Ira M. Cushing

The dynamo here described is designed to light 14-volt miniature series lamps connected in multiple; driven at a speed of 2800 R. P. M. it will give 5 amperes at 15 volts. The accompanying illustration gives the outline and important dimensions.

The field frame and base should be made of a good grade of cast iron and cast in one piece. The frame and bearings should be made and assembled as described for the 150 Watt dynamo in the December number of Amateur Work. The bore of the field should be a scant 2 1/8 in. or 2 3/32 in. The closer to the latter figure the builder can come, the more efficient the machine will be.

There is only one difference between the bearings for this and the 150 Watt machine. The shaft for this generator is .3125 in. diameter in the bearings. The brass tube for bearing lining should therefore have an internal diameter of less than 5/16 in. in order to bore it out for a running fit.

Another way to make a good bearing would be to run melted babbit or bearing metal into the bearing in place of the brass tube and when cool bore out for the shaft. In boring use a size smaller drill than required and ream to a running fit.

For lubrication, an oil hole should be drilled through the top of the bearing. If the generator is to be run very long at a time this hole can be tapped and fitted with a grease cup or a wick feed oil cup.

The shaft is aproximately 10 in. long and .3125 in. diameter in the bearing. The enlarged portion begins 4 3/8 in. from the commutator end and is 7/16 in. in diameter. This part is 2 1/2 in. long. At the commutator end of the large part is cut a No. 14 thread for about 5/16 in. The nut and retaining washer should be 1/4 in. in diameter and assembled as on the 150 Watt dynamo, the edge of the washer to be 1/4 in. from the pulley end of the enlarged portion.

The armature core is made up of enough sheet iron punchings to fill tightly the space of 1 1/2 in. The punchings are 2 1/16 in. diameter with 12 5/16 in. holes in the periphery, with their centers on 1 3/4 in. diameter circle. The assembling and insulation of the core should be as for the 150 Watt dynamo.

The commutator and brush holder rigging described and illustrated for the 150 Watt machine, will be correct for this generator with a slight change. The brass tube upon which the commutator is built should have small enough inside diameter to be bored out for a driving fit on the shaft. Otherwise the yoke, studs, brush holders and brushes will be of the same size as before described.

Wind the armature with No. 20 D. C. C. magnet wire putting 34 wires in each slot. Since this core has the same number of slots as the 150 Watt dynamo the winding scheme will be the same. There will be six coils with 34 turns in each coil. Calling the slot No. 1 where the first coil starts the six coils will fill the slots as follows:-Coil 1, 1-16; coil 2, 11-4; coil 3, 9-2; coil 4, 7-12; coil 5, 5-10; coil 6, 3-8.

Much care should be taken in winding to lay the wires straight, even and tight. If the finances of the builder would allow I would recommend double silk covered wire. This would give more leaway for getting all the wire in the slots, as the double silk does not take up as much room as the double cotton. Single cotton covered wire can be used but great care must be exercised in winding not to break the insulation. Insulate the shaft each side of the core; at the pulley end with tape and at the commutator end with a fiber tube 3/8 in. long and 5/16 in. inside diameter. Carefully test each coil for a ground with the core as it is wound.

Testing each coil as wound may save rewinding the whole or part of the armature to remove a ground. This test should be made with a voltage of at least 30 volts, but 110 volts is better if at hand. Have a lamp in series with the testing wires when using 110 volts, as otherwise the worker may get a severe burn in case of a ground. Besides, the lamp will readily indicate the ground.

The commutator should be assembled on the shaft and connected to the windings as on the 150 Watt dynamo, connecting the end of a coil to the segment next beyond. In winding and making connections neatness is a very important factor. It not only gives a better appearance to the machine but it also results in a better working machine. Wires that are laid evenly, straight and tight will stay in place very much better than wires laid haphazzard.

The field should be wound with 714 turns, or more, of No. 20 S. C. C. magnet wire to each pole. Wind the poles as full as possible; if more than the above number of turns can be put on the more efficient will be the dynamo. The same method of winding should be followed on this machine as described for the 150 Watt generator. This and the 150 Watt dynamo are shunt wound machines, and so the connections of field and armature will be the same.

Use a 1 1/2 in. pulley with a 1 1/2 in. face and drive with a 1 or 1 1/4 in. flat belt. Determine the direction to drive the dynamo by applying current from 10 or 12 dry cells, or any source having 15 volts. It will run as a motor in the same direction as driven for a generator.

The field winding will require approximately 4 lbs. of No. 20 wire and the armature will require about 1/2 lb of No. 20 wire.

75 Watt Dynamo