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# hw7 - torque using your expression for current Although...

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EE321 Spring 2008 Homework 7 Problem 30 – Separately Excited DC Machine A separately excited dc machine has a armature resistance of 0.2 Ohms, a field resistance of 4 Ohms, L af of 200 mH. We wish to determine the currents (armature and field) in such a way as to achieve a torque of 2 Nm, while minimizing the loss. What should the armature and field current be ? What will the machine loss be at this point ? Problem 31 – Series Connected DC Machine Consider a series connected DC machine connected to an AC source with rms amplitude t V and radian frequency e ω . Assuming the speed is constant, compute the average torque in terms of r ω , t V , e ω , aa L , af L , ff L , a r , and f r . Note: the first part of this problem is to find the current, which can be done using phasor analysis. The second part of the problem is to find the average
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Unformatted text preview: torque, using your expression for current. Although this problem may seem strange at first, it is actually used in practice. This type of machine is commonly referred to as a universal machine (since it can be run from a dc or ac source), and is sometimes used in handheld tools such as corded variable speed electric drills. Problem 32 – PM DC Machine Parameters When spun at a speed of 1000 rad/s, the open circuit armature voltage of a PM DC machine is 100 V. Next, the sped of the machine is reduced to 900 rad/s, and a 100 V source is applied to the machine terminals. The armature current is 20 A. What are the electrical parameters of the machine? Note: this is basically a slight variation of the procedure we considered in class....
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