ELECTRIC MOTOR 



1996 



ELECTRIC MOTOR 



ring, wound with insulated wire; but these 

 armatures are no longer used for commercial 

 purposes, because the magnetic field acts only 

 on the outer parts of each coil. The inner 

 part is of no use in producing motion, and the 

 ring armature requires about twice the length 



FIGURE 4 

 A drum armature. 



of wire and therefore offers twice the resistance 

 of the drum armature in general use, as shown 

 in Fig. 4. 



The simplest electric motor has an armature 

 with two poles and a two-part commutator. 

 But such a motor is not made for practical 

 use. Som'e motors have four poles, and large 

 ones have more; to all these the name multi- 

 polar is given. 



Counter Currents. When the armature moves 

 it causes a current that flows in the opposite 

 direction and becomes an opposing force, which 

 weakens the current causing the motion. The 

 greater the speed of the armature, the greater 

 will be this counter force, but at the same time 

 that this counter force is increased by the 

 speed of the armature, the direct electromotive 

 force is proportionally increased so that there 

 is little or no loss of power from this source. 

 There are, however, two direct causes of loss 

 of power. These are friction and the resistance 

 offered by the conductor along which the cur- 

 rent flows. No machine can transmit all the 

 power it receives, for some of it must be used 

 in the operation of the motor itself. Every 

 conductor offers more or less resistance to the 

 current it carries, so considerable power is lost 

 in transit. For these reasons electric motors 

 are smaller than the dynamos supplying them 

 with power. See ELECTROMOTIVE FORCE. 



Types of Motors. Various types of electric 

 motors of all sizes are on the market, but all 

 have grown out of the fundamental types first 

 invented. Chief among these are the following : 



The Series Motor. This is the simplest type 

 of motor. The current passes through both the 

 armature and the field coils. Its course can be 

 traced by the arrows in Fig. 5. The power of 

 this motor depends upon the power of the 

 armature current and upon the strength of 

 the field. Since an increase of current increases 



FIGURE 5 

 A series-wound motor. 



both the armature current and the strength of 

 the field, a slight increase in current produces 

 a large increase of power. The rule for the 

 series motor is: For light loads the power 

 increases as the 

 square of the cur- 

 rent strength. 

 For heavy loads 

 the increase in 

 power is nearly 

 in proportion to 

 the increase of 

 current strength, 

 because the mag- 

 nets become satu- 

 rated and the 

 armature alone is 

 affected by the 

 current. 



The Shunt 

 Motor. In the 

 shunt motor the 

 current is divided, one part going through the 

 armature and the other through the field coils. 

 The direction of each branch is shown in Fig. 

 6. The coils of the field magnets consist of 

 many turns of 

 fine insulated 

 wire. Since the 

 magnetizing e f - 

 feet of a current 

 d e p en d s upon 

 both its strength 

 and the number 

 of times it flows 

 around an iron 

 coil, the field is 

 much stronger in 

 the shunt-wound 

 than in the series- 

 wound motor. 

 Because of the 

 length of wire in 

 the field coils 

 they have such 

 high power of resistance that changes of load 

 which strongly affect the armature produce 

 comparatively slight effects upon the field ; con- 

 sequently the shunt motor runs with a more 

 uniform speed than the series motor. 



The shunt motor is used where uniformity 

 of speed is required, as in the operation of 

 machines. The series motor is used where 

 great power is required in starting and con- 

 siderable variation of speed is necessary, as on 

 electric cars. 



FIGURE 6 

 A shunt-wound motor. 



