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Published byElinor Hines Modified about 1 year ago

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DC MOTORS SEE 3433 ELECTRICAL MACHINES

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DC MOTOR - Shunt motors - Separately excited - Series (we’ll look briefly)

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DC MOTORS

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+ +- - DC motor DC MOTOR

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+ +- - DC motor Mechanical Load: fans, blowers, Compressors, +Va-+Va- +Vf-+Vf- TmTm T load Load torque opposing the motor torque DC MOTOR

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+ +- - DC motor +V-+V- +V-+V- F = mg TmTm T load

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DC MOTOR Hoist

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DC MOTOR - Some applications require the control the speed - Some applications require the control the torque - In order to control the torque or speed we need to know the T- characteristics of the motor and the mechanical load Intersections between the two characteristics will determine the operating point

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DC MOTOR Shunt motor V t = I a R a + E a I t = I a + I f E a = k +Vt+Vt RaRa R cf R cw ItIt IfIf IaIa T e = k I a TeTe T load Mechanical load k = V t - I a R a

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Three possible methods of speed control: Field flux Armature voltage V t Armature resistance R a

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TeTe TLTL Vt ↓Vt ↓ Varying V t Requires variable DC supply

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TeTe TLTL Vt ↓Vt ↓ Varying V t Requires variable DC supply

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TeTe Varying V t Requires variable DC supply TLTL Constant T L

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VtVt Varying V t Constant T L

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Introduction TeTe Ra ↑Ra ↑ TLTL Varying R a Simple control Losses in external resistor

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TeTe TLTL Varying ↓ ↓ Not possible for PM motor Maximum torque capability reduces

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For wide range of speed control 0 to base armature voltage, above base field flux reduction Armature voltage control : retain maximum torque capability Field flux control (i.e. flux reduced) : reduce maximum torque capability TeTe Maximum Torque capability Armature voltage control Field flux control base Method of speed control in DC motor drives

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TeTe Maximum Torque capability base

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TeTe Constant power Constant torque base 0 to base armature voltage, above base field flux reduction P = E a I a,max = k a I a,max P max P max = E a I a,max = k a base I a,max 1/ P

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0 to base armature voltage, above base field flux reduction

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+Vt+Vt RaRa R cf R cw ItIt IfIf IaIa BUT there are problems !

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0 to base armature voltage, above base field flux reduction +Vt+Vt RaRa R cf R cw ItIt IfIf IaIa Controlling V t will also affect I f Controlling I f via R cf caused losses I 2 R

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0 to base armature voltage, above base field flux reduction Separately Excited DC motor DC supply for armature DC supply for field What if we have an AC supply ?

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Separately Excited DC motor AC to DC converter AC to DC converter 3-phase AC source Armature voltage controlField voltage control + V dc - + V dc -

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