DC Motor Drives by ewghwehws

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									   DC MOTORS

      SEE 3433
ELECTRICAL MACHINES
         DC MOTOR




- Shunt motors
- Separately excited
- Starter
DC MOTORS
      DC MOTOR



+ -   + -




  DC motor
           DC MOTOR
                                     Load torque opposing
+                 +                  the motor torque
Va                Vf
-                 -

     + -   + -


                       Tm    Tload




       DC motor


                            Mechanical Load: fans,
                            blowers, Compressors,
          DC MOTOR
+                +
V                V
-                -

    + -   + -


                     Tm   Tload




      DC motor




                                  F = mg
        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
                                          Vt = IaRa + Ea
                If             It
                                          It = Ia + If
         Rcf             Ia         +
                              Ra          Ea = k
                                    Vt
                                          Te = kIa
         Rcw
                                   
               Te                        k = Vt - IaRa

                                              Vt  IaR a
               Tload                     
                                                 k
     Mechanical
       load                                   Vt   Ra
                                                   2
                                                        Te
                                              k (k)
Three possible methods of speed control:

          Field flux

          Armature voltage Vt

          Armature resistance Ra




                                   Vt   Ra
                                          Te
                                   k (k) 2
                         Vt   Ra
                              2
                                   Te
                         k (k)
                                             Varying Vt
      
 Vt
kT                                   TL



                                               Vt ↓




                                               Te

               Requires variable DC supply
                          Vt   Ra
                               2
                                    Te
                          k (k)
                                              Varying Ra
      

 Vt                                    TL
kT

                                               Ra ↑




                                               Te

               Simple control
               Losses in external resistor
                         Vt   Ra
                              2
                                   Te
                        k (k)
                                                   Varying 


 Vt                                   TL
kT

                                                    ↓




                                                    Te

               Not possible for PM motor
               Maximum torque capability reduces
               Method of speed control in DC motor drives

Armature voltage control : retain maximum torque capability
Field flux control (i.e. flux reduced) : reduce maximum torque capability
For wide range of speed control
0 to base  armature voltage, above base  field flux reduction

                                                            Armature voltage control
                                                            Field flux control
          Te

                                                       Maximum
                                                       Torque capability




                                    base                           
Te

             Maximum
             Torque capability




     base              
  P Te      Constant torque         Constant power



   Pmax




                                 base                          


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

  P = EaIa,max = kaIa,max          Pmax = EaIa,max = kabaseIa,max
                                               1/
0 to base  armature voltage,   above base  field flux reduction
0 to base  armature voltage,    above base  field flux reduction


                        If           It


                  Rcf        Ia             +
                                   Ra
                                            Vt

                  Rcw
                                            




                 BUT there are problems !
0 to base  armature voltage,    above base  field flux reduction


                        If           It


                  Rcf        Ia             +
                                   Ra
                                            Vt

                  Rcw
                                            




              Controlling Vt will also affect If

              Controlling If via Rcf caused losses  I2R
0 to base  armature voltage,   above base  field flux reduction

                  Separately Excited DC motor




    DC supply                                        DC supply
   for armature                                       for field




               What if we have an AC supply ?
             0 to base  armature voltage,       above base  field flux reduction

                              Separately Excited DC motor
3-phase AC




                  AC to DC       +                               +
  source




                                                                          AC to DC
                  converter      Vdc                             Vdc      converter

                                 -                               -


                       Armature voltage control            Field voltage control
                             Starter in DC Motor


• At stand-still,  = 0  Ea = 0


                        Ra
                  Ia
            +                                         Vt
                                               Ia 
                                                      Ra
            Vt

            –                            eg, Vt = 100, Ra = 0.1  Ia = 1000 A !
                                    
                                 Starter in DC Motor


• We can limit Ia at start-up by:

           1) Controlling Vt using variable supply – e.g. using
              power electronics converter

           2) Adding external resistor  known as starter



                           Ia     Ra
                     +                                            Vt
                                        +              Ia                 When Ea = 0
                                        Ea                    R a  R st
                     Vt
                                        –
                     –       Rst




              • As speed builds up (so too Ea), Rst is gradually reduced
                       Starter in DC Motor


• As speed builds up (so too Ea), Rst is gradually reduced

                               Ia
 Starter circuit
                              Imax 1   2     3    4


                              Imin
   1    2     3    4

                                                             t (s)

                          speed




                                                              t (s)
                      Starter in DC Motor


Practical Starter circuit

								
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