Hey everyone, My professor just posted this review question: “A motor connected

Question

Hey everyone,

My professor just posted this review question:

“A motor connected to a 120 V DC voltage source develops 320W of useful mechanical power. At the same time 22 W is lostthrough friction. The current from the voltage source is 3A. (a) What is the electric power supplied by the motor? (b)What is the motor’s efficiency? (c) What is the motor’sresistance?

He wrote that the answers are:
7. a. 360 W b. 89% c. 2

Unfortunately, I don’t understand how he got those. Couldsomeone please explain this to me? I will definitely rate high ifyou give a good explanation. Thanks so much!

Explanation / Answer

   a.   Total power drawn fromthe source   P   =   V* I   =   120 *3   =   360   Watt    b.   Efficiency       =   (Usefulpower output / Power supplied) * 100%                                     =   (320/ 360) *100%   =   88.8888%   ˜   89.0%    c.    Power loss due tobackEMF    Pback   =   (P- Pout) - mechanicalloss   =   360 - 320 -22   =   18   W       =>   Also   Pback   =   I2* R          18   =   32* R          Resistance ofcoil   R   =   18 /9   =   2       =>   Also   Pback   =   I2* R          18   =   32* R          Resistance ofcoil   R   =   18 /9   =   2

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