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MOTOR CONTROL STARTER

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MOTOR CONTROL STARTER Powered By Docstoc
					    Motors & Motor Starters
Prepared By:
                  Erik Redd
                      &
               Jeremy Roberts
                         Motors
                    AC-Motors
Parts of an Electric Motor
       A. Stator : Stationary Frame
       B. Rotor : Revolving Part

The rotary motion in an ac-motor is caused by the
       fundamental law of magnetism.
This law states that like poles repel and unlike poles attract.
       Diagram of an ac-motor
  This shows a three
  phase, two pole stator.

Where A, B, and C are
 the three phases
          Diagram of the Three Phases
Fig. 13-2 Pg. 244
Poles 1 and 4 are at their greatest magnetic field at
  time equal to one, because phase A (red line) is
  connected to those poles, and the same for the
  other poles when their corresponding phases are at
  maximum current magnitude.
        Synchronous Speed
Speed at which it takes the motor to go one cycle and
  one revolution.

S=[120*frequency}]
     (# poles)

Example:
For a three-phase, 60 Hertz, 2 pole motor:
S=[120*60]/2=3600 revolutions per minute
           Polyphase Squirrel-Cage
               Induction Motors
•   The most common three-phase
    motor
•   Does not have solid poles
•   Instead, it has laminations:
    numerous flat sheets held together
    in a package. They are insulated
    from each other (this reduces Eddy
    currents) making up the stator
•   The difference between induction
    and synchronous motors is that the
    rotor for an induction motor can
    travel at a different speed than the
    stator. This is called Slip.
•   slip= Syn. rpm – Motor rpm *100
                   Syn. rpm
Example.
A 2 pole, 60 Hz motor runs at a full-load
 speed of 1760 rpm.
What is the slip?
Ans. %slip= 3600-1760*100
               3600
=51.1%
        Single-Phase Motors
 Supplied by single source of ac voltage
 Rotor must be spun by hand in either direction,
  does not have a starting mechanism
 Has no starting torque
 Three different types of single-phase motors: split-
  phase, capacitor start, permanent split-capacitor,
  and shaded-pole motors
    Resistance Split-Phase Motors
   Has a start winding and a main
    winding
   Winding currents are out of
    phase by 30 degrees, this
    produces a flux field that starts
    the motor
•   Main winding current (IM) and
    start winding current (IS) lags
    supply voltage (VL)
   Start (inrush) current is high
   Needs centrifugal starting
    switch or relay to disconnect
    the start winding (protects it
    from over heating)
   Efficiency is between 50-60%
          Capacitor-Start Motors
   Has the same winding and
    switch mechanism arrangement
    as split-phase but adds a short
    time-rated capacitor in series
    with the start winding
   The time shift phase between
    the main and start winding is
    close to 90 degrees
   IS leads VL
   Efficiency is between 50-65%
   Capacitor controls the inrush
    current
        Permanent Split-Capacitor
               Motors
   Winding arrangement is the
    same as the capacitor and split-
    phase motors
   Capacitor can run continuously,
    rated in microfarads for high-
    voltage ratings
   No centrifugal switch is needed
   IM lags VL, while IS leads VL
•   Efficiency is between 50-70%
             Shaded Pole Motors
   Simple construction, least
    expensive
   Has a run winding only,
    shading coils are used instead
    of the start winding
   Stator is made up of a salient
    pole, one large coil per pole,
    wound directly in a single large
    slot
   A small shift in the rotor causes
    torque and starts the motor
   Efficiency is between 20-40%
                   DC Motors
•   Consists of an armature winding and a stator
    winding
•   Armature windings act as the rotor
•   Has three different classifications: constant torque,
    constant horsepower, or a combination of the two
•   Standard industrial dc motors are shunt wounded
•   Modifications of the dc motor are: shunt wound,
    stabilized shunt exciting fields, compound wound
    motors, and series wound motors
    Armature Voltage Control
 Is used for motor speeds below base speed
 Output torque= T=k*ø*IA
  k is machine constant
  ø is the main pole flux
  IA is the armature current
        Shunt Field Control
 Is used for motor speeds above base speed
 Horsepower, (HP)= Torque*rpm
                         5252
        Where torque is in lb-ft
           Speed Regulation
   Speed Regulation
    (IR)= no load rpm- full load rpm
                   full load rpm
       Brushless DC Motors
 Three phase ac power is converted into dc
  by the input side of the motor to charge up a
  bank of storage capacitors
 These capacitors are called the Buss
 The purpose of the buss is to store energy
  and supply dc power to transistors in the
  output side as the motor requires the power
  to start up
         Brushless DC Motors
   Figure 13-21, page 264 shows the input power
    section
   It consists of three fuses, six diodes, a choke, and
    two capacitors
   The fuses protect the diodes
   The choke protects against line transients
   The motor control may run at very low speeds at
    very high torques while drawing little current from
    the ac line
         Brushless DC Motors
   This picture is a
    representation of the
    encoders (rotor part of
    the motor) telling the
    corresponding
    transistors (stator) to
    turn on in order to get
    maximum torque from
    the motor
Picture of a Brushless Motor
       Motor Control Starters
 Motor will draw high inrush current while the
  starter will slow current down
 Starter reduces the amount of torque needed to
  start the motor
       Magnetic Motor Starter
 Normally open contacts
 Not always possible to control amount of
  work applied to the motor
 Has overloads
    – Motor may be overloaded resulting in damage
      to the motor
    – Open due to excessive motor current, high
      temperature, or a combination of both
         Full-Voltage Starter
 Contains one set of
  contacts
 Motor is directly
  connected to the line
  voltage
     Reversing Motor Starter
 Contains two starters of equal size
 Two starters connect to the motor
 Interlocks are used to prevent both starters from
  closing their line contacts at the same time
 Figure 14-4A
    Reduced-voltage Motor Starter
 Applies a percentage of the total voltage to start
  (50% - 80%)
 After motor rotates, switching is provided to apply
  full voltage
 Torque will be reduced when starting
 Four types:
       1) Autotransformer
       2) Primary Resistance
       3) Wye – Delta
       4) Part Winding
       Autotransformer Starter
   Two contactors are used:
      1) Start contactor
         - Closes first and connects motor to the line
           through an autotransformer
         - Deenergizes
      2) Run contactor
         - Motor switches to this contacter which has
           full voltage
      Primary Resistor Starter
   Two contactor
      1) Line contactor
         - First to energize connecting motor to the
           line voltage through a resistor
         - After preset time, contactor opens
      2) Accelerating contactor
         - Energizes
         - Causes smooth acceleration to full voltage
           Wye – Delta Starter
   Three contactors are used
     1) Line contactor and start contactor
         - Energizes first and connects motor in wye
           putting about 58% of line voltage across
           each motor phase
         - Contacts open after preset time
     2) Run contactor
        - Energizes connecting motor in delta and
           putting full voltage on the motor
            Part Winding Starter
   Starter supplies about 48% of normal starting torque
   Not truly a reduced-voltage means
   Two Types
    1) Two-Step - one winding connected to
        full voltage line and, after a preset time,
        the other connects
    2) Three-Step – one winding is connected in series
        with a resistor to the voltage line; after interval, resistor
        is shorted out and then second line is connected to
        full voltage line
    Solid-State Motor Starter
 For lower starting
  torque and smooth
  acceleration
 Used on conveyors,
  pumps, compressors,
  etc.
Standard Modes of Operation
 Motor voltage gradually increases during
  acceleration
 Creates a kick start pulse of 500% of full
  load amperage for high friction
 Used when necessary to limit current
 Used when motor requires a full voltage
  start

				
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posted:2/23/2012
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