A shunt de motor has the following characteristics: Vt=640 V, Ra=0.6 Ω, Rf=120 Ω, and Ka ϕ=0.46

step 1 In this problem first time we'll draw the magnetization curve. So just look at it carefully. The

A shunt de motor has the following characteristics: Vt=640...

A shunt de motor has the following characteristics: $V_t=640 \mathrm{~V}, R_a=0.6 \Omega, R_f=120 \Omega$, and $K_a \phi=0.46 I_f$. If the motor is used to drive the wheels of a subway train, find the torque available for starting if a $20 \Omega$ resistor is connected in series with the armature.

Key Concepts

Effect of Series Resistor in Armature Circuit

Adding a resistor in series with the armature increases the total circuit resistance, which in turn reduces the armature current available at startup. This reduction in current affects the starting torque, since the torque is directly related to the armature current. The concept underlines the trade-off between protecting the motor or controlling the starting current and maintaining adequate starting torque for performance.

Starting Torque in DC Motors

Starting torque refers to the maximum torque a DC motor can generate when starting, i.e., when the motor’s speed is zero and the back EMF is absent. The torque produced is directly proportional to the product of the armature current and the magnetic flux, with a constant of proportionality determined by motor design. Accurately computing the starting torque is important for applications requiring high initial torque, such as driving heavy loads from a standstill.

Armature Circuit Analysis

Armature circuit analysis involves evaluating the electrical path that supplies current to the motor’s armature. This includes considering the applied voltage, inherent resistances (like the armature resistance), and any additional resistors in series. At startup, when the motor is stationary, the back EMF is zero, so the current is determined solely by the applied voltage and total resistance. This analysis is fundamental for calculating the initial armature current, which directly affects the starting torque.

DC Shunt Motor

This concept refers to a type of DC motor where the field winding is connected in parallel with the armature. In a shunt motor, the field flux remains relatively constant, which leads to a stable speed under varying loads. Understanding the configuration of a shunt motor is crucial as it influences the behavior of the motor, particularly in terms of how it develops torque and maintains speed during different operating conditions.

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