International projectionist (Jan-Dec 1945)

called coil sides. The coil is wound so that one coil side is under a north pole when the other coil side is under a south pole. The flux will be distorted in such a manner that a tendency towards rotation will exist. Thus, if the thrust on one conductor is upward, the thrust in the other conductor will be downward. Under such conditions the coil will rotate one-half of a revolution. The coil would then come to rest because a north pole due to the coil would be placed opposite a' south pole of the main field on one side of the armature, and a south pole due to the coil would be placed opposite the north pole of the main field. The magnetic field due to the coil would tend to lock with the main magnetic field. This would not provide complete motor action since only one-half of a revolution would result. In order to provide complete motor action in a direct current machine it becomes necessary to have some means of reversing the current through the movable coil (armature) . Construction of a Motor The commutator is a device that reverses the direction of current through the armature of a motor every time that the coil moves from a north to a south pole. In a two-pole single coil motor (not practical) this reversal would take place every one-half revolution. In a four-pole motor the current would reverse every quarter turn. The commutator is made from slotted, curved sections of copper. These sections are known as commutator segments and are placed around the circumference of the motor shaft but are carefully insulated from the shaft and from each other. A soft grade of mica is used for insulation. Figure 3 shows how the commutator is connected to the coil. If the current through the coil is traced it will be seen that its direction through the coil sides changes every half-revolution. Under these conditions the north pole due to the coil will always be near the main coil FIGURE 2. Fleming's left-hand rule. commutator brushes FIGURE 3. Coil commutator segments, and brushes. field's north pole, and the south pole due to the coil will be near the south pole of the main field. These conditions are perfect for continuous motion because of the force of repulsion always existing between the armature and the main field. Reference to Figure 3 shows that the current from the line is fed into the armature by a pair of carbon conductors called brushes. These brushes provide a convenient means for making an electrical connection to a rotating load. In practice the brushes are held in a housing called a brush holder. They are pressed against the commutator segments with moderate pressure, which is provided by a spring. In a small motor the brush holder is fixed to the motor frame and the brush slides into the holder. A spring is inserted into the holder and presses against the brush providing the necessary pressure between brush and commutator. In a large motor the brush is tightly held by the holder which is mounted on a swivel. A heavy spring is attached to the holder. The other end of the spring is fastened to the motor frame, and this spring presses the holder and brush assembly toward the commutator. The motors that have been used here for illustrative purposes have been assumed to have a single-coil armature. Such a motor would not be practical because there are times during each revolution when the coil would be rotating through MARCH QUESTIONS AND CORRECT ANSWERS 1. (Q.) Why shouldn't an ohmmeter be left on the "low-ohms" range for a long period? (A.) Most ohmmeters have a batteryconnected directly across the meter when it is on the "low-ohm" scale, and if left in that position the battery will run down. 2. (Q.) What type of meter is recommended? (A.) A vacuum tube voltmeter is most useful. 3. (Q.) Why isn't the d.c. wattmeter very practical? (A.) The d.c. wattmeter must be used in conjunction with a separate voltmeter and ammeter, and is somewhat erratic. a field of zero flux. The torque that such a motor would develop will be pulsating. A two-coil armature would eliminate this condition, but the torque developed would still be somewhat pulsating in character. The best results are obtained when a large number of coils is used. In armatures of this type only a small portion of the total number of coils is undergoing commutation at any one time, therefore, the variation in the number of active conductors is so slight that the torque developed is substantially constant. Counter Electromotive Force The armature resistance of a 3-horsepower, 110volt motor is about 0.5 ohms. If this armature were connected across 110 volts, the current by ohm's law would be 110 220 amps. 0.5 The rated current of this motor can also be found by means of Ohm's law. A 3-horsepower motor takes 2238 watts. There are 746 watts in one horsepower, therefore, 3 horsepower would equal 3 times 746 or 2238 watts. The rated current of such a motor may be found from the formula P = E x I. Solving for I. we obtain the formula 2238 110 = 20 amps approximately. Since this motor is rated at 20 amps, a current of 220 amps for the armature is not only excessive but unreasonable. When a motor is in operation, it is obvious that the current through the armature is not determined by its ohmic resistance alone. The conductors on the motor, in addition to carrying current and developing torque, are also cutting magnetic flux. Whenever a conductor cuts flux a current will be induced in the wire. This current is always in opposition to the current that the line tries to send into the armature. Thus, the armature current is bucked out to some extent. Under normal operation the resultant current through rheostat j — A/WAA shunt g field ^armature FIGURE 4. Shnnt motor. APRIL 1945 19