Consider a shunt-connceted de motor that has the...

Consider a shunt-connceted de motor that has the magnctization curve shown in Figure 16.19 on page $786 .$ The de supply voltage is $V_{T}=200 \mathrm{V},$ the armature resistance is $R_{A}=0.085 \Omega,$ the field resistance is $R_{F}=10 \Omega$, and the adjustable resistance is $R_{\mathrm{ad}}=2.5 \Omega$. At a speed of $1200 \mathrm{rpm}$ the rotational loss is $P_{\text {tot }}=1000 \mathrm{W}$. If this motor drives a load that demands a torque of $T_{\text {out }}=200 \mathrm{Nm}$ independent of speed determine the motor speed and efficiency.

Consider a shunt-connceted de motor that has the magnctization curve shown in Figure 16.19 on page $786 .$ The de supply voltage is $V_{T}=200 \mathrm{V},$ the armature resistance is $R_{A}=0.085 \Omega,$ the field resistance is $R_{F}=10 \Omega$, and the adjustable resistance is $R_{\mathrm{ad}}=2.5 \Omega$. At a speed of $1200 \mathrm{rpm}$
the rotational loss is $P_{\text {tot }}=1000 \mathrm{W}$. If this motor drives a load that demands a torque of $T_{\text {out }}=200 \mathrm{Nm}$ independent of speed determine the motor speed and efficiency.

Key Concepts

Shunt-Connected DC Motor

This concept involves a DC motor where the field winding is connected in parallel (shunt) with the armature winding. In such motors, the field flux is relatively constant over a wide range of loads due to the voltage being applied directly across the field winding. This characteristic makes shunt motors well-suited for applications requiring nearly constant speed despite load variations.

Magnetization Curve

The magnetization curve represents the relationship between the field current and the magnetic flux produced in the motor. It is essential for understanding how variations in field current affect the flux and, consequently, the developed back EMF and motor speed. The shape of this curve can significantly influence the performance of the motor, especially under varying load conditions.

Armature and Field Circuits

This concept pertains to the analysis of the separate electrical circuits within a DC motor. The armature circuit involves the current-carrying conductors where voltage drops due to resistance affect performance, while the field circuit provides the magnetic field necessary for operation. Understanding how resistances in these circuits (such as adjustable or series resistances) affect current distribution is crucial for modeling motor behavior.

Motor Speed and Torque Relationship

In DC motors, the motor speed is inversely related to the field flux and directly influenced by the back EMF. The developed torque is a function of both this flux and the armature current. Analyzing the balance between the load torque demand and the motor’s ability to produce torque, while considering losses, is key to determining the operating speed and efficiency under a given load.

Losses in Electrical Machines

Losses in DC motors include copper losses (due to the resistance in the armature and field circuits) and rotational losses (which encompass friction, windage, and core losses). These losses must be accounted for when determining the net power available for doing useful work, and they have a direct impact on the overall efficiency of the motor.

Efficiency Calculation in Motors

Efficiency in DC motors is determined by the ratio of mechanical output power to the total electrical input power, after accounting for various losses. Calculating efficiency requires a careful analysis of all forms of losses present in the motor, as well as an understanding of how the supply voltage and resistive drops affect the available power for mechanical work.

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