Thanks for the question.
No, the alternator load is not kept constant, the load variations are handled by the power network.
In order to keep the frequency constant at keying, additional torque is given by the drive motor at key-down to match the increased power to the antenna. The drive motor is an induction motor. The supply lines to the motor are equipped by transductors, i.e. choke coils with variable inductance, so the voltage to the drive motor can be adjusted between 1500 and 2300 V. The rotor winding of the drive motor is connected to variable liquid resistors, so the rotor resistance can be varied over a wide range. At key-down the inductance of the transductors is reduced and also the rotor resistance is reduced, both measures contributing to increased torque. The additional torque can easily be adjusted by the liquid resistors to match the increased load. A German radio amateur has analyzed the frequency variation at keying and found it to be within +- 10 Hz.
Ola Hernvall /AlexanderSAQ