Can an electrostatic motor produce constant torque at varied speed?
There are many types and configurations of electromagnetic motors. Many of those configurations can be operated in a way that the motor has the capacity to produce constant torque with variable speed over a substantial speed range.
Electrostatic motors are generally duals of various types of electromagnetic motors. As a dual, various aspects of an electrostatic motor are reversed with respect to their electromagnetic duals. In electromagnetic motors, speed is generally proportional to armature voltage and torque is generally proportional to armature current. In electrostatic motors, armature current is proportional to speed and armature voltage is proportional to torque. Electrostatic motors have some capability for constant torque with variable speed.
There are technical papers describing current and recent electrostatic motor research. Some of them may be openly available. I will see what I can find.
I believe these are open source:
Aditya N. Ghule and Daniel C. Ludois, High Torque Density Macro-scale Electrostatic Rotating Machines: Electrical Design, Generalized d−q Framework, and Demonstration IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 55, NO. 2, MARCH/APRIL 2019
Aditya N. Ghule, Peter Killeen, and Daniel C. Ludois, Synchronous Electrostatic Machine Torque Modulation via Complex Vector Voltage Control With a Current Source Inverter IEEE JOURNAL OF EMERGING AND SELECTED TOPICS IN POWER ELECTRONICS, VOL. 8, NO. 2, JUNE 2020