Converters#2

[WXYZCIENCE]

This picture is of a Brush Type converter. The unit output is a 60 hz square wave and is powered by 24volts dc. Its efficiency is around 95% at 100w and 85% at 300w. Over 300w the efficiency drops off because of the brush and other related losses.

I was amazed to see the output the first time it was run. The 24v servo motor is powered to only half 12v as you see, just a little over 60hz. This next picture is the servo motor.

The brush unit was selected because of number of brushes and commutator segments. A close up of the commutator is pictured here.

We short 2 segments together then using a jumper wire connect to 2 other segment 90 deg. out of phase. The next 2 segments are left open. There are two sections wired the same. Operation:>> 1. We apply power to the servo motor. 2 After the motor is in motion we apply power to one set of brushes, the commutator switches the dc and this is fed to the windings of the transformer through the second set of brushes. Most 12 volt battery chargers are wound with 2 low side windings. We chose to series the windings and drive the unit with 24v dc. In this way we reduced the current through the brushes. The AC output is then applied through a RFI filter you find in most computer supplies, removing some of the unwanted brush noise. This unit is in an ongoing development. We have had much success with the design. By varying the speed of the servo motor and the voltage, we can run a 70 watt Taco pump (Taco is a registered trade name) down to 40 watts with good results. This unit was first designed as a fail safe pump driver for our wood fired boilers. Joe.  

[oztules]

Well Joe,

You have successfully tried for me what I have been pondering for some time. While rewinding a starter motor for a friend some months back, I had pulled apart a car starter motor (the rewind was for a large grader..didn't want to experiment on that till I felt comfortable with the car one first). With the windings stripped off and a very solid commutator looking at me I started wondering about using it for switched commutation. The problems I expected were related to sparking between the isolated (by two segments) copper strips, as they were being switched at full potential.

Did this (or does this) cause excessive arcing. If so, did you (have you) considered capacitors soldered between segments to help mitigate this, or is it a problem that didn't occur.

I have also considered high voltage (240v) switching in this manner, but the arcing if it isn't too bad at 24 volts, may get completly out of hand at 240v. Have you fiddled with this problem yet?

For 1kw at 240v switching would be low current, but high voltage.

One other solution for this may be in submersing in oil, and using a custon commutator with low hydraulic drag and turbulance...it too will have to wait.

You have unexpectedly touched on a project I have in waiting...(till I'm allowed to start it...house not finished yet).

It would be nice to be able to present a non-electronic inverter for mechanically inclined but electronically uncomfortable people.

You have already just done this. Hats off to you and another alternative uncovered.

.........oztules

[WXYZCIENCE]

We do have caps across the 24 volt side brushes and this eliminates most of the sparking. The limitations to this design, when we exceed what the brush can handle we do get arcing problems around 500 watts. My brother William built the unit. He had the idea and started with an old chop saw commutator. We found some small motors last year and they fit. Four brushes already installed. The commutator has 16 segments. He used 8 for connections and the other 8 are open. Also William has tried many configurations, including restive step, inductive coils. Simple is best in this case, only two jumper wires. He plans to fuse the 24v supply and add a centrifical power switch. Also some simple motor speed controls, ie. regulate the servo motor supply. We will continue to post results. One of the reasons William built this converter because those cheap FET units blow at the drop of a hat. A little more noise though. A small trade off I guess. Joe.    

[oztules]

Thanks for your explanation, I think I will use the starter motors as they are high current  four brushes  and very very gettable....even over here.(a dead car has nowhere to go on an island)

Another possibility may be to use several commutators pressed onto one shaft and graft the extra brush holders on as well.

I also have an ohio pm motor with two sets of four brushes already installed (high current motor), and this may be good for testing the theory without extra machine work.

"One of the reasons William built this converter because those cheap FET units blow at the drop of a hat. A little more noise though. A small trade off I guess. Joe."

Truer words were never spoken.

..........oztules

[willib]

what kind of a camera takes such great closeups ?

http://www.otherpower.com/images/scimages/5769/brushtype.jpg

i bet its a canon?

[WXYZCIENCE]

Fujifilm S3000, it has a good lens and macro focus. Joe.

[zap]

A magnifying lens will usually work well for closeups if your camera's macro isn't up to the task. Just experiment with where you need to hold the lens. Lighting will usually be the toughest part to deal with but it's usually not too difficult, a lamp or even a flashlight from the side will usually suffice.

Even mediocre optics can turn a marginal camera into quite a nice piece of equipment.

For these shots I used the eye piece from a discarded set of binoculars. Binoculars can also be used for telephoto lenses.