Drill Motor Speed Control

I connected Power Controller (black box with Power In, Load Out and Potentiometer) to a hand drill.
If the speed is reduced below 50% the rotation is jarky but if I use the same controller to dimm lights it is working fine.
How can I rectify this problem

You may have problems with false triggering caused by pulses from brush arcing and/or other commutation ..
Try to connect a snubber circuit consisting of 1kΩ resistor and 0.1µF (AC rated) non-polar capacitor in parallel with the power triac .. see picture below (R3+C3) ..

Rgds,
IanP

With snubber I still have jarky rotation at very low speed, less then 5% of the maximum speed.
Is there anythibg else that can be done to stop it?

Can you describe that "black box" [power controller] in greater details?

Rgds,
IanP

Drill controllers just reduce voltage to the coils by chopping off the AC waveform and the same jerking effect will occur when these are used on inductive loads such as in your case. As in case of the bulb, it is a resistive load and the controller works well. If possible you may try firing the triac with refernce to a Zero cross detector circuit.
On the other hand there is a innovative product which starts the drill from 0 to full speed in 4 seconds every time the drill is switched on using this controller. The 4 second time is sufficient to position ourself and get redy for the work and too the soft starting effect will protect the bushes,bearings and gears inside the driller. I am using the same manufactured by United 7 technologies. www.united77.com

I use KEMO's M012 Power Controller.
**broken link removed**

Because you use an off-the-shelf item there is no much you can do to modify it ..
In other circumstances I would encourage you to build your own “power controller” based on two back-to-back SCRs (to avoid problems that you have with this most likely triac-based controller) ..
However, you can try to connect LC filter, consisting of an inductor (at least 1mH) and >220nF (@250Vac) capacitor, between the output of your black box and the load ..
First, using an ohm-meter identify which connection loops-through the black box from input to output, then install inductor on the other output wire, and the cap in parallel with the load .. see picture below ..

Rgds,
IanP

Yeah, with 1mH/4A and 220nF/630V it is working fine.
Thanks