Servo - November 2008

Photo 3. I hung my IDEAL 400 AC clamp meter around the universal motor’s AC line lead (black lead) with a 90 degree TRIAC triggering angle dialed in. The universal motor’s no-load AC amperage draw varied from 6.92 to 7.00 amperes.

that converts the triac trigger angle to a trigger delay time:

//**********************************************
//* CONVERT TRIGGER ANGLE TO TIME
//**********************************************
unsigned int angle_to_time(char trigger_angle)
{

unsigned int angle2time_val;
if(trigger_angle == 0)
{

angle2time_val = 0;

}
else
{

motor loose. Photo 3 is a real time shot of my motor’s no-load AC current draw with a triggering angle of

90 degrees.

If you’re having problems getting your arms around the triac triggering angle concept, visualize them as beginning with the rising half cycle of an AC signal. Half way through the first positive half cycle is 90 degrees, which is also the peak positive-going voltage of the AC cycle. As you move past 90 degrees, you will move towards the next zero crossing point at 180 degrees. The firmware will reset the triggering angle to relative zero at each zero crossing event and trigger the device at the same relative time in the negative cycle as it did in the positive cycle of the AC signal.

For instance, let’s assume we trigger the triac at 90 degrees into the positive half cycle. It will conduct until the AC signal crosses zero at 180 degrees into the cycle. The next trigger pulse will be issued 90 degrees after zero crossing, which is actually 270 degrees into the AC cycle. At the next zero crossing, the firmware will reset the trigger timing reference to zero and the triac will again commutate. The result is that we have energized the universal motor for 50% of the full AC cycle with a 90 degree trigger angle. Since the motor will run very slowly at a 140 degree trigger angle and run faster as the trigger angle is decreased, we can conclude that the angle is inversely proportional to the motor speed. Here’s the code

angle2time_val =
 0xFFFF - (trigger_angle 0x2E);
 //each degree 46us at 60 Hz

}
return angle2time_val;

Understanding how the triac trigger timing is determined is the key to understanding the rest of the universal motor controller firmware. Each degree of trigger angle delay time is equivalent to approximately 46 µs assuming a 60 Hz AC signal. If you’re reading this in a 50 Hz mains country, each degree of delay time is approximately 56 µs. Thus, the controller hardware and firmware will work with 50 Hz AC systems. All you have to change is the time delay associated with each degree of triggering angle.

A PIC18F2620 timer uses the angle2time_val to kick off a 400 µs TRIAC trigger pulse at the same relative time in every AC half cycle. At every zero crossing event, the current trigger angle is converted to a value that is fed into the PIC18F2620’s triggering delay timer. To prevent the motor from lurching at every speed change, the firmware includes a routine that ramps up or down to the new trigger angle. The firmware also contains a soft-start routine that smoothly ramps the motor up to speed upon initial power-up.

Hands On ... Carefully, Please ...

Photo 4. It ain’t as pretty, but it works just as well as the production universal motor controller you see in Photo 2. The smaller 470 nF X2 filter capacitor is directly above the varistor and fuse. The much larger 1.0 µF X2 transformerless capacitive power supply blocking capacitor is just left of the PIC18F2620.

SOURCES

The best way to get your hands around driving the universal motor is to do it yourself. Photo 4 is a shot of the breadboard version of the controller I assembled before committing to a printed circuit board (PCB). However, you don’t have to build this from scratch as the controller download package contains an ExpressPCB file that you can use as-is to fabricate your own PCB. The firmware included in the download package was created using the HI-TECH PICC- 18 C compiler and is ready to run out of the box. I’ve included a set of interrupt-driven RS-232 routines in the code for those of you that want to experiment with controlling the controller serially. All that’s left for you to do is to collect the necessary electronic components, solder them down, and fire up your universal motor. All it takes is an email to solicit my help with this project. I had bunches of fun with this and I’m sure you will too. SV