MOTORS. is usually a much smaller machine-thus a dynamo of 1,000 horse power will furnish the necessary force for nine or ten 100 horse power motors. In construction, the dynamo and motors are essentially the same, but in their action are exactly the reverse. Motors depend for their operation on the tendency to motion in a magnetic field. If the conducting wire, while situated in the magnetic field, is actually conveying an electric current (from whatever source) it experiences a side thrust, tending to move it forcibly, parallel to itself, across the magnetic lines and so enables it to exert power and to do work. This is illustrated in Fig. MOTORS. 140, where the small arrows exhibit the movement of the current and the large arrow the resulting push or magnetic force. This action is the principle of the dynamo used as a motor. Two points are vital to the right understanding of the action of electric motors: (1) the propelling drag, (2) the counter electromotive-force. The first is that the real drivingforce which propels the revolving armature is the drag which the magnetic field exerts upon the armature wires 'through which the current is flowing (or, in the case of deeply-toothed armatures, on the protruding teeth): the second is that the revolving armature generates a counter electromotive force as its moving wires cut the magnetic lines. FIG. 140. The Propelling Drag.-In a generator the drag acts in a direction which opposes the rotation, and is, in fact, a counterforce or reaction against the driving force. In a motor the drag is the driving-force, and produces the rotation. The Counter Electromotive-force.-Let it be remembered that wherever in an electric circuit, current flows through some portion of the circuit in which there is an electromotive DYNAMO AND MOTOR BELTS. force, the current will there either receive or give up energy according to whether the electromotive-force acts with the current or against it. This will be made clearer by Fig. 139, representing a circuit in which there are a dynamo and a motor. Each is rotating right-handedly, and therefore gen. erates an electromotive-force tending upwards from the lower brush to the higher. In each case the upper brush is the posi tive one. But in the dynamo, where energy is being supplied to the circuit, the electromotive-force is in the same direction as the current; whilst in the motor where work is being done, and energy is leaving the circuit, the electromotive force is in a direction which opposes the current. DYNAMO AND MOTOR BELTS. "Points" relating to Dynamo and Motor Belts.-No subject relating to motors will be found of more practical interest than the following: 1. Dynamo belts should make a straight run through the air and over the pulleys without wabbling; they should maintain an even and perfect contact with that part of the pulley with which they come in contact. In order to do this they should be kept soft, pliable, and have no abrasions or rough places the belt should be first-class—as near perfection as possible, for they must do their work so the light burns with DYNAMO AND MOTOR BELTS. out flicker. When belt fasteners give way there is too much strain upon belt. The greatest amount of slack in a belt is found when it leaves the driving pulley, hence the tightene should be near the driving pulley, as it takes up the slack, prevents vibration and diminishes strain on belts and bearings. More than 110° of heat is injurious to belts. 2. The double belt should always run with the splices, and not against them. One-quarter turned belts should be made of two-ply leather, so as to avoid so much side strain. Slowmotion belts should be made of two-ply leather, as they receive hard labor and strains. 3. The electric generators of the alternating system require special belts, as they are run at great velocity. Belts for the alternating system should be endless, perfectly smooth, even in texture and thickness. If too much power is added to the sticking or adhesive qualities of a belt the friction will cause loss. 4. Friction is greatest when the pulleys are covered with leather. Friction depends upon pressure, but adhesion depends upon the surface contact; for instance, two square feet of adhesion will hold twice as much as one square foot, hence, the more a belt adheres to pulley surface without straining, through too much tightening, the better the driving power. Wet days produce slipping because the leather absorbs dampness. 5. A leather-covered pulley will produce more resistance than polished or rough iron ones. A good belt dressing makes a smooth, resisting surface, and as it contains no vils, which DYNAMO AND MOTOR BELTS. create a slippery surface to belts, it increases belt adhesion. The friction of leather upon leather is five times greater than leather upon iron. 6. Moisture and water distend the fibres, change. the properties of the tanner's grease and softening compounds. Repeated saturation and drying will soon destroy leather. Leather well filled with tanner's grease or animal oil, if allowed to hang in a warm room for several months without handling, will dry out, become harsh, and will readily crack. 7. Many things have been used to make belts stick to the pulleys, some of considerable value. A careful study of all the parts that work together is required in order to get full power transmission. Suitable belt dressing will overcome many serious questions that arise, but it must be properly applied. 8. A running belt is stretched and relaxed at different times, and unless there is perfect elasticity in all its parts there will not be uniform distension. Whatever relieves the strain upon belts prolong their life. There should be 25 per cent. margin allowed for adhesion before a belt begins to slip. 9. The adhesion between the surface of belt and pulley must produce more friction than the pull or tension. When great tension or stretching is required it evidences the fact that the belt is not properly proportioned, or that it is oil soaked and there is too much oil on the pulleys. 10. An endless belt will always give the best results, as lacing produces a momentary flicker in the lights at each revolution. |