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eeet-241/_Homework/hw15.md

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Skyler MacDougall

Homework 15: due 4/22/2020

  1. A 230V shunt motor has a nominal armature current of 60A. If the armature resistance is 0.15\Omega, calculate the following:

    1. The counter-emf [V]
    2. the power supplied to the armature [W]
    3. the mechanical power developed by the motor, [kW] and [hp]
    1. In the problem above, calculate the initial starting current if the motor is directly connected across the 230V line.
    2. Calculate the value of the starting resistor needed to limit the initial current to 115A.

  3. A separately excited DC motor turns at 1200r/min when the armature is connected to a 115V source. Calculate the armature voltage required so that the motor runs at 1500r/min, and at 100r/min.

  4. The following details are known about a 250 hp, 230V, 435 r/min DC shunt motor:

    • nominal full-load current = 862A
    • insulation class = H
    • weight = 3400kg
    • external diameter of the frame = 915mm
    • length of frame = 1260mm
    1. Calculate the total losses and efficiency at full-load.
    2. Calculate the approximate shunt field exciting current if the shunt field causes 20% of the total losses.
    3. Calculate the value of the armature resistance as well as the counter-emf, knowing that 50% of the total losses at full-load are due to the armature resistance.
    4. If we wish to attain a speed of 1100r/min, what should the approximate exciting current be?