OBJECTIVE
To learn how to use the Thyristor Firing Unit
DICSUSSION
The Thyristor Firing Unit is a module which detects a sinusoidal voltage passes through zero and, following an adjustable delay, provides a firing pulse. This pulse is designed to trigger, that is to cause, thyristor to conduct when the anode-cathode voltage is positive. The delay is set by a voltage applied to the ANGLE CONTROL INPUT of the module. The Thyristor Firing Unit is built around a microcontroller which in fact a microcomputer in a single integrated circuit. Therefore, it is fully digital and very accurate. F2-1 shows a simple single-phase, half-wave controlled rectifier built with a Thyristor Firing Unit and a Power Thyristor Module. Notice that the DC SOURCE shown in F2-1 is part of the Thyristor Firing Unit.
F2-1: A single-phase half-wave controlled rectifier.
F2-2 shows the waveforms related t the circuit shown in F2-1. The time interval tα represents the delay from the zero-crossing of the sinusoidal voltage waveform to the start of the pulse which triggers the thyristor. Usually, this delay is expressed as an angle and is called the angle of retard, or firing angle, α. It is defined as an angle interval, expressed in degrees, by which the starting point of conduction is delayed in relation to the starting point of the same circuit if the thyristors were replaced by diodes. The delay variables α and tα are related by the equation:
α = (tα/T) × 360˚ (1)
where T is the period of the sinusoidal supply voltage, expressed in the same units as tα.
When the Thyristor Firing Unit is used to control thyristors in a circuit powered by a source whose frequency is 60 Hz, equation (1) becomes:
α = (tα/16.67 ms) × 360˚ (2)
when the Thyristor Firing Unit is used to control thyristors in a circuit powered by a source whose frequency is 50 Hz, equation (2) becomes:
α = (tα/20 ms) × 360˚ (2)
F2-3 shows the front panel of the Thyristor Firing Unit. It is divided into five sections which are described in the following subsections.
F2-3: Front panel of the Thyristor Firing Unit.
FIRING ANGLE section
In this section, one find a small screen which displays the firing angle, expressed in degrees. The firing angle, expressed in degrees. The firing angle can vary between o and 180˚ (in practice it cannot exceed 179.3˚) in steps of 0.7˚. The FIRING ANGLE display flashes on and off when there is no synchronization signal applied at SYNC. INPUT.
ANGLE CONTROL section
The section allows the firing angle of thyristors to be controlled. It is divided in two subsections. The INPUT terminal receives a voltage which can vary from -10 to +10 V to make the firing angle vary between 0 and 180˚. The COMPLEMENT and ARC COSINE push buttons allow the relationship between the firing angle and the voltage applied at the INPUT to be selected.
When neither the COMPLEMENT nor the ARC COSINE push button is depressed, the firing angle varies linearly from 0 to 179.3˚ when the input voltage varies from -10 to +10 V.
When the COMPLEMENT push button is depressed, the firing angle varies linearly from 179.3 to 0.7˚ when the INPUT voltage varies from -10 to +10 V.
When the ARC COSINE push button is depressed, the firing angle varies as a function of the arc cosine of the input voltage. For example, if the INPUT voltage is equal to +5 V, the firing angle is equal to arc cosine (5 ÷ 10), that is, 60˚. As a result, the firing angle varies from 173.0 to 0˚ when the INPUT voltage varies from -10 to +10 V.
When both push buttons are depressed, the firing angle varies as a function of the complement of the INPUT voltage arc cosine. As a result, the firing angle varies from 7.0 to 179.3˚ when the INPUT voltage varies from -10 to +10 V.
SYNChronization INPUT section
The terminals in this section must be connected to the single-phase or three-phase voltage source which supplies power to the thyristor circuit to be controlled. This input is used to detect when the ac network voltage passes through zero. The input signal must be sinusoidal, its frequency must be 50 or 60 Hz, and its voltage must be between 10 and 450 V ac. This synchronization input is electrically isolated from the Enclosure/Power Supply by means of an opto-coupler and a dc-to-dc converter which uses an isolating transformer.
When a single-phase voltage source supplies power to the thyristor circuit to be controlled, the “live” and neutral conductors must be connected to SYNC. INPUT terminals 1 and 2, respectively. When a three-phase voltage source supplies power to the thyristor circuit to be controlled, the connections to SYNC. INPUT terminals 1 and 2 are always shown in the circuit diagrams given in the section procedures.
DC SOURCE section
This section provides a dc voltage which can be set between -10 and +10 V. This voltage can be used to control the firing angle. The OUTPUT voltage is -10 V when the control knob is set to the MIN. position. The OUTPUT voltage is +10 V when the control knob is set to the MAX. position.
FIRING CONTROL section
The output signals of this section are pulses which are used to trigger the thyristors in the Power Thyristors module.
The pin configuration of the OUTPUTS connector is shown in F2-3. Pins 1 to 6 provide 0-5 V firing pulses and are related to thyristors Q1 to Q5 of the Power Thyristors module, respectively. Pin 7 provides a synchronization signal and pins 8 and 9 are connected to the common point of the Thyristor Firing Unit.
The MODE push button allows the Thyristor Firing Unit to operate in the single-phase (1-) or three-phase (3-) mode. In the single-phase mode, firing pulses are sent only to thyristors Q1, Q2, Q4 and Q5.
Option
Two miniatures switches (SW 1 and SW 2) located on the printed circuit board of the module allow some option to be selected.
When switch SW 1 is set to the 1 position, the isolated synchronization signal produced from the signal injected at the SYNC. INPUT is interrupted. This allows an isolated synchronization signal coming from another module to be connected to a miniature banana jack located near switches SW 1 and SW 2 on the printed circuit board of the module.
Note: The miniature switch is set to the 1position when the switch rocker is depressed on the 1 side. The miniature switch is set to the 0 position when the switch rocker is depressed on the 0 side. The 1 and 0 side are indicated on the plastic cover which protects the circuitry in the Thyristor Firing Unit.
When switch SW 2 is set to the 1 position, the Thyristor Firing Unit becomes a very different apparatus. This new apparatus is a control signal generator for an autonomous inverter. In this case, the display on the Thyristor Firing Unit indicates the frequency of the control signal generated. This frequency can vary between 0 and 127 Hz in both directions of rotation. Reversing the direction of rotation reverses the phase sequance of the control signals generated. The ANGLE CONTROL INPUT allows the frequency to be varied. The COMPLEMENT and ARC COSINE push buttons as well as the SYNChronization INPUT are not used. The operation of the DC SOURCE is not affected by SW 2. The FIRING CONTROL section sends out the control signals generated to the power module (Power Thyristors, MOSFET Chopper / Inverter etc.) used to implement an autonomous inverter. The MODE push button allows 120˚ or 180˚ full-wave modulation control signals to be selected. When the MODE push button is depressed, 120˚ full-wave modulation control signals are selected. When the MODE push button is not depressed, 180˚ full-wave modulation control signals are selected.
Note: Switches SW 1 and SW 2 must always be set to the 0 position unless otherwise specified in section procedure.
To operate the Thyristor Firing Unit, you first set switches SW 1 and SW 2 to the 0 position (to allow operation as a firing unit), and then install the module in the Enclosure / Power Supply. Next, you apply a synchronization signal (such as 120 V, 60 Hz) to SYNC. INPUT terminals 1 and 2, and connect the DC SOURCE OUTPUT to the ANGLE CONTROL INPUT. The FIRING CONTROL OUTPUTS can be connected to a power module (Power Thyristors) after the FIRING CONTROL MODE is selected (signal-phase or three-phase).
F2-4 is a functional block diagram which shows how the Thyristor Firing Unit is built. This diagram clearly shows that the SYNChronization INPUT is fully electrically isolated from rest of the circuitry in the Thyristor Firing Unit, thereby isolating the SYNChronization INPUT from the common point of the Enclosure / Power Supply. On the other hand, the output signals are not electrically isolated. The output signals must be amplified and isolated to trigger the thyristors. From this diagram, one can see that the microcontroller is the main element of the Thyristor Firing Unit.
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