Author Topic: Efficiency and practicality: 120v AC generator vs. 24v DC alternator?  (Read 1666 times)

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Hey Everyone,

I am tinkering with a small scale hydro project. As I am working through the various aspects of the project I had to ask myself: "Self, once you generate this power, how are you going to use it?"

And Project Scope Creep began....

Well, I can charge batteries and run my (limited) DC loads. As I expect that I may be producing as much as 12kwh daily I will never use that much running my 12v loads.

How about an inverter connected to the batteries so I can run some 120v AC loads as well? Sounds good!

Wait!! We have Net Metering... I need to get a grid tie inverter. Settled.

As I was considering this the power went out. Again.... (We have spent over a week total in the last three months with no utility power) So I pulled on my boots and went out and flipped the breakers on all of my high current loads, fueled & started my generator and then hit the transfer switch. About 5 minutes later the breaker on the generator tripped... Damn! What did I forget to turn off? I reset the breaker and started my quest for the offending load. Turned out to be my Wife had turned the microwave oven on.

This is becoming a routine. My generator is not large enough to support my minimum loads so I have been looking for cost effective alternatives. I found a deal on a new surplus water cooled 12hp diesel engine with an 1800 rpm governor. I thought that I might couple that with a 120v AC generator head and build myself a proper backup power system. I started designing a transfer panel and the thought hit me:

Perhaps instead of running a 120v AC generator and using a 120v to 24v battery charger to maintain my batteries why not run a 24v alternator to charge the batteries and a large enough grid tie inverter to run my basic loads?

What is the difference in efficiency?

I know that a typical automotive alternator is roughly 50-55% efficient and the inverters are in the 90% range. Assuming that I am producing and consuming all available power. No batteries in the equation. That would yield around 45% system efficiency - Mechanical to electrical.

I have not been able to find similar information regarding the 120v AC generator heads.

What do you all think?


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Re: Efficiency and practicality: 120v AC generator vs. 24v DC alternator?
« Reply #1 on: February 14, 2016, 08:13:05 PM »
One of the things I have not gotten around to doing, is measuring the efficiency of a 5kw generator head, its brushed, no regulator at all.
I believe they achieve regulation through saturation of the rotor's core.

Anyhow I suspect they are about 75% efficient at full load because it has a 9hp engine, @9x746watts/5000.

I achieved about 75% efficiency from a 2 hp induction motor that I turned into a synchronous generator, generating about a kilowatt worth of power.

far as i can tell you should be able to exceed the nameplate efficiency of an induction motor by converting it to synchronous, but the real gain is from reducing the volts per hz, reducing core loses.

at nameplate volts per hz my 2hp induction motor consumed something like 240 watts from the outlet, running backwards as a synchronous motor at unity power factor.

I suspect a 5KW generator head would be on the order of 1 hp of losses at no load, but i hope its not that much.

if you want to make a 24v generator, i would consider rewinding an ac generator head for 3 phase, 24vac. its only 24 volts so you won't need that many turns per coil which makes it much easier. then rectify to dc, and reduce the rpm of the generator to charge the batteries at something like 2/3rds nominal rpm, 1200 or 2400 rpm.


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Re: Efficiency and practicality: 120v AC generator vs. 24v DC alternator?
« Reply #2 on: February 19, 2016, 03:42:56 PM »
We didn't have electricity for 3 weeks afte Hurricane Wilma. Found out quickly that the smallest generator that would run a common washing machine was 5 hp. Cheap washing machines using the grid simply brute force spin the tube of water when it's full. That requires a lot of sustained power. So I went on line and bought an electronic washer from WalMart for about $450. It's apartment sized and has a dc stepping motor. To spin it bumps and coasts, bumps and coasts. That washer will run from a cheap 600 watt inverter cabled to my car.

It has about 3/4 the capacity of the other washer, but I don't have to run a generator to use it.

My fridge is a 12v Engel. They just keep getting more expensive, but with a compressor and freon and a thermostat keep meat, milk and cheese fresh as well as a much larger upright that runs on the grid.