I have a drill press but the speed of rotation depends on the torque output of the motor. It means that if I put more load, the output speed will go down. I want to have a simple way to keep a constant speed of rotation and make it independent of the load torque. The above circuit might work. What is your idea? For now, I have already think of several issues with this design.
1. The RC constant must be quite large to accoumodate for the minimum rotation speed of the motor, unless I use a rotation encoder or high gears instead of a simple magnet. This means that the feedback loop control will be hard to do or there will be overshoot or undershoot at transient.
2. There might be capacitive coupling between the magnet (ground) and the coil. I might need a good electostatic shield but not magnetic shield.

 

That appears to be a capacitor-start/induction motor, if so that is not a recommended way of controlling an induction motor, especially such as a drill.
Are you saying this controller is already built in?
If so, confirm the motor spec's.
Many controlling a motor such as a drill stand use ex T.Mill DC motors.
Max.
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The drill press is purchased (for about USD40), and it works okay. I just want to modify it so that the torque will not affect the rotation speed. The speed controller which is already built-in the drill press is using a variable resistor dial to control the speed. So if I drill something hard and strong, the speed goes down, but once I finish drilling, the speed goes back up. If I have to manually keep the constant speed, I would turn the dial up during drilling, and turn it back down after drilling. This makes the drill difficult to use. I want it to run at a more constant speed independent of the torque output. This is a low cost drill press and it uses a brsuhed AC motor at 220V. Since the speed controller is just a simple triac with a variable resistor dial, I think it would be easy just to add a few components to strap across the capacitor, and to automatically control the power based on the actual speed, which should be derived from the changing magnetic field pick-up by the coil. I have not built that part of the circuit, but I just want ot see if it is even worth the effort doing it, or just to go buy a more expensive drill press instead.

 

Someone who was a member here had designed a PID controller for this type of Universal motor.
But designed for the router type motor with a min speed of 5krpm.
http://www.vhipe.com/product-private/SuperPID-Home.htm
It requires PID feedback control.
Max.