I've also added some information of topics that are frequently addressed. If you're only concerned with the Oven Control Build you can skip down to that discussion.
Warning – All wiring projects involving AC voltage are inherently dangerous and can cause serious injury or death if done improperly. You assume all responsibility for the electrical work that you perform and the dangers involved therein. If you are at all unsure about completing any aspect of this or any other wiring project, consult a qualified electrical contractor to perform the service(s) for you.
Always follow electrical code requirements specific for your location. Before undertaking any electrical project contact your local electrical authority and your insurance company to ensure that you comply with all policies, regulations and authorities concerning this work.
Electrical Service Considerations:
I've seen many physical oven builds started without first looking to insure the availability of a suitable power source. While there are several wattage calculators available a simple wattage to cubic foot comparison is all that's necessary. IMO something in the vicinity of 150 watts per cubic foot is optimal, assuming good construction of the oven shell and good insulation.
Single Phase Service:
Most homes, home based garages and small business locations will probably be serviced with single phase power. Voltages in the U.S. are 120V phase to neutral and 240V phase to phase. It's very important that you look closely to determine what your service panel or sub panel amperage ratings are. You need to have adequate capacity and the availability to add a double pole breaker. The oven circuit needs to be a 3 wire with ground if you wish to run any components such as a light and/or re-circulating fan. Under no circumstance is it acceptable to use the ground as a neutral in existing or new 240V 2 wire with ground circuits commonly used for clothes driers.
Three Phase Service:
While 3P power is a welcome addition to any business/industrial site, there are a couple of issues to be noted. Using 3P power has no advantage in billing for a resistive heat application(as used on ovens). Wattage is the billable factor so using 3P power for
an oven does not result in any dollar savings. The efficiency of 3P motors themselves is the driving force for power consumption savings in installations using large HP motors.
If you have access to 3P service it can certainly be incorporated into the oven build. While there are true 3P heating elements available they tend to be rather pricey so use of single phase household oven elements is usually the acceptable alternative. It's best to balance the element loads over the 3 phases so wire your elements in this manner. If the 3 phases are identified as A, B & C, then wire the first element using feeds from the A & B source, the second element from the B & C source and the 3rd from A & C. If you do not have elements in combinations of 3 just balance the load as best you can. The transformer will be less efficient but everything still works.
3P voltages come in many varieties, with common voltages being 120/208, 120/240 and 277/480. If using a 208V 3P service elements rated at 240/250 volts will heat but just at a 25% reduction of wattage(lower input voltage equals less wattage output). If using a 120/240 3P service the elements will produce their stated wattage. Using a 277/480V 3P service is possible but you would need elements rated at this voltage. Placing 480V on elements rated for 240/250 volts would be a hazard and result in very short life expectancies of the elements. If you are not familiar with 3P service and it's nuances contact a qualified electrician to assist you.
Component Purchasing Considerations:
When purchasing, the first consideration is to make sure all components are compatible. Before purchasing components determine what capabilities you desire in the operation of your oven. Some considerations are:
Do you need ramp/soak capabilities from the PID controller which would allow you to control the rate of temperature rise in the oven ?
Do you desire a built-in timer function that will turn off the oven elements after the cure cycle is complete?
Do you want internal lights and or a recirculating fan, high temperature or timer expiration alarms?
Do you prefer an SSR or Contactor relay to control the oven elements?
Have you determined the wattage necessary for the planned oven size?
These are all questions you need to answer before purchasing components and starting construction of the control unit. In the build portion of this document I discuss only the specific components I've used. You are certainly welcome to modify the controls any way you wish, but I cannot assist with that effort.
PIDs - Most of the lower priced PIDs, but not all, are capable of driving either an SSR or contactor and many have 2 sets of output contacts for driving a relay and adding alarms such as for high temperature conditions. Use caution in purchasing the lowest priced Asian offerings as often they provide little to no documentation. I suggest reading through their manuals prior to purchase.
Ramp/Soak controllers or ramp to set point models of the Watlows such as the 93's, SD's and the newer EZ-Zone are all industrial strength professional controllers. The Watlows are all configurable in many designations, so before purchasing one of these models be certain that it is configured to suit your requirements. One nice advantage of using a Watlow is that they are made in the USA and can be repaired if they malfunction, unlike the Asian offerings.
Most PIDs, regardless of the model, can support at minimum (2) SSRs or (4) contactors if multiple relays are necessary for your configuration.
SSR’s and contactors - The decision of which type relay to use is a personal choice imo. Some sort of relay is necessary as the PID controller internal circuitry is not capable of directly switching the amperage loads of a typical oven heating element(s). In this build I am using an SSR so the below discussion concerning contactors is informational only.
SSRs by design are capable of switching multiple times per second and have a switching life usually in the millions of cycles. They have no moving parts and accomplish the switching by use of semiconductors. The switching process generates heat so a good heat sink is necessary(don't forget to add dialectic grease between the ssr and heatsink). Computer cpu heatsink/fan combinations work well for cooling but these fans will require a 12VDC power source(discussed more in the build section). They can fail either open or closed so use of a high temperature alarm in the pid should be considered.
SSR’s typically switch only one leg of a 240 volt power requirement for elements so be certain to disable the electrical mains power supply prior to working on any of the oven components. SSRs can be found in either AC or DC input voltage models, with the DC version being the most common.
Contactors are electro-mechanical relays that open and close contacts using an electromagnet coil. Their switching produces an audible click/clack sound and their life expectancy is normally in the hundred thousand cycles. Contrary to commonly mentioned characteristics, contactors can fail closed(relay contacts weld themselves together) so use of a pid's high temp alarm is always a good consideration.
Contactor models can be found with input coil voltage requirements in many variations of AC or DC voltages so be certain to get one that matches your intended power supply. A 120VAC coil contactor is probably the most commonly used type in Powder Coating ovens.
Contactors also come in various pole or contact sets. For a single phase 240 volt oven a 2 pole is most commonly used. If you have a higher amperage load requirement the element loads can be broken up and switched over a 4 pole contactor of a lesser amperage rating than what would be required if using a 2 pole model. For 3 phase applications look for a 3 pole model of the appropriate voltage.
When using a contactor, the PID parameter controlling cycle time must be increased to slow the on/off cycling process or the contactors life expectancy will be significantly reduced.
Timer, toggles, alarms, etc. - In the build section I will discuss a number of components I have found to work well. This is not an all inclusive list and there are many choices of components. At the very end of the build I will list some sources I’ve found for various components.