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

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

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. 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
CYCLE T ON
OFF
Typical 1 cycle to
256 cycles
approx 5 secs
At 50% TON = TOFF
Long cycle time can
be few mins
TCYCLE = TON + TOFF

7. Single Cycle Firing - Analog I/P
Cycle Time is 2 cycles at 50%
CYCLE
50% Power T T
CYCLE
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. TE 200, TC2000 & TC2001
Less Expensive than three leg control but when the
unit is off the non-controlled leg is at line potential
3 Wire Star
3 Wire Delta
TC2000, TE200 or
TC2001
TC2000, TE200 or
TC2001

9. Phase Angle - Analog I/P
Firing equally delayed for each 1/2 cycle
Shown as 50% On
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
SSDS / RSDA
TE 10S
425S / 450
TC1027

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

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