Exam5Solutions-Spring08

Exam5Solutions-Spring08 - EEL 3211 Test 5 May 1, 2008 Time...

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EEL 3211 Test 5 May 1, 2008 Time limit: 50 minutes, Closed note, closed book, one (1) 8½ x 11 sheet of notes allowed. 1. A separately excited DC motor is shown below. The field current and terminal voltage V T are assumed constant. a. (10 %) Assuming that the motor is initially stationary. If the switch closes at t = 0, describe in words, clearly and briefly, using equations to support you explanation, how the motor achieves its steady state speed under no-load conditions. Neglect all losses and armature reaction. Be sure to specify the behavior of armature current I A and motional emf E A in your description. Brevity, clarity, and neatness count! When the switch is closed a current will flow in the loop. Since the loop is initially stationary, the induced voltage A E will be zero and maximum current (equal to T A V R ) will flow. This current produces a rotor magnetic field which interacts with the field of the magnet producing a torque RM BB t ∝× rr and the rotor begins to turn. But as the rotor begins to turn it induces a voltage A E causing the current to decrease since TA A A VE I R - = . As the current decreases so does the torque, and, with no load, the machine winds up in steady state with zero net torque and the induced voltage equal to the battery voltage.
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This note was uploaded on 07/28/2011 for the course EEL 3211 taught by Professor Staff during the Summer '08 term at University of Florida.

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Exam5Solutions-Spring08 - EEL 3211 Test 5 May 1, 2008 Time...

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