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Wound-Rotor Motor

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Torque-slip (speed) curves for a wound-rotor motor

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Curve 3 – T D,max @ s=1 Curves 4, 5 – T D,max @ s>1 s>1 – “plugging” – reversal of a motor before it comes to rest by interchanging 2 of the 3 line leads going to the stator. n r is negative!

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Example 5.10 page 198 A family of torque-speed curves for a y- connected, 400-hp, 2300-v, 14-pole, 60- Hz, wound-rotor induction motor is shown in Figure 5.10. Curves A and D show the extremes of rheostat adjustment. Determine –a) the range of rotor speeds available by rheostat adjustment, assuming 100% rated torque load on the shaft:

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Example 5.10 page 198 (continued) –b) the rheostat resistance required to obtain 260% rated torque when starting. The ratio of stator turns per phase to rotor turns per phase is 3.8, and the motor parameters, in Ω/phase are R 1 =0.403, R 2 =0.317, X 1 =1.32, X 2 =1.32, X M =35.46.

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Example 5.10 page 198 (continued) Solution for part (a) n r = n s (1-s) Determine n s –n s = 120f/P = (120)(60)/14 = 514.29 r/min Look at the intersections of the curves with the 100% torque line –for the low-speed curve, D, s = 0.395 –for the high-speed curve, A, s = 0.02

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Torque-speed curves for Ex 5.10

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Example 5.10 page 198 (continued) for the low-speed case, –n r = n s (1-s) = 514.29(1-0.395) = 311 r/min for the high-speed case, –n r = n s (1-s) = 514.29(1-0.02) = 504 r/min

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Torque-speed curves for Ex 5.10

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Example 5.10 page 198 (continued) for part (b) Curve B has locked-rotor torque of 260% T Dmax occurs at s = 0.74

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