John is quite right, running the motor at such a low speed (relative to its design speed) is likely to destroy the router motor. Various effects come into play when one reduces the speed of the motor.
For example, the motor usually has a fan to draw air through the motor, thereby expelling heat from the rotor. As the fan slows down, it draws less air through. Heat is still being generated. The rotor heats up. Heat destroys. The windings are likely to go first -- the insulation breaks down and the windings short out. Time to throw out the router and get another one.
For such low speeds as 50 to 100 rpm, one should be using something that "geared down" (using either pulleys or gears.) I would look to using either a drill press or a hand drill.
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