In this article we will discuss about:- 1. Thyristor Control of AC Regulators 2. Variable Voltage and Variable Frequency Control 3. Variable Current Variable Frequency Control 4. Thyristor Control of Cycloconverters 5. Reduced Voltage Starting (Soft Start) 6. Rotor Resistance Control 7. Slip-Power Recovery Scheme.
Thyristor Control of AC Regulators:
An ac regulator converts a constant ac voltage into a variable ac voltage of the same frequency. No doubt, ac voltage level can be changed by auto-transformer, tap-changing transformer, saturable reactors etc. These devices have been in use for long time and still are in use. But ac regulators using thyristors and triacs are becoming more and more popular because of their high efficiency, fast control and compact size. However, ac regulators using thyristors and triacs introduce objectionable harmonics in the circuits. AC regulators are classified as single- phase or three-phase. Each of these may be half-wave (i.e., unidirectional) or full-wave (i.e., bidirectional).
Since the input to an ac regulator is ac, it is always line commutated. Therefore, forced commutation is not required. As such the circuits of ac regulators are quite simple. Two types of controls are used in ac regulators. These are known as integral cycle control and phase control.
In an integral cycle control, also known as on-off control, the thyristors are used as switches to connect the motor to the supply source for a certain number of cycles of the source voltage and then to disconnect it for another certain number of cycles. Each of the on- and off-times consists of an integral number of cycles. Thyristors are switched on by gate pulses at zero voltage crossing of input voltage.
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In phase control, the thyristors are employed as switches for connecting the motor to the supply for a certain portion of each cycle of the supply voltage. Most ac regulators use phase control. The power circuit configurations for integral cycle control and phase control do not differ in any manner.
Smooth variation of three-phase ac voltage can be realized by means of different configurations of the power circuit.
The three-phase regulators, may be half- wave or full-wave. Circuit of three-phase half-wave regulator for delta-connected motors or star-connected motors in which neutral point is not accessible is shown in Fig. 3.34. This circuit uses three thyristors and three diodes. Though half-wave ac regulator illustrated in Fig. 3.34 affects a saving in the cost of semiconductor devices and does not give rise to dc components in any part of the system but it introduces more harmonics into the line current than does the full-wave regulator. Half-wave circuit is not used in actual practice.
Figure 3.35 shows a 3-phase full-wave regulator. It uses 6 thyristors, 2 for each phase. The input transformer may or may not be employed. As regards the heating of motor windings are concerned, star-connected motor supplied through a full-wave ac regulator is preferred over a delta-connected motor supplied through a full-wave ac regulator. This is so, because any third harmonic voltages generated by the motor back emf can cause circulating currents in case of delta-connected motor.
For delta-connected load circuits in which each end of each phase is accessible, the arrangement illustrated in Fig. 3.36 is employed. This arrangement has the advantage of reducing the current of the device as it has to carry now 1/√3 of the current if they were connected in the line of the delta winding. Once the phase current wave is known, the line current wave can be constructed by superposition.
For star-connected load circuits in which neutral point is accessible and can be opened, the arrangement illustrated in Fig. 3.37 may be used. In this arrangement, the number of thyristors required reduces to three and control circuitry becomes considerably simplified. The power consumption of the motor may be as high 100% greater than that with sine wave voltage control especially at reduced speeds.
Variable Voltage and Variable Frequency Control:
If only frequency is changed and stator voltage is kept constant, the stator flux will not be at its rated value. The operation with flux below or above the rated value is not desirable. For constant flux operation, it is necessary that the induced emf increases or decreases linearly with applied frequency. At higher voltages and at high frequency operation stator drops are very small and thus constant flux operation is obtained by keeping V/f ratio constant.
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The variable stator voltage and frequency can be obtained from the systems illustrated in Fig. 3.38 or in Fig. 3.41, known as square-wave inverter and pulse width modulated (PWM) inverter respectively.
A square-wave inverter power circuit is illustrated in Fig. 3.38. The three-phase ac supply is converted into dc by a controlled rectifier. The output of the rectifier is supplied to the filter circuit to remove the harmonics. The dc output from filter is fed to a controlled inverter which provides variable voltage variable frequency output. This supply is fed to the stator of the 3-phase induction motor whose speed is to be controlled.
Figure 3.39 shows the phase voltage waveforms VAN, VBN, VCN and line- to-line voltage waveforms VAB, VBC and VCA. Each line voltage waveform is displaced in time phase by 120 electrical degrees from one another and is of quasi-square wave with 120° width. The noteworthy point is that the inverter thyristors are force commutated because induction motor is a lagging power factor load. The feedback diodes help in circulation of load reactive power with filter capacitor and maintain the output voltage waves fixed to the level of dc link voltage.
Desired voltage-frequency relationship of induction motor is illustrated in Fig. 3.40. When the frequency is less than the normal frequency, the voltage is reduced by the same proportion so as to keep V/f constant. At very low frequencies, as the reactance drop becomes smaller in comparison to stator resistance drop (ω L < R), additional voltage is required to be applied to compensate this effect. It means a higher V/f ratio. When the frequency exceeds the normal frequency, the torque decreases with the decrease in air-gap flux and the motor now operates in the constant power region, as illustrated in Fig. 3.40. This is equivalent to field weakening mode of speed control of a dc motor.
When operating at reduced voltage the converter voltage decreases and, therefore, the commutation capability of the capacitor also decreases. So the inverter is usually provided with an auxiliary constant voltage dc supply for commutation purposes.
The inverter mentioned above cannot return power back to the ac supply lines unless another phase controlled rectifier is added to form a reversing system. This method of speed control is used in low and medium size 3-phase induction motors where the speed ratio is usually restricted to 10 : 1.
The noteworthy point is that electrical machine is designed corresponding to near saturation point on the magnetization curve (or B-H curve). This is done from the point of view of full utilisation of the core. If the stator frequency is reduced, keeping the stator voltage constant, motor will operate in the saturation region and therefore, the motor will draw large magnetising current causing increase in core and stator copper losses and therefore, decrease in motor efficiency. However, if only supply frequency is increased keeping the stator voltage constant, the motor will operate at low flux density and thus motor capacity will be under utilised.
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The variable voltage variable frequency pulse width modulation (PWM) control circuitry for an induction motor is illustrated in Fig. 3.41. This is recent method and supersedes the square-wave inverter circuit discussed above.
Pulse width modulated (PWM) inverters use the chopping or pulsing technique for control of alternating voltage output of a static inverter. The dc link voltage is kept uncontrolled by a diode rectifier. The square or step-wave output voltage is rapidly switched on and off several times during each half cycle so that a number of pulses of equal amplitude are formed. Each pulse has the amplitude of the inverter input voltage Vdc. The magnitude of the fundamental output voltage is controlled by variation of the total on-time during a half cycle. By commutating one side of the bridge several times during a half cycle, the output voltage of the waveform shown in Fig. 3.42 (a) can be obtained.
Simple PWM inverters can be easily made to produce a waveform with only two pulses per half cycle within a six step envelope, as illustrated in Fig. 3.42 (b). Significant fifth and seventh harmonics are present in such waveform and they cause appreciable deterioration in the low-speed performance of the ac motor. For elimination of low order harmonics, more refined techniques of PWM are employed, in which high frequency pulsing occurs throughout the half cycle.
In sophisticated PWM systems, the pulse width is varied throughout the half cycle in a sinusoidal manner, as illustrated in Fig. 3.43. Actually, the pulses should be regularly spaced and the pulse width at a particular position should be proportional to the area under the sine wave at that position. In the PWM waveform, the lowest harmonic frequency is at the pulse repetition frequency and, if this is much higher than the fundamental frequency, adequate filtering is provided by the inductance of the machine.
Such waveforms are usually produced by means of a control circuit in which a high frequency triangular waveform is mixed with a sinusoidal waveform of the desired frequency. Voltage control is obtained by varying the widths of all pulses without affecting the sinusoidal relationship.
Figure 3.44 illustrates sinusoidal PWM technique where an isosceles triangular wave is compared with the sinusoidal wave signal and the points of commutation are determined by the crossover points. If modulation index is found to be less than unity, only carrier frequency harmonics with fundamental frequency related side bands appears at the output.
Such waveform generates less harmonic heating and torque pulsation in comparison to that of the square wave. When the modulation index exceeds unity, maximum voltage is obtained in the square-wave mode. Thus, PWM mode is applicable in the constant torque region while in the constant power region the operation is similar to that of the square- wave mode.
The transistorized PWM control, shown in Fig. 3.45, is used for control of low to medium size motors. No doubt power transistors cost much more than that of the thyristors of the same capability but savings due to elimination of commutation circuit and corresponding commutation losses, the circuit proves to be more economical and efficient. Also, the transistors operate faster PWM is possible at higher frequency. This cuts down further the machine losses.
Variable Current Variable Frequency Control:
A variable current variable frequency control circuit for an induction motor is illustrated in Fig. 3.46. Variable dc voltage provided by a phase controlled rectifier is converted to a current source by connecting a large inductor in series. The large inductance maintains the current constant. The voltage available at the stator terminals of the 3-phase induction motor is almost sinusoidal with superimposed voltage spikes due to commutation. The converter employed is a line commutated while the inverter is forced commutated because induction motor operates at lagging power factor. The phase controlled converter can be replaced by a diode rectifier followed by a dc chopper.
The circuit has the following advantages:
(i) Since the input current is constant, misfiring of devices and short circuits do not pose any problem.
(ii) Less number of components in inverter circuit and less commutation loss.
(iii) Rugged and reliable power circuit.
(iv) Simpler and more reliable control circuit. This is because only 6 thyristors are to be controlled.
(v) Peak current of devices is limited.
(vi) It can handle reactive or regenerative loads without freewheeling diodes.
Disadvantages:
(i) Somewhat sluggish response of the drive.
(ii) Somewhat bulky and expensive inverter. This is due to large size of the inductance and the commutation capacitors.
(iii) Low frequency range of the inverter.
(iv) It cannot operate under no-load condition. This is because some minimum load current is necessary for the satisfactory commutation of the inverter.
Thyristor Control of Cycloconverters:
A cycloconverter converts ac at one frequency to an ac of another frequency. Cycloconverters can be classified as single- phase to single-phase, three-phase to single-phase and three- phase to 3-phase devices. They can also be classed as step-up and step-down cycloconverters. A step-up cycloconverter provides an output whose frequency is higher than that of input while a step-down cycloconverter provides an output of frequency lower than that of input.
Step-down cycloconverter uses line or natural commutation. Cycloconverters were developed initially for electric traction systems operating at 25 Hz and 16 2/3 Hz. In their early stages of use, the cycloconverters used mercury-arc rectifiers. With the development of thyristors the applications of cycloconverters have increased.
The basic power circuit scheme of a 3-phase cycloconverter is shown in Fig. 3.47.
Independent control of output frequency and voltage is obtained with only one parameter variation, i.e., viz., by variation of the firing points of the controlled rectifiers. The frequency of the output voltage is controlled by the rate at which the firing points are varied about the quiescent point and the output voltage is controlled by the maximum excursion of the firing points from the quiescent point. The cycloconverter, with its associated firing circuit, produces an output voltage that is replica of the reference voltage.
The operation of cycloconverter is characterized by several features. They are, generally, employed as step-down frequency converters. There is no fixed minimum ratio of input to output frequency; however, the output frequency is restricted typically to one- third or one-half of the input or line frequency. Below these ratios, the efficiencies of both the cycloconverters and motors supplied by them start falling significantly.
Reversibility is another feature of cycloconverter drive systems. A cycloconverter fed ac motor drive will respond to a change in polarity of the input signals by changing the direction of rotation of the motor without the use of contactors to reverse phase sequence.
The ability of cycloconverter to handle power flow in either direction is another important feature. This, together with the above mentioned reversibility feature, provides an induction motor drive capable of operating in any of the four quadrants of the motor’s speed-torque curve.
While the cycloconverter has many attractive features from a theoretical point of view, there are several limitations because of which they have not gained popularity. It needs more power semiconductors than an inverter. For example, the three-phase cycloconverter needs 18 thyristors, whereas the rectifier-inverter combination (Fig. 3.38) needs only 12 thyristors.
Cycloconverters can produce only a sub-frequency output. Line pollution with harmonics and low power factor can also be problems with cycloconverters of high power rating. However, recent advancements in fast switching devices have resulted into devices known as forced commutated direct frequency changers (FCDFC) which operate at high efficiency and have low harmonic content.
Cycloconverters drives are normally used for large size motors as the cost and complexity of the power and control circuits prohibit their use for general applications. Cycloconverters have been employed in diesel electric locomotives where a high frequency alternator coupled to the engine shaft provides power at the input. These have also been employed in gearless cement mill or ball mill drives.
Reduced Voltage Starting (Soft Start):
The starting line current at full voltage of an induction motor may be about 6 times the rated full-load current. Such a high current may cause severe voltage dip in the network supplying the induction motor.
The circuit illustrated in Fig. 3.48 can be used for feeding a reduced voltage at the start. As seen it is a 3-phase ac regulator. By proper control of firing angle the regulator provides a low output voltage which is supplied to the induction motor. When the motor attains the full or rated speed, the regulator can be short circuited by mechanical contactor so that the motor operates normally at rated voltage. Moreover, if the motor is employed for a constant speed drive it is possible to operate the motor at reduced voltage when the mechanical load is small. Operation at reduced voltage causes decrease in power losses in the motor and thus results in energy saving.
Rotor Resistance Control:
The conventional method of connecting resistances across the slip rings of a wound rotor induction motor is a form of rotor voltage control. The main drawback of this method of speed control is its poor efficiency because of power wasted in the external resistors.
Figure 3.49 (a) shows a 3-phase diode rectifier and gate- turn-off thyristor (GTO) connected in the rotor circuit of a wound rotor induction motor. The GTO operating as a chopper varies the resistance R as per the duty cycle α. The effective resistance Re is given by-
Re = R (1 – α) …(3.59)
Thus the speed of a wound rotor induction motor is controlled by varying the ratio of on-time to off-time.
Slip-Power Recovery Scheme of Thyristor Control:
Figure 3.49 (b) shows a scheme known as slip-power recovery Fig. 3.48. The rotor terminals are connected to the 3-phase ac supply mains through two fully controlled thyristor bridges. Bridge 1 acts as a rectifier (or converter) and bridge 2 acts as an inverter. Power output from rotor can be supplied back to the supply source. Since the frequency of rotor currents is slip frequency, this method is known as slip-power recovery scheme. By controlling the firing angles of the two bridges, the rotor power output can be varied.
Thus the motor slip and speed (for the same torque) will also change. However, the drawback of this scheme is that both the bridges draw reactive power from the supply mains. Therefore, the overall power factor of the motor is poor. If speeds only below synchronous are desired, bridge 1 may be uncontrolled and, thus, may consists of diodes. If both bridges are controlled, the operation of the two bridges can be reversed also to get speeds above synchronous one.
Infact, the slip-power is either returned to the supply network as in Scherbius scheme or used to drive an auxiliary motor which is mechanically coupled to the induction motor shaft, as in Kramer scheme.
1. Static Scherbius Drive:
The static Scherbius drive also uses the principle of slip-power recovery. The schematic diagram is shown in Fig. 3.50. For achieving both sub- synchronous and supersynchronous speed control, converters 1 and 2 must be fully-controlled thyristor bridges, one functioning at slip frequency as a rectifier or inverter while the other operating at supply system frequency as an inverter or rectifier. The cost of converters is quite appreciable and a slip-frequency gating circuit is also required.
Moreover, at speeds near synchronous, when the slip-frequency emfs are quite small for natural commutation, special connections for forced commutation methods are required. If converter 1 is taken uncontrolled one (Diode Bridge) the converter cascade and the control unit would become economical and simple, but only sub-synchronous speed control would then be available.
The 3-phase transformer between the supply source and inverter 2 is meant to bring the rotor circuit voltage up to the value corresponding to the voltage of the supply source. The main drawback of sub-synchronous cascade drive is its poor power factor particularly at reduced speeds.
This drive has applications in large power fan and pump drives which need speed control in a narrow range only. Power ratings of diode bridge inverter and transformer is just maximum slip times the power rating of the motor resulting in a low cost drive. This drive provides a constant torque control. Constant power control can be obtained from Kramer drive discussed below.
2. Static Kramer Drive:
Figure 3.51 shows the schematic diagram of the Kramer cascade with static converter. The rotor circuit of a slip-ring induction motor feeds the slip power, rectified by a diode bridge, to the armature of a separately excited dc motor mechanically coupled to the induction motor. Speed control is achieved by varying the field current of the dc motor. An emf proportional to the back emf of the dc motor may be considered to be injected into the rotor circuit of the induction motor to cause variation in the speed of the system.
For achieving larger speed range, Diode Bridge will need replacement by a thyristor bridge. With thyristor bridges speed can be controlled up to standstill.
Static Kramer drive possesses no line-commutated inverter, it takes less reactive power and introduces smaller harmonic contents of currents than a static Scherbius drive. However, it has maintenance problems which are posed by commutator and brushes of the auxiliary dc motor. It has also drawback of large moment of inertia.
Static Kramer drive systems are used in large power pumps and compressor type loads where speed control is within narrow range and below synchronous speed.