dpack.html

D-Pack Electric Motor

This is an AME d-pack electric motor. I selected it because it was 12 volts, could make over 2-Hp, and cost $50. They are no longer available. You can get a data sheet here.

The tank uses one of these for each track. I've noticed that after a few minutes of use, this motor gets very hot. I was under the impression that it is a continuous duty motor, but I don't believe it. It gets so hot you can't touch it after only 30 minutes of use.

Where I run it the motor is only about 75 to 80% efficient. This means that 270 to 290 watts of heat needs to be dissipated - much more than I had expected.

I'll need to install some sort of active cooling to prevent a burn-out. Only 30 min of 'tanking' is not acceptable.

I decided to take the motor apart to explore what was inside. Two long bolts are all that hold it together. These are the end caps. On the left is the shaft end, the right is the terminal end. The 4 brushes are visible.

This is the armature. It has 16 'teeth' with coils wound around sets of 4. The wire used is 17 awg. Considering the amount of current this thing can consume, I was expecting heavier stuff.

The shaft end has a 0.675" shoulder which is accepted by the end cap - it rides on a brass oil-impregnated bearing. The heat has darkened the oil, and I found an oil film all over the permanent magnets.

The commutator end has a 0.336" shaft also riding on a brass oil-impregnated bearing.

The commutator. There are 16 contacts, each with 4 wires attached, except one, which has 6.

The stator. Two sets of two permanent magets, each with 2 poles, for a total of 4 poles.

On these magnets, the material is magnetized through the thickness. If you consider the left magnet, above the orange mark the inside is say the north pole, and the case side is the south pole. Below the orange mark the inside is the south pole, and the case side is the north pole. At the mark, the material is not magnetic.

Some close-ups of the armature. Each coil is only 3 turns of wire, looped around 4 teeth.

A close-up of the brush assembly. Opposite brushes are connected to each other. At any given time, two or 4 sets of contacts are made on the commutator.

Given all the heat, I'm glad the brushes and commutator faired so well.

I decided to add a ball bearing to the pulley end of the motor, and to ventilate it. A new aluminum end cap was machined to accept a bearing and to allow air flow. The other end had a hole machined into it for an air intake.

I was surprised with the tolerances in these motors. The air gap between the armature and stator magnets was one or two hundredths of an inch. This was apparent when I first assembled the new endcaps and ball bearings. The bearings have a 0.75" bore, but the shaft is 0.667" - I made an aluminum collar to fit over the shaft, but I had machined it a few thousands too much - I needed to use some brass shim stock to recenter the shaft to prevent the armature from rubbing.

The motor was remounted and a copper tube was attached to the intake. Here a sponge gasket is installed.

A household bathroom fan was modified to fit in the available space. The fan is 110 volt, so a small power inverter was added. The fan outlet mates with the sponge gasket to force air thru the motor. The airflow is not great, but it is much better than before, which was none.

After a few tests it became clear that there was not enough air flow, so I removed the fans and replaced them with a vacuum cleaner turbine.

I cannabolized a $15 auto vacuum cleaner (Target) and put the turbine in a small wood box. I used a spare shop-vac hose to pipe the air to the motors.

Here it is mounted with the hoses routed to the motors. An initial test showed that much more air is being pumped through with this setup.

Next time out I'll see how well this works.

The turbine uses a 90 Watt motor, far more powerful than the battery powered hand held units sold in most stores. This was also the cheapest one I found!

The motors are very nicely cooled with this new setup, but the radio control's range dropped to about 10 feet!

A little EMC fix goes a long way - three 0.1 uF caps and two ferrites cleaned up the noise, and the radio control works as before.