# AC Rectification and Hertz

Discussion in 'The Projects Forum' started by olet95, Apr 26, 2006.

1. ### olet95 Thread Starter New Member

Apr 26, 2006
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I am working on a project and need some help with how to provide a means of adjusting the hertz frequency of the output. The unit uses a transformer secondary winding to supply voltage to a bridge rectifier (full wave) this DC voltage then powers a rotor which induces voltage into a stator. I have 128 vollts of AC but at about 97 hertz how can I get to around 60 ?

2. ### Papabravo Expert

Feb 24, 2006
10,338
1,850
A bridge rectifier is by definition full wave.

The power company controls the frequency of the AC by controlling the speed of the generator. Don't you need to slow the rotor down. I don't know if droping the DC voltage will do that.

3. ### olet95 Thread Starter New Member

Apr 26, 2006
2
0

If I slow the rotor to get anywhere near the 60 hertz my AC voltage drops to an unacceptable level. Do you know anything about tapped transformers ? This is how the rectified voltage is supposed to be changed to the rotor. All taps on the transformer produce about the same results.

4. ### windoze killa AAC Fanatic!

Feb 23, 2006
605
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Could you supply a circuit diagram of this. I am having a little trouble picturing what you are talking about.

5. ### Papabravo Expert

Feb 24, 2006
10,338
1,850
Yes, I know something about tapped transformers. I know that you will have a difficult time changing the frequency of the AC output with a tapped transformer.

6. ### n9352527 AAC Fanatic!

Oct 14, 2005
1,198
4
Let me restate what you are trying to do first so we are all on the same page. You have a transformer connected to mains and you rectified the secondary voltage to a DC voltage. Then, you use this rectified DC to power a generator (rotor and stator in your terms) and generate an AC voltage. You get 128VAC at the output of the generator with frequency of 97Hz. If you drop the rectified DC voltage to slow the generator down then the output voltage dropped to an unacceptable level. Is this right?

This is a symptom of using incorrectly specified generator and load.

Is this 128VAC an unloaded voltage?

The first thing you should do in a system like this is to fix the generator speed so that you get the correct frequency. There are two easy ways to do this, the first is to control the DC voltage so that at a _given_ output load and voltage the generator produces 60Hz. The second way is to control the amount of load drawn at the output at the desired voltage and a _constant_ DC drive voltage so that the generator produces 60Hz.

If your load is fairly constant, then you just need to adjust the DC drive once and you'd get somewhat close to 60Hz. It wouldn't be exact but there's no problem there unless your application needs it to be exact.

If your load is not constant, then basically your have a bigger problem. You need to dynamically control, either:

1. The DC drive, maybe by using a variable transformer instead of a fixed ratio one. This is the easiest method, you need a control circuit that is able to detect the output load voltage and varies the transformer ratio. The frequency would follow from that.

2. The load, so that the total amount of load is somewhat constant. The way to do this is to use banks of dummy loads which you connect or disconnect accordingly. For example if your maximum load is 1000W and you have banks of nine 100W dummy loads, one 50W dummy loads and two 25W dummy loads and the actual load is 220W, you'd connect seven 100W dummies, one 50W dummy and one 25W dummy to give you a total of 995W. Close enough to 1000W.

This is not the easiest method, but particularly useful if the actual power that turns the generator is difficult to control or varies, for example this was the method I used for easy to install small hydrogenerators for remote villages. One trick is to sense both the voltage and the frequency to control the total amount of load so that variation in water flow (due to installation differences or stream water debit) does not affect the output voltage and frequency that much, up to a point.

3. There are a third method, you could put a mechanical brake on the shaft as a dummy load. Not really recommended though unless you are able to maintain the brake frequently.