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Chapter 1 of 12 тАв Page 1 of 248ЁЯФТ Protected PDF тАв Watermarked
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Back emf of synchronous motor depends on
Speed
Load
Load angle
All of above
Load angle
In a synchronous motor, the magnitude of the back EMF (EbтАЛ) depends solely on the rotor excitation (field current), while the load angle (╬┤) determines the stability and power transfer. Although EbтАЛ is fixed for a constant field current, the load angle changes to maintain equilibrium as the mechanical load changes.
In a synchronous motor, the magnitude of the back EMF (EbтАЛ) depends solely on the rotor excitation (field current), while the load angle (╬┤) determines the stability and power transfer. Although EbтАЛ is fixed for a constant field current, the load angle changes to maintain equilibrium as the mechanical load changes.
EbтАЛ=4.44f╬жTkwтАЛ тАФ Back EMF depends on frequency, flux per pole, and windings
PmechтАЛ=XsтАЛVEbтАЛтАЛsin(╬┤) тАФ Power developed depends on the load angle ╬┤
The synchronous motor operates at a constant speed dictated by the supply frequency. When a load is applied, the rotor poles experience a retarding torque, causing the rotor to shift its position relative to the rotating magnetic field of the stator. This angular displacement, known as the load angle ╬┤, adjusts the internal phase balance between V and EbтАЛ to accommodate the mechanical power demand without changing the magnitude of EbтАЛ itself.
The back EMF EbтАЛ is generated by the DC excitation current in the rotor field windings.
Since the rotor speed is fixed by synchronous speed NsтАЛ=120f/P, the speed does not affect EbтАЛ unless frequency changes.
The load angle ╬┤ is defined as the angle between the terminal voltage V and the back EMF EbтАЛ.
Constant speed operation
Ability to control power factor via excitation
Requires separate DC excitation
Not self-starting without additional equipment
Power factor correction
High-power constant speed industrial drives
The speed of a synchronous motor is strictly tied to the supply frequency and number of poles (NsтАЛ=120f/P).
Option A is incorrect because speed is fixed for a constant frequency.
Option B is incorrect because load causes a change in ╬┤, not the intrinsic magnitude of EbтАЛ itself.
C is correct тАФ The load angle ╬┤ is the phase shift that adjusts to balance the input and output torque, while EbтАЛ is primarily dependent on the excitation current (IfтАЛ).
Always remember that in synchronous machines, the magnitude of EbтАЛ is controlled by the field excitation, while the internal phase angle (╬┤) is controlled by the mechanical load.