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õî Chapter (4) D.C. Motors Introduction D. C. motors are seldom used in ordinary applications because all electric supply companies furnish alternating current However, for special applications such as in steel mills, mines and electric trains, it is advantageous to convert alternating current into direct current in order to use d.c. motors. The reason is that speed/torque characteristics of d.c. motors are much more superior to that of a.c. motors. Therefore, it is not surprising to note that for industrial drives, d.c. motors are as popular as 3-phase induction motors. Like d.c. generators, d.c. motors are also of three types viz., series-wound, shunt-wound and compound- wound. The use of a particular motor depends upon the mechanical load it has to drive. 4.1 D.C. Motor Principle A machine that converts d.c. power into mechanical power is known as a d.c. motor. Its operation is based on the principle that when a current carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force. The direction of this force is given by Fleming’s left hand rule and magnitude is given by; newtons BI F l = Basically, there is no constructional difference between a d.c. motor and a d.c. generator. The same d.c. machine can be run as a generator or motor. 4.2 Working of D.C. Motor Consider a part of a multipolar d.c. motor as shown in Fig. (4.1). When the terminals of the motor are connected to an external source of d.c. supply: (i) the field magnets are excited developing alternate N and S poles; (ii) the armature conductors carry ^currents. All conductors under N-pole carry currents in one direction while all the conductors under S-pole carry currents in the opposite direction. Suppose the conductors under N-pole carry currents into the plane of the paper and those under S-pole carry currents out of the plane of the paper as shown in Fig.(4.1). Since each armature conductor is carrying current and is placed in the.
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[Audio] The rotation of the armature in an anti-clockwise direction is driven by the combined forces acting on each conductor. These forces result in a driving torque that sets the armature rotating. As the conductor moves from one side of a brush to the other, its current reverses while being influenced by the next pole's opposite polarity. Consequently, the direction of force on the conductor remains the same..
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[Audio] In a DC motor, the presence of back electromotive force, or back e.m.f., plays a crucial role in regulating the armature current. Back e.m.f. ensures that the motor draws just enough current to produce the required torque for the load. When a motor is running with no load, the armature current is small, and the back e.m.f. is close to the applied voltage. However, when the motor is loaded, the decrease in back e.m.f. allows for a larger current to flow through the armature, resulting in increased driving torque. Conversely, when the load is decreased, the back e.m.f. causes the armature current to decrease, allowing the motor to accelerate. This automatic adjustment of armature current by back e.m.f. enables the motor to regulate the flow of current and meet the load requirements. The voltage equation for a DC motor considers the applied voltage, back e.m.f., armature resistance, and armature current..