Generator project

This is a picture of the generator described in the above webpage. Mine dosen't look so smart. At the left you can see the capacitors, in the middle, the motor and at the right a "Briggs and Straton" Lawnmower engine. The engine used in this example is from a rotary mower, so it's driveshaft is vertical. My engine was from a cylinder mower, so my driveshaft is horizontal.

My neighbour wanted to make one of these generators as well, so we decided to have a joint effort. He is a retired mechanic and has a collection of all sorts of mechanical bits and pieces. He donated an old lawnmower engine to the project (from a "Suffolk" lawnmower) and I welded a frame together.

THE OLD LAWNMOWER

To generate power we need to turn an electric motor with the engine. The sort of electric motor needed is called a "squirrel cage" motor as the rotor looks like a cage for exercising small animals, hamster cage might have been a better name. These motors are so common that 1/3 of all the electricity generated world-wide is used to power them. They are in lots of things, including pumps / fans / washing machines - however some of them are more complicated than others.

I already had one squirrel cage motor and my neighbour had another three. However two of his motors were not suitable as they had too many windings for me to figure out (some induction motors have separate windings for starting/running and can have more windings for different speeds/reverse). None of the motors had any makers plates on them - so we didn’t know their HP or RPM.

When they are running, there is a rotating electric field inside a squirrel cage motor, this field rotates at a speed dependant on the number of poles in the motor and the number of AC cycles per second. The formula to work out the speed of the field is speed = (120 * Frequency) / poles. The fastest speed that a squirrel cage motor can spin at is therefore 3000RPM, with more poles reducing this speed. American motors can spin slightly faster, as their mains frequency is higher.

My original motor had two poles so I reckoned the field in my motor would turn at 3000 RPM, when running as a motor. The motor would run slightly slower than this due to "slip" which is the technical term to describe how the rotor cannot quite keep up with the rotating field (Due to friction - and / or load). To use the motor as a generator my engine would have to spin the motor slightly faster than this (As slip works both ways).

To start generating the motor must have a capacitor connected in parallel with it's windings, this is to provide a load for the generating process to begin. As we have a higher mains voltage than America, we are able to use much smaller capacitors than were used in the original machine. For motor in the picture below I used a 25uF capacitor.

I tried using an electric drill to turn the induction motor to test it's ability as a generator. At first the drill turned the motor quite easily, but when it started to generate there was a huge increase on the load to the drill. In fact the drill went from being nearly free running on no load, to a full load instantly. This was with a 25 uF Cap in parallel with the motor. I was able to power a 100watt light bulb with this setup, but not for too long as my drill was starting to get hot.

I then fitted the motor to the frame, which I had previously welded and connected it to the engine via a V belt. This set-up generated electricity at the first try. I adjusted the speed of the engine until I had about 230 volts and then tried running a 100watt lamp, which worked fine. However the frequency was measured at 90Hz ! After experimenting with different pullys and capacitors, I was able to produce 230V 50Hz.

This is my original motor coupled to the engine. This one worked fine, but could only give an output of about 300 watts. The engine can be moved from left to right, to adjust the tension of the drive belt.

 I then got a different motor from an old electric pump, I welded up a bracket for it so that it would fit in the frame shown above.

New motor and bracket.

This was a four pole motor, so it ran at 1500 RPM. With a 50uF Capacitor I got 230V 50Hz and can generate about 500 watts of power.

Generator with new motor fitted. It's running the 300watt lamp in front, notice the wheels which had to be fitted as the generator is quite heavy.

I put the capacitor into a plastic box and made up a meter to show the output voltage. Also on the box is an on/off switch to control the output.

This shows the control box, sockets and lamp (which the generator is powering).

The finished generator will run for about 2 hours on the little petrol tank on top of the engine. After this time the motor is still quite cool to the touch.

Don't forget that 230 Volts can be dangerous - take the same care with power from these generators that you would with the regular power in your house !