SPEED CHARACTERISTICS. 



39 



APPENDIX I. 



SERIES MOTOR. 



18. Operation. A series* motor is distinctly a variable speed 

 motor. Its characteristics are shown in Fig. 3. The speed increases 

 rapidly as the load is decreased, becoming dangerouslyt great if the 

 load is removed or reduced too much. The series motor, therefore, 

 cannot be run at no load 

 and normal voltage; it 

 can be run at no load 

 with a series resistance 

 in circuit. 



The series motor, be- 

 sides being used for trac- 

 tion,^: is used for hoists, 

 etc. For such service it 

 is well adapted. The im- 

 portant characteristic is 

 that by slowing down 

 under heavy load, it can 

 increase its torque with- 

 out requiring a corresponding increase in power ; for torque = power 

 -4- speed ( 3b). If the speed did not decrease with load, it is seen 

 that the power would have to be greatly increased to give the same 

 torque. This would require a much larger motor. 



* ( i8a). For the purpose of comparison with the shunt, compound and 

 differential motor, the characteristics of the series motor are here described, 

 although its test is not usually to be included as a part of the present 

 experiment. When the test is made, it is well to combine it with efficiency 

 measurements, 33, Exp. 2-B. 



t(i8b). In the laboratory, be prepared to shut down quickly if ex- 

 cessive speed is reached. With a belted load, there is danger of the belt 

 flying off; with a brake, there is danger of an unintentional sudden de- 

 crease in load. 



$ ( i8c). In traction, the controller is usually so arranged that two 

 motors can be connected in series or in parallel with each other for speed 

 control, thus giving each motor half or full voltage. The series resistance 

 is likewise used for control and for starting. In starting, the resistance 

 and both motors are all in series. 



AMPERES 



FIG. 3. Characteristics of a series 

 operated at constant voltage. 



motor, 



