In any power control situation, the first step is to determine the current and voltage requirements and decide whether the application is to be three-phase or single-phase. Single-phase controllers will be less expensive that three-phase controllers. Figure 1 shows the calculations to determine power and current.
The second step is to determine the type of control mode (zero-cross or phase-angle) and select the basic features required to enhance performance. Load type will dictate the control mode and features. For example, transformer-coupled loads, fast-responding loads, and loads that change resistance with age or temperature typically require phase-angle control. Table 1 can help you ascertain the type of control mode to be used. If your process can accept it, zero-cross control typically is less expensive than phase-angle.
The third step is to choose a reliable and serviceable controller. Expect more than the obvious. The features highlighted can increase the reliability and serviceability of SCR power controllers.
Adding a filter in the zero-cross detection circuit helps reject electrical noise and is acceptable for some applications. Others -- controllers intended for transformer-coupled loads, for example -- require a precise, noise-immune technique. For these applications, request a phase-locked digital circuit to determine the zero-cross by a voltage integration technique. Transformer load applications require that all SCRs be turned on for precisely the same amount of time to avoid applying DC. The phase-locked self-adjusting digital clock technique provides the means to achieve accurate timing and can be used as is on 50 or 60 Hz applications.
Surge Current.Certain loads -- transformers or loads with a low cold resistance, such as infrared lamps -- can have a large surge current. Therefore, the controller's surge current rating must be adequate to protect the SCRs from damage if they are turned on by transients. Any surge current that may occur is dependent upon the load and the source resistance and is difficult to determine. However, a surge current rating of 12 times rated current usually will provide satisfactory reliability.
Transient Protection.Rapid voltage changes (Δv/Δt) and high voltage transients are common. Under such conditions, the SCRs can turn on, causing undesired surge currents or outputs if proper protection is not provided. Unprotected SCRs can be destroyed by repeated transients.
To prevent damage or failure due to high voltage transients, SCRs should have a high blocking voltage (1,400 V for 480 VAC) and should be protected with fast-acting metal oxide varistors (MOVs). Further protection is provided by a series resistor and capacitor circuit connected across the SCRs to provide a shunt for high frequency transients.
Gate Drive.SCRs are turned on by the injection of current into the gate. The gate's input impedance changes drastically during turn on. Therefore, a gate drive circuit, particularly on large SCRs, is required to inject the desired current independent of gate impedance.
Cooling. An 18°F (10°C) increase in semiconductor temperature will decrease reliability by 50%. In most applications, it is difficult for the user to determine if adequate cooling is provided. Characteristics of adequate cooling design include:
- The ability to operate continuously in a 131°F (55°C) ambient environment with no derating.
- Extremely smooth surfaces on which the SCRs are mounted.
- The use of a thermal compound on all electrical connections to aid thermal conduction and to prevent oxide formation.
- Proper containment of the cooling air such that all of the air provided by the fan passes over the heat sink.
Single Plug-In Circuit Board. Using a single circuit plug-in circuit board greatly improves serviceability. With this setup, circuit cards can be switched from one controller to another when diagnosing problems. Also, use of a plug-in board eliminates the possibility of miswiring during service. An added benefit is if the circuit can be used on all controllers (independent of current rating), only one card is required for spare parts.
Status Indicators. Consisting of small LEDs, status indicators provide information regarding the magnitude of the command signal, magnitude of the load current, and status of features such as overcurrent trip and shorted SCR detection.
Power Connectors. Connectors used to join the supply leads and load leads should be designed to remain secure when field wiring is installed. (The typical installer will overtorque the connectors. If the connectors become loose under these conditions, problems will develop.) The connectors also should be located in a position that it is convenient to use a wrench and apply the right torque during installation. Finally, a design that allows air from the semiconductor cooling fans to pass over the large current connectors will improve reliability.
Long Life Fans. Fans often are required to cool the semiconductors in SCR power controllers. Fans with ball bearings or extended-life sleeve bearings provide better performance and longer life.
Spring-Loaded Connectors. Using spring washers on all electrical junctions ensures a good connection. The resistance of one conductor or bus bar to another generates heat in electrical connections. This heat causes expansion and deformation of the connection and, in turn, can cause the resistance to increase, generating even more heat. The spring applies a constant force to the junction, allowing for expansion. In addition, all connections should have an electrical compound applied to improve thermal and electrical conductivity.
Force-Indicating Clamps. By design, "hockey puck" SCRs and diodes must be squeezed between heat sinks with spring-loaded clamps to ensure electrical and thermal conductivity. To facilitate field replacement and serviceability of the semiconductor, the spring clamps should have a force indicator to determine the proper force.
The next time you need an SCR power controller, consider specifying some of these features. They can add functionality and increase reliability, reducing the chance of unplanned production shutdowns.
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