The PR3 range of thyristor stacks provides full seamless control of three
phase resistive loads, using two thirds control technique. Signal control is by a
DC signal. These burst firing control stacks use fast pulse zero volts switching
technology, to minimise flicker and eliminate RFI problems. They also
incorporate an automatic resetting temperature trip, integral semiconductor
fuses and heatsink. The two models in this build includes the 54kW model,
which is forced-air cooled. All have easy access to signal & power terminals
for simple installation.
Suitable for 3-wire, 3-phase floating-star or closed-delta configured resistive
loads. This includes the Heating, Ventilating and Air Conditioning (HVAC)
market for air
Alarm relay functions (3-way terminal – V/free alarm “NO_NC_C”)
The alarm circuit has ‘voltage free’ relay contacts and are rated up to 2A @ 125V
ac (RMS) load. Connection is via the PCB terminal.
For alarm relay status options see SPECIFICATIONS.
Note: The internal supply to the relay is obtained from the transformer via two 20mm 1A fuses. These are
connected to the Black (L2) and Grey (L3) phases and therefore the relay and LED can only energise when
there is an over-temperature condition, a sensor fault, or a phase loss on L1 phase. See Remote Supply
Over temperature protection
When a heatsink temperature of above 90ºC is detected by the sensor, the alarms relay changes state and the status LED indicator flashes
continuous rapid pulsing. The power to the load will be disconnected and will not return until the temperature drops to 85ºC. The 54kW unit is
also fitted with a cooling fan that switches ON when the heatsink reaches approximately 55ºC.
Temperature sensor loss
The Status LED indicator changes to continuous equal ON/OFF pulsing if the sensor fails.
Phase loss with auxiliary supply
When any one of the three phase inputs are missing, the relay changes state and the LED flashes with
ON/OFF bursts of 1.5 seconds.
This is only functional with a remote supply (see below).
The default setting of DIL switch (SW1) is in the ON position, the alarm relay will be energised under a fault condition. Changing SW1 to the off
position will energise the alarm relay continuously until a fault condition occurs.
The unit will be factory set for an internal supply (SW4 in the “INT” position). If there is a requirement for the alarm relay and LED to energise
when a phase fault condition occurs, then there is provision for an external 24V ac or dc supply (SW4 in the “EXT” position).
NOTE – If the remote supply is utilised, the main (L1, L2 and L3) supply must come on before this supply is switched on.