. Farm machinery and farm motors. e of sandpaper on the commutator while it is inmotion 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 motor is just the reverse; it is a machinefor converting electrical energy into mechanical. Anymachine that can be used as a dynamo can when suppliedwith electrical power be used as a motor. Dynamos andmotors 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
. Farm machinery and farm motors. e of sandpaper on the commutator while it is inmotion 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 motor is just the reverse; it is a machinefor converting electrical energy into mechanical. Anymachine that can be used as a dynamo can when suppliedwith electrical power be used as a motor. Dynamos andmotors 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 thatwhen 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, , if a current is passed through the 488 FARM MOTORS I coil, the coil will tend to rotate. This is the principle ofthe electric motor: instead of taking a current off of thearmature, one is put into it and at the same time sentthrough the fields. The current passing through the. FIG. 367—MULTIPOLAR MOTOR fields induces magnetism in them; the lines of force pro-duced by this magnetism draw on the armature and causeit to revolve. By stud3nng Fig. 356 it will be noticedthat the coil will revolve until the plane of the coilis parallel to the lines of force, and then stop. This samecondition would take place in the motor if it were not forthe commutator. Just at the instant the coil is broughtto the position to stop, the commvitator changes the di- ELECTRICAL MACHINERY 489 rection of the current and the turning efifect is thrown tothe other side and the armature moves on. Counter electromotive force of a motor.—The armaturewires of a motor rotating in its own magnetic field cutthe lines of force as if the motor were being driven as adynamo, consequently there is an induced inthem. The direction of this induced , is op-posite to that of the applied pressure. Such an is known as counter electromotive fo
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