Slide 1 by tLGoNhJ


									           Synchronous Motors

• Constant-speed machine
• Propulsion for SS “Queen Elizabeth II”

  – 44 MW
  – 10 kV
  – 60 Hz
  – 50 pole
  – 144 r/min
   Synchronous Motors (continued)

• Construction

  – Stator identical to that of a three-phase
    induction motor – now called the “armature”

  – Energize from a three-phase supply and
    develop the rotating magnetic field

  – Rotor has a DC voltage applied (excitation)
   Synchronous Motors (continued)

• Operation

  – Magnetic field of the rotor “locks” with the
    rotating magnetic field – rotor turns at
    synchronous speed
           Cylindrical (Round) Rotor

Constructed from solid steel forging to withstand large
centrifugal stresses inherent in high-speed operation
Used for high speed, low inertia loads (low starting torque)
Salient-Pole Rotor
                 Excitation Windings
                  Salient-Pole Rotor
            with shaft-mounted DC exciter
Need carbon brushes to make
contact with the commutator
Salient-Pole Rotor with brushless excitation
       Synchronous Motor Starting

• Get motor to
  maximum speed
  (usually with no load)

• Energize the rotor
  with a DC voltage
The VARISTOR or resistor in shunt with the field winding prevents high
voltage from being induced during locked-rotor and acceleration.
The induced current helps to accelerate the rotor by providing additional
Brushless Excitation
                   How it works

• Frequency-sensitive Control circuit
  –   Looks at emf induced in the field
  –   fr = sfs
  –   At locked-rotor, s=1, fr = fs
  –   Close SCR1 – block current from field
  –   Open SCR2 – connect discharge resistor across the
         How it works (continued)

• As the speed approaches ns, fr gets small,
  fr = sfs approaches 0
  – Open SCR2 – disconnects the discharge resistor
  – Close SCR1 – allows field current to flow
        Salient-Pole Motor operating at
      both no-load and loaded conditions

Angle δ is the power angle, load angle, or torque angle
  Rotating Field Flux and Counter-emf

• Rotating field flux f due to DC current in the
  rotor. A “speed” voltage, “counter-emf”, or
  “excitation” voltage Ef is generated and acts in
  opposition to the applied voltage.
• Ef = nsfkf
        Armature-Reaction Voltage

• Rotating armature flux, ar is caused by the
  three-phase stator currents. The induced speed
  voltage caused by the flux ar cutting the stator
• Ear = nsarka
Armature-Reaction Voltage (continued)

• Ear = nsarka
• ar proportional to armature current Ia
• Ear = (Ia)(jXar)
  – where Xar = armature reactance (Ω/phase)
Equivalent Circuit of a Synchronous Motor
         Armature (One Phase)

        V  I R  I jX  I X  E
          T       a           a       a            l   a   ar   f

         X X X
              s           l           ar

        V  E  I (R  jX )
          T           f           a            a       s

        V E I Z
          T           f           a        s
Phasor Diagram for one phase of a
  Synchronous Motor Armature
 Synchronous-Motor Power Equation

• In most cases, the armature resistance is
  much smaller than the synchronous
  reactance, so the synchronous impedance
  Zs is approximately equal to jXs .
The Equivalent-Circuit and Phasor Diagram

                          IaXscosθi = -Efsinδ
  The Synchronous-Motor Power Equation

• VTIacosθi = -(VTEf/Xs)sinδ
• VTIacosθi = power input per phase = Pin,1Φ
• -(VTEf/Xs)sinδ = magnet power per phase
             developed by a cylindrical-rotor motor
             (a function of Ef and δ)
• Pin,1Φ = -(VTEf/Xs)sinδ is the synchronous-
                  machine power equation
• For three phases,
   – Pin = 3(VTIacosθi)  proportional to Iacos θi
   – Pin = 3(-VTEf/Xs)sinδ  proportional to Efsinδ

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