When to use Phase Angle Control vs Time Proportioning, Soft Start and Line Voltage Compensation

Phase angle controllers should be used when the following conditions exist:

  • The load requires voltage control
  • The load may not be able to tolerate full line voltage so a phase angle w/voltage limit may be employed
  • The load has high inrush characteristics and required soft start
  • The load is inductive
  • The load is sensitive to the thermal shock of time proportiong (burst firing)
  • The load is very fast
  • The load is a phase controllable motor or vibratory feeder

  Burst Firing (time proportioning or PWM) at the zero crossings is a good method to control the load power when the following conditions exist:

  • The load is a heating load and has sufficient thermal mass that the on and off pulses will not ripple through the process
  • Low EMI is required

Soft Start is a method of limiting the rate of rise of the output of a phase angle controllers power output generally during start up only.  Soft Change is a method of limiting both the rise and fall of of a phase angle control’s power output.  Both of these methods are used mainly to limit the load’s inrush due to either the resistance temperature coefficient of a heater lamp or because the load is inductive.  Soft start is useful on high inrush heaters such as Quartz, Molybdenum, Tungsten, or Graphite heaters.  Soft Change can be a good choice for phase angle controlling inductive loads, where rapid changes in the phase angle can induce a DC imbalance in the in load and thus cause misfiring, heating in the inductor, or blown fuses.

Soft start phase angle control limits inrush

Line Voltage Compensation in a Manually Controlled Heater Process:  In a simple manually operated phase angle control arrangement the user sets the power level of the phase angle controller and this delivers a percentage of the full power to the heater.  Ruling out any other process disturbances, the line voltage can change at any time affecting the power delivered to the heater dramatically (don’t forget P=E^2/R).  Line voltage compensation effectively cancels out the change in line voltage keeping the power delivered to the load exactly where the user sets it.

Line Voltage Compensation in a Closed Loop Heater Process:  In a closed loop heater process, a temperature controller such as a CAL 3200 sets the output level of the phase angle controller.  The temperature controller (if it is a PID controller) will correct for changes in line voltage, but these changes have to ripple through the process lags before they arrive at the controller.  This can affect the product quality, because often times the sensing point of the controller isnt perfect.  Line voltage compensation cancels the affect of line voltage changes before they arrive at the temperature controller. 

Line Voltage Compensation improves process stability

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