# Low voltage - high current AC motor speed control

What we are trying to accomplish is digitally controlling the speed of a 18VAC motor, which can easily pull 1A (specifically old H0 Märklin locomotives, which can reportedly pull 5A at peaks). The supply power in our case is fixed at 18VAC.

Ideas we considered:

• Voltage divider with digital potentiometer which is probably not feasible due to the high current requirement.
• Using a DIAC+TRIAC dimmer circuit (with a digital potentiometer), but the existence of low-breakover-voltage DIAC's seems to be a myth. I found one 15V SIDAC but that still seems to high for our 18VAC motors.

Dimmer circuits for AC with voltages below 30V don't seem to be popular so I hope one of you can come up with/knows a circuit able to do this. Or perhaps you know an alternative for the DIAC? Not sure if this helps but a microcontroller with PWM capabilities is available.

simulate this circuit – Schematic created using CircuitLab

Figure 1. Modified circuit.

Try leaving out the DIAC. It's main purpose is to give a well defined breakover voltage to even out the differences between the positive half-cycle switching point ('first quadrant' in datasheets) and the negative half-cycle switching point ('third quadrant'). Without it you may see a difference in trigger point on a scope but it's unlikely to cause trouble with your model train. You will also be able to get much closer to 0° trigger angle than you could with a DIAC in circuit.

Figure 1. Triac phase control for dimmer application.

Figure 2. Triggering modes. Quadrants, 1 (top right), 2 (top left), 3 (bottom left), 4 (bottom right). Source: Wikipedia TRIAC.

simulate this circuit

Figure 2. Onboard triac control.

For onboard control you can trigger the triac directly from the micro-controller (through a suitable current limiting resistor).

• Rectify, smooth and regulate the AC input.
• Tap off a zero-cross signal from your AC supply and feed this to the micro. Make sure that however you do this that any high-voltage transients can't damage the input. (Diode from input pin to V+, for example.)
• Let the micro figure out when to pulse the triac.
• You can now trigger the triac on first and fourth quadrant. It will be happy enough and it should really simplify your electronics.
• Simple but effective, this is the kind of answer I was looking for. I'm going to simulate this and I'll get back to you. FYI: We are not digitizing the track, the supply on the track will be constant. We are trying to digitize each individual locomotive. Commented Aug 29, 2016 at 9:12
• So to see if I understand this correctly: use some zero-cross detection circuit/component and feed its output (with some safeguards) to the MCU? And then after a delay (which varies based on the require speed) pulse the triac with the MCU? Commented Aug 29, 2016 at 10:05
• So this: microcontrollerslab.com/… Commented Aug 29, 2016 at 10:12
• Zero-cross detection added. Now we need to get the trigger signal back to the triac which is on a different "ground". More later. Commented Aug 29, 2016 at 10:36
• +1 2019 :-) ...... Commented Nov 2, 2019 at 0:41

You should look for advice that is specific to the type of Marklin locomotive that you have. You have not just the motor to consider, but also the mechanism that reverses the motor. It appears that, depending on the age or specific model, there is either a solenoid that operates a mechanical method of reversing or a relay that switches the field coil. The solenoid or relay apparently requires a higher voltage pulse to operate or operates when voltage is removed and reapplied. It may not work properly if you ramp the voltage up and down.

The "AC" motor is not strictly an AC motor, but a universal motor. A universal motor is like a DC motor. It has a armature (rotor) with winding coils, a commutator and brushes plus a stator field that also has a winding coil. The armature and field are connected in series with each other. The motor is called "universal" because it can with either AC or DC power, but the performance differs. To reverse rotation, the field coil polarity needs to be reversed with respect to the armature. This is apparently done by using two windings that are wound or connected in opposite directions.

• About the reversing mechanism: that pulse is generated by our supply at the moment and we will no be looking at digitizing that part yet. We want to start small by just controlling the speed, and I will be careful to see if such a pulse would break the speed control circuit that we choose to use. About the Universal motor part: that is an interesting idea since that would mean switching to DC is not that difficult and would make our quest to digitize the entire system a lot easier in the long run. Commented Aug 29, 2016 at 9:08

Consider getting a dimmer intended for transformer-input lamps. The combination of such a dimmer and a 12V transformer provides dimming for a 12V halogen lamp system, but the substitution of a 20V transformer would suit your needs. The dimmer operates at full line voltage, and the transformer steps down and isolates the power that goes to your train (isolation is an important safety feature). Lutron Magnetic Low Voltage dimmer And available in voltage-controllable series, rated for motor use: Grafik EYE interface

• That would require a high-voltage source. We want to digitize individual locomotive while there is 18VAC on the tracks. Commented Aug 29, 2016 at 9:14
• I was assuming a single-engine setup, and controlling the track voltage. AC control at low voltage is hard (triacs are inefficient). Motor speed control implies a universal motor, it would run on DC. Commented Aug 29, 2016 at 10:05