Category: DC MACHINES GENERATORS AND MOTORS

As per the characteristics of DC motors, different types of DC motors are applied for different jobs as follows:

There are two types of compound-wound DC motors, namely cumulative compound motors and differential compound motors. Cumulative compound motors are most common. The characteristics of these motors lie between the shunt and the series motors. The N − Ia characteristics, T − Ia characteristics, and N − T characteristics are shown in Figure 11.53(a), (b), and (c), respectively. Fig. 11.53  (a) N−Ia, (b) T−Ia and (c) N−T characteristics of shunt…

The conventional diagram of this motor is shown in Figure 11.45. In these motors, the shunt field current Ish = V/Rsh remains constant since the supply voltage V is constant. Hence, the flux in DC shunt motors is practically constant (although at heavy loads, somewhat flux decreases due to armature reaction). Fig. 11.45  Conventional diagram of a DC shunt motor 11.23  CHARACTERISTICS OF SERIES…

The performance of a DC motor can be easily judged from its characteristic curves known as motor characteristics. The characteristics of a motor are those curves that show relation between the two quantities. On the basis of these quantities, the following characteristics can be obtained: The following important relations must be kept in mind while…

It has been seen that the same machine can be used as a DC generator or as a DC motor. When it converts mechanical energy (or power) into electrical energy (or power), it is called a DC generator and when it is used for reversed operation, it is called a DC motor. Table 11.1 gives the comparison…

In the case of motors, the mechanical power (H.P.) available at the shaft is known as brake horse power (BHP). If Tsh is the shaft torque in Nm and N is speed in rpm, then useful output power = ωTsh = 2π NTsh/60 W

In DC motors, whole of the electromagnetic torque (Te) developed in the armature is not available at the shaft. A part of it is lost to overcome the iron and mechanical (friction and windage) losses. Therefore, shaft torque (Tsh) is somewhat less than the torque developed in the armature. Thus, in the case of DC…

We know that when a current carrying conductor is placed in the magnetic field, a force is exerted on it that exerts turning moment or torque (F × r) (see Fig. 11.38). This torque is produced due to electromagnetic effect, and hence it is called electromagnetic torque. Fig. 11.38  Force exerted on a single conductor Let P = number of poles…

The current flowing through the armature is given by the relation:   When mechanical load applied on the motor increases, its speed decreases that reduces the value of Eb. As a result, the value (V − Eb) increases that consequently increases Ia. Hence, motor draws extra current from the mains. Thus, the back emf regulates the input power as per the extra…

The function of a commutator in DC motors is to reverse the direction of flow of current in each armature conductor when it passes through the MNA to obtain continuous torque. 11.16  BACK EMF It has been seen that when current is supplied to the armature conductors, as shown in Figure 11.36(a), placed in the main magnetic…