In this article we will discuss about:- 1. Introduction to Thyristor Control of Electric Motors 2. Thyristor Control of DC Series Motors 3. Thyristor Control of Three Phase Induction Motors 4. Thyristor Control of a 3-Phase Synchronous Motor 5. Special Features 6. Advantages and Disadvantages.
Introduction to Thyristor Control of Electric Motors:
Electric motors are employed in a very large power range, from a few watts to thousands of kilowatts. Many applications require very precise position adjustments (as in robotics). In many applications optimum performance and efficiency are the main concern. Variable speed drive (VSD) systems help in optimisation of process so as to reduce investment costs, operational costs and maintenance costs. Energy saving is another big advantage of variable speed drives.
The advent of thyristors capable of handling large currents has revolutionized the field of electric power control. Thyratrons, ignitrons, mercury-arc rectifiers, magnetic amplifiers motor-generator sets have all been replaced by solid state circuits employing semiconductor diodes and thyristors. Thyristor controlled drives employing both dc and ac motors find wide application in industry as variable speed drives.
In the 1960’s, ac power used to be converted to dc power for direct control of drive motors with solid state devices (high power silicon diodes and silicon controlled rectifiers). Initially saturable reactors were employed in conjunction with high power silicon rectifiers for dc drives. Nowadays, thyristors are extensively used for ac-to-dc conversion.
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General Configuration of a Motor Drive:
Figure 3.2 shows a block diagram illustrating the control of a motor drive. The main constituents are power electronic converter, motor, processes, process control computer and controller.
The required characteristics of the motor drive are determined by the process. The process may need a variable- speed drive or a servo drive etc. The motor is selected on the basis of these requirements. The power electronic converter converts ac single phase or 3-phase input into a supply suitable to provide desired characteristics of the motor. The process control computer receives a feedback from the process as regards the extent to which the requirements are met. This feedback may be of speed, position etc.
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The computer feeds an error signal to the controller which, in turn, does the corrective action. The corrective action may be, say, the change in the firing angle of thyristors of the power electronic converter. In some situations the accuracy and response time of the motor to adjust the new speed is of utmost importance. In some other situations the requirements may not be so critical.
Thyristor Control of DC Series Motors:
1. Fully Controlled Rectifier:
Figure 3.32 (a) illustrates the circuit diagram for a dc series motor supplied from single-phase ac mains through a fully controlled rectifier. The motor armature and field windings have resistance as well as inductance. Since the output current of a rectifier is not a perfect dc, the role of inductance also comes in. R is resistance of armature including that of field and L is inductance of armature including that of field. During positive half cycle, thyristors TH1 and TH2 are forward biased and start conducting at ωt = α. The load current flows through TH1, motor and TH2. At ωt = π, the supply voltages reverses.
Because of inductance L thyristors TH1 and TH2 continue to conduct beyond ωt = π. From ωt = π + α to ωt = 2π, thyristors TH3 and TH4 are forward biased. When TH3 and TH4 are triggered at ωt = π + α, thyristors TH1 and TH2, are subjected to reverse bias and are turned off by natural commutation. Load current is transferred from TH1 and TH2 to TH3 and TH4. This mode of operation continues till TH1 and TH2 are triggered in the next positive half cycle.
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The quadrants of operation and waveshapes are shown in Figs. 3.32 (b) and 3.32 (c). If inductance L is sufficiently large, the motor current is more or less constant. The value of α should be such that when thyristors are triggered the instantaneous value of ac input voltage Vmax sin ωt is more than back emf Eb. This sets a lower limit on firing angle α.
Average output voltage of converter,
For continuous motor current Ia, we can write-
Since the field current is also equal to Ia, the back emf Eb can be written as K1IaN where K1 is a constant. In writing Eb as K1IaN we have neglected the residual magnetism in the motor.
Substituting value of Ia from Eq. (3.54) in Eq. (3.55), we have-
Using above Eq. (3.56) speed-torque characteristics for different values of a can be plotted, as illustrated in Fig. 3.32 (d).
2. Semicontrolled Rectifier:
A dc series motor supplied from single phase ac supply mains through a semicontrolled rectifier is shown in Fig. 3.33 (a). The rectifier has two thyristors TH1 and TH2 and two diodes D1 and D2. The freewheeling diode DFW, helps in conduction of current when thyristor is not conducting. R is the resistance of armature and field and L is the inductance of armature and field windings.
During positive half cycle thyristor TH1 is triggered at ωt = α and starts conducting. The value of α should be such that Vmax sin α > Eb. The current flows through TH1, motor and diode D1 from ωt = α to ωt = π. At ωt = π the input voltage becomes negative and freewheeling diode DFW is forward biased. Thus at ωt = π, thyristor TH1 and diode D1 stop conducting and current is transferred to DFW.
During negative half cycle thyristor TH2 is forward biased and when it is triggered at ωt = π + α, the freewheeling diode DFW stops conducting and current is transferred to TH2 – D2 combination. TH2 and D2 conduct from ωt = π + α to ωt = 2π. At ωt = 2π, thyristor TH2 and diode D2 get turned off and current freewheels through DFW from ωt = 2π to ωt = 2π + α. At ωt = 2π + α, thyristor TH1 is triggered again and next cycle of operation starts. Thus the circuit operation is: DFW, conducts for 0 ≤ ωt ≤ α; TH1 and D1 conduct for α ≤ ωt ≤ π; DFW conducts for π ≤ ω t ≤ π + α; and TH2 and D2 conduct for π + α ≤ ωt ≤ 2π.
The quadrant of operation and wave-shapes are shown in Figs. 3.33 (b) and 3.33 (c) respectively.
The average output voltage of semiconverter is given as-
The back emf Eb can be written as K1IaN neglecting residual magnetism-
Substituting the value of Ia from Eq. (3.57) in above equation, we have-
Using above Eq. (3.58) speed-torque characteristics for different values of α are plotted, as illustrated in Fig. 3.33 (d).
During operation of dc separately excited and dc series motors, it is observed that the region of discontinuous current is smaller in case of dc series motors. The use of semiconverter provides continuous current over almost the entire operating range except when the load torque is very small. If inductance is employed in the armature circuit, continuous armature current can be obtained over the entire range of operation.
DC series motors are normally employed for constant output power applications. However, speed-torque characteristics do not correspond to the constant output power for a given firing angle and, therefore, in order to obtain constant output power over the entire range of speeds, the firing angle α has to be adjusted. The semiconverter system, because of its freewheeling action, helps in maintaining continuous current and thus provides better performance to motors in comparison to full-converter system. It has also been observed that dc series motor along with semiconverter provides better performance.
Phase controlled converters have poor power factor especially when the output voltage is less than the maximum, i.e., when firing angle α is large. Semiconverters provide better power factor in comparison to full converters even though the improvement is marginal.
Thyristor Control of Three Phase Induction Motors:
The speed of an induction motor is given as-
Thus the speed of an induction motor with fixed number of poles depends upon the supply frequency f and the slip s which in turn depends upon the voltage or current supplied to the motor.
Following methods are available for speed control of 3-phase induction motors using thyristors:
1. Stator voltage control or variable voltage constant frequency control.
2. Variable voltage and variable frequency control.
3. Rotor resistance control.
4. Secondary foreign voltage control.
Stator voltage variation is accomplished by means of ac regulators, which control the rms value of the ac voltage applied to the motor by introducing thyristors connected back-to-back in each supply line. Variable frequency power is obtained by means of a cycloconverter, which directly converts fixed frequency ac to variable frequency ac or by an inverter which converts dc to ac.
The effective value of an external resistance introduced into the rotor circuit can be controlled by connecting a high frequency chopper across the resistance and varying the time for which the chopper is on during the cycle. Static frequency converters are being employed for replacing the auxiliary machines in the Scherbius system. The Kramer scheme has also been changed by making use of a diode bridge rectifier in place of rotor converter, but a dc motor is still required for converting the rectified slip power to mechanical power.
Thyristor Control of a 3-Phase Synchronous Motor:
The synchronous motor is a constant speed motor and it develops torque only at synchronous speed which is directly proportional to supply frequency. Therefore, variation of frequency of ac supply is a convenient method to control the speed of a synchronous motor. Before the advent of power electronic devices change of frequency was a very difficult task.
However, the power electronic circuits provide a simple method of changing the frequency. The 50 Hz ac is converted into dc by a controlled rectifier. A variable frequency inverter converts dc into variable frequency ac. To keep the flux density in the motor constant voltage and frequency must be varied in the same ratio so that voltage/frequency ratio is constant.
Figure 3.52 shows a system of speed control of a synchronous motor. The 3-phase 50 Hz ac is rectified by a controlled 3-phase rectifier. The filter removes harmonics from the output of the rectifier. The variable frequency inverter feeds a variable frequency ac to the synchronous motor. Thus the speed of a synchronous motor can be controlled. The dc output from filter is also used to excite the rotor winding.
In low-speed high power applications it is also possible to use a cycloconverter to convert 50 Hz ac into variable frequency ac. The cycloconverter does the conversion without an intermediate dc link. The maximum output frequency is limited to about one-third of the frequency so that the output wave from has low harmonic content.
Load Commutated Inverter Drive For Synchronous Motor:
For large sized motors (motors having output rating of more than 750 kW), load commutated synchronous motor drive becomes competitive with induction motor drive in adjustable speed requirements. Figure 3.53 shows the circuit for a load commutated inverter drive. The three-phase ac supply is rectified by phase controlled 3-phase bridge converter. The dc supply is passed through a filter to improve waveform and then fed to the load commutated inverter.
Each phase of the synchronous motor is represented by an internal induced emf in series with an inductance of stator winding. The frequency and phase of stator currents are synchronised to the rotor position. The current commutation, in the load commutated inverter to supply currents to the stator phases in an appropriate sequence is provided by the stator induced emfs. The presence of 3-phase induced emf in the motor stator winding causes the commutation of the thyristor. A control of firing angle of converter controls its dc output voltage and, therefore, the current.
A complete circuit of load commutated inverter drive controller is shown in Fig. 3.54. The input voltage to stator of a synchronous motor is measured to calculate the rotor field position as function of time. The measured voltage is rectified to provide a dc signal proportional to instantaneous speed of synchronous motor. The turn-off time Toff available to thyristor in the inverter is kept constant.
Keeping field current and Toff constant, the actual speed is compared with the reference speed. The error signal is amplified to provide reference ld. If actual current Id is less than reference, the rectifier increases the voltage supplied to inverter, thus increasing Id and the motor torque. Based on current Id and measured voltage, the fire pulses to the gates of thyristors of inverter are provided to keep Toff constant.
Special Features of Thyristor Drive Motors:
The most commonly used dc motors for thyristor drive are separately excited and series excited dc motors. The thyristor drive motors usually differ from conventional dc motors in construction. Before discussing the special features incorporated in thyristor drive motors, discussion on the effects of thyristor power supply on performance of a dc motor is necessary.
Effects of Thyristor Power Supply on the Performance of a DC Motor:
1. The output voltage from thyristor converter consists of a dc component and ac harmonic components.
2. The output voltage can change very rapidly in comparison to that of a motor-generator set owing to the absence of field time constants associated with the generator.
3. An abnormally high value of armature current may rise in the event of thyristor fault when operating in the inverter mode.
4. The magnitude of harmonics decreases with the increase in frequency of harmonic for all converters. The magnitude of the harmonics increase as the firing angle is increased and dc voltage is reduced for all bidirectional converters. The higher the armature inductance, the lesser will be the harmonic currents.
5. Torque is developed by the dc component of the current whereas heating is developed by the effective (or rms) value of current. The form factor (ratio of effective value to average value) for half-wave three-phase thyristors may be taken as 1.2 while for full-wave three-phase thyristors it is 1.1. This increases electrical losses and, therefore, heating is 5 to 7 per cent more for three-phase full-bridge converters while for three-phase half-bridge converters it is from 15 to 20 per cent.
6. The commutating ability is seriously affected by the presence of harmonic currents. The peak value of the current is increased, interpole flux will be reduced in magnitude and a time lag will be introduced between the interpole current and the flux due to eddy currents generated in the iron path of the inter polar flux.
The other effects of thyristor power supply on motor performance are heating of interpole winding, saturation of interpole magnetic circuit, transformer voltage at the brushes and increase in voltage commutator segment.
Special Features of Thyristor Drive Motors:
The thyristor drive dc motors are designed with the following special features in order to improve their performance:
1. The thyristor drive dc motors are made with larger diameter armature and larger size poles of reduced height.
2. The commutators are made larger in order to provide extra insulation to withstand larger and rapid voltage fluctuations.
3. The yoke as well as the main and commutating poles are laminated to reduce the eddy current effects.
4. Low inertia armatures are employed for improving the response.
5. Compensating windings are used in large motors to reduce armature reaction effect.
6. Split brushes of good commutating quality are used for reducing the effect of transformer voltage in the coils undergoing commutation.
7. The use of a laminated yoke instead of a solid yoke improves the commutation to a greater extent.
8. The use of a large number of commutator bars reduces the voltage between commutator segments and improves the commutation.
9. The use of an octagonal, rather than circular, shape for the frame accommodates more material and gives a larger rating for the same frame size.
10. The current densities used for the armature and interpole winding are reduced compared to conventional dc motors of the same frame size and rating in order to reduce the effect of heating armature and interpoles.
11. A better class of insulation (class F materials as standard insulation) is used to allow higher temperature rise and dissipation of more losses from a given frame.
12. The ratio pole arc/pole pitch is reduced in order to reduce the ratio of commutating zone to neutral zone.
13. Armature inductance is increased to reduce the ripple current. It may also increase reactance voltage. The number of turns per armature coil is kept at the minimum since the reactance voltage is proportional to the square of the turns per coil. Use of dummy coils is avoided.
14. Forced cooling by an auxiliary ac motor is widely used for improving cooling of the motor at reduced speeds.
15. Great care is required to be taken for accurate construction- spacing of brushes, poles and manufacture of commutators.
Advantages and Disadvantages of Thyristorized Control:
Thyristorized control has the following advantages and disadvantages:
Advantages:
1. The response of the control device is faster as it eliminates the time lag introduced by the inductances of the generator field and the armature.
2. Due to low voltage drop across the thyristor, the efficiency of the control system is high.
3. The control device is smaller in size, lighter in weight, cheaper in cost, requiring less space and minimal maintenance.
4. Simple and reliable operation.
Disadvantages:
1. Because of higher ripple content in the converter output, motor heating and commutation problems are serious.
2. Due to switching action of thyristors and non-sinusoidal nature of current, there is more possibility of interference with the communication networks.
In all variable speed drive systems power electronic converter acts as an interface which accepts electric power from the existing source and converts it in a controlled manner into a suitable form compatible with the particular load or the process for which it is employed. The main sources for electric power are: Single or three-phase 50 Hz ac from utility systems and dc from storage batteries or solar cells. The four basic forms of power conversion required are- ac to dc, dc to dc, dc to ac and ac to ac.
The modern converters are compact, cheap, reliable, durable, flexible, and completely controllable. They need reduced maintenance too. They are suitable for all the four basic forms of power conversion mentioned above through rectifiers (ac/dc), choppers (dc/dc), inverters (dc/ac) and cycloconverters or ac regulators (ac/ac).
For dc motor control, controlled dc power from a constant voltage ac supply is obtained by means of controlled rectifiers (usually termed as converters) using thyristors and diodes. The control of dc voltage is achieved by varying the phase angle at which the thyristors are fired relative to the applied alternating voltage waveform. This scheme of control is known as the phase control.
In another control system, known as the integral cycle control, the current is gated to flow from the ac supply for a number of complete cycles and is then quenched for a further few cycles, the process being repeated continuously. Control is applied by adjusting the ratio of on and off durations. This method is suitable for the control of fractional kW output dc motors.
Phase controlled converters are simple to operate and are less expensive as they do not require additional circuitry for commutation process. In such converters natural commutation is achieved, i.e., when an incoming thyristor is turned on, it immediately reverse biases the outgoing thyristor and turns it off.
Phase control and integral cycle control methods are also applicable for ac motors wherein the converter circuitry is not required.
Control of dc motors supplied from dc supply is achieved by means of a thyristor switching circuit called the chopper. In chopper circuits, the control of the average voltage is achieved by changing the on-to-off time duration ratio for which the dc supply voltage is applied to the motor. This provides an efficient and stepless control of dc motors.
The motor can also be made to operate in the regenerative braking mode. Instead of converter circuits, it is possible to use an uncontrolled rectifier, which provides a constant direct voltage, followed by a chopper to provide a variable mean direct output voltage. The chopper controller needs forced commutation of the thyristor.
For the control of ac motors supplied from dc supply, thyristor, transistor or MOSFET based inverters are used. Such switching circuits transfer energy from dc supply to ac load of variable frequency and/or variable voltage. Because of switching operation, the ac voltage waveforms are stepped, the harmonic contents of which are filtered out. As the power supply is normally ac, the complete scheme for obtaining variable-voltage and frequency power involves the use of both an inverter and a converter.
A cycloconverter is a control unit for providing variable- voltage and frequency power directly from a fixed frequency supply without the necessity of an intermediate dc stage. The mechanism of voltage and frequency control is a combination of those employed in the phase-controlled converter and the pulse-modulated inverters.
Cycloconverters inspite of their attraction of direct ac to ac conversion, suffer from certain drawbacks because of which they did not become popular. Some of these drawbacks are: cycloconverters can generate only a sub-frequency output; they produce output with large harmonic content and have a low input power factor. Cycloconverters are employed for low-speed drives and for controlling linear motors in high-speed transportation systems.