and they have the output of the motor connected to a rechargeable battery or capacitor and then that goes to an LED or light bulb.

5. ### ELECTRONERD Senior Member

May 26, 2009
1,146
16
Ok, I took de' pictures. The first one represents a standard AC waveform. The next one is with a rectifier diode with the cathode to ground. Finally, the third picture is with the cathode to the positive input of the signal generator. Hope that helps you.

Note that in the second picture the peak of the wave is a little lower than the first sine wave. This is due to insertion loss; you see the diode isn't perfect and so it takes of 0.4V or something like that. If you look at a datasheet this is what it takes to forward bias the diode.

File size:
138.6 KB
Views:
25
File size:
143.1 KB
Views:
22
File size:
138.6 KB
Views:
20
6. ### count_volta Thread Starter Active Member

Feb 4, 2009
435
24
Ahhh sweet. I'm jealous. How much was your scope? Thanks.

Yea the forward drop of the diode is usually 0.7V.

We did learn about this last semester, and we even designed power supplies for our homework.

7. ### ELECTRONERD Senior Member

May 26, 2009
1,146
16
Hey sweet! Building power supplies is very good! I learned about power supplies with my scope.

I got my scope for free. It is an incredibly old oscilloscope...but hey...it works. So don't be jealous; in fact, never be jealous. Someone else will always eventually have something better than you or I have. I would advise you to get a MUCH newer one... Since no one wanted the old junker, and since no one knew if it worked, i got it. I hope to get a nicer one someday. I recommend that you do get a power supply, especially if your in college. You could find a cheap one online at ebay or craigslist. You might be able to find one at a amateur radio swapfest (I'm not sure where you live though). You can check when upcoming swapfests will happen at http://www.arrl.org/hamfests.html They are either called swapfests or hamfests. Yeah, I was 0.3V off of the voltage drop, eh? Ah well...I guess I'm dreaming of the future...

May 26, 2009
1,146
16
9. ### rjenkins AAC Fanatic!

Nov 6, 2005
1,015
69
How smooth the output is from a DC generator (or motor as a generator) would largely depend on the number of segments on the commutator.

You'd only get the stronly pulsed 'rectified AC' waveform with an extremely basic two pole motor with a two segment commutator.

Toy motors often have three or five segments, you would get a rather smoother output with dips between the peaks.

Going up to things like car windscreen wiper motors, you probably have nine or more segments, the ripple will be quite low and by the time you get up to big industrial motors with with a hundred or more segments you would have a job telling it was not from a regulated DC supply.

The current a motor draws when running depends on the difference between the back EMF it is generating and the voltage it's supplied from. (Plus it's internal resistance, cable resistance and a few other things).

If the back EMF is not 'smooth', the current drawn will vary and in turn the torque out will vary. This does not matter much in toys, but as you get to better quality and higher power equipment it becomes critical for battery life in small items and power consumption, vibration and heat in industrial gear.

ps. a Wheatstone bridge and a Bridge Rectifier are different animals..
http://en.wikipedia.org/wiki/Bridge_circuit

10. ### ELECTRONERD Senior Member

May 26, 2009
1,146
16
Yep, I forgot momentarily about the difference. Sorry about that.

11. ### Amberwolf Member

May 2, 2008
28
2
Rather than getting the hand-cranked generator from a radio, you could just use a bicycle generator. Although it would require creating the hand-crank and gearing, if you are intending to use it as a hand-crank device.

But if you are going to drive it from some other source with a higher RPM, you might not need any of that anyway.

Mar 24, 2008
20,772
2,540
13. ### count_volta Thread Starter Active Member

Feb 4, 2009
435
24

This is hilarious. You force yourself to bike faster by knowing that if you do, the fan will spin faster and you will cool down more. Its equivalent of hanging a piece of fruit in front of a hamster on a wheel.

14. ### ELECTRONERD Senior Member

May 26, 2009
1,146
16
That's pretty good! I know of someone who hooked up a mini TV instead of the fan.

15. ### Amberwolf Member

May 2, 2008
28
2
Yeah, if you don't pedal fast enough, it changes channels to an infomercial.