Frequently Asked Questions
- 1. How does an SCR power controller work?
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Phase angle controllers work like a light dimmer. The load voltage is varied by varying the time within each electrical half-cycle during which voltage is applied to the load.
Zero-cross is analogous to a very fast contactor that "makes" only at the beginning of each electrical half-cycle and always "breaks" at the end of the electrical half-cycle.
- 2. When should phase angle be used and when should zero-cross be used?
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Phase angle can be used to control all loads. Phase angle must be used to power transformer coupled loads and fast heating loads like lamps.
Zero-cross has the advantages of being lower in price, slightly more reliable and generates substantially less harmonics than phase angle controllers.
- 3. What are the advantages of SCR controllers verses mechanical contactors?
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- Infinite resolution
- No inherent wear out modes
- Often less expensive
- Voltage compensation, current limiting and over current trip are often in the SCR controller design.
- 4. How should a controller be fused?
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Control Concepts recommends that a class "T" fuse be used to protect the SCR. Control Concepts has found that the class T fuse is an excellent compromise between protection and premature fuse failure.
- 5. How much heat does the SCR controller generate?
Approximately 1.2 watt's per amp switched per power line controlled by an SCR.
- 6. What command signals will the SCR controllers operate on?
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All of the commonly available command signals of the various temperature controllers and PLC outputs such as; analog signals 4-20mA, 0-5Vdc, 0-10Vdc, Potentiometer and on/off 3-32Vdc, 0-24Vdc, 0-120Vac or 0-240Vac.
Other command signals can be ordered to match with older control signals like 2-12mA, 12-20mA, 1.2-6Vdc.
- 7. Can I put parallel loads on one controller?
In most cases yes, as long as the total load current is not more than the SCR controller is rated to handle.
- 8. What size of fuse should I use on a 30-amp single-phase load?
Size the fuses to be 25% over the full load current. In this case, fuse the SCR controller at 40 Amps with a very fast fuse, "Type T" or faster.
- 9. Do controllers generate harmonics on the electrical lines?
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- Yes they do generate harmonics on the power lines.
- A Zero cross controller generates most of its harmonics below 50/60 cycles.
- Phase angle controllers generate harmonics above 50/60 cycles with the largest distortions at 3rd, 5th & 7th harmonics for single phase controllers. For three phase, phase angle controllers, the third, and multiples of the third, are not created.
- 10. What are the effects of harmonics and should I be concerned?
Harmonics can cause overheating of inductive equipment, such as transformers, motors, etc. However, actual occurrences of this problem from harmonics are essentially non-existent.
- 11. Are the controllers listed or recognized by a national testing lab?
Most controllers manufactured by Control Concepts have been tested by Underwriters Labs to UL and CSA standards and carry the UL and CUL marking.
- 12. How do I derate Control Concepts, Inc SCR Power Controllers?
While derating is necessary with most manufacturers controllers, starting at as little as 25 degrees C (room temperature), our controllers are rated at 55 degrees C (130 degrees F). So in most cases, no derating is required. Consider the cost savings: with many controllers, in order to control 120 amps at 55 degree C, you would need to order a 200 amp controller (because of their derating curve). With our controllers, you can order a 120 amp controller and actually control 120 amps.
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