1983 Porsche 911 SC Targa

Wednesday, April 25, 2012

Transmission Maintenance

There are four transmission seals that I replaced – the input drive shaft, two output drive shafts, and the gear selector shaft. The seals must prevent gear oil from leaking out and allow the shafts to rotate freely. I also changed the gear oil. The throw out bearing fork came in my clutch replacement kit.
Old worn out parts that were replaced - clutch spring, oil seals, clutch release fork, and jack port covers.

New clutch release fork installed in the transmission bell housing.  I had to file down some of the casting flash at the top and bottom of the bore to get it to fit.
Because I am keeping the transmission in my EV conversion, I cannot eliminate all of my oil use with this car, but usage is greatly reduced because the oil in the gear box is not changed nearly as frequently as oil in the crank case. There are AC electric motors capable of going without a gear box (direct drive or fixed gear). They are used in most commercial electric vehicles. AC is cost prohibitive to most home hobbyists converting their own cars. The DC electric motor I selected has a wider torque band than the gasoline engine, but not wide enough to eliminate the need for a transmission. I expect to use 2nd gear to accelerate up to 45 MPH (70 km/hr) and 3rd gear at greater speeds. The reverse gear will also be required. In addition to being more economical, the DC motor will have more torque available at low RPM compared to AC electric and gas engines – peak torque is available from 0 to 2500 RPM.

Friday, April 20, 2012

Motor Break-in: Seating the Brushes

The series wound DC motor doesn't contain any permanent magnets.  There is a set of coils attached to the motor body in an assembly called the stator, so named because the stator coil is stationary.  There is another set of coils attached to the motor shaft in an assembly called the armature.  These coils rotate with the motor shaft.  The two sets of coils are wired together in series.

When current passes through the coils, a magnetic field develops, and the armature and motor shaft  rotate until the magnetic field in the armature aligns with the fields in the stator. The electric motor will not continue to spin unless the magnetic fields change. The current flow in the coils must be reversed. This switching process is done mechanically inside the motor. Electricity flows to the armature coils through brushes that are pressed against a cylinder (commutator) that is composed of electrically isolated segments.  Each segment is electrically connected to the armature coil.  As different segments of the commuter come into contact with the brushes as the motor shaft rotates, the current in the coil changes in a way that the magnetic field in the motor advances.  The motor shaft will contiue to chase the moving magnetic field, allowing the motor to continuously spin.

The electric motor is capable of drawing over 1000 amps of current, so it is important that the brushes make good electrical contact with the commutator to minimize electrical resistance and heat problems. The brush seating process entails operating the motor without a load at a relatively low 12 volts, for 40 hours to finely shape the brushes against the commutator.

The electric motor is strapped down securely for seating the brushes with a 12 volt run-in procedure.

Saturday, April 7, 2012

EV Adaptor Plate, Hub and Motor Arrive

My adaptor plate and hub arrived. In 1978, Porsche switched the design of the flywheel from a 6 bolt to a 9 bolt design. The supplier had yet to encounter a order for a 9 bolt version, so I sent them my flywheel so they could measure the proper dimensions for the hub. The adaptor plate connects the bodies of the transmission and electric motor. The hub connects the rotating shaft of the electric motor to input shaft of the transmission.

Now that I have the flywheel back, I took it to an automotive machinist to have it resurfaced and balanced. The machinist was an interesting man. He works on Cosworth racing engines and he restores T-28 Trojan aircraft – a US military plane used to train Air Force and Navy pilots in the 1950’s. The shop was located in an aircraft hanger, and it was packed with machine tools, engine blocks, and several aircraft in various states of restoration.
The motor bolts to this side of the adaptor plate.  The hub bolts to the motor shaft.

The transmisiotn bolts to this side of the adaptor plate.

The Warp9 electric motor is beautiful.

Out with the gas and in with the electric.