I want to speed control of a dc motor using scr triggered by ujt pdf ...plz help ...
control of a dc motorcontroL
A semi converter uses two diodes and two thyristors and there is a limited control over the level of dc
output voltage. A semi converter is one quadrant converter. A one-quadrant converter has same polarity
of dc output voltage and current at its output terminals and it is always positive. It is also known as twopulse
converter. Figure shows half controlled rectifier with R load. This circuit consists of two SCRs T1
and T2, two diodes D1 and D2. During the positive half cycle of the ac supply, SCR T1 and diode D2 are
forward biased when the SCR T1 is triggered at a firing angle ωt = α, the SCR T1 and diode D2 comes to
the on state. Now the load current flows through the path L - T1- R load –D2 - N. During this period, we
output voltage and current are positive. At ωt = π, the load voltage and load current reaches to zero, then
SCR T1 and diode D2 comes to off state since supply voltage has been reversed. During the negative half
cycle of the ac supply, SCR T2 and diode D1 are forward biased. When SCR T2 is triggered at a firing
angle ωt = π + α, the SCR T2 and diode D1 comes to on state. Now the load current flows through the
path N - T2- R load – D1 -L. During this period, output voltage and output current will be positive. At ωt
= 2π, the load voltage and load current reaches to zero then SCR T2 and diode D1 comes to off state since
the voltage has been reversed. During the period (π + α to 2π) SCR T2 and diode D1 are conducting.
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The UJT, capacitor, and the resistors make up an oscillator that supplies an ac voltage to the SCR's gate. From my understanding, the SCR wouldn't turn off when the gate pulse become zero. It will only turn off when negative pulse is applied to it. However in this situation, the gate pulse is fed by the Base1 of UJT which is shown in the diagram. The Base1 pulse is only from level 0V to certain positive level voltage (no negative voltage level). My question is, how will the pulse turn off the SCR and eventually control the speed of motor? Thanks in advance...
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This is speed motor controller circuit of 12V DC motor. as SCR DC motor speed control circuit using IC-CMOS. You can adjust the speed of rotation of the spindle motor from 5-60 cycles per minute.
The work of circuit, The IC1-(CD4081) is nand gate ic type. It accepts the voltage from the bridge diode BD1. But there is no filter to smooth current.The other in the pin 2.The VR1, C1 and R1 is a phase shift or time delay to slow down,voltage from pin 3 to triggers pin the gate of SCR1 to work. Or conductors to the motor, causing it to rotate. Speed of the motor, can be achieved by adjusting VR1.
The power supply input pin 14 of IC1 is filtered to smooth the current, by through D2 and C2. The D1 prevents the noise from the motor. and D3 is reverse-voltage protection of motors. This can cause circuit damage. If the circuit is used with a motor that consumes more power. Heat sink should be attached to BD1, D1 and SCR1, because heat up. May cause equipment damage.
reference;
http://slidesharezunaibali/to-control-the-speed-of-dc-motor-simple-project