Useful as an upgrade or a spare for Hegner scroll saws. 

Single speed machines have a standard motor with a double pole  ON/OFF switch.

Variable speed machines have a different motor with a shaft encoder and V/S control

With the exception of the industrial rated Polycut machine the motor and switching is identical across the range of Hegner scroll saws

Single speed motors/machines  cannot be upgraded to variable without replacing the motor

Hegner scroll saws are one of these variants

Single Speed  - with switch under the table

Variable Speed  - with dial up controller situated under the work table

Variable Speed - with the speed control integrated into a footswitch controller