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

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

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

Firing Modes Logic Burst / Whole Cycle Single Cycle Phase Angle

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

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

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

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

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.

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

Load Types

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

Single Phase - Variable Load Resistance TE10A 425A 460 TC1028

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

Three Phase - Variable Load Resistance TC3001 TC2001

Continental SSDA RSDA RS3DA

TU Series

TC Series

TE10S / TE10A

450 / 460

425S / 425A