Generators #5: The Permanent Magnet Rotor

I glued a couple magnets onto a wooden dowel with a screw through it to make a permanent magnet rotor. The magnets aren't particularly strong, and my first attempt had the poles opposite one another: that is, the north on one magnet mated with the south on the other, so the field was even weaker. (This all fit into the field coil from the Dirt Devil motor.) At 3,100 rpm, I could output 53 Hz AC at 2.7 volts open, 2.1 volts into 51 ohms (0.086 watts) and 0.7 volts into 3 ohms (0.16 watts).

While this isn't particularly promising, I purchased a pack of 50 0.25" diameter neodymium rare-earth magnets from K&J Magnetics. They're supposedly rated at N45 which I guess is almost as strong as you can get.

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.