Assembling a heat control system involves many decisions-some dictated by the application, others the choice of the user. Belonging in the first category are the ac input voltage, the number of phases, and the system's maximum load carrying capacity (amps). These depend on the application and are generally not matters of choice. Items that must be decided include the type of control device to use, the control firing method (On/Off, Proportional, and/or Phase Angle); the control input (0-10Vdc, 4-20mA, etc.); and the needed options, such as current limiting, soft start, zero/gain adjust, line voltage regulation, digital communications interface, and retransmit of output drive signal.
Once those decisions have been made it's time to decide how much of the system to buy pre-assembled, and how much to build in-house. We'll come to that presently, but first let's look at some basic electrical decisions.
Choosing the Control DeviceDepending on the size of the load and the control precision you need, your choices are SCR (silicon control rectifier) modules or solid state relays (SSRs). If your power requirements are fairly low (50 Amps or less) and you don't need sophisticated control, an SSR is a good low-cost option. While generally thought of as simple on-off devices to replace contactors, SSRs are also available with phase angle or proportional control (Fig. 1). They can't do things like soft start, and they don't come with variable time base control, but they're inexpensive, self-contained, and easy to apply. While low-end SSRs may use triacs as switching elements, others use SCRs, and these can be enhanced if necessary. For example, it's possible to get an analog input board that allows a plain-vanilla SSR to accept 4-20 mA inputs and do proportional control.
Choosing the Type of ControlThere are three basic types of control: On/Off, proportional, and phase angle. On/Off control, the simplest, is good for applications that don't need a great deal of precision and have a fair amount of thermal inertia. Many heat trace systems, for example, turn the heaters on when it gets cold and turn them off when it warms up. On/Off control can be done with a small control board and either contactors or SSRs. SSRs have no moving parts, they don't wear out, and they do zero-point switching, which cuts down on electrical noise.
Other System PartsOnce you've chosen the power device and how to control it, you'll need to consider such things as flexible wiring options, touch-safe design, fuse and power distribution connections, fuse replacement cost, built-in controller protection, power connections and related maintenance issues, and agency approvals. You'll also have to choose the options you need. These include shorted SCR detection, current limiting, soft start, zero/gain adjust, line voltage regulation, digital communications interface, and re-transmitting the output drive signal.
Deciding How Much to Do YourselfHow much of the job should you do yourself?
Your choices range from doing everything to buying a complete custom power control panel. You may well end up somewhere in between: buying subassemblies, mounting them to a panel, and running the wiring.
Designing and building your own controller has some advantages. Of primary importance is that it gives you everything you want - the number of relays, the inputs and outputs where you want them, etc. It might be quicker, since all the parts needed are probably in distributor stock. If one brand of solid state relays doesn't do quite what you want or is sold out, there's another brand sitting on the next shelf. There are no engineering charges to pay, or lead time for the system to be designed, assembled, tested, and shipped.
Despite all this, there are some compelling advantages to buying rather than building.
How good are you at doing thermal calculations? You'll need to calculate heat sink size, plus select the appropriate fans, while attempting to keep the cabinet size under control. Those who design controllers for a living know how to keep things cool without making them unnecessarily large.
Don't forget bus bars. They take up space, they can be expensive, and once they're in place, moving other components can be difficult. It's also a good idea to minimize the total number of connections in high-power circuits. One way to do that is to use a controller that permits direct connection of the distribution sub-circuits and the bus bars. This permits you to eliminate costly sub-circuit terminal blocks, which add connections in the power path. Another benefit of having the power distribution block directly on the power controller is it lets you fuse each load circuit independently with lower-amperage fuses. Consider that a 400A fuse can run $117 and be hard to find quickly, while an 80A fuse goes for $33 and is available off the shelf.
Also, regarding power connections, there is the choice of supply cable lugs. Make sure these are copper, preferably the crimped-barrel, tin-plated type. It's a good idea to use two-hole lugs, because they can't twist and loosen. Loose lugs can cause like hot spots and shorts to adjacent conductors.
Concerning overheating problems, you'll probably use an SCR controller with a thermal shutdown, as well as the appropriate I2T fuses, but you can help prevent unexpected downtime by making sure the controller gives some sort of signal (LED or a software alarm) if it approaches its temperature limit. It may also be worthwhile to get a controller that alerts the operator if one of the SCRs shorts. This last isn't always needed, but it's worth knowing about.
The problems that come up the first time you apply power are in a separate category. Even if everything goes as anticipated, assembling the unit will probably take longer than you expected. And if it doesn't work right, it's your fault.
Buying a custom control panel from a reputable manufacturer shouldn't take any longer than assembling one yourself. In fact, it can be faster, because the manufacturer doesn't have to send out for parts. In addition, the panel is designed by those who design them for a living and who know how to avoid troublesome errors. Assembly is performed by professional panel builders, not your maintenance people, who don't build panels every day and who would probably be best utilized doing something else-like maintenance. The manufacturer also has access to the latest and most advanced parts.
Even if you decide to build your own, try to get as much as possible pre-assembled. SCR modules and solid state relays, for example, are available with everything put together, including the heat sink. The alternative is to order the power handling devices, heat sinks, DIN rail mount, and fan separately, then use time and money assembling it. It's better to buy them as one unit and just drop it into place.
If you plan to sell the unit, rather than use it in your own shop, you'll have to get all sorts of agency approvals. That's enough trouble, but what if you want to sell it in Europe? Do you really want to go through the process of getting the CE mark? How much will things like electromagnetic compatibility testing cost you, and how much will it cost to do a redesign if you fail the first time?