Generators #4: A New Hope — AC generators

I figure I can easily convert a DC motor to an AC motor by tying one side of the rotor windings to both sides of the brush assembly rotor thing, and the other side to the shaft. That way I can step up the output voltage to a useable level.

I went to the basement and grabbed all the motors. I decided to try converting the drill motor since it generated the highest current (next to the original) and it's the largest. I decided against the air pump motor because, although it can source a lot of current, the output voltage is so low it's almost not worth it … then again, that's based on running it up to 3,100 RPM on the drill press, not the huge speeds from spinning on the rubber wheel.

Anyway, I took apart the gold-colored motor and rewired it. I started by cutting a groove around the commutator and soldering the original poles together so I ended up with a 2-pole slip-ring instead of a 3-pole commutator. I ground down the brushes so each would contact one pole of the slip-ring. I disconnected the three poles, undid the connection that tied the windings in a loop and connected each end to the slip-ring contacts.

When I reassembled it and tested it, I roughly zero volts out. I had successfully demonstrated that the motor was wired efficiently and Kirchoff's laws still apply: of course there was no current flowing at that point — otherwise, when run as a motor, it would short-out. Dummy.

I took it apart again and wired it for 3-phase. Two phases were wired to the brush output, and the third was soldered to the shaft bushing so I could use the case as the third phase. It sort-of worked, but then the wire I was using to tie the third leg to the shaft came loose and wedged between the rotor and the magnets.

I reassembled the motor with a stiffer wire. I found that the brushes would bounce on the imperfect rings I had so they wouldn't transfer the power. I could get up to about 1 amp of output on any one coil and up to 1.5 volts or so. I cleaned up the rings and such but couldn't get them round enough with the tools I had so I gave up on it.

However, I also had an AC brush motor from a Dirt Devil hand-held vacuum and figured I could make a rotor with permanent magnets to induce current in the outer coil. Finally a break in all this: the rotor shaft was 5/8" which is the same size (well, 8mm) as the the bearings I bought for the Savonius rotor. Even better, the magnets in the gold-colored permanent magnet motor are very close to the right size.

Generators #3: Experimentation

I tested some of the motors at high-speed to see how much they'd generate and to see if any would be suitable. Note that the motor speed was dictated by the shaft diameter; the drill motor, 6V motor, and original motor had larger shafts and ran slower.

Motor

open
voltage

voltage
@ 3 ohms

power
@ 3 ohms

drill

> 4.7

4.6

7.0 watts

gold

> 4.5

4.6

7.0 watts

silver

> 3.0

3.7

4.6 watts

6V motor

> 6.5

4.6

7.0 watts

original

> 14.0

7.5

18.8 watts

For the stepper motor, at 1,100 rpm, using both phases through a bridge rectifier, it would source 0.63V into 3 ohms or 0.13 watts. One phase through a 5:1 step-up transformer frustratingly yielded the same results: 0.6V. Both phases in series yielded 0.67 volts, and at 1,720 RPM, 0.8 volts. The open voltage on the transformer secondary was 11.75 volts.

I also took apart one of the pumps that I used to use for the air horns in the car and got its motor separated. It could source 3 volts into 3 ohms and had a short-circuit current of 5 amps at 3,100 RPM.

Generators #2: Stepper motors suck as generators

I decided to go buy a motor so I went to Glenwood Sales (549 Hague St.) and browsed for a while before eventually settled on a 30 V stepper motor with a 48 ohm coil. It apparently has 4 phases, and each phase would ordinarily consume 0.625 A or 18.75 watts.

The stepper motor could generate as much as 50 volts open-circuit (it's wired with two coils center-tapped.) Through a 51-ohm load I found it could get as high as 5 volts, and through a 280-ohm load, 20 volts. The good news is it's pretty linear from about 1,000 RPM through 3,000. The bad news is those two ratings yield only 0.5 watts and 1.4 watts respectively.

Using a full-wave bridge on each side and tying the outputs together and running at 3,100 RPM, I could get 9 volts into 51 ohms and 34 volts into 280 ohms: 1.6 watts and 4.1 watts respectively. Open, it would generate 90 volts. Indeed, none of the motors would source more than a half a volt at 3,100 RPM; one that I hadn't tested would generate 0.5 V and 1.5 amps, so at least that's promising.

Starting from the original 3-pole rotor.

The third phase is connected to a wire ...

... that is soldered to the bushing, electrically tying it to the motor case.

Method of mounting the rotor bearings; Generators #1: These generators suck

I found that I have some 1" thin-walled tubing from an exercise machine that has a nearly 22mm diameter — the outside diameter of the bearings I bought. I figure I can weld a piece of tubing to the ends of the windmill frame then cut a slot and use a seat-post clamp to grip the bearing.

I also tested the motors I had selected as candidates for a 10-watt generator. None of them performed better than the motor I had originally selected.

Based on my former calculations, the rotors would only run at 300 RPM in a 10 MPH wind, so a 7x gear-up would only yield 2,100 RPM which wasn't enough to generate any appreciable power. To get any power out, I had to run the baby carriage wheel in the drill press at 620 RPM with a 0.33" gear, so the ratio to the 7 3/4" wheel (23.5x) means I ran the motor at 14,500 RPM. If this were 20 mile-per-hour wind, then a 70 mile-per-hour wind would overcrank the motor to over 50,000 RPM and probably blow it up. I'll have to see how slow I can get the motor to run and still get decent power. Well, I know that 3,100 RPM is not enough: the motor, when hitched directly to the drill press, would only generate about 3.4 volts open and 1.9 volts into 3 ohms (only about 1 watt.)

Ok, so let's say I limit the motor to 15,000 RPM at maximum wind speed of 70 MPH, or 2,100 RPM. The ratio, therefore, is 7:1 … bummer. I think I'm going to need to get a different motor. If I use 21,000 RPM as the cap, I can get to a ratio of 10:1, so in a 10 MPH wind, the rotors would only run at 300 RPM driving the motor at 3,000 RPM which isn't enough.