I am trying to 'automate' the control of an electric drill motor that I'm taking from a garden variety variable speed drill (from Harbor Freight.)

It has a universal AC motor which I basically understand, a DPDT switch to reverse direction and a TRIAC circuit to vary speed.


  1. I see four wires going to the 'field/stator coil(s)'. Are there two separate field/stator coils here? Is this to provide low/high power?
    I think in theory, you only need one coil with two wires and that coil is wired in series with the brushes. Is that right?
    How does this circuit work with two coils?
  2. How does this variable speed circuit work?
    Is the wiring diagram correct? It doesn't show a variable resistor and it doesn't show what drives the control input of the TRIAC.
  3. Why are there two switches in the thing? Does switch two close at high speed and thus shunt around the TRIAC?

Please see the attached 4 diagrams:

drill pic

drill pic2

drill pic3


2 Answers 2


Question 1 . i see 4 wires going to the 'field/Stator coil(s)'. Are there 2 seperate field/stator coils here? is this to provide low/high power. I think in theory , you only need 1 coil with 2 wires and that coil is wired in series with the brushes... is that right? how does this circuit work with 2 coils.

The switch-box connected to the brushes also needs to be supplied. But it is neither connected to AC, nor directly to the trigger box. Finally, it connects to the cables from the trigger box at the terminals of the field coil. You can already see on the picture that a white and a black cable are (very likely) connected to the same terminal of the coil on the right. It's not so obvious on the left side. But of course the switch box somehow must be supplied.

Question 2. How does this variable speed circuit work? Is the wiring diagram correct (it doesn't show a variable resistor and it doesn't show what drives the control input of the triac? Why are there two switches in the thing? Does switch 2 close at high speed and thus shunt around the triac?

This is not a complete schematic, it is only a sketch. AC should be connected to terminals 1 and 3, field coil and switch box to 2 and 3.

The triac is used for a phase-fired controller. The trigger will contain a variable resistor between 4 and the "the black-dot-terminal". Depending on the trigger position, the current will increase above the triac threshold at a certain point within the first 90° of a half-wave, and the triac will fire. Maybe, the trigger contains even more electronics to fire the triac somewhere over the full 180° range.

Even when you pull the trigger to the maximum, the trigger will still cut a little from the half waves, and in general, some voltage will drop over the triac. Therefore, the switch between 4 and 2 will short cut the triac in this case, allowing the full AC voltage to pass to the motor.

On the other side, you do not want to power the motor / triac when you don't touch the trigger. In this case, the switch between 1 and 4 will be open.


According to your comment:

Here is a more complete schematic of your electric drill:


simulate this circuit – Schematic created using CircuitLab

As you want to change the rotation direction of your drill, you need to be able to swap the polarity of either the field coil or the rotor coil. Here, the power-regulated AC from the trigger is connected to two terminals of the field coils via the black cables. The white cables are connected to the same terminals, feeding the direction switch, which powers the brushes via the blue cables.

Field and rotor coil are connected in parallel, which increases the power of the motor (each gets the full voltage from the trigger box, they don't have to share it)

And keep in mind my variable resistor also is just a model on how the trigger may actually work. See above.

  • \$\begingroup\$ thanks! but im still confused why there are 4 leads going to the field coil(s). in a simple motor.. the field coil is wired in series with the rotor coil via the brushes.. thus you only supply your AC power to 2 leads.. lead #1 would be to one side of the coil and lead #2 would be one side of the brushes. thus does my motor have an 'extra' field coil for 'added pwer' \$\endgroup\$ May 2, 2015 at 15:52
  • \$\begingroup\$ There are 4 leads because the motor has a separate field winding on either side of the stator. These windings could simply be joined together to make a single coil, but putting them in a balanced configuration either side of the armature may reduce voltage stress on the insulation and help to suppress brush arcing. The field coil is almost certainly wired in series with the armature, not parallel (you can verify this by tracing the wires and/or measuring the windings with an Ohmmeter). \$\endgroup\$ May 2, 2015 at 20:20
  • \$\begingroup\$ @BruceAbbott I agree there. That is a typical single phase "Universal" motor, and the fields have to be in series with the armature. A Universal motor is just a plain DC motor at its heart, but running off of AC. By using a series configuration for the field, they have very high starting torque, as the magnetic flux builds with current, which is at maximum at stall or load. In parallel, you can't get the magnetic field boost when the shaft is loaded. \$\endgroup\$
    – R Drast
    May 4, 2015 at 14:06
  • \$\begingroup\$ I'm still not sure about the serial connections due to the wires. It seems absolutely clear on the picture that a white and black cable are connected on the left terminal. Black is supply, white connects to the direction switch. I have no idea how this can be a serial wiring. I'd check if this wires are really connected to a single terminal by inspection / measurement of the resistance. \$\endgroup\$
    – sweber
    May 5, 2015 at 12:48
  • \$\begingroup\$ thanks everyone for your help! I think my armature is wired in series with the field coils. it appears that 'thyristor controlled power' is fed into "Side A" of upper field coil. "side B" of upper field coil then feeds thru DPDT into the armature coil. arm coil flows back out thru the DPDT to " Side B" of lower field coil. SideA of lower field coil completes circuit to ground. does this make sense? \$\endgroup\$ May 11, 2015 at 6:47

Just a small error in the schematic. The way you had it would short circuit the system. enter image description here


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