Heater Power Regulator

Digital Type(SPR-D Series)

 A. General comparisons of SCR power controllers

1) SCR Switching control type
   This type can not control the angle of thyristor (SCR) as below mentioned
   phase angle control and zero-crossing,but only turns on/off thyristor (SCR)
   at sending control signals to SCR.This type has more efficiency and fewer
   harmonics than SCR phase angle control type, and can produce low cost.
   But the resistance element temperature experiences very large temperature
   swings.

2) SCR Phase angle control type
  This type is phase angle control base which turns on/off thyristor (SCR)
  at a certain angle with the controlling input of waveform 0~180 degree
  of load power.This type is effective to control a specific voltage and
  current,and to use the control of inductive load. This type generates
  harmonics,because of producing wave distortion. So this type takes troubles
  to be surrounded electronic machines and tools, and reduces the efficiency
  of the transformer because of generating ineffective power during turning
  off SCR.


3) SCR Zero-crossing(Zero-voltage switching) control type
  This type is time control base which turns on/off thyristor (SCR)
  at zero point of ac power sine wave, and this does not produce
  wave distortion. Thus, this type does not generate harmonics and
  ineffective power. Today, this type is  better for controlling a big
  heater and heaters surrounded with sensitive machines to noise
  than SCR phase angle control type. But this type cannot control
  induction heater load. The Zero-crossing control type can use
  fixed time base zero-crossing,variable time base zero-crossing,
  and mixed(phase angle+variable time zero-crossing) control.
 (1) Fixed Time Base Zero-Crossing control
     A fixed time base means that the “cycle time” on the SCR power
     controller is permanently set or fixed(not adjustable).
    During this time period(1 second, for example), the SCR power controller
    will turn the SCR ON and OFF the provide the correct percentage of power
    to the heater. For example, the  SCR has a fixed time base of 1 second,
    and the power supply is 60Hz. If the process output demands a 40% power
    requirement,the SCR will be turned on for 24 cyles(40% of 60 cyles) and
    turned off for 36 cyles.

 
  (2) Variable Time Base  Zero-Crossing control
      A SCR power controller with a variable time base changes th time base
      according to the power requirement, and uses the smallest possible number
      of ac power cycles to deliver the required percentage power to the heater.
      If the process output demands a 40% power requirement, the SCR will be
      turned on 2 cyles out of every 5 cyles.For  90% power requirement,
      the SCR will be turned on 9 out of 10 every cyles
 

4) SCR  Mixed Control (Phase angle + Variable Zero-Crossing control)
   This type  combines the best features of the phase angle and variable time base
   zero-crossing firing method, and is utilized on the power control of variable
   resistive heater loads(for example, halogen , molybdenum,tungsten heater).
   This mixed type is under phase angle control during initial power application
   and then switch to variable time base zero-crossing. So, this ensures maximum
   controllability when needed, and then revert to RF interference free zero-crossing
   control after process stabilization.

 


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