Have you looked up how a dynamo operates?
Permanent magnet is the field so it sets initial flux
and voltage generated is flux X a constant for that particular machine X RPM
so. If increased voltage causes the machine to deliver more power to its load, well of course it'll require more power at the shaft.
In different words,
The work required is determined by the load power consumed. Power out = power in * efficiency
Stronger magnets mean you may not have to turn as fast to get the same power out. But, turning slower, you need more torque (have to push harder to produce the same power in), which may be desired, or not desired (depends on the drive system).
Depending on the design, different magnet strengths will affect the efficiency, but I'd consider that a secondary effect. (wow , effected a sentence using both effect and affect :) )
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