Thyristors, also known as Silicon Controlled Rectifiers, are three-junction PNPN semiconductor devices that are applicable in switching of heavy electrical loads ( Bradley, 2017 ). Thyristors can also be regarded as two interconnected transistors. Below is the symbol of a Thyristor.
Characteristics
Thyristors are semiconductor devices that can operate only in the switching mode. Thyristors have four layers (P-N-PN) with three PN junction series ( Bradley, 2017 ). The thyristor is a unidirectional device thus only conduct current in one direction, therefore, cannot be used for amplification. The thyristor is a three-terminal device. The terminals are labeled: Anode, Cathode, and Gate. The three PN junctions in Thyristors can be switched “ON” and “OFF” at a breakneck rate ( Bradley, 2017 ). The devices are operated by current whereby a small Gate current controls a larger Anode current. They also conduct current only if the forward biased and triggering current applied to the Gate. Once it is triggered “ON,” thyristor acts as a rectifying diode. To maintain conduction in the thyristors, the Anode current must be higher than the holding current. The thyristors also block current flow when reverse biased, even if Gate current is applied.
Delegate your assignment to our experts and they will do the rest.
Operation
The operation of Thyristors can be best elaborated through the assumption that it is made up of two transistors which are connected back-back as a pair of complementary regenerative switches.
https://www.electronics-tutorials.ws/power/thyristor.html
The figure above shows the current flow in the two transistors equivalent circuit (Thyristor). There are two transistors; NPN transistor TR2 and PNP transistor TR1. The collector current of transistor TR2 feeds directly into the base of transistor TR1 ( Bradley, 2017 ). Likewise, the collector current of TR1 feeds into the base of transistor TR2. It shows that the two interconnected transistors rely on each other to operate thus until of them is given some base current, nothing can take place even in the presence of Anode-to-Cathode voltage (( Chen et al., 2016 )). When the Anode terminal of the thyristors in negative concerning the Cathode, the center N-P junction becomes forward biased while the outer P-N junction becomes reversed biased thus making it behave like an ordinary diode.
Thyristor will, therefore, block the flow of reverse current until at some high voltage level. A reverse action happens when the Anode terminal is made positive concerning Cathode. This time the forward current is blocked. Once the thyristor has been turned “ON,” it passes the current in the forward direction triggering the latching action of two internal transistors. The latching action causes the gate signals to lose all control. Once it is “ON,” the thyristor can only be turned “OFF” by either removing the supply or voltage or reducing Anode to Cathode current.
Application
Thyristors are usually applied where high voltages and currents are present. The principal types of thyristors are diacs and triacs. Diacs are mainly employed as a solid-state triggering device for other semiconductor switching devices such as SCR’s and triacs. On the other hand, triacs are commonly used in applications like motor speed controllers and lamp dimmers.
Types of Continuous Duty Motors
The “Brushed “DC Motor
Characteristics
A conventional brushed DC Motor has got two parts, the Stator and Rotor. The Stator is the stationary body of the motor while the Rotor is the inner part which rotates producing the movement. The stator comprises of electromagnetic coils which can be electrically connected with the armature of the motor ( Chen et al., 2016 ). The figure below shows a series and shunt connected DC Motor.
https://www.electronics-tutorials.ws/io/io_7.html
Operation
The current carrying conductors in the rotor connect at one end to commutator which allows the making of electric connection through carbon brushes to an external power supply as the rotor rotates ( Chen et al., 2016 ). The rotor creates a magnetic field which tries to align itself with the stationary stator filed making the rotor to rotate on its axis. The rotor’s rotational speed depends on the strength of the rotors magnetic field hence an increase in voltage will increase the rotational speed. Therefore, varying applied DC voltage can also vary the speed of motor rotation.
https://www.electronics-tutorials.ws/io/io_7.html
The “Brushless” DC Motor
Characteristics
The brushless DC motor resembles permanent magnet DC motor except that it does not have any brushes. Brushless DC motor rotor develops little heat increasing the motor’s life. The motors substitute the need for brushes with more complex circuit drive. The rotor’s magnetic field is a permanent magnetic field and always in synchronization with stator field. This enhances a precise speed and torque control.
https://www.electricaltechnology.org/2016/05/bldc-brushless-dc-motor-construction-working-principle.html
Operation
Brushless motors incorporate some ways of detecting the rotors magnetic poles which are needed to produce the feedback signals necessary for control of semiconductor switching devices. One of the most common magnetic pole sensors is the Hall Effect Sensor. These sensors are used to switch the polarity of the electromagnets ( Chen et al., 2016 ). After switching poles, the motor is then synchronized to a digital clock signal which provides precise speed control.
The DC Servo Motor
Characteristics
A servo motor consists of the built-in gearbox for speed reduction. The motor is capable of delivering high torques directly. Its output shaft does not rotate freely. It also comprises of a positional feedback device and error connection ( Yao et al., 2014 ).
Operation
DC Servo Motor Block Diagram
https://www.electronics-tutorials.ws/io/io_7.html
A positional input device or reference signal applied to the positional feedback device controls the motor speed or the magnetic field pole. The error detects amplifier checks the input signal and compares it with the motor’s feedback signal to determine whether motor output shaft is in an error condition ( Ibrahim et al., 2014 ).
Application of the Continuous Duty Motors
The three motors can be applied in series motors, shunt motors, compound motors, separate excited DC generators, shunt wound generator, and series wound generators.
References
Bradley, D. A. (2017). Power electronics . Routledge.
Chen, W., Liu, C., Tang, X., Lou, L., Cheng, W., Zhou, Q., ... & Zhang, B. (2016). High peak current MOS gate-triggered thyristor with fast turn-on characteristics for solid-state closing switch applications. IEEE Electron Device Letters , 37 (2), 205-208.
Ibrahim, H. E. A., Hassan, F. N., & Shomer, A. O. (2014). Optimal PID control of a brushless DC motor using PSO and BF techniques. Ain Shams Engineering Journal , 5 (2), 391-398.
Yao, J., Jiao, Z., & Ma, D. (2014). Adaptive robust control of DC motors with extended state observer. IEEE Transactions on Industrial Electronics , 61 (7), 3630-3637.
