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					                              BAE 4353                      12/3/2002

Electric Motors
• Classification / types
   –   DC Motors
   –   AC Motors
   –   Stepper Motors
   –   Linear motors
• Function
   – Power conversion - electrical into mechanical
   – Positional actuation – electrical signal to position




                                                                        1
                                BAE 4353   12/3/2002

DC Motors
  – DC Motors
     • Fundamental characteristics
         – Basic function
     • Types and applications
         –   Series
         –   Shunt
         –   Combination
         –   Torque characteristics
     • Modelling




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                                     BAE 4353                       12/3/2002

Fundamental characteristics of DC Motors

                            Stator                                      Stator
               S                Stator                   S                  Stator
               N                Coils                                       Coils
                                                    N           S
    N                       N                   N                    N
        S               S

             Rotor                              S       Rotor       N

   SN                  N
                            S                   S                   S
                                                    N           S
                S
                N                                         N

            End view                                 End view
            Time 0                                   Time 0+



        Shifting magnetic field in rotor causes rotor to be forced to turn
                                                                                     3
                              BAE 4353                         12/3/2002

Nature of commutation
• Power is applied to armature                            V+
  windings                                       Stator
                                                               V+
   – From V+                                                         Brush
                                                     N               Assembly
   – Through the +brush
                                             S
   – Through the commutator          Rotor
     contacts                                         N

   – Through the armature (rotor)            S
     winding                                          N               Comutator
                                                                V-
   – Through the – brush                         Stator

   – To V-                                                V-
• Rotation of the armature
  moves the commutator,
  switching the armature winding
  connections
• Stator may be permanent or
  electromagnet


                                                                                  4
                                                               BAE 4353                                          12/3/2002

DC motor wiring topologies




                                                                                        Shunt Field
                                                                                                            Shunt

                            120

                            100              Sh u n                                                   Series Field
   Percent of rated Speed




                                                      t

                            80
                                                                                                        Series
                            60                                Co
                                                 Se                m
                                                      rie           po
                                                          s              un
                            40                                                d
                                                                                                      Series Field
                            20




                                                                                        Shunt Field
                             0                                                                         Compound
                                      100        200          300                 400
                                  0




                                       Percent of Rated Torque


                                                                                                                             5
                              BAE 4353                          12/3/2002

Series Wound DC motors
     • Armature and field connected in a series circuit.
     • Apply for high torque loads that do not require precise speed
       regulation. Useful for high breakaway torque loads.
         – locomotives, hoists, cranes, automobile starters
     • Starting torque
         – 300% to as high as 800% of full load torque.
     • Load increase results in both armature and field current increase
         – Therefore torque increases by the square of a current increase.
     • Speed regulation
         – Less precise than in shunt motors
             » Diminished load reduces current in both armature and field
                resulting in a greater increase in speed than in shunt motors.
         – No load results in a very high speed which may destroy the motor.
             » Small series motors usually have enough internal friction to prevent
                high-speed breakdown, but larger motors require external safety
                apparatus.


                                                                                 6
                               BAE 4353                          12/3/2002

Shunt wound DC motors
     • Field coil in parallel (shunt) with the armature.
         – Current through field coil is independant of the armature.
             » Result = excellent speed control.
     • Apply where starting loads are low
         – fans, blowers, centrifugal pumps, machine tools
     • Starting torque
         – 125% to 200% full load torque (300 for short periods).




                                                                             7
                              BAE 4353                        12/3/2002

Compound wound DC motors
     •   Performance is roughly between series-wound and shunt-wound
     •   Moderately high starting torque
     •   Moderate speed control
     •   Inherently controlled no-load speed
          – safer than a series motor where load may be disconnected
              » e.g. cranes




                                                                          8
                                                                            BAE 4353               12/3/2002

Permanent magnet DC motors


                           120

                           100             Pe                                                           Permanent
  Percent of rated Speed




                                                rm                                     Permanent
                                                  an                                                     magnet
                                                       en                               Magnet
                           80                               tM                                            poles
                                                              ag
                                                                   ne
                                                                        t
                           60

                           40


                           20

                            0
                                 0   100          200                       300    400
                                      Percent of Rated Torque


                                                                                                                    9
                               BAE 4353                     12/3/2002

Permanent Magnet DC Motors
   – Have permanent magnets rather than field windings but with
     conventional armatures. Power only to armature.
   – Short response time
   – Linear Torque/Speed characteristics similar to shunt wound
     motors. Field magnetic flux is constant
       • Current varies linearly with torque.
   – Self-braking upon disconnection of electrical power
       • Need to short + to – supply, May need resistance to dissipate heat.
   – Magnets lose strength over time and are sensitive to heating.
       • Lower than rated torque.
       • Not suitable for continuous duty
       • May have windings built into field magnets to re-magnetize.
   – Best applications for high torque at low speed intermittent duty.
       • Servos, power seats, windows, and windshield wipers.


                                                                          10
                              BAE 4353                                                                           12/3/2002

Modeling DC motors
• A linear speed/torque curve
  can be used to model DC
  motors. This works well for                                           n   No load speed

  PM and compound designs                                     120
  and can be used for control
                                                              100
  models for narrow ranges for




                                     Percent of rated Speed
                                                                                       Id
                                                                                          e
  the other configurations                                    80                              al
                                                                                                   lin
                                                                                                         ea
• Model will assume!                                          60
                                                                                                              rm
                                                                                                                od
                                                                                                                     el
   – Linearity
                                                              40
   – Constant thermal
                                                                                                                                 Stalled rotor
     characteristics                                          20
                                                                                                                                    torque
                                                                                                                                 s
   – No armature inductance                                    0
   – No friction in motor                                           0          100             200                        300   400
                                                                                 Percent of Rated Torque




                                                                                                                                         11
                                    BAE 4353                         12/3/2002

  DC Motor modeling
                                                   T,

   From the circuit
                                                   Armature
    V  IR  Eb




                                +
                                    V          R     E (back emf)
   Motor equations
      Eb  K e                         I

                                                                     Power is:
      T  Kt I
                                                                               R  2
  Substituting the above:   And no-load speed                                  K K T
                                                              P  T  T n       
      T                                                                        e t
  V      R  K e                     V
      Kt                         n 
                                       Kt                           Max power is:
     V       T
              R          In terms of no-load speed
     Kt Ke Kt                                                                V2
                             torque/speed equation is:               Pm ax 
                                                                             4R
For stalled rotor torque
                                        R                         Units:
          KV
      Ts  e                            K K T
                                n       
                                        e t                         K e  [Vs / rad ]
           R
                                                                      K t  [ Nm / A]

                                                                                        12
                               BAE 4353                      12/3/2002

Application
• Use motor voltage and no-load speed to calculate Kt
• Kt = Ke in SI units
• Use stalled rotor torque, V, and Ke to find R
    – Note, R varies with speed and cannot be measured at rest
• See web download for explanation of Kt, Ke:
 http://biosystems.okstate.edu/home/mstone/4353/downloads/
Development of Electromotive Force.pdf




                                                                         13
                             BAE 4353           12/3/2002

DC motor control – H-bridge
• Switches control direction                12V
   – “A” switches closed for
     clockwise
                                        A                   B
   – “B” switches for counter-
     clockwise                              M
• PWM for speed control                                     A
                                        B
   – “A’s” duty cycle for clockwise
     speed
   – “B’s” duty cycle for counter-
     clockwise speed
• Can be configured to brake
   – Bottom “B” and “A” to brake



                                                                14
                             BAE 4353                 12/3/2002

H-Bridge implementation
• Elements in box are
  available as single IC
                                                  M
                                                DC Motor

                           Vsupply



                           PWM
                                        Input
                      Direction
                                        Logic
                           Brake
                                                       H-Bridge Circuit
                       Ground




                                                                          15
                                BAE 4353                            12/3/2002

Brushless designs
• Commutation is done
  electronically
                                              +V                             +V
   – Encoder activated switching
   – Hall effect activated switching
   – Back EMF driven switching
• PM armature
• Wound/switched fields                                                              Field


• Application
   – Few wearing parts (bearings)
                                                                                  Armature
   – Capable of high speed
   – Fractional HP
       • Servos                            Optical Encoder

       • Low EMC
                                                      Encoder activated switching




                                                                                             16
                                BAE 4353                         12/3/2002

Stepper Motors
• Description
   – Generally a two phase motor
   – permanent magnet rotor and wound fields
   – Rotor normally has many poles
       • 200 poles = 1.8 degrees per step
   – Used primarily for position or velocity control
   – Typically no position feedback
       • Torques are managed so that an intended step is always achieved
           – Accelerations, decelerations and loads must be managed intelligently

• Two general types of windings
   – Unipolar
   – Bi-polar



                                                                                    17
                            BAE 4353                     12/3/2002

Winding configurations
• Bi-polar design
                                          12V




   – 6 wire
                                          N
                    12V




                                          S




                                        12V




                                              H-Bridge




• Unipolar design                                N
                             H-Bridge




   – 4 wire
                      12V




                                                 S




                                                                     18
                               BAE 4353             12/3/2002

AC Motors
• AC Motors
  – Fundamental characteristics
  – Types
     • Fractional horsepower (single phase)
     • Integral
         – Single phase (Cap start Induction run)
         – Three phase
  – NEMA Torque characteristics
  – Modelling




                                                                19
                              BAE 4353                       12/3/2002

Fractional horsepower designs
  – Shaded Pole (low starting torque, simple, cheap)
      • uses a short circuited coil embedded in face of field to cause one
        side of field to be magnetized before the other
  – Split phase (low starting torque)
      • Two windings (2-phase), one with high resistance hence different
        RL and phase
      • Centrifugal switch on starting winding
  – Capacitor Start Induction Run (medium starting torque)
      • Two windings (2-phases)
      • Capacitor used on second winding to create leading phase
      • Centrifugal switch on starting winding
  – Universal? (intermittent use, brushes!)
      • DC motor with inductance managed to allow AC operation
  – Synchronous (clocks, synchronization)
      • Permanent magnet rotor always in phase with AC
                                                                             20
                        BAE 4353                                           12/3/2002

AC motor model
• See Siemens AC motor                  Rs              Ls            Lr

  info for modeling info.
                                      Lm (Magnetizing Inductance)   E Iw         Rr


                                       E                                    E - Magnetizing voltage
                             Im                  
                                                     E
                                                               T  kI w    Im - Magnetizing current
                                    2 f Lm          f                      f - Frequency
                                                                            T - Torque
                                     Is  Im  Iw
                                           2    2
                                                                            Iw - rotor current
                                                                             - Magnetic Flux, rotor




                                                                                                 21
                              BAE 4353                        12/3/2002

AC Motors
• Relationship between number of poles and motor synchronous
  speed
                        Poles        Synchronous
           120 f                        Speed
      Ns 
             P                          (RPM)
                             2                   3600
                             4                   1800
                             6                   1200

• Squirrel cage motors must operate with some slip .5 to 8% to allow
  the rotor to be magnetized.
   – Actual speed is synchronous speed reduced by the slip.
                                 (100  %slip)
                          N  Ns
                                      100
                                                                          22
                       BAE 4353   12/3/2002

Squirrel Cage Rotor




    Seimens AG, 2002




                                              23
                               BAE 4353                    12/3/2002

Inducing magnetism in the rotor
• Difference between
  angular velocity of rotor
  and angular velocity of
  the field magnetism
  causes squirrel cage
  bars to cut the field
  magnetic field inducing
  current into squirrel cage
  bars.                                   Rotor
• This current in turn
  magnetizes the rotor
                                     N                                 S
                                            Difference in
                                           rotation of field
                                           magnetism and
                                            rotor rotation




                                                                           24
                                                       BAE 4353                                      12/3/2002

Torque/speed curve

      % of Full-Load Torque
                              250                       Pull-up Torque           Breakdown
                                        Locked rotor                               Torque
                                          torque
                              200

                              150                                                            Full-Load Torque



                              100
                                                                         Slip (Full load)
                              50

                               0
                                    0      20          40        60            80           100
                                                % of Synchronous Speed
                                                                                                                 25
                                                      BAE 4353                            12/3/2002

Typical starting current

                                    700

                                    600
           % of Full-Load Current
                                    500
                                            Locked Rotor
                                                                      Full-Load Current
                                          (Starting Current)
                                    400

                                    300

                                    200


                                    100

                                     0
                                                               Time


                                                                                                      26
                            BAE 4353                  12/3/2002

Motor characteristics
• Enclosure / frame          60 Hz     50 Hz
                             115        380
• Voltage / frequency        200        400
                             230        425

• 3 or 1 phase               460       220/380
                             575
• Poles / speed
• Service factor
   – Fraction of rated HP that motor can be operated at
• Insulation class/ Temp rise
   – (operating temperature compatible)
• NEMA Design A,B,C,D, etc. (Torque curve type)
   – See next page
• Efficiency
                                                                  27
                           BAE 4353                     12/3/2002

NEMA Torque characteristics summarized


                        BREAK-              FULL
 NEMA STARTING STARTING DOWN                LOAD        TYPICAL
DESIGN TORQUE CURRENT TORQUE                SLIP      APPLICATIONS
  A     Normal      High          High       Low     Mach. Tools, Fans
  B     Normal     Normal        Normal     Normal Same as Design "A"
                                                     Loaded compressor
  C      High      Normal             Low   Normal
                                                      Loaded conveyor
  D    Very high    Low           -------    High    High Punch Press




                                                                     28
                                 BAE 4353                            12/3/2002

NEMA Motor Characteristics
 Design    Locked    Pull-up    Breakdown     Locked          Slip         Efficiency
            Rotor    Torque       Torque       Rotor           %
           Torque     % FL         % FL       Current
            % FL                               % FL

A         70-275    65-190     175-300      NA          0.5-5            Med-High


B         70-275    65-190     175-300      600-700     0.5-5            Med-High
(most
common)
C         200-285   140-195    190-225      600-700     1-5              Med



D         275       NA         275          600-700     5-8              Low


E         74-190    60-140     160-200      800-1000    0.5-3            High




                                                                                    29
                             BAE 4353                                     12/3/2002

PWM Variable Frequency Drives
• Variable frequency drives use AC to DC converter then a
  DC to AC converter (inverter)
   – Inverter frequency and voltage output can be varied to allow
     motor speed to be varied.
   – Very efficient and cost effective variable speed for 1 HP and up
                                       650 V




                                               Control Logic
          L1
     480V L2                                                               M
          L3




                Rectifier     Filter                           Inverter
                                                                                      30

				
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