User`s manual

Burst Fire
Overview
Variable, time-base burst firing from the Series 982 provides the most
even distribution of power with the lowest level of noise generation (RFI).
An SSR or SCR firing card translates a command signal into a burst of AC
cycles. The output is zero-cross fired, which always allows at least one full
AC cycle to pass within the variable time base. Burst firing is the preferred
mode to control resistive loads.
The Series 982 detects when the AC sine wave of the load will cross the 0-
volt point. It uses this information to switch the load ON or OFF only at a
0-volt point, minimizing RFI.
The burst fire time base in the Series 982 varies from a maximum 1.66-
second time base (1-percent output; 1 cycle ON, 99 OFF) down to a 33.3-
millisecond time base (50-percent output; 1 cycle ON, 1 OFF). The graphs
on the next page show how the time base varies with the percent output.
Requirements
• This feature only works with zero-cross solid-state devices. It will not
function correctly with random-fire devices.
• To enable burst fire the Series 982 must have an open collector or solid-
state relay output: controllers with option “B,” “C” or “K” selected for
output 1 (98_C-__*_-____), output 2 (98_C-___*-____), output 3 (98_C-
____-*___) or output 4 (98_C-____-_*__).
• In single-phase applications the controller should be on the same phase
as load.
• The time burst is appropriate for fast loads or very tight control. It pro-
vides advantages only for PID control, not for ON/OFF control.
• The short time bases used by burst fire makes it incompatible with the
heater current feature. The heater current option requires a minimum
of 300 milliseconds ON time to get a reading.
• The Series 982 has built in zero-cross detection circuitry, eliminating
the need for external firing circuitry to trigger SCR’s. The controller will
not allow burst fire to be selected if its zero-cross detection circuitry is
not functioning.
• The feature is enabled by selecting burst fire [brSt] at the cycle time
prompt for the appropriate output in the PID Menu — [Ct1A], [Ct1b],
[Ct2A] or [Ct2b].
• Only the 982 and 981 can use the burst fire feature. The low-voltage
units (984 and 983) cannot use burst firing.
9.2 WATLOW Series 982 User’s Manual
Software Features, Chapter 9
Software
ç
CAUTION:
This feature only
works with zero-
cross solid-state
devices. It will not
function correctly
with random-fire
devices.
ç
CAUTION:
In single-phase
applications the
controller should
be on the same
phase as load.