Control Concepts’ experts can help you prevent and fix these mistakes. Review your design today to ensure it will run smoothly, reliably, and at peak performance.
Mistake 1. Incorrectly Sizing Your Controller
One of the most common design mistakes is improper controller sizing. While an undersized controller may experience excessive heat, reduced lifespan, component stress, or frequent failures, an oversized controller can lead to higher costs, reduced control resolution, and unnecessary system complexity.
Control Concepts flat rates controllers for the full frame current at 50ËšC and 6000 ft. altitude. Consider the following requirements to improve both reliability and performance:
- Maximum current
- Ambient temperature
- Duty cycle requirements
- Safety margins
- Future needs
Mistake 2. Lacking a Mechanical Disconnect
SCRs are not mechanical disconnects and do not provide electrical isolation from the line voltage, even when off. Forgetting to include a mechanical disconnect may be fatal.
Mistake 3. No Electrostatic Shield in an Isolation Transformer with an Ungrounded Load
Without the electrostatic shield, AC current can couple across the transformer due to inter-winding capacitance. This results in measuring a potentially lethal secondary high voltage to earth ground.
Mistake 4. A Mechanical Contactor on the Load Side of the SCR
Customers often desire to switch loads using a mechanical contactor for safety or load trimming. When a mechanical contactor opens and closes, the contacts aren’t made simultaneously. If the contactor is opened under power, kickback voltages may be sent to the SCR, which can cause premature failure. Furthermore, when an SCR controller uses power or current feedback, the controller may momentarily apply full power to your load when the contactor is switched on, causing damage to the load, saturation of a downstream transformer, or blown fuses.
Mistake 5: Choosing the Wrong Firing Mode
SCR firing methods have varying trade-offs. Phase Angle Firing provides fast response and precise control at the cost of increasing harmonics, electrical noise, and power quality concerns. Zero Cross firing provides reduced harmonics, better power factor, and improved power quality. For many industrial heating applications, Zero Cross firing offers the best balance between performance and electrical efficiency.
To choose the right firing mode, consider the following design requirements:
- Load characteristics
- Process requirements
- Utility considerations
- Power quality objectives
Mistake 6: Poor Load Configuration Design
Load configuration directly impacts system performance, phase balance, efficiency, and operating costs. For example, unbalanced three phase loads cause uneven current distribution, while incorrect delta/wye selection may result in improper voltage delivery. Furthermore, load configuration design should account for future expansion.
Verify:
- Voltage requirements
- Load balance
- Power distribution strategy
Mistake 7: Missing Fuses
Protection devices like fuses are critical elements in the design because SCRs require rapid fault protection. Without proper protection, faults can destroy SCR modules and cause expensive fixes or lengthy downtime.
Mistake 8: Overlooking SCCR Requirements
Many facilities discover SCCR problems only during inspections or commissioning, leading to noncompliance, safety risks, and costly redesigns. To prevent this mistake, consider available fault current and the SCCR ratings of controllers and fuses during system design.
Mistake 9: Ignoring Thermal Management
Heat is the enemy of power electronics. Even properly sized SCR controllers can fail prematurely if thermal management is inadequate. Common thermal issues range from restricted airflow from poor enclosure layout to undersized cooling systems and elevated ambient temperatures. Warning signs to watch for: frequent alarms, unexpected shutdowns, or shortened controller life.
To best address this issue, consider cabinet ventilation, heat sink sizing, ambient conditions, and cooling requirements during the design phase.
Mistake 10: Missing Communication Requirements
Facilities increasingly require the following integration capabilities:
- EtherCAT
- Modbus TCP
- EtherNet/IP
- PROFINET
Plan communication architecture early in the project to improve visibility and system control, and to prevent costly redesigns.
Mistake 11: Poor Power Quality
Power quality problems can develop slowly and remain unnoticed for months. Common problems include harmonics, low power factor, and transformer heating. Learn more
Mistake 12: Designing for Today Instead of Tomorrow
The best SCR system designs balance current requirements with future possibilities, including increased production capacity, additional heating zones, new automation requirements, or different communications need. To prevent redesigns, retrofits, and downtime, consider how your needs may change in the future.
