# How to specify an electric alternator...

1. Aug 5, 2016

### Rakesh bhatoa

how we can specify an alternator (which converts rotational energy into electricity) .
how we can calculate the charge given out by alternator in its one revolution .

2. Aug 5, 2016

### Staff: Mentor

Most generators are efficient, so the easiest approximate calculation is electric power out = mechanical power in.

Charge is not power. Electric power is measured in watts and calculated from voltage times current. Mechanical power is calculated from torque times speed.

3. Aug 5, 2016

### Rakesh bhatoa

If we gives the supply of alternator ( which should fitted in a bike) to a heating coil ,then is it possible to produce 250 joule of heat in 4 revolution of alternator

4. Aug 5, 2016

### Staff: Mentor

One watt of power is one joule per second. So if your bike pedal rotates once per second, 4 seconds in 4 revolutions, then 250 joules/4 seconds is 62.5 watts. Yes that is possible. See https://en.m.wikipedia.org/wiki/Bicycle_performance

But the alternator is probably connected to the bicycle by a belt or chain so that the alternator makes several revolutions per one bike pedal revolution.

5. Aug 8, 2016

### Rakesh bhatoa

Thanks Sir for reply , I have one another question ,if i use 24 volt 45 ampere alternator then what will be the resistance should the heating coil be provided with , so that it should produce 250 joule of heat in four revolution of alternator, please show me the calculations also sir.

6. Aug 8, 2016

### Staff: Mentor

62.5 watts / 24 volts = 2.6 amps.

24 volts / 2.6 amps = 9.2 Ohms resistance

But I suspect that my answers are not helpful because you asked the wrong questions. Please tell us what you are trying to accomplish. We will try to help. For example, why 250 joules in 4 revolutions?

7. Aug 9, 2016

### Rakesh bhatoa

Sir, i want to supply 250 joule of heat to the air (at constant volume) by electric means, in maximum 4 revolution of an alternator.This is the part of my project about which i m just thinking

8. Aug 9, 2016

### Rakesh bhatoa

Sir,why did you say "my answers are not helpful for you" and what is the meaning of "45 ampere " in alternator specification(in my question)?

9. Aug 9, 2016

### Staff: Mentor

45 amperes is the maximum current the alternator can supply. You don't need that much.

250 joules in 4 revolutions is a strange requirement. Most people need either
1. specific energy (a joule is a measure of energy) to do something like charge a battery. The number of revolutions is not relevant.
2. Or a specific power (measured in joules per second) to do something like supply a light. Pedaling continues as long as power is needed.
Can you explain why 250 joules in 4 revolutions is your requirement?

10. Aug 10, 2016

### Rakesh bhatoa

sir,i m trying to prepare an engine that will run on air only..

11. Aug 10, 2016

### Rakesh bhatoa

sir, Is there an effect of time of completion of one revolution on heat energy produced in one revolution.Example ,Is the one revoluiton taking place in 4 sec will give the same heat energy that is provided in one revolution which is taking place in half ( .5 ) second ?

12. Aug 10, 2016

### Averagesupernova

13. Aug 11, 2016

### Rakesh bhatoa

Sir read the patents of guy negre on compressed air engine

14. Aug 11, 2016

### Rakesh bhatoa

I m using guy negre's concept

15. Aug 11, 2016

### Rakesh bhatoa

16. Aug 11, 2016

### Staff: Mentor

You need only a small fraction of the rated output. Minimum speed for many alternators is about 1000 RPM. So, yes there is a time effect, one revolution should be completed in less than 0.06 seconds.

Once again, your requirements and your numbers sound unusual and far from reality. The quality of the answers we are able to give you are proportional to the information you are willing to share with us.

Below is a sample alternator performance curve. Note that the key input parameter is not revolutions but revolutions per second, and the key output parameter is not joules but joules per second (watts=joules per second=volts*amps).

17. Aug 11, 2016

### Rakesh bhatoa

Thnx sir for helping me to understand alternators

18. Aug 12, 2016

### Rakesh bhatoa

sir, the project i m thinking is an engine that works on air only .in this engine the air is sucked in the down stroke of the piston ,the air is compressod in the later stroke (air becomes at high pressure),now this pressurised air is supplied to a pre chamber ,the pre chamber will then fully closed , the heat is supplied to the air at the constant volume in the pre chamber ,thus the air is further pressurised upto desired pressure,then this high pressurised air will then supplied to the main engine which give us the rotaional power which is used to rotate the tyres of the vehicle ,Now the power of an engine is also supplied to an alternator whose power is supplied to heating element which is fitted in pre chamber to heat the air ,the gear ratio b\w engine and the alternator is set such that in one revolution of engine there are four revolutions of alternator. I required the heat upto 250 joules in the pre chamber in one revolution of engine (or in four revolution of an alternator).Sir this is the thing i want to do........so sir just help me that ,how can i produce this 250 joules of heat (in four revolution of an pertucular aternator)

19. Aug 12, 2016

### Rakesh bhatoa

sir, in graphs that you send what is the meaning of 12volt

20. Aug 12, 2016

### Staff: Mentor

Oh dear, I'm afraid that your project resembles "perpetual motion or free energy ideas", because it would violate the second law of thermodynamics. Those topics are forbidden here on Physics Forums, and the mentors may close this thread.

21. Aug 12, 2016

### Averagesupernova

Somehow I had a feeling that is where this was going. Look into a sterling engine. These are fairly efficient external combustion heat engines. The heat source does not matter. However, you will NEVER get more heat energy out of a generator running a heating element than what the engine requires to turn said generator. Look into these engines since there is a lot of information out there in real actual numbers about their efficiency that you can apply to real actual numbers about the efficiency of alternators. From here you will see that what you are trying to do is not possible. I am surprised this thread was not closed before I posted this.

22. Aug 12, 2016

### RonL

I for one think you are looking in the correct area of compressed air, but I feel you need to expand your knowledge base about "heat from electric" best wishes in your thoughts. Lots of study material on the net, wiki might be the best start and as you learn, go to more detailed sites.

23. Aug 12, 2016

### Rakesh bhatoa

ok sir , but what is meant 12 volt in graph?

24. Aug 12, 2016

### Averagesupernova

It means exactly what it says. 12 volts. What good is a specification sheet if there is no reference to voltage? According to the graph, when the shaft speed hits 5000 RPM the alternator is able to supply about 90 amps at 12 volts. This is about 1080 watts. If the voltage were specified at 5 volts then the output would only be about 450 watts. The voltage regulator that exists in the alternator is responsible for maintaining the voltage at around 12 volts. Typically an automotive alternator regulates at about 14.25 volts.

25. Aug 13, 2016

### Rakesh bhatoa

Sir ,it means voltage varries as the rpm varries (1000 to 5000) or alternator does not supply the current at single voltage (12 v) with varrying rpm?