Homebrew Grid Tie Inverter

The RC car wheel was a disater. I could not reliably mount it to the generator shaft. I then got a plastic wheel with hard rubber tyre on it. Initially I feared that it would get no traction on the iron flywheel but with enough pressure from the generator mount it works very well.

The generator is connected to a three-phase bridge. The funny thing about this circuit is that there is only a little ripple. The output is connected to a fat electrolytic capacitor (12 mF/40 V) to get rid of the ripple and to store the energy needed during the work phase. The initial testing took place with a dummy load attached.

The actual inverter consists of a power supply, two timers, a driver and output stage. The PSU supplies the timers (8 V) and the driver (12 V). It also generates a zero crossing signal. This triggers the first timer (half a HEF4538). C3 is a filter capactor of about 0.001 uF. The timer sets the time for the sine of the mains to reach its crest (POS), about 4 ms. Just before that the second timer is triggered (DUR) which is the work phase, about 2 ms. During this time the V(in) voltage from the generator stored in the capacitor is connected across the output transformer to feed the generated power into the mains grid.

This pulse is fed to the driver where it is split in the positive and negative phase (Q1 and Q2). The pulse is blanked by the transformer voltage to prevent the MOSFET from firing at the wrong phase. This is done with the four NAND gates. The output then goes to the MOSFET driver IC TC4426A, which contains two high current drivers.

The output stage has two MOSFETs with a high current capability of 75 A. This is the reason the high current drivers are needed because the IRFB3307 devices have a high gate capacitance that a CMOS gate is unable to drive. The sources go to ground via a current sensing resistor. This gives a nice linear measure of feed-back power. The drains of the MOSFETs are connected to the toroidal transformer. Liberated from old equipment it had to be modified to get two more or less equal windings. It is rated at an estimated 200 VA and has two 11 V windings. It never runs warm. Nor do the transistors warm up. Only the reducer unit of the generator has losses and gets warm. The generator itself generates about 15 VDC just before it produces power.

Finally, the bike is put onto the wooden plate with the generator mount. A piece of wood keeps it in place and pushed against the friction wheel.