Unit 4.0 Synchronous Motor

4.1 Working Principle & Starting of Synchronous Motor

• Working Principle:

  • A synchronous motor runs at synchronous speed, i.e., speed is locked with supply frequency.

  • Stator: supplied with 3‑phase AC → produces a rotating magnetic field.

  • Rotor: excited with DC → produces constant magnetic poles.

  • Synchronism occurs when rotor poles lock with stator’s rotating field.

• Starting:

  • Synchronous motors are not self‑starting.

  • Methods:

    • Damper winding / squirrel cage winding → motor starts as induction motor, then DC excitation applied.

    • Auxiliary motor starting → external motor brings rotor near synchronous speed.

    • Variable frequency starting → supply frequency gradually increased until synchronism.

1

Understand Construction

Rotor excited with DC, stator with 3-phase AC.

• Stator: 3-phase winding

• Rotor: DC excitation

• Produces constant poles

2

Starting Methods

Motor requires external help to reach synchronous speed.

• Damper winding (induction start)

• Auxiliary motor drive

• Variable frequency supply

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4.2 Equivalent Circuit & Phasor Diagram

• Equivalent Circuit: Similar to synchronous generator but current direction reversed.

• Phasor Diagram: Shows relation between terminal voltage (V), induced EMF (E), and armature current (I).

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4.3 Power Angle Characteristics

• Power developed (P):

$$P=\frac{EV}{X_s}\sin \delta$$

where $E$ = induced EMF, $V$ = terminal voltage, $X_s$ = synchronous reactance, $\delta$ = torque angle.

• Curve shows maximum power at $\delta ={90}^{\circ }$.

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4.4 Effect of Change in Excitation at Constant Load – V & Inverted V Curves

• V‑Curves: Plot of armature current vs. field current at constant load.

  • Shows minimum current at unity power factor.

• Inverted V‑Curves: Plot of power factor vs. field current.

  • Demonstrates how excitation affects power factor (lagging → unity → leading).

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• V‑Curve:

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• Inverted V‑Curve:

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4.5 Hunting and Its Prevention

• Hunting: Oscillations of rotor about synchronous position due to sudden load changes.

• Causes: Sudden load variation, system disturbances.

• Effects: Loss of synchronism, mechanical stress, power oscillations.

• Prevention:

  • Use of damper windings.

  • Proper system design and load management.

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4.6 Applications of Synchronous Motor

• Synchronous Phase Modifier: Operates at leading power factor → improves system PF.

• Constant Speed Drives: Maintains exact synchronous speed → used in textile mills, compressors, pumps.

• Other uses: Power factor correction, timing devices, industrial machinery.

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✅ Key Takeaway

Synchronous motors are unique for their constant speed operation and ability to operate at leading power factor, making them vital for power factor correction and industrial constant‑speed drives.