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Synchronous Motor - with Load Detailed Description II

Load Condition of synchronous motor:

If the synchronous motor is to be loaded, the following conditions need to be satisfied. The conditions are:

  • Motor should be able to run continuously at its synchronous speed even if it is loaded
  • Torque angle  δ should increase with increases in Load
  • Magnitude of excitation voltage Vo and field excitation Ef should remain constant as
                          V = Ef + Ia Xs
  • Armature current Ia drawn from the supply should increase
  • Phase angle ϕ should increase in the lagging direction

As the load on shaft increases the speed of rotor decreases. After sometime it increases by increasing the power from line and the speed of rotor increases. Such that rotor starts to rotate at Synchronous speed as the rotor slips back in space to increase the torque angle. As the torque angle increases the induced torque increases.

Above conditions are derived from the equation:

    P=V Ef sinδ /Xs = V Ia cos ϕ 
      Ef sinδ = Xs/V x P = KP
Where,
      K = Xs/V = constant

From the equation it is clear that increases in torque increases the rotor speed. After sometime motor regains it's synchronous speed with increases in torque angle

As the load increases the torque angle δ increases until the Condition called pull out of synchronism. After that motor will stop running.

Pull out torque:
It is defined as the maximum torque at which the synchronous motor can develop rated voltage and frequency without losing synchronism. The value of synchronous motor varies from 1.5 to 3.5 times of full torque.

Hunting/ Surging/Phase Swinging:

These factors are mainly caused by:

  • Varying Load
  • Pulsating supply frequency

If Load on shaft increases, it's rotor falls back by "beta". If Load increases further beta increases and cope with the changed Load. If Load increases further beta increases and attain the value again to maintain synchronism. If Load decreases beta decreases and oscillates like pendulum.

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