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DC System Riddle No.7 - Microprocessor based controlling of dc motor
how can i explain microprocessor based speed control of separately excited D.C motor using thyristor?
Author : pramod kushwaha - From: India
Sat, July 5th, 2014 - 16:58
Some technical limitation should be considered for thyristor control as mentioned below.

The circuit waveforms for a phase controller with an inductive load or an active load (for example, a motor) are more complex than those for a purely resistive load. The circuit waveforms depend on the load power factor (which may be variable) as well as the triggering angle. For a bidirectional controller (i.e triac or pair of anti-parallel thyristors),maximumoutput, that is, sinusoidal load current, occurs when the trigger angle equals the phase angle.
When the trigger angle, δ, is greater than the load phase angle, φ, then the load current will become discontinuous and the triac (or thyristor) will block some portion of the input voltage until it is retriggered.
If the trigger angle is less than the phase angle then the load current in one direction will not have fallen back to zero at the time that the device is retriggered in the opposite direction. This is shown in Fig. 1. The triac fails to be triggered as the gate pulse has finished and so the triac then acts as a rectifier. In Fig. 1 the triac is only triggered by the gate pulses when the applied supply voltage is positive (1+ quadrant). However, the gate pulses which occur one half period later have no effect because the triac is still conducting in the opposite direction. Thus unidirectional current flows in the main circuit, eventually saturating the load inductance.
This problem can be avoided by using a trigger pulse train as shown in Fig. 2. The triac triggers on the first gate pulse after the load current has reached the latching current IL in the 3+ quadrant. The trigger pulse train must cease before the mains voltage passes through zero otherwise the triac will continue to conduct in the reverse direction.

Reference: Power Semiconductor Applications Philips Semiconductors
Author : Hamid - From: Iran
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