Final answer:
The initial current drawn by the motor can be calculated by dividing the total voltage (120 V) by the total resistance (90 Ω), resulting in an initial current of 1.33 Amps when first starting the motor.
Step-by-step explanation:
The initial current drawn by a 120-V series-wound dc motor that has field resistance of 80 Ω and armature resistance of 10 Ω can be calculated using the basics of electric circuits and Ohm's law. When the motor is initially started, there is no back emf generated since it is directly proportional to the speed of the motor, which starts at zero. Therefore, the total voltage supplied is opposed only by the internal resistance of the motor.
To calculate the initial current (I), Ohm's law is applied which states that the current through a conductor between two points is directly proportional to the voltage across the two points (V) and inversely proportional to the resistance (R), mathematically represented as I = V / R. The total resistance of the motor can be found by adding the armature resistance (10 Ω) to the field resistance (80 Ω), resulting in a total of 90 Ω.
Using the given values:
I = V / R = 120V / 90Ω
Which gives us an initial current of 1.33 Amps.
It's important to remember that this value of current is only when the motor starts, and as it reaches its operating speed, a back emf will develop which will cause the operating current to be different from the initial current.