You’re a safety engineer reviewing plans for a university’s new high-rise dorm. The elevator motors draw 20 A and behave electrically like 2

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You’re a safety engineer reviewing plans for a university’s new high-rise dorm. The elevator motors draw 20 A and behave electrically like 2.6-H inductors. You’re concerned about dangerous voltages developing across the switch when a motor is turned off, and you recommend that a resistor be wired in parallel with each motor.
1) What should be the resistance in order to limit the emf to 70 V?
2) How much energy will the resistor dissipate?

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  1. Answer:

    a) R `= 3.5 ohms

    b) energy decipated =   560J

    Explanation:

    V = I . R

    R = V / I

    R `= 70 / 20

    R `= 3.5 ohms

    2)energy decipated = 1/2ij²

    energy decipated = 1/2 x 2.8 x (20)²

    energy decipated =   560J

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