# Controlling a 240V AC Universal Motor with Microcontroller, Optoisolator & TRIAC

I got an old washing machine motor(universal) laying around, that I'd like to speed control. I'm new to almost all the components mentioned above, but here is what I currently use:

• Microcontroller: Arduino Nano
• Optoisolator: MOC3041
• TRIAC: 2N6071AB (Replace? Thinking of BTA24-600B)
• Universal Motor: UOZ 112 G 55

But after a long search I came across this circuit: http://imgur.com/d9nEwLQ

So first off; I've tried to hook the TRIAC (2N6073AB) to 240 VAC, it scattered into two pieces after about 5 sec. It says it's ment for 400 VAC, so I'm a bit confused. I didn't have a heat sink attached though, but still... Maybe someone got something to add to this behavior? This is how I connected the TRAIC for testing purposes:

simulate this circuit – Schematic created using CircuitLab

Second; for the circuit above, I bought 1/4 watt resistors, and I cannot understand how they can handle 240 VAC, seems really strange to me. Or maybe they can't? Like the capacitor in series with the 39 ohm resistor, how is that even possible?

Third; What role does the 330 ohm resistor play, why is this one needed? And it also says "for highly inductive loads, change this value to 360 ohms", what value? Is it the 39 ohm resistor? And why change it, is because of the high start current for motors?

Fourth; for the 0.01 microFarrad capacitor, where does this value come from? And from what I've softly read, the snubber circuit is to prevent the phase shift between voltage and current caused by the motor, right? Will these capacitors do: Blue Ceramic Disc Capacitors 1KV 1000V 103PF 0.01uF?

Final question; I've been reading some PDFs about thyristors (inc. TRIAC), and it said:

"The output of most microcomputer input/output (I/O) ports is a TTL signal capable of driving several TTL gates. This is insufficient to drive a zero-crossing TRIAC driver."

I guess that's not the case with Arduino Nano since it uses PWM signal? Or do I still need the NAND-gate? And if someone would like to explain why the zero-crossing TRIAC driver doesn't accept certain signals, I would be grateful.

I'm sorry about all the questions, and I thank you in advance. And I'm very surprised of how little I seem to know about the subject. I have a great interest in electronics, and from an educational point of view, should I have far more knowledge than it seems that I currently have. But I'm very eager to learn, so I would appreciate well written answers. My thanks.

Kind regards MrMongoloid

• Pop the motor details into the question (rather than spread through the comments) so that all the relevant info is there. +1 for a very well written first question. – Transistor Jul 31 '16 at 18:49
• Controlling a Universal motor this way won't do quite what you expect. You'll need to separate its field and rotor windings, and keep the field winding connected to the full supply voltage (assuming it's shunt wound). Otherwise the speedup (from field reduction) and slowdown (from rotor voltage reduction) will cancel out. – Brian Drummond Jul 31 '16 at 18:58
• @Brian Drummond Universal motors are series wound by definition. – Charles Cowie Jul 31 '16 at 19:06
• @CharlesCowie "if it were shunt wound, it wouldn't work at all with AC power" what stops it working with AC? Both rotor and field reverse polarity at the same time, just as in the series form. Is it the phase shift as the field winding is almost purely inductive? – Brian Drummond Jul 31 '16 at 21:28
• I believe the R/L would need to be much different because the armature and field would need to be designed to operate at the same voltage but draw much different currents. Also, the back emf controls the armature current. Those two things would cause a phase difference between the armature and field. With a series motor, there is only one current path and thus no possibility of phase difference. – Charles Cowie Jul 31 '16 at 22:03

1. So first off; I've tried to hook the TRIAC (2N6073AB) to 240 VAC, it scattered into two pieces after about 5 sec.

Without a schematic of your wiring we can't say. It sounds as though you mis-wired it or switched it on to a dead-short between mains and neutral. There's a schematic button on the editor toolbar if you wish to update your question.

1. For the circuit above, I bought 1/4 watt resistors, and I cannot understand how they can handle 240 VAC, seems really strange to me. Or maybe they can't? Like the capacitor in series with the 39 ohm resistor, how is that even possible?

Figure 1. Carbon film resistor with exposed carbon spiral (Tesla TR-212 1 kΩ). Source: Wikipedia Resistor.

There are three main specifications to watch when using resistors:

• The resistance value. This is obvious.
• The voltage rating. In Figure 1 we can see a spiral resistance track around a ceramic core. The track is probably about 20 to 25 mm long if unwound. Above a certain voltage electrical breakdown will occur across the resistor - perhaps jumping between turns on the spiral. Typically they're good to 200 to 250 V but mains voltage can peak at $\sqrt {2} V_{RMS}$ so when we use them on 230 V mains we generally use two in series.
• The power dissipation has to be kept below the rating of the resistor. This can be checked using $P = \frac {V^2}{R}$ or $P = I^2R$.

It may help to consider capacitors as two layers of foil separated by an insulating film. All that is required is to make the insulation layer thick enough to withstand the applied voltage.

1. What role does the 330 ohm resistor play, why is this one needed? And it also says "for highly inductive loads, change this value to 360 ohms", what value? Is it the 39 ohm resistor? And why change it, is because of the high start current for motors?

There are three resistors in your circuit.

1. For the 0.01 microFarrad capacitor, where does this value come from? And from what I've softly read, the snubber circuit is to prevent the phase shift between voltage and current caused by the motor, right? Will these capacitors do: Blue Ceramic Disc Capacitors 1KV 1000V 103PF 0.01uF?

Covered above. I don't know what the 103PF means. The Littlefuse article should give you enough detail on this.

1. "The output of most microcomputer input/output (I/O) ports is a TTL signal capable of driving several TTL gates. This is insufficient to drive a zero-crossing TRIAC driver."

You don't quote a source for this but it looks a bit out of date. Most of the micros can now switch 20 mA and this is plenty for an opto-isolator LED.

1. I guess that's not the case with Arduino Nano since it uses PWM signal? Or do I still need the NAND-gate? And if someone would like to explain why the zero-crossing TRIAC driver doesn't accept certain signals, I would be grateful.

PWM isn't used with triac control circuits. This is explained in my answer to Activating SSR for an AC motor via PWM input.