70 ELEMENTS OF ELECTRICAL ENGINEERING. 



Electric motors for use in very dusty places are usually built 

 with an enclosing dust-proof case. Such an enclosed motor is 

 shown in Fig. 60. This type of motor is usually called a mill 



*"* 





Fig. 60. 



type motor. Street car motors are also enclosed. A typical street 

 car motor is shown in Fig. 9 1 . 



A given motor carrying a given load runs much cooler when 

 it is entirely open to the air than when it is enclosed, or, for a 

 given running temperature it will carry a heavier load if it is open 

 than if it is enclosed. 



39. The dynamotor. It is often necessary to take current from 

 lighting mains at, say, no volts for use in charging a few cells 

 of storage battery. To do this a large resistance may be con- 

 nected in series with the storage battery. The objection to this 

 is that a great deal of power is lost in the resistance. A method, 

 which is usually more economical, is to operate a standard 1 10- 

 volt motor with current taken from the iio-volt mains and use 

 this motor to drive a generator which is designed to give the low 

 voltage required for charging the storage battery. This combina- 

 tion of motor and generator is called a motor-generator set. 



