The "speed" setting that is part of the trigger in a drill controls the average voltage delivered to the electrical motor within it.
Power Control - The Trigger Button
This is accomplished by a PWM (Pulse Width Modulation) circuit, which pulses the voltage rapidly, say at several kHz. The position of the trigger button controls average width of the "on" portion of the pulse - the duty cycle-, which in turn determines the average voltage applied to the motor and thus its power setting.
This electrical power translates to mechanical torque.
There are two charts that tell us how to relate trigger setting, RPM and torque to the task at hand, e.g. drilling, hoisting etc....
Torque and RPM
The first chart graphs RPM and Torque for various power settings. Have a look:
The curves above shows two important aspects:
- Torque and RPM are related by the power setting. They are not constant, but they are related. Follow any of the blue lines for some power setting, e.g. 60%: if the torque is high, the rpm is low and vice versa.
- At a given RPM, the torque can be increased by increasing duty cycle (from say 60% to 100%). This increases the electrical power and thus increases the RPM if the torque is kept the same, as in a load that is being hoisted. Or, it increases torque if the operate keeps the the RPM the same, e.g. by pressing a drill harder into the workpiece while increasing the power setting.
Mechanical Power is Torque and RPM
Mechanical power is determined by the product of torque and RPM. What does that mean in these charts? Select a power setting (select a blue line), determine your RPM, read the torque, and multiply the two.
You can also do this graphically, multiplying RPM -a horizontal distance on the chart- with torque -a vertical distance- means you are calculating the area of a rectangle at that operating point.
That is, select a point one of the electrical PWM duty cycle lines, determine RPM and Torque on your point, and multiply the two. Multiplying two numbers is how you calculate the area in a rectangle. So this power calculation is effectively the area of a rectangle in the curve, and the rectangle's top right corner is determined by the selected operating point.
When you select a higher power setting you are riding a higher blue line which will draw larger rectangles.
When do you get Maximum Power?
When is the mechanical power maximum? Is it at high RPM and less torque, or at high torque and less RPM?
The maximum mechanical power is delivered when the product of torque and RPM is maximum.
(I'll spare you the details, but after some math involving triangles and linear equations, it turns out that the area is maximum when the sides of the rectangle are equal. It's intuitive too, which helps: a thin narrow triangle under the blue line has less area than a square.)
To get the maximum torque you will get the lowest RPM. And to get high RPM you need to relax the torque. Notice how the maximum drill power is not delivered when RPM is maximum or when the torque is maximum.
Drilling or Hoisting - Not the Same Load
What determines the RPM of your drill is however not just determined by the PWM duty cycle as set by the trigger button, but also by the load. At the same power setting (duty cycle), the RPM will vary depending on the torque load: press hard while drilling and the RPM drops because the load increases. For a hoisted weight it's different, the load is the weight of the hoisted object and the weight (obviously) does not depend on the RPM.
These effects are described by "load torque speed characteristics". Different operations have different characteristics. For instance, for drilling, friction increases with RPM and so does the torque load.
Here are some example load characteristics, where "w" is the greek letter omega and it represents the RPM:
The one on the left is typical for friction loads. Notice it starts at zero: with zero RPM there is zero friction (zero load), and Friction increases with RPM.
A hoist load on the other hand is relatively constant: the RPM does not make the load heavier (acceleration aside), but there is a bit of friction in the pulleys or gears that may increase with RPM.
Note that these curves describe how torque and RPM relate under the same load condition; that's for the same drill pressure or same hoist weight.
If you increase or decrease the drill pressure or change the workpiece material, you get a different curve (drawn lower or higher) but with similar shape.
Putting the two together: Equilibrium
To understand what torque and RPM you get for the task at hand, you need to put these two curves together.
If you keep the drill presssure constant (not pressing light or harder on the drill bit) the drill RPM will settle where the load torque from the friction and RPM equals the motor torque delivered at that RPM.
Here's a graph that includes two load characteristics in brown, one for pressing hard and one for pressing lightly.
If you go from pressing lightly to pressing hard (while you keeping the same power setting), the RPM will drop, but the torque will increase. Try to follow this on the curve.
Now you can hopefully visualize what happens with torque and RPM when you increase & decrease the power setting, or increase & decrease the pressing force.
Don't burn your workpiece
As an aside, friction load increases with the square of the RPM. Double the RPM means quadrupling the friction torque. This is in fact why RPM in many tasks is limited. Sure it makes drilling or sawing go faster, high RPM also quickly overheats your work piece or cutting bit.
Overheating the tool will cause it to soften and wear faster. Overheating the workpiece will cause undesirable damage.