CHAPTER II. 

 DIRECT CURRENT MOTORS. 



EXPERIMENT 2-A. Operation and Speed Characteristics of a 

 Direct Current Motor, (Shunt, Compound and Differential). 



PART I. INTRODUCTORY. 



i. Generators and Motors Compared. Structurally a direct 

 current generator and a direct current motor are alike,* the 

 essential elements being the field and the armature. The same 

 machine may accordingly be operated either as a generator or 

 as a motor. 



Operating as a generator, the machine is supplied with me- 

 chanical power which causes the armature to rotate against a 

 counterf or opposing torque; this rotation of the armature gen- 

 erates an electromotive force which causes current to flow and 

 electrical power to be delivered to the receiving circuit. 



Operating as a motor, the machine is supplied with electrical 

 power which causes current to flow in the armature against a 

 counterf or opposing electromotive force; this current creates a 

 torque which causes the armature to rotate and mechanical power 

 to be delivered to the shaft or pulley. 



*(ia). Since generators are built in much larger sizes than motors, 

 one generator being capable of supplying power for many motors, there 

 may be a difference in design due to size. Moderate size machines, gene- 

 rators or motors, are built with few poles, four being common in small 

 motors. On the other hand, very large machines that is generators are 

 built with many poles. 



In all direct current machines, generators or motors it is common 

 practice to use a stationary field and a revolving armature ( i, Exp. i-A). 



f ( ib). There is no counter torque in a generator until current flows 

 in the armature; there is no counter electromotive force in a motor until 

 there is rotation of the armature. 



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