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Variable Time Base Burst-firing (zero cross) controllersare also available with a variable time base. The on and off time is proportional to the command signal, but the time base changes according to the demand. At 50 percent it’s a two cycle time base, one cycle on and one cycle off. The example of 40 percent is twocycles on and three cycles off.

W A T L O W

• Fixed Time Base Burst-firing (zero cross) controllersare available with either a one second or four second time base. In either case, the SCR is turned on for a time proportional to the command signal. In the example just mentioned (assuming 60 cycle ac current), the 40 percent would be 24 cycles on and 36 cycles off with a one second time base.

Application Guide catalog - 7347 Application Guide

B.Burst Firing or Zero Cross

Power Controllers

AC power alternates plus and minus60 or 50 times per second (depend- ing on power generation standards),thus 60/50 cycle . An SCR can accu- rately control each cycle. Burst firingprovides a proportional output to theheater by turning on for a number ofcycles and then remaining off for a number of cycles. The proportion onand off is according to the tempera-ture controller’s command signal. A zero to 5V

Methods Of Firing SCRs

Zero cross (also known as burst firing)provides even output power with thelowest levels of noise generation (RFI). Zero cross is the preferredmethod for controlling a resistive load.The power controller determines whenthe ac sine wave crosses the 0-voltspoint, then switches the load, minimizing RFI. Fixed Time BaseRef. 99 50 Percent Variable Time Base, 1 Cycle On,1 Cycle OffRef. 10040 Percent Variable Time Base, 2 Cycles On, 3 Cycles OffRef. 101 Phase Angle FiringRef. 102 Soft StartRef. 103 Current LimitingRef. 104 1.Zero Cross Î (dc) signal with the outputat two volts would have the SCR on 40 percent of the time. There are two types of burst-firing controllers: • Soft Start is accomplished by delay-ing the turn on, and slowly increasing the on time in subsequent cycles with less delay. This allows heaters that change resistance with temperature toturn on slowly. This typically occursover a 10 second period. A.Solid State Contactor Temperature controllers with a “time proportioning” output proportion heat to the process with an on and off command signalto thepowercontrol- ler. Proportioning is accomplished by turning the heat off for a longer time period as the actual temperature approaches set point. This is not to be confused with an on-off temperature control, where there is no proportioning.SCR controllers designed to respondto this on or off signal are solid state contactors. They are capable ofoperating from a cycle time as short as one ac cycle. This rapid response to a fast proportioning signal produces excellent process control. Current Limiting utilizes the samedelay, and also has a transformer to sense current. A limit to the accept- able amount of current is established and the logic in the controller will notallow current above this predeter-mined value. However, if a short in the heater occurs during this on time, the resulting high current cannot be restricted from passing through theSCR. The only protection possible for the SCR is an I
24 Cycles ON36 Cycles ON Temperature Controllers 2 t fuse that will blowwithin the ac cycle. 3.Combination Burst firing is also called “zero crossfiring” or “cycle proportioning.” Utilizing the advantages of phaseangle firing to turn heater loads on and switching to burst firing to main-tain power appears to offer the best of both worlds. This has been tried where burst firing cannot be used at turn on. Unfortunately, the bursts of power have also been found to short-en the life of silicon carbide elementsor transformers that the SCR is controlling. 2. Phase Angle Firing The SCR, once turned on, latches onand only turns off when the polarity changes. If the turn on point is notzero, but is delayed inside the sinewave, then the amount of powerallowed to pass through the SCR can be controlled. This is called “phaseangle firing.” 109