# AC household generators

1. Dec 17, 2015

### deckart

I'm only familiar with a gas powered generator that I use at home when the power is out. But I'm designing a contraption that will be able to spin a generator with considerable torque. But the RPM will not be consistent. I can limit the upper end but it will often be below max RPM.

Am I correct in assuming that lower RPM will affect the 60 Hz frequency generation? If so, are there ways to maintain 60 Hz with a varying input RPM?

2. Dec 17, 2015

### Staff: Mentor

Generate a DC voltage and use an inverter to make the appropriate 60Hz AC Mains output voltage.

3. Dec 17, 2015

### deckart

Aha, that's the way to do it! Thanks.

4. Dec 18, 2015

### CWatters

How will your "contraption" regulate it's output power?

My understanding is that when the load on a generator increases (because someone turned on a light) they open the throttle to increase power/torque and maintain rpm. If your contraption can't do this then perhaps consider using a battery to decouple the load from the generator?

5. Dec 18, 2015

### deckart

From what I understand, excess power can be put back into the grid. This is one of the benefits of household solar. If the home owner isn't using the excess his meter actually begins to go backwards.

6. Dec 18, 2015

### dlgoff

I wouldn't consider selling power back due to the equipment cost. As it is, here in the US, you'll need a Automatic Transfer Switch (ATS).

Last edited by a moderator: May 7, 2017
7. Dec 19, 2015

### deckart

"But why would utilities go through the trouble of purchasing this energy?
Well, because the government says so. In the U.S., the Public Utility Regulatory Policy Act (PURPA)dictates that electric utilities on the traditional power grid must purchase the excess electricity that renewable energy systems generate. It's a way of encouraging renewable energy production without requiring utilities to invest in expensive renewable systems themselves." http://science.howstuffworks.com/environmental/energy/sell-electricity-back-grid.htm

I don't believe I need to purchase an ATS.

8. Dec 19, 2015

### dlgoff

I don't think you can get around this. Yes utilities are required to purchase excess when the equipment is properly attached to the grid; that and they're not going to pay you at the same rate as they are selling it for and usually for quite a bit less than selling price.

9. Dec 19, 2015

### deckart

I understand that you get paid at a lesser rate, but there are also rebates, tax incentives, etc.
As far as the hardware required, I'm asking a buddy who is an electrical contractor. He installs solar array systems and knows details. I'll post what he has to say on it.

10. Dec 19, 2015

Caution: Be SURE to check the 'fine print' in your local power companies regulations or rules or whatnot regarding what you actually have to do to sell power back to them. I looked into this once but decided not to go this route because not only did they pay very little per kWh, they also charged a $75 monthly fee for this service. I'd have had to generate several times my homes power consumption just to break even with that fee. 11. Dec 19, 2015 ### deckart Ok, so if I sell power back to the grid, it could cost me more than if I didn't? 12. Dec 19, 2015 ### Drakkith ### Staff: Mentor It's possible, yes. Like I said, check with your power company. 13. Dec 19, 2015 ### CWatters That's certainly possible here in the UK. The meter doesn't go backwards (most are digital these days) you either have a separate export meter or they assume that 50% of what you generate will be exported. 14. Dec 19, 2015 ### Nugatory ### Staff: Mentor The ATS requirement is a bit of a red herring here. An ATS automatically and completely disconnects your house from the grid whenever your generator is working, so it's not part of the solution if you're planning to return excess power to the grid. To connect your generator to the grid, you need the power conditioning equipment to ensure that not only are you generating at 60 Hz (North America), but also that it is in phase (the peaks and valleys of your 60 Hz power match up with those of the grid) and the waveform is at least vaguely sinusoidal and spike-free (the output of some light-duty farm/home/shop generators can be amazingly dirty). There also has to be an automatic lockout so that the power company can shut you down whenever they need to work on the power lines in your neighborhood. This equipment is becoming increasingly available and affordable when you're starting with the DC output of photovoltaic cells, but I'm not sure that there's a commercially available solution for a repurposed home generator, let alone one that would be acceptable to the local power company. 15. Dec 19, 2015 ### russ_watters ### Staff: Mentor It might. Have you calculated how much it will cost for you to generate your own electricity? It shouldn't be hard to find out the price per kWh the utility will pay you and compare with what it is costing you to make it. There's a reason people don't do this very often and I'm not sure you understand what you are getting yourself into (or even what it is that you are going to be doing). In general though, when you buy electricity, you are paying for three separate things: the generation at the plant, the transmission from the plant to the local grid and the distribution of the power from the local grid to your house. Each of those is about a third of the cost, so if, for example, you pay 15 cents for your electricity, you'd only be able to sell it back for about 5 cents because the only thing you are doing for the utility is the generation. 16. Dec 19, 2015 ### anorlunda Phrasing it that way makes it sound like there must be a back channel communication from the utility to your house. That's not reliable enough for safety purposes. It also sounds like disconnect is used only when the utility truck is in your neighborhood. That's insufficient In addition to protecting utility workers you must not attempt to power your whole neighborhood, because that could put neighbors at risk. The same disconnect must also protect you from galvanic DC currents that could fry all your stuff when an unbalanced fault exists. That means ground fault detection on the customer side. For safety purposes, the disconnect automation must use only local information, and it must act instantaneously when the grid point you connect to loses voltage. Only devices certified to meet applicable standards may be used to interconnect a home to the grid. 17. Dec 19, 2015 ### deckart It may cost an excess of$20-30k USD to get this project up fired up. And that's just a very early estimation. It's a prototype and there will be a lot of refinements to be worked out after it is already up and running. I don't expect it to pay for itself, actually, if it does it will likely be more than a decade of use before it does. But, it will be capable of generating far more power than your typical home based solar array.

The generator will be DC and convert to AC using an appropriate inverter.

I'm researching the details with my local utility company and there is a limit to the amount of power you are allowed to put back on the grid. But you do get a good rate from them: "If your system generates more electricity than you use, the excess power delivered to the PUD’s distribution system is credited to your account at the same rate class at which you would buy power from the PUD. Other requirements of the rate schedule apply, including the daily system charge. Unused credits carry over each month until April 30 of each year. If any net metering credits remain, they will be granted to the PUD as described in Washington State Law. (RCW 80.60)"

18. Dec 19, 2015

### deckart

However, at this utility company you only receive a credit. No money to be made from this power company out of a home based generator. But there are other incentives.

Last edited: Dec 19, 2015
19. Dec 19, 2015

### dlgoff

My thought concerning the need for the ATS was: During power outages, times when you start the generator, you certainly don't want it attached to the grid. This is what's dangerous for anyone working to restore the power.

20. Dec 19, 2015

### deckart

It's required and would be part of the system.
• A disconnect switch with a lockout function, allowing PUD 3 linemen to disconnect your solar array from the electric grid so that they can work on utility lines without fear of energy back-feeding from your system into the grid.
The electric company has complete access to shut it down without notice.