Saturday, August 24, 2019


I paid $3 for this drill at the local garage sale. The battery was dead (as you can expect from several years old NiCd battery) but the motor was OK.  I decided to fix it with an external power supply. The battery output was 14.4 V. It is hard to find any supply with such a voltage, but 12 V sources are ubiquitous.   Inside e-waste bin at my job I found one with 6 A current output. That should be enough to power the drill, right? So I removed the battery from the unit, dissembled terminals from it, attached barrel jack to terminals, and assembled unit back. Now time to test. Alas, the result of the test was not exactly   I expected.  Indeed drill worked, but only if I pull the trigger slowly. Make it fast and behavior of drill becomes strange. Its chack starts to rotate and then immediately stops. On the clip below you can see it with the voltmeter showing voltage applied to the drill.
This clip shows that voltage drops to zero unless I squeeze the trigger slow enough. Is there an explanation for such an effect? Could it be fixed?
I believe I have answers to both these questions. Two things influence the drill start. First, there is an inrush current, which is the maximal instantaneous input current drawn by an electric motor when it first turned on. While under normal load this drill needs 3-4 A of electrical current, at the moment of start it may draw twice or even thrice more. So for some time (till motor reaches stable rotation), current may exceed  6A for which supply is rated. And now the second thing comes into the picture:  power supply is sophisticated enough to protect itself from the overload. As a result of that protection, it shuts itself down. As soon trigger is released, voltage is restored. 
But why does it work if the trigger is pulled slowly? Modern drill trigger is a variable speed device. It not just switch but power regulator. If it is engaged partially only partial voltage is applied to the motor, so inrush current is not that big. While rotor speed increases created by the rotation back EMF (electromagnetic force) limits the current through the motor winding.
Here is the remedy to the fast start problem: limit electrical current at the initial stage. To test this statement I tried resistor with 0.3 Ohm resistance and 10 Watt power rate. Indeed fast start effect is gone but  I did not like how it worked. Under the normal load, when current reaches 3 A, more than one volt dropped on the resistor and less than eleven volts applied to the drill.  And the resistor becomes hot.
So It tried another approach: inductor.  Inductor possesses big resistance to variable current but small resistance to direct current. Means, during start when current jumps from zero, the inductance will limit current but it will have no effect during stable rotation. Here is Toroid Inductor I bought on Amazon (several pieces for $2 all).  You can see it next to a penny  (for size comparison).

This inductor is rated for 100 micro Henry of inductance and 6 A of current. Current is OK, sure,  but what about inductance? The test shows it works as needed: drill could be turned on fast enough and no voltage drops when the drill is up to the speed.

Some more details regarding this project and how I build it you can find on Instructables site .