In a synchronous motor if the back emf generated in the armature at no load is approximately equal to the applied voltage, then the
Right Answer is:
Excitation is 100%
SOLUTION
Effect of Change of Excitation on No-Load in Synchronous Motor
To find how the excitation affects the working of the synchronous motor, let us assume that it is an ideal machine having no armature winding resistance and no rotational losses. Let us also assume that the motor is connected to an infinite bus so that its terminal voltage and the supply frequency can be taken as constant.
The ideal motor on no-load requires no armature current. This is possible only when the applied voltage is equal and opposite to the excitation emf. As the infinite bus frequency remains unchanged, the ideal motor runs at a constant speed and so the excitation emf can be changed only by changing the excitation (or field) current.
When the excitation current is adjusted so as to make the excitation emf, Eb, equal to the applied voltage, V. the excitation is said to be normal or 100% excitation. Since no power is developed by the motor, the power angle, φ is zero.
If the excitation current is increased above the normal value, the excitation emf, Eb, increases and becomes greater than the applied voltage, and the motor is said to be over-excited. In over-excited condition Eb > V
If the excitation current is decreased below the normal value, the excitation emf, Eb, decreased and becomes less than the applied voltage, and the motor is said to be under-excited. In under excited condition Eb < V