The value of ? for which the mechanical power developed is maximum can be obtained as,
When Ra is negligible, ? = 90o and cos (?) = 0 hence,
Solving the above quadratic in Eb we get,
is completely dependent on excitation, the equation (8) gives the
excitation limits for any load for a synchronous motor. If the
excitation exceeds this limit, the motor falls out of step.
Assume load constant hence ? constant.
but ? = ? for Pm = (Pm)max
Substitute cos? = Ra/Zs
This is the required condition of excitation.
Note : Note that this is not maximum value of but this is the value of foe which power developed is maximum.
The corresponding value of maximum power is,