Suggested by Sprague in the 90's documentation, this circuit applies to AC motors. The documentation does not indicate the maximum engine power that can be controlled.
This circuit was found in a 1992 publication, using an operational amplifier not very common...
This 13.8 V transistorized power supply with a current capacity of 20 A has a protection system...
This amplifier with 6 or 9 V power supply was sold in Japan in the 1960s. Its power was fractional watt...