USING AN AC motor as an alternator.

He needs a magnetic field. I don't think such a motor has permanent -- or electro- -- magnets.

you can do it, but the performance sucks.

I've made generators using induction motors attached to engines, just to see it work.

they act really weird, and work at some weird RPM nowhere near the value of the faceplate produce around their rated input voltage at about 60Hz.

There's usually enough residual magnetization in the rotor or laminations to self excite, but not always. Runnign 12 volts across the windings from my engines starter battery for a second would always work when they didn't.

The last test with a 3/4 HP 1725 RPM motor with about 200uF of caps across the power cord that would usually run it seemed too be able to run about

400 watts of light bulbs, but only when run at 2200 to 2800 RPM. The output frequency did stick to about 60Hz, somehow. The output voltage regulation wasn't that bad, and was probably "regulated" by some sort of saturation or leakage in the motor design itself.

I can't explain how the whole thing worked, but trying to drive an inductive load would make the thing stop acting as a generator, and the engine would then rev up as the load seemed to go away.

Also, if you're dealing with single phase motor used as generators this way with 1:1 speed coupling to the engine, you get a very strange flicker or beating to to output voltage. I could see light bulbs pulsate in intensity, which was horribly obnoxious.

then I realized that it was because I was using a 4 cycle engine, so the power stroke was brighter than the rest of them. a 15Hz flicker seemed about right. It still doesn't really jive with what the tachometer said for 2200 to 2800 RPM. I was using a wrap-around the ingnition cable unit from Sendec.

Anyways, it was an interesting project.

The secret to a successful engineering project is the ability to generate many ideas and QUICKLY eliminate the bad ones. Most ideas are so incredibly bad that you don't need a detailed analysis to reject them. Gross approximations will do just fine.

So, let's head down that path. Stop when you get an answer you can't live with.

Assume that a 40hp motor can produce 40hp worth of electricity. That's about 30kW. Will 30kW do the job? Is it in the right format? Is 3-phase power what you want? Remember that you may need much more than 30kW peak to start a large machine. If the diesel dies or you have a brown-out under the peak load, things get ugly.

Pick a round number. Say that it takes 90kW worth of diesel fuel to produce 30kW of electricity. I don't know what that works out to in dollars per kilowatt-hour, but does that number compare favorably with your other electrical energy alternatives? The good news is that, if you can capture it and it's at a temperature you can use, you have 60kW of heat for the factory, summer and winter.

By this point, it should be obvious that the generator costs way more than power from the grid. I think my estimates were very generous.

So, if you can stand the cost and the power is in the format you can use, you get to determine exactly how the motor is wound and start trying to separate out the field and power it.

I'm betting you don't have to do all that work to reject the idea.

It seems alot more efficient when the grid is down.

J

What *kind* of motor? If it's a typical squirrel-cage induction motor, your friend is SOL.

--
Rich Webb     Norfolk, VA

The off-grid hydroelectric guys do this often. You have to put large capacitor banks across the 3-phase leads to provide the self-exciting current to build up the rotor field. You have to run the motor above synchronous speed to get 60 Hz out, and the frequency regulation will be pretty poor, and load-dependent. You will likely have to start the alternator with no load except for the capacitor bank, and then connect the load. Starting a large motor may cause a collapse of the excitation and it won't restart until the load is removed. Voltage regulation may not be real good, either, but can be adjusted by adding/removing caps for the cap bank.

So, it DOES work, but it is not like a commercial generator with electronic frequency and voltage regulation.

There was a guy who had river rights on the Colorado River, and used a huge resistor bank to regulate the alternator, both for voltage and frequency. Of course, wasted load didn't bother him a bit, as he wasn't feeding it Diesel fuel.

Jon

this exactly matches what I came across, with a single phase induction motor. do you know why such a great speed overrun is needed to hit 60Hz and why the motors seems to try to output only it's rated Hz and input voltage?

My generator project was going to have a marine alternator to make 24 volts, but somehow I ordred the wrong type (12 volts) and shelved the project after my bank of batteried from a charger mishap.

and why the motors seems to try to output only it's rated Hz and input voltage? "

The field actually rotates in the rotor. It does the same thing when it is a motor, when you load it and it pulls more current, you are fighting the m agnetic field's desire to not rotate in the rotor. At 60 Hz it should be 1,

800 RPM but it isn't. It's 1,750. That is actually nominal but it is accept ed as the normal loss for motors that run machines rather then say, clocks.

This is the same shit only opposite. It's also a hell of alot less eficient when not made for the purpose. In a way it's like running an internal comb ustion engine backwards, just not as bad.

J

Rather THAN say

A typical AC induction motor only develops torque at less than synchronous speed. This speed is called the "Slip Speed" . The slip speed is lower the more the motor is loaded. The motor will draw more current the more it is loaded until it is overloaded when it stops developing torque and stops. It will then be drawing maximum current. The same motor used as a generator needs to be spun faster than synchronous speed in order for there to be slip. The more the generator is loaded the faster it must spin until it is overloaded and it stops generating. Eric

The rotor field slips, just as when run as an induction motor. So, if

60 Hz on a 2-pole motor gives 1725 RPM, slip equals 75 RPM. You'd have to provide 1800 + 75 RPM to get about 60 Hz at the same output current. I'm not sure a single-phase motor works well as an induction alternator. The rotor field may collapse between poles. Possibly a capacitor-run motor with suitable caps would work, though.

Jon

Not at all. But exciting it and regulating the output is a bit tricky. Common as wind power systems.

?-)