Libratherm Instruments Private Limited

Libratherm offers single phase zero cross over SCR firing cards model LZF-1 for AC single phase operated electrical resistive or inductive loads. This card is basically developed to replace the electro-mechanical contactor with the solid state switch using silicon controlled rectifiers (SCRs). When the electrical load is operated through contactors – the AC cycle is switched ON or OFF at any point of 0 to 180 degree phase angle, this leads to generation of arcing on the contacts and may generate harmonics, which can cause electromagnetic disturbance in the power lines and can also cause malfunctioning of other electronic instruments on the panel. The card featured here eliminates generation of voltage transients because of zero cross over switching technique, also no arcing will be observed during switching operation – since no mechanical contact movement is involved in the solid state device.

The card LZF-1 accepts logic pulse of 3 to 36VDC or potential free contact or analog signal from temperature controller or from PLC and generates single phase gate cathode triggering pulses for a pairs of back to back connected SCRs. The analog signal is pulse width modulated to generate variable duty cycle on-off pulses. Hence, one can control AC single phase load in time proportional mode.

Using LZF-1 card and external back to back connected SCR modules (such as Semikron SKKT27, SKKT57, SKKT106, SKKT162, SKKT 273, SKKT 573) one can build solid state relays rated for 25A to 500A @ 230VAC.

Description:

Libratherm offers three channel SCR firing cards model SCRF-3N and LZF-3N for AC single phase and three phase operated electrical resistive or inductive loads. These cards are basically developed to replace the electro-mechanical contactors with the solid state switch using silicon controlled rectifiers (SCRs). When the electrical load is operated through contactors – the AC cycle is switched ON or OFF at any point of 0 to 180 degree phase angle, this leads to generation of arcing on the contacts and may generate harmonics, which can cause electromagnetic disturbance in the power lines and can also cause malfunctioning of other electronic instruments on the panel. The card featured here eliminates generation of harmonics because of zero cross over switching, as well as no arcing will be observed during switching – since no mechanical contact switching is involved in the solid state device.

SCRF-3N accepts 3 independent or common logic pulse of 3 to 36VDC or potential free contact from temperature controller or from PLC and generates 3 independent gate cathode triggering pulses for 3 pairs of back to back connected SCRs. Hence, one can control 3 independent single phase loads or single 3 phase load in star or delta configuration.

LZF-3 accepts 3 independent or common 4-20mA or 0-5V or 0-10V linear input from temperature controller or from PLC or from potentiometer and generates 3 independent gate cathode triggering pulses for 3 pairs of back to back connected SCRs. Hence, one can control 3 independent single phase loads or single 3 phase load in star or delta configuration. The linear signal is pulse width modulated to generate pulsating output with variable on/off duty cycle time of 2 to 30 seconds using on card potentiometer.

Our product SCR Firing and Triggering Cards using Phase Angle Control Technique For Single / Two or Three Phase heater control and DC rectifier. Using synchronized phase angle control technique.Libratherm offers SCR triggering cards model LTC-12, LTC-13 and LTC-15 and LTC-18 , which are designed to fire or trigger 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 gate/cathode triggering pulses G1K1 + G2K2. These pulses can be used to trigger back-to-back connected SCRs upto 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. Many times it is required to limit the current flowing through the load. Such current limiting feature is available in LTC-13 – for single phase and two phase load. Whereas for 3 phase load, common three phase current limiting is achieved by LTC-18 card. The load current or Line current feedback is derived from the external CTs. This feature is useful for transformer load or inductive loads or for heating elements like Silicone Carbide and Molybdenum Disilicide, which exhibits significant change of resistance with increase in temperature. This feature will restrict the maximum load current to the value set using the on card current limit (CL) potentiometer. On a sudden increase in the current the current trip feature will over-ride the current limit feature and the firing of the thyristor will stop instantly. The trip contacts are available on the terminals.

Features:

• All these cards can be mounted inside the control panel.

• Digital and Linear circuit based.

• Smooth Control On card setting to adjust Ramp Up and Ramp Down.

Libratherm offers single phase zero cross over SCR firing module model LZF-10 for AC single phase operated electrical resistive or inductive loads. This module is developed to replace the electro-mechanical contactor with the solid state switch using silicon controlled rectifiers (SCRs).  When the electrical load is operated through contactors – the AC cycle is switched ON or OFF at any point of     0 to 180 degree phase angle of single sinusoidal AC cycle, this leads to generation of arcing on the contacts and may generate harmonics, which can cause electromagnetic disturbance in the power lines and can also cause EMI to other electronic instruments on the panel.  The module featured here eliminates generation of voltage transients because of zero cross over switching technique, also no arcing will be observed during switching operation – since no mechanical contact movement is involved in the solid state device.
 
This LZF-10 module accepts logic pulse of 12 to 36VDC from temperature controller or from PLC and generates single phase gate cathode triggering pulses for a pairs of back to back connected SCRs.

Using LZF-10 module and external back to back connected SCR modules (such as Semikron make SKKT27, SKKT57, SKKT106, SKKT162, SKKT 273, SKKT 573) one can build solid state relays rated for 25A to 500A @ 40 to 250VAC. We also offer required RC snubber as per the details given below.

Description:

Libratherm offers Thyristor Switch Card model TSC-303 which is specially designed for APFC (Automatic Power Factor Correction) applications to switch ON and OFF, 3 phase capacitor banks, based on the command from the APFC relay module. These cards are developed to trigger solid state switch or silicon controlled rectifiers SCR/SCR or SCR/Diode Modules. The gate/cathode triggering pulses are generated using zero cross over firing techniques. Libratherm offers 2 versions of Thyristor switch card. TSC-303 based on Anodic fiiring technique and TSC-306 based on cathodic firing technique. The two sets of gate / cathode pulses are galvanically isolated using the oncard ferrite core pulse transformers in TSC-309 and in TSC-303, gate / cathode triggering pulses are generated using zero cross over optically isolated integrated chips (ICs).

Each card can simultaneously trigger 2 sets of back to back SCR+SCR modules or 2 sets of back to back SCR+Diode modules to switch 3 phase capacitor banks. TSC-306/TSC-303 accepts control signal in the form of relay contact or DC pulse from APFC relay unit. When command is ON, capacitors banks are connected to the power lines and when command is OFF capacitor banks are disconnected.

Each TSC-309/TSC-303 card has the provision to accept 2 external thermostat contacts, which can be used to operate the cooling fan mounted on the heat sinks and to trip the circuit in case of over - heating. On card LED indicates the on/off status of incoming DC power supply, ON/OFF status of capacitor banks and status of heat sink temperature.

Normally, each of the power factor controllers gives 6 to 12 outputs to select that many capacitor banks of different KVAR depending on the total KVAR demand to maintain the unity power factor. Hence, for each output from APFC it will be required to use 6 to 12 nos. of TSC-309/TSC-303 cards.

The APFC panel builders can use TSC-309 or TSC-303 cards as an independent firing card and can wire the SCR modules separately mounted on the heat sink in the panel. The physical isolation of electronic cards and SCR modules makes the overall system safer, reliable and it becomes easy to maintain and service. Libratherm has supplied more than 5000 cards, which are installed on the field giving satisfactory performance for power factor correction application.