Phase angle control, TRIAC and AC motor control

Ultimate goal: Use PID to control a condenser fan in a small refrigeration setup.

I am a HVAC service tech. I have all of the required knowledge and tools required to safely (and ethically) handle refrigerant.

Background info: In compression refrigeration systems, the condenser coils typically are designed to operate at 30F (standard efficiency) above ambient temperatures(This is known as a 30F Coil TD). Most AC/R equipment, metering devices etc are designed to work optimally at a specific pressure differential across the metering device. Design day temperature corresponds to a 90F ambient temperature. This means that the saturated refrigerant vapour inside the condenser coil is 120F and a pressure that varies depending on the refrigerant you are using. In a closed loop system, where VOLUME does not change, the pressure of a saturated refrigerant vapour corresponds to the temperature. This means that if you can control the temperature of the refrigerant, you can control the pressure, and the saturated condensing temperature can always be calculated by measuring the discharge pressure off the compressor.

Since refrigerant heat exchangers are designed to operate at a fixed temperature above ambient, the ambient temperature directly effects the discharge pressure of the compressor. If you do not have a high pressure differential between your evaporator and condenser, you simply cannot move enough refrigerant across your metering device. If you can't move enough refrigerant across the metering device, you are going to be missing out on cooling capacity.

What I want to do.

I want to control the airflow across my coil to maintain an optimal head-pressure. I am using an R-22 system, which has a design day pressure of 260psig. I want to control my condenser fan speed to maintain that pressure. A pressure sensor will be used as the fan control feedback. The arduino will monitor the pressure and increase or decrease the fan speed to maintain the head pressure.

The fan motor is a 1/2 HP cap star, cap run AC motor. It is using a potential start relay. I want to use a phase angle controller to trigger a triac. I want to trigger the PAC with the arduino, and let the PAC turn the triac on and off. I've seen setups that control RPM for CNC motors, but the details are typically quite sparse.

Ultimately, I would like to be able to also control the compressor speed as well. This way I could change the cooling capacity of the compressor based on the cooling load demand. I will tackle that later though.

I recognize that there are off the shelf solutions. They are expensive, and I use them fairly routinely in a commercial environment. I want to DIY it, so I can learn and understand how the controls are doing what they do. This is just a hobby project...because I can.

MY QUESTION: Does anyone know of previous AC motor speed control projects that are well documented? Any good resources for reading about triacs and phase angle control?

All the stuff I have seen drives the Triac directly. Or through an opto isolator. I would think your project will depend an awful lot on the PAC. You are basically programming to the PAC interface, correct?

yup, that was the plan.

I think I might actually go more towards ECM control. Use a brushless DC motor with an ESC that will accept pwm inputs. ECM/inverter compressors are definitely going to be the way of the future.

How do you drive the triac directly through an opto? I didn't think triacs have zero sensing built in.