Libratherm Instruments Private Limited

3 PHASE – 12 PULSE SCR TRIGGERING CARD (PHASE ANGLE CONTROL)  

 Description: 

Libratherm offers new version of SCR triggering cards model LTC-24 which is designed to fire 12 independent SCRs or six back-to-back connected SCRs using synchronized phase angle control technique. The card accepts analogue control signals of (0-5) / (0-10) VDC or (4-20) mA and provides proportional 12 x gate/cathode triggering pulses G1K1 to G12K12.  These pulses can be used to trigger SCRs up to 1000Amps with gate current upto 300mA. The card also provides built in regulated 5VDC supply to connect external potentiometer to manually control the SCR firing. The on card ramp up and ramp down settings allows gradual rise and fall of the voltage levels across the load.  The max/min power settings allow to increase or decrease AC/DC voltage level to the load. The current limit and over current trip settings allow user to limit the max. current through the load and in case of over current the trip relay is energized and thyristor firing is shut off.  

 The LTC-24 is useful for heating control applications to control three phase AC power across the heating coils or to control the star or delta connected transformer loads. The 12-pulse card has advantage over the 6-pulse card since for higher load carrying capacity, instead of using higher ratings of SCRs, two lower ratings of SCRs can also be used. For example, in case of AC to DC bridge rectifiers requiring 1000A DC current, instead of using six nos. 1000A SCR, twelve nos. of 500A SCRs can also be used. Similarly for AC to AC control applications two parallel configurations can be made using six pair of back-to-back connected SCRs in star or delta configuration

 For current limit and current trip features, LTC-24 accepts two external Load Current:5A CT inputs to monitor three phase-load current. These current values are internally compared for limit and trip features.

• Latest SMPS design
• Stable and regulated 24VDC
• Protected against Industrial RFI/EMI interferences.
• Wide input supply of 90 to 240VAC
• Built in overload protection.

• Highly accurate T Rh sensor from Rotronic-Swiss
• Duct Mount, Wall Mount and Vacuum version

PRODUCT DESCRIPTION:

Libratherm offers a 3-phase heater status module (HSM-3). This module is customised and designed using a high speed 32-bit microcontroller to monitor the AC current flowing through 3-phase heaters. The actual current flowing per phase is displayed on a 3-digit display, which will scan all 3 phases in 3 seconds scan mode with the respective R Y B LEDs turning ON. The module accepts the current feedback from the external 3 CTs having a 5A ratio or can directly accept heater current per phase of max.25A. HSM-3 has built-in 3 x CTs to measure the heater current from each phase. The input heater/CT connectors are rated for 30A.

The built in 32-bit microcontroller processes the heater current for the digital display and convert it to the MODBUS RTU format for transmitting the data over the RS-485 serial port.

The front panel keyboard allow user to program the Slave ID of each module, high and low current set point for each phase and scale the current based on the external CT ratio. The front panel provides various LEDs indicating the per phase Heater current in Ampere, TX/RX indicating the serial data transmission, the status of input contact (DI) from the external temperature controller and status of output relay contact (DO) in case of heater current deviates from the high and low limits to indicate drop in the current as the heater failure condition. Under heater fail condition, the phase current will always reduce so low alarm can be used to detect the status. Whereas high alarm condition can be used, if the heater current exceeds the high set value. 

How the Heater fault is detected?

The above features make this model suitable to detect the heater status to identify whether heater is OK or gone open. For example, per phase 3 heaters each of 2KW @ 230VAC, is connected in parallel totalling 6KW per phase and 18KW @ 3 phase. Under normal condition when heaters are turned ON, the phase current will be 26A. If one of the heaters fails, the current will reduce by 1/3^rd that is by 8.7A through that phase and reading will be 26A-8.7A = 17.3A. If the low set point is set to 20A, the internal relay will energise since the phase current is now less than 20A. This relay contact can activate the external audio-visual alarm to indicate that one of the heaters out of 9 heaters has failed or gone open or this DO can be used as DI to the external PLC. The heater current is logically compared only if the input contact (DI) is closed.

The three phase heater current values, status of DI/DO are also transmitted over                  RS-485/MODBUS protocol for external PLC/HMI to interpret the heater failure condition. MODBUS address registers are shared with the user in the operation manual.

The module HSM-3 works on an external (18-36) VDC power supply and can be wall-mounted inside the panel. The electronics PCB is enclosed in a suitable powder-coated metal enclosure of size 115(H) x 195(W) x 75 (D) in mm. 

Where this Module can be used?

The HSM-3 module finds its applications in continuous production machines incorporating several 3-phase heater banks and where failures of heaters affect the process line output and it is required to pinpoint the heater failure location and to allow the operator to replace the failed heater in the shortest possible down-time. It detects heater failures by allowing online monitoring of the current flowing through each heater over RS-485 on the PLC/HMI screen or by providing the digital output (potential free contact) as described above.