Why Thyristor Power Control Units Rather Than Contactors?

1. Why Thyristor Power Control Units Rather Than Contactors? • Better Temperature Control - Deliver short bursts to match power demand • Eliminates Temperature Fluctuations - Sense changes in supply and adjust power delivery to prevent power fluctuations • Increased Production - built in diagnostics and partial load failure detection allow preventative maintenance before total failure and costly shut downs of process • Lower Maintenance Costs - no moving parts and plug in design increases heating element Lifetime - reduced thermo-mechanical stress • Process Protection - Current and Power limits protects process and heating elements • Lower Energy Costs - Allows plant wide energy monitoring which reduces peak demand

2. Firing Circuit Will only conduct one way Gate Anode Cathode Thyristor Device Parallel Pairs Thyristor Power Control

3. Control Circuit Firing Circuit Isolation Fuse Snubber MOV Protection MOV - Metal Oxide Varistor - Resistance changes on voltage, as voltage rises mainly on spikes it will absorb the excess voltage Snubber - Resistor/ Capacitor for elimination of spikes and false triggering particularly on transformer loads. Fuse - Protecting against over current draw, will blow faster than the thyristor operates. Eurotherm Thyristor Unit

4. Firing Modes • Logic • Burst / Whole Cycle • Single Cycle • Phase Angle

5. Logic Firing - Logic Input To minimise interference the thyristor unit switches on at the first zero crossing of the mains voltage after the the logic input goes On • On at Voltage zero. • Off at Current zero. Logic O/P from controller Typically Off <2Vdc On >5Vdc

6. T T ON OFF T CYCLE Whole Cycle Firing/Burst Firing - Analog I/P For lesser disturbance on mains you may increase the cycle time • Cycle Time giving for 50% output Typical 1 cycle to 256 cycles approx 5 secs At 50% TON = TOFF Long cycle time can be few mins TCYCLE = TON + TOFF

7. CYCLE T CYCLE Single Cycle Firing - Analog I/P • Cycle Time is 2 cycles at 50% 50% Power T 33% Power • Silicon carbide • When element new the resistance is low and full voltage must only be applied for short bursts. • As the elements heat up and or age the voltage can be sustained for longer periods. • To prevent problems on new elements a power limiting must be used otherwise the element can shatter

8. TC2000, TE200 or TC2001 TC2000, TE200 or TC2001 Less Expensive than three leg control but when the unit is off the non-controlled leg is at line potential TE 200, TC2000 & TC2001 3 Wire Star 3 Wire Delta

9. On Phase Angle - Analog I/P • Firing equally delayed for each 1/2 cycle Shown as 50% Not widely used in Europe as there are problems with CE requirements.

10. Load Types • Fixed Resistance - Any firing mode • Variable Resistance - Current limit or single cycle • Transformer Coupled - Phase angle or burst firing with delayed start. This reduces surge currents. Will switch on at zero crossing point of current. • Inductive as above

11. Load Types

12. Single Phase - Constant Load Resistance 425S / 450 TC1027 SSDS / RSDA TE 10S

13. Single Phase - Variable Load Resistance 425A 460 TE10A TC1028

14. Three Phase - Constant Load Resistance TC300 TC3000 RS3DA / RS3AA TC2000 TE200S / TE200A TC2001

15. Three Phase - Variable Load Resistance TC3001 TC2001

16. Continental SSDA RSDA RS3DA

17. TU Series

18. TC Series

19. TE10S / TE10A

20. 450 / 460

21. 425S / 425A