Published 7/1/2002

Selecting SCR Power Controllers

While it is more geared to the manufacturer of SCR controllers, this article can give finishers some good tips on what to look for when purchasing or building their own SCR power controller…

Michael Hardy, Chromalox

Electrical Specifications

A power controller must have specifications matched to the application. Major specifications include:

AC input voltage and number of phases;
SCR control firing method (on/off, proportional and/or phase angle);
Maximum load carrying capacity (amps);
Control input (0-10 v, 4-20 mA, etc.); and
Options.

Pre-Assembled Components

SCR power controllers come in a variety of configurations and options. You can even build one yourself. However, to design and build an SCR controller may not be cost efficient, since packaged systems are readily available in a variety of configurations and options. As with any system, installation is faster if the system components come pre-assembled with all the options needed for your application. Commonly, an SCR controller includes the following components:

Firing card - controls the gating of the SCRs;
SCRs - silicon controlled rectifiers for load switching;
Bus bars - conduct power from the SCRs to the load connection terminals;
Current transformer - for measuring current to detect partial load failures and facilitate soft starting of the load;
I2T fuses - fast acting fuse that protects the SCRs;
MOV protection - metal oxide varistor circuit that protects the SCRs from excessive transient voltage;
Heat sink - typically, a finned aluminum extrusion on which SCRs are mounted to conduct heat away from them;
Cooling fan - forced convection cooling of the heat sink;
Connection lugs - load connection terminals and grounding lug; and
Mounting - plate or panel for mounting the SCR assembly in an enclosure.

Physical Size

The SCR power controller is one of the larger components of a heat control system. A power controller with a small footprint reduces panel size requirements, saving money by allowing use of a smaller enclosure. A compact power controller also increases space for other components and makes assembly easier by giving the installer more space to work in. With the continual shrinkage of semiconductors and other electronic components, you can find 100-amp, single-phase SCR power controllers in package sizes smaller than 0.5 ft3.

Flexible Wiring Options

A significant challenge in panel building is running the AC power wiring into and out of the SCR controller. Some users want power conductors to enter at the top of the controller and exit at the bottom. Others may want the power to enter and exit on the same side. This depends on how AC power lines enter the control panel enclosure. Because many SCR controllers have fixed power connections and few options on their location, a panel builder or installer may be forced to bend conductors in a u-turn and run them around the inside of the panel to meet entry and exit specifications. With high amperage-power conductors, this may require such a large bending radius that an oversized enclosure is needed. Besides the wasted space, these wiring practices are time consuming, add component cost and may increase the heat within the enclosure.

The solution is an SCR power controller with a design that allows power connections in any direction without large radius bends. Typically, this means a controller with a bus bar design that acts as an integrated power distribution block, allowing multiple connections in any direction. This type of design also eliminates the need to purchase and wire a separate distribution block, reduces panel construction costs and lowers installation costs by allowing installers to quickly and easily connect power directly to all circuit legs.

Touch-Safe Design

Most SCR controllers are open systems that expose the technicians to potentially high voltage parts. It is common practice to install a Plexiglas cover over the power components as a way to add protection, but of course this requires additional labor and materials. Some suppliers provide SCR power controllers in a touch-safe design, which eliminates the need for Plexiglas.

Fuse and Power Distribution Connections

In some installations, the panel builder must wire the SCR controller to a power distribution block before installing fusing. This step can be avoided if the SCR controller allows sub-circuit power distribution connections right on the output bus bars. Direct cable runs between the controller outputs and fuse blocks reduce both wiring and component costs. Remember, sub-circuit terminal blocks can range in price from $45-120.

Moreover, fewer high-power connections mean that there are fewer potential hotspots and points of failure. The higher the amperage rating, the more important this becomes. A temperature related insulation failure, resulting in an electrical short, can easily destroy the entire control panel.

Fuse Replacement Cost

Another benefit of selecting an SCR power controller with a built-in power distribution block is easier division of output circuits into multiple legs. This arrangement is often required in a multi-circuit temperature control system and may be desirable in many other applications. For example, circuit division allows the use of lower amperage fuses, which are less expensive and more readily available from the distributor's shelf stock. This reduces your inventory cost and reduces downtime in emergency situations. Over the lifetime of a panel, these cost savings can be significant. A typical 400-amp fuse costs $117 while a typical 80-amp fuse costs $33.

Built-In Controller Protection

In addition to fuses that protect the external power circuits and equipment, the user normally wants to protect controller SCRs, which are expensive to replace. Many power controllers include I2T fuses and a thermal shutdown feature. In case of thermal overload, it generally is preferable to have advanced warning of an impending shutdown so that steps can be taken to avoid the loss of work-in-process material. Therefore, the protection circuit should provide an LED or software alarm before temperatures reach the automatic shutdown point. Some power controllers only have a thermostatic cutout that shuts down the system without warning.

Another form of protection is a feature that detects a short in the SCR, which can help shorten troubleshooting and down time costs. While this feature is desirable for reason of cost, it may be omitted for less critical applications. This feature can be an option in the form of a retrofittable plug-in board. This allows the user to specify only the components needed for a specific application, but upgrade the panel later if its use changes.

Power Connections

To reduce maintenance problems, do not overlook power cable connections. Since controller input and output configurations vary by manufacturer, it is common practice for makers to supply cable lugs that fit their designs. You may find designs that use aluminum lugs, but these should be avoided with copper conductors. The different thermal expansion rates of the two metals can result in the connectors loosening over time. This is especially true for a high power heating system that uses on/off control and will be worse for lugs that have a bolted split-barrel connection to the conductor. The most secure and reliable copper conduction is a crimped barrel, tin-plated copper lug. (It is anticipated that Underwriters Laboratories will soon require all copper connections before issuing a UL product listing.)

Also, a two-hole lug provides the most reliable connection to a power distribution block. A single-hole lug is more likely to rotate during installation or if it becomes loose under thermal or mechanical stress. This creates a higher potential for shorts and thermal problems in nearby circuits. A two-hole lug greatly reduces this possibility and helps minimize maintenance costs.

CE Mark

When building power control panels for equipment sold to European markets, a CE mark may be required on all major components to signify compliance with EEC directives. Purchasing an integrated power controller with the CE mark simplifies sourcing and vendor management compared to individual component purchases. Not having those CE marks can eliminate the panel builder from consideration as a supplier or could require extensive rework on a previously built panel. Since you may not know where a panel will end up, it is advisable to standardize on controllers and other components that have the CE mark.