Page:Popular Science Monthly Volume 90.djvu/807

 Popular Science Monthly

��791

��Armature Winding for Small Series Motors

ON large generators and motors, the armature coils are wound and shaped on a form. This method is applicable to

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��Measuring the length of an armature coil with wire solder, and the pattern for the form

any slotted core. Consider in detail the winding of small series motors such as are used to run vibrators, massage outfits and small grinders. These motors are usually very small and a higher degree of heating must be allowed than in larger machines. The resistance of the motor must be high to prevent too much current flowing through and dangerously overheating its windings. These little motors are too small and inefficient to generate much counter force, so high internal resistance is a necessary feature. The current through the motor is only about .1 ampere. With so small a current as this the motors heat too much to run for long periods at a stretch. Also, the radiation of heat from the windings is very poor, due to the large percentage of cotton insulation in small wire windings. The wire is very fine in the motor — about No. 32 double-cotton- covered on the fields and as fine as No. 36 d. c. c. on the armature.

As these motors are bi-polar, the arma- ture coils go through slots 180 deg. apart. Commutators usually have at least 12 segments, so as to keep the pressure across adjacent commutator bars below 10 volts. Some have more bars. With high voltage between bars, sparking will be bad and

��metal particles will easily cause adjacent bars to short-circuit.

For example, in winding a 14-slot core that has a 14-bar commutator, there will be 14 coils and the size of wire will be No. 36 d. c. c. Before the form is made we must get an idea of the shape and size to make the coils. Remember that an arma- ture winding is symmetrical; that is, the coils are just alike, having the same shape, the same position in the slots and the terminals all coming out similarly. One side of a coil is put in the bottom of its slot and the other side of the coil goes in the top of the slot on the opposite side of the core. Now with a piece of wire-solder or lamp-cord you can make a pattern of the coil and see how it will work when done. See Fig. i. Measure the length of the top and bottom sides of the coil. Say the top is 2 in. and the bottom is i3^ in. Now take the pattern out of the slots, pull it out straight and measure it. Say it measures 73^ in. Deducting 33^ in. for the two sides, there is left 2 in. each for the two ends of the coil. Now make a form with these figures for its dimensions. A piece of thin

��board }/^ in. thick will do. Cut out the side pieces for the center form and screw all three together. See Fig. 2 and 3. This can be mounted in a lathe or anything else you have with which to turn it.

To determine the number of turns f)er

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��Placing insulating material in slots, insert- ing the coils, covering and setting the bnishes

coil, cut short lengths of the wire and crowd the slot as full as you can, with say 720. Take % of that, or 480, as the proper

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