Science universe: Physics articles

SHUNT MOTOR A shunt motor is connected in the same way as a shunt generator. The field windings are connected in parallel (shunt) with the armature windings. The circuit for a shunt motor is shown in figure 2-4. Figure 2-4. - Shunt-wound dc motor. Once you adjust the speed of a dc shunt motor, the speed remains relatively constant even under changing load conditions. One reason for this is that the field flux remains constant. A constant voltage across the field makes the field independent of variations in the armature circuit. If the load on the motor is increased, the motor tends to slow down. When this happens, the counter emf generated in the armature decreases. This causes a corresponding decrease in the opposition to battery current flow through the armature. Armature current increases, causing the motor to speed up. The conditions that established the original speed are reestablished, and the original speed is maintained. Conversely, if the motor load is ...

COUNTER EMF While a dc motor is running, it acts somewhat like a dc generator. There is a magnetic field from the field poles, and a loop of wire is turning and cutting this magnetic field. For the moment, disregard the fact that there is current flowing through the loop of wire from the battery. As the loop sides cut the magnetic field, a voltage is induced in them, the same as it was in the loop sides of the dc generator. This induced voltage causes current to flow in the loop. Now, consider the relative direction between this current and the current that causes the motor to run. First, check the direction the current flows as a result of the generator action taking place (view A of fig. 2-2). (Apply the left-hand rule for generators which was discussed in the last chapter.) Using the left hand, hold it so that the forefinger points in the direction of the magnetic field (north to south) and the thumb points in the direction that the black side of the armature moves (up). Yo...

DIRECT CURRENT MOTORS LEARNING OBJECTIVES Upon completion of this chapter you will be able to: State the factors that determine the direction of rotation in a dc motor. State the right-hand rule for motors. Describe the main differences and similarities between a dc generator and a dc motor. Describe the cause and effect of counter emf in a dc motor. Explain the term "load" as it pertains to an electric motor. List the advantages and disadvantages of the different types of dc motors. Compare the types of armatures and uses for each. Discuss the means of controlling the speed and direction of a dc motor. Describe the effect of armature reaction in a dc motor. Explain the need for a starting resistor in a dc motor. INTRODUCTION The dc motor is a mech...