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DC_examples

# DC_examples - DC Motors Numerical examples 1 A dc...

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DC Motors Numerical examples 1. A dc separately excited motor of 37.3 kW, 250 V, 1200 rpm has armature resistance R a = 0.2 . Its rated field current is I f = 2.17 A, and field winding resistance is R f = 115 . The motor when operating at full load (rated torque) is drawing an armature current I a = 200 A. The rotational losses are 4700 W. a) Find the rotor speed at no load. b) What is the current and the torque at standstill ( ϖ = 0). c) Draw the speed-torque characteristic of the motor. Solution: a) The rotor angular speed a a m E f V R I K ϖ - = Φ . If f f I Φ : , then E f f f K K I Φ = . At no load: m f f V K I ϖ = Electromagnetic torque: em T f a T K I = Φ Since f f I Φ : , then em f f a T K I I = . Full (rated) torque: n n n P T ϖ = Rated power (on the shaft) of the motor: 37.3 n P kW = Since the rated angular speed: 2 2 1200 125.7 / 60 60 n n n rad s π π ϖ = = = The rated torque: 37300 296.8 125.7 n T N m = = The electromagnetic torque at full load: 37300 4700 334 125.7 n m em n P P T N m ϖ + ∆ + = = = The coefficient 2 334 0.77 2.17 200 em f f a T N m K I I A = = = Speed at no load: 250 150 / 0.77 2.17 NL rad s ϖ = = , and

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