ELECTRICAL MACHINERY 487 



working the brushes back and forth until the proper position 

 is located. 



2. Brushes not spaced according to commutator bars. The com- 



mutator bars should be carefully counted and the brushes 

 accurately set between them. 



3. Brushes do not bear against commutator with sufficient 



pressure. 



4. Brushes do not bear on the commutator with a perfect surface. 



5. Collection of dirt and grease which prevents good contact of 



the brushes on the commutator. 



6. A high or low commutator bar which causes poor contact. 



7. Commutator not worn perfectly round, consequently poor con- 



tact with the brushes. 



657. Repairing a dynamo. If the insulation breaks 

 down, a wire burns out or the commutator becomes worn 

 out of round, an expert should be called in, and generally 

 the defective part will have to be sent to the factory for 

 repairs. Sometimes a good machinist can put the arma- 

 ture in a metal lathe and turn it down round. A good 

 man with a file can work down a high bar, and holding 

 a piece of sandpaper on the commutator while it is in 

 motion will clean it of all oil and dirt. 



MOTORS. 



658. Comparison with a dynamo. A dynamo is a ma- 

 chine for converting mechanical energy into electrical. 

 An electrical motor is just the reverse; it is a machine 

 for converting electrical energy into mechanical. Any 

 machine that can be used as a dynamo can when supplied 

 with electrical power be used as a motor. Dynamos and 

 motors are convertible machines; thus the various dis- 

 cussions will apply as well to the motor as to the dynamo. 



659. Principles of the motor. It has been shown that 

 when a coil of wire is placed in a magnetic field and ro- 

 tated an electrical current is produced. If the oppo- 

 site of this is done, i.e., if a current is passed through the 



