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Chapter: Electrical machines : Synchronous Motors

Torque and Power Relations

Motor Torque, Mechanical Power Developed

Torque and Power Relations

Motor Torque


Gross torque, T =9.55 Pm/Ns N-M where Pm = Gross motor output in watts = Eb Ia cos(d - ф) 

Ns = Synchronous speed in r.p.m.

Shaft torque, Tsh = 9.55 Pshout/Ns N-M


It may be seen that torque is directly proportional to the mechanical power because rotor speed (i.e., Ns) is fixed.


Mechanical Power Developed


Neglecting the armature resistance Fig: 2.25 shows the phasor diagram of an under-excited synchronous motor driving a mechanical load. Since armature resistance Ra is assumed zero. tanq= Xs/Ra = ¥ and hence q =90°.


Input power/phase = V Ia cos ф

Since Ra is assumed zero, stator Cu loss (I Ra)2 will be zero. Hence input power is equal to the mechanical power Pm developed by the motor.


Mechanical power developed/ phase, Pm = V Ia cos ф, referring to the phasor diagram in Fig: .

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