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 



