Thursday, December 5, 2013

Bird Bath Defroster Update

Continuing the Bird Bath Defroster build after a busy holiday break for Thanksgiving.

My first goal was testing the output of the stepper motor at various speeds with output rectified through 2 bridge rectifiers, since this is how I envisioned setting it up outside.
After thinking about how to go about doing this I remembered that my drill press has speeds listed for different pulley configurations.

Placing the axle of the stepper motor in the chuck of the drill press and securing the motor I was able to run it at different speeds and see what voltage and current it can produce.  I tested it with the output open as well as testing it across the heating element from the coffee maker as a load. Testing the heating element with my Multimeter I found that it's resistance was 16.6 ohms. 

RPM VLoad I (mA) VOpen
760 1.9 112 4.75
1150 2.89 166 7.22
1630 3.83 219.9 10.16
2180 4.91 283.4 14.3
3070 6.18 362.7 21.43

Running the stepper at high speed, and connecting the heating element, I checked to see if it would warm up after a few minutes – no matter the speed I did not feel any warmth from the element. I checked the casing of the coffee maker for specifications and realized that it is rated for 900watts – the stepper couldn't generate even 10% of that at the highest speed I tested. 

I also tested the stepper with blades attached to find out what it could output at 15 mph wind speed. I mounted the motor on a wooden plank and held it out my car window while driving at 15 mph (to simulate 15 mph wind speed). The motor was wired through the rectifiers and attached to my Multimeter.  At 15 mph the motor produced 8-10 volts (no load attached) so from previous measurements I assume that it spins at 1150 – 1630 rpm in a 15 mph wind.

Why 15 mph? Checking out wind speed data for Plainfield, Illinois I found that the average annual wind speed is 16.5 mph, with highest speeds in fall and winter and lowest over the summer months. Also, the speedometer on my car is analog not digital so it's easier to estimate 15 (than 16.5).

From this data it definitely appears that I need a different heating element. Searching around my workbench I was able to find the hair curler that I disassembled previously and after some work with a hacksaw, and a screwdriver serving as a pry bar, I removed the heating element:

The element appears to be Ni-chrome wire wrapped around mica and sandwiched between mica (at least I believe it is mica – it looks like it and I know that it is used as a dielectric in some types of capacitors so it is a good electrical insulator).  My next step with the Ni-Chrome wire will be:
  1. measure its thickness/wire gauge
  2. determine resistance per unit length (ohms per cm or per 10 cm)
  3. minimum current at 9 volts to heat wire
If the current needed to heat the wire is low enough to be met by the stepper then I will proceed to making a heating element by encasing the wire in the mica (to electrically insulate it) and then placing that inside aluminum (to transfer the heat to the water).  I do plan on sealing the aluminum closed with epoxy to water proof the wiring (brazing or welding would likely destroy the Ni-Chrome wire).

I will post tomorrow with updates on results.