SCR speed control of universal motor
I just ran across this post and since I know how these circuits work, I thought I'd explain some of the details for those who are interested.
AC Routers, drills, dremel tools, and similar equipment have what's called universal motors in them. Universal motors are actually dc motors. The SCR component in the third picture works like a diode, it conducts during half of the ac cycle, and does not conduct during the other half cycle. It cuts on and off at the frequency of the ac voltage.
At full speed it acts just like a diode. It turns on when the voltage crosses zero, and remains on untill the voltage crosses zero in the opposite direction.
Now unlike a diode, the SCR has a third lead called the gate which controls whether the device conducts at all or not. If there is a sufficient voltage applied to the gate then the device conducts dc like a diode as explained. If the gate signal is below a certain level, then the device is like an open switch and it does not conduct at all.
The speed of a dc motor depends on the (average) voltage applied. At full speed, the average voltage of one whole half sine wave is applied to the motor. If we turn the speed control knob down, it adds some resistance to the SCR gate signal so that the SCR turn on point is delayed untill some time after voltage crosses zero. With the added resistance, the gate signal must rise to a higher voltage (which occurs later in the cycle) to reach the threshhold gate voltage where it turns on. So it turns on later in the cycle.
By nature, the SCR then conducts (with or without a gate signal) untill the voltage again crosses zero, where it automatically cuts off (like a diode). When the turn on point is delayed (the turn off point does not vary) the effective average voltage is thus reduced so the motor runs slower.
So the SCR cuts on and off at the frequency of the AC, and the speed control knob affects the duty cycle (and therefore the average voltage) of the (pulsing) dc applied to the motor.
An interesting feature of this type of circuit (and motor) is that the gate signal is also affected by the back emf of the motor in such a way that it serves to automatically (try to) maintain a constant speed. If you set a drill to run real slow and try to slow it down with your hand, it will try to maintain speed by increasing its torque. This has to do with the peculiar way that universal motors are wound. But that's another subject... That's also why these speed controls only work with universal motors. Due to the way the universal motor is wound (with 2 coils per pole), it does affect the SCR gate voltage, which makes the SCR speed control possible. Normal DC motors (with one coil per pole) cannot produce a usable signal to trigger the SCR gate.
Incidentally, back emf is relative to the actual motor speed. It is the motor turning which produces back emf in the first place. So you can imagine how a motor turning at a specific speed can have a specific effect on an SCR gate
to lock the speed at a specific rate. And how adjusting the knob can retard the effect of the back emf on the gate to slow down the motor.
The variable speed control in a drill is the same type of circuit as for a router. Router speed controls are rated for tens of amps whereas drill speed controls are only good for a few amps.
