How to design an SCR Power Controller

 

SCR unit is a semiconductor device which acts as a switch formed by two thyristors in antiparallel.

Each thyristor is like a one way valve able to go in conduction only when are satisfied these conditions:

To form a real switch for alternating current is necessary to use two thyristor in antiparallel. Remember that thyristor does not work with d.c. voltage power supplyThe thyristor that is in conduction does not stop to conduct up to when the voltage go to zero. This also if we remove the gate signal after to have switch ON it.

 

Here below we can see how the thyristor unit work

 

Gate signal ON When L1 is positive the current flow in A versus and thyristor Th1 goes in conduction. When L1 is negative The current will flow in B versus and the thyristor Th2 goes in conduction. If we compare the thyristor unit to a switch in this condition is ON Gate signal OFF   Both thyristor does not go in conduction If we compare the thyristor unit to a switch in this condition is OFF

Gate signal ON when voltage is not zero

When L1 is positive the current flow A versus only if gate signal is ON. Now we can image to give a gate signal when voltage is not zero

How is possible to understand changing the firing angle θ is possible to have a variable load voltage.

 

VOLTAGE PROTECTIONS

Snubber and varistor are used for voltage transient protection. Hight speed semiconductor fuses with proper I2t are used to don’t demage SCR. External fuses are offered with fuse holder capable to break the circuit from 15 to 110A in one phase units and 15 to 55 in 2 and 3 phases units. Over this value the unit will have internal fuses. CD Automation offers as an option H.B. circuit plus SCR short circuit alarm.

CURRENT PROTECTION WITH FUSES

Fault current may result from short circuit in external connections. Thyristor due their small thermal mass have a very limited overload capacity if compared to motors, transformers etc. Basically I2 t of the fuse must be less than I2t of thyristor. Overload protection is demanded to a thermal overload sensor mounted on heatsink to quench thyristor automatically when temperature can damage the unit. For thyristor protection don’t use circuit breaker because its speed to open circuit is too slow and thyristor will be damaged in case of short circuit.

 

ADVANTAGES OF THYRISTOR UNITS

The benefits of thyristor units (also called Thyristor Power Controller and SCR Power Controller) compared with electromechanically contactors are numerous:

• No mechanical parts moving
• Thyristor offer a reliable long term solution
• Very fast switching
• Life of thyristor unit can be evaluated from 5 to 10 year depending on how the thyristor are stressed. The normally electrical life of contactors is 1,5 million of operations that can be reached in few months.
  Ex: Imagine a furnace with contactor driven by a temperature controller with a cycle time of 20 sec. At 50% power demand is 10sec ON+10sec OFF.
  That means that contactor is switched ON every 20 sec. During one working day over 3 turns(24 hours) the contactors life is given by:
  1Hour = 180 switching of contactor
  1Day = 180×24=4820 swicting
  Calculation life = 1.500.000 / 4.820 = 347 day
  This means that every years the contactors must be changed with labour cost and stop of production.
• Better temperature control due to very fast switching and better resolution in controlling the power. Save money for reduction of scrap
• Better quality because keep constant the temperature with no fluctuation
• Increase the heating element life because with very fast switching there are no thermal stresses.
• Lower maintenance cost for contactor and for heating elements substitution.
• Protection of thermal elements and building parts of furnace or machine with current limit, power limit to remain within limits specified
• Reduction of cost because is possible to establish how many elementary loads has to be fired on same time. Avoid to have peak demand power
• Capability to monitor in a control room all the loads via the communication bus available on CD Automation thyristor units.
• Possibility to diagnostic via serial link faults on heating elements or on thyristor units
• Communication can be used to control for power set point and reading of voltage, current, power, and load status. Recipe facility are olso available with digital communication.

 

27 May 2024