# DC MOTORS SEE 3433 ELECTRICAL MACHINES. DC MOTOR - Shunt motors - Separately excited - Series (we’ll look briefly)

## Presentation on theme: "DC MOTORS SEE 3433 ELECTRICAL MACHINES. DC MOTOR - Shunt motors - Separately excited - Series (we’ll look briefly)"— Presentation transcript:

DC MOTORS SEE 3433 ELECTRICAL MACHINES

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

DC MOTORS

+ +- - DC motor DC MOTOR

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

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

DC MOTOR Hoist

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

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

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

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

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

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

VtVt  Varying V t Constant T L

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

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

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

TeTe  Maximum Torque capability  base

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

0 to  base  armature voltage, above  base  field flux reduction

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

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

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 ?

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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