# Can an electrostatic motor produce constant torque at varied speed?

The idea of this question came from a DC motor that produces less torque by greater speed.

1. Can an electrostatic motor produce constant torque at varied speed?
2. How can I make a mathematic model of this type of motor (for comparison with a DC motor?)

For question #2 as far as I know $$\ F = \frac{k Q_1Q_2}{r^2} \$$ and the current will flow while charging/discharging the rotor.

Note: Referece motor for my question youtube video. THis is picture capture from video.

• A YouTube video is not a proper source for data. Find a data sheet for the device. Aug 28, 2020 at 17:38
• @Andyaka I just want to show what electostatic motor that I means to. I believe there are no datasheet for electostatic motor. Aug 28, 2020 at 17:40
• @Andyaka: The motor is a home made electrostatic motor that runs on high voltage. The video shows it running from a Van de Graaff generator, a 30kVDC power supply, and a Wimshurst machine. I don't think there's a datasheet to be had.
– JRE
Aug 28, 2020 at 17:44
• Then post a picture cause I'm not watching some potentially dodgy video on trust at my age with "her indoors" leaning over to see what I'm up to. Too much explaining = not worth the bother. Aug 28, 2020 at 17:53
• @Andyaka sorry for your inconvinience. I capture the image from video and added to my question. Aug 28, 2020 at 18:03

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

• "speed is generally proportional to voltage and torque is generally proportional to current" Wow, it seem like invese of DC motor property. I add the video of themotor that I refered to. Hope it's help to specify motor type. If your have any research or keyword I very appreciate. Aug 28, 2020 at 18:06
• I revised my answer to say armature current and voltage. The proportionalities mentioned are included in textbooks describing motor speed vs. torque capabilities.
– user80875
Aug 28, 2020 at 18:21
• This is a good answer. I say it an alumnus of the cited research lab (WEMPEC). Aug 28, 2020 at 19:44
• @SunnyBoyNY, as a retiree of an industrial member of WEMPEC, they let me visit and see what they are up to.
– user80875
Aug 28, 2020 at 20:18
• @CharlesCowie Was sad to see that Tom Lipo died recently, worked with him many years ago on a 200kW inverter. Aug 29, 2020 at 5:51

SEMs have declining torque with increasing RPM just like magnetic ones but can be only 5% of the stall power at the same torque at 0 RPM.

But what they consider “medium voltage” 5kV for electrostatics we consider Ultra High conductors for semiconductors , so speed control or constant torque using transistors in thus range are worth a hundred bucks because of the overkill on current ratings.

So your torque is not constant with RPM.

I wonder if the e in SEM’s are excited or electrostatic. (Lol) but 5kV would be exciting with partial discharge sparks flying thru the dust. It could kill a thousand mosquitoes per second.

Ref ixys/IXTL2N470/

• You so funny LOL. Aug 28, 2020 at 18:43

The inversely proportional relationship between torque and speed is universal and can never change.

Torque = Power/Speed (T=P/ω)

At constant power, there would be increasing torque with decreasing speed and vice versa.

In any motor, torque would remain constant as the speed is reduced, should the power also be simultaneously reduced, and vice versa.

A VFD controlled induction motor is one such example. 'V/f' would remain constant up to the rated speed after which 'V' would remain constant at the rated voltage and 'f' would continue to increase. Thus there would be a linear increase in power, with the torque remaining constant up to the rated speed, after which the power would remain constant and the torque would drop.

The behaviour would be the same with a DC motor having 'armature control' up to the rated speed and 'field weakening' after that.