I was recently given the dismantled remains of an ancient magneto by one of our electricians, informed that it had belonged to his grandfather, and asked if I could make it work again. Stupidly I agreed to have a go.
I'll try to describe it as I see it - if I use any non-magneto terminology, then please bear with me as it was built at least 40 years before I was born.
The item is housed in a mahogany box with a glass viewing window in the lid (more later). Two electrodes are brought out, and there is a hole for a crank handle to be connected to the mechanism.
Internally, the unit has a pair of bobbin coils of a few hundred turns of cotton-covered wire, each wound on a steel former approx. 45mm dia. x 45mm long . The bobbins are mounted parallel to each other on a steel yoke approx. 60mm long. The yoke is carried on a split shaft on which the whole assembly rotates in insulated (wooden!) bearings.
The two bobbins are connected in series, and the outer ends of the windings are brought out to either side of the split shaft carrying the assembly. Spring contacts take the output of the winding to the external connections. A simple switch shorts the coil twice per revolution.
The bobbins rotate in the field of a large, though not especially powerful, horseshoe magnet, forming a closed magnetic circuit through the bobbins and yoke twice per rotation. Obviously as the yoke rotates the polarity of the magnetic field relative to the bobbin reverses. The magnet has a sliding keeper bar, which drives a dial indicator showing 0-100% through the viewing window. I guess this effectively weakens the magnet as it moves closer to the pole faces.
The bobbin assembly is rotated by a hand crank and geared drive and then pulley system, giving a ratio of about 30:1 step-up. The fastest I think I have managed to get is about 3000rpm, and I got the glorious output of 3.2V at 120Hz or so.
So... my questions:
What is the reasonable output voltage that I might expect from this magneto? I can see that the voltage is generated by the L.di/dt as the switch makes and breaks current.
Am I correct in thinking that the contact timing and duration are critical to how this thing works? How to adjust them, other than trial and error?
What on earth would you use one for? Someone in the office suggested that 'electric shock therapy' used to be fashionable. Guess I missed out! Any other ideas?
I'll try to describe it as I see it - if I use any non-magneto terminology, then please bear with me as it was built at least 40 years before I was born.
The item is housed in a mahogany box with a glass viewing window in the lid (more later). Two electrodes are brought out, and there is a hole for a crank handle to be connected to the mechanism.
Internally, the unit has a pair of bobbin coils of a few hundred turns of cotton-covered wire, each wound on a steel former approx. 45mm dia. x 45mm long . The bobbins are mounted parallel to each other on a steel yoke approx. 60mm long. The yoke is carried on a split shaft on which the whole assembly rotates in insulated (wooden!) bearings.
The two bobbins are connected in series, and the outer ends of the windings are brought out to either side of the split shaft carrying the assembly. Spring contacts take the output of the winding to the external connections. A simple switch shorts the coil twice per revolution.
The bobbins rotate in the field of a large, though not especially powerful, horseshoe magnet, forming a closed magnetic circuit through the bobbins and yoke twice per rotation. Obviously as the yoke rotates the polarity of the magnetic field relative to the bobbin reverses. The magnet has a sliding keeper bar, which drives a dial indicator showing 0-100% through the viewing window. I guess this effectively weakens the magnet as it moves closer to the pole faces.
The bobbin assembly is rotated by a hand crank and geared drive and then pulley system, giving a ratio of about 30:1 step-up. The fastest I think I have managed to get is about 3000rpm, and I got the glorious output of 3.2V at 120Hz or so.
So... my questions:
What is the reasonable output voltage that I might expect from this magneto? I can see that the voltage is generated by the L.di/dt as the switch makes and breaks current.
Am I correct in thinking that the contact timing and duration are critical to how this thing works? How to adjust them, other than trial and error?
What on earth would you use one for? Someone in the office suggested that 'electric shock therapy' used to be fashionable. Guess I missed out! Any other ideas?
