# Magnetic generator power rating

1. Jun 11, 2012

### barendfaber

Hi there,

I have recently downloaded this document on magnetic generators. I would just like to confirm a most probably very standard thing. One of the generators discussed is a 2.5 kW at approx 250 rpm. To confirm, is the 2.5 kW delivered per minute or per second at the rated speed?

#### Attached Files:

• ###### Permanent magnet generator design - 2kW Wind turbine.pdf
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Last edited: Jun 11, 2012
2. Jun 11, 2012

### Staff: Mentor

KW is a unit of power, which is energy per unit time. In this case, kJ/sec.

3. Jun 11, 2012

### barendfaber

Thank you! So this means the generator supplies 2.5 kW per second, over 1 hour this is 2500 watts x 3600 seconds = 9,000,000 J or 9 MJ? Can I also relate this to 9,000,000 watts?

4. Jun 11, 2012

### Staff: Mentor

No, you can't say kW per second. That would be redundant/wrong since the seconds are already contained in the definition of the unit.

Your math is correct, though typically electrical energy is expressed in kilowatt-hours, not joules.

And the way 9 MJ relates to 9 MW should be obvious from the calculation you did: 2.5 kw for an hour = 9MW for a second.

5. Jun 11, 2012

### barendfaber

Thanks for this, and sorry for all the questions. I appreciate your help.

I am a bit confused here with the last comment. If I run the generator for 1 hour (3600 seconds), should it not be 9MW for the 1 hour in total, instead of 9 MW for a second? Or am I missing something here? I am trying to learn this on my own, so if I sound stupid, could you point me in the right direction?

Lets say I run my generator for just 1 second, that gives me 2.5 kW. Now I want to use this power to lift a 10 kg object 1 meter into the air over 1 second. If my lift cost me (10 kg x 1 meter x 9.8 ms2 gravity) 98 J or W, am I correct in assuming that I will have 2.402 kW left?

Last edited: Jun 11, 2012
6. Jun 11, 2012

### Staff: Mentor

You are continuing to make the same mistake of confusing power and energy by inventing the nonexistent unit of kw/s. Kw is just kw. 2.5 kw for a second? 2.5 kw. For an hour? Still 2.5 kw. You need to pay very close attention to what the units are telling you.

Perhaps until you get it straight, you should do all the calculations in terms of joules.

7. Jun 11, 2012

### barendfaber

Do apologise, can see how I am getting confused. So to confirm, the generator is rated at 2.5 kW, this is power. If this generator runs for 1 hour, I will have 2.5 kWh, or 2.5 kW x 3600 seconds as calculated above, this is energy? This gives me energy over an hour. What happens if I run my generator for only 15 seconds, how much energy will I get?

8. Jun 11, 2012

### Staff: Mentor

Correct.
2.5 kJ/sec * 15 sec = ....?

9. Jun 12, 2012

### barendfaber

So energy over 15 seconds is 22.5 kJ. If I use some of that energy to lift the 10 kg object 1 meter in the air over 1 second (98 J), I will have 22.402 kJ left to use for something else.

I can start a new thread, but wanted to ask if you are good with magnetic generators? I now have the basics ready, and would like to find out the strength of the magnets and the gauge of wire used in the generator described in the pdf document. Can you help with that?