1983 Porsche 911 SC Targa

Saturday, April 19, 2014

Brushes and Motor Balance

I’ve been working on the car nearly every day for the last 5 weeks. I feel like I should have more to show. There are lots of small items in progress and I will report on them as I complete them.

To paint my battery racks (the front rack is done, the back racks are in progress), I’ve had to take a lot of components out of the car – 60 battery cells, the controller, wiring, and battery charger, not to mention the battery racks themselves. As long as the car was stripped down, I decided to inspect my motor, and it is a good thing I did. I was surprised to find that the brushes were very heavily worn down.

After doing a little bit of research, it turns out my motor shipped with H49 brushes. H49 are great at transmitting heavy electrical loads but they tend to wear very quickly. Perfect for a drag racing application, but not so good for street use. After 3,500 miles (5,600 km), my brushes were half the original length, or about 80% used up, because you can’t run the brushes all the way down to the embedded pigtail wires. I was expecting to go 50,000 miles (80,000 km) between brush replacements.

New H60 brush on the left. Old H49 brushes in the center and right. I'm not sure why there is uneven wear. A new brush is 1.5 inches (38 mm) long. 

Here is one of 4 brush holders, with a pair of brushes. The brushes are pressed against the commutator with coil springs.

The brushes have worn shallow grooves in the commutator. This is something I've seen reported by others with the H49 brushes.

I ordered a set of H60 brushes. The motor takes 8 brushes. These brushes are supposed to be better for street use. They are also supposed to be a little more efficient that the H49 brushes, reportedly around 10%. When new brushes are installed, even though they come preformed with a radius, only a small portion of the brush face is in direct contact with the commutator. The surface of the brush will eventually seat after several hours of run time. The motor will not be capable of full power until the brushes are seated. The motor was run on the bench with a 12 volt battery and charger for 32 hours to accelerate the seating process.

Also, while the motor was out of the car, I decided to address a small vibration from the motor. At 3,000 rpm I could feel a small vibration in the gear shift knob. The car didn’t vibrate and there was no audible indication of a problem. When I first pulled the gas engine, I took the flywheel to a machine shop for balancing. I should have also brought the clutch disk and pressure plate, to be balanced as an assembly. With just the flywheel, spinning the motor during my brush break-in procedure was vibration free. But when I bolted on the pressure plate, vibration was present above 2000 rpm. The motor spins at about 2400 rpm at 12 volts with no load. The vibration was easily solved by adding extra washers to the pressure plate bolts. Start with a single washer. If the vibration gets worse, move the washer to another position. Once you find the correct position to add mass, keep adding washers until the vibration is gone. I didn’t want a stack of washers all on one bolt. I ended up with one washer each on a grouping of three adjacent bolts. Vibration problem solved.

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