In this article we will discuss about:- 1. Classification of Power Converters 2. Matching Power Electronic Converter and Motor 3. DC Motor Control.
Classification of Power Converters:
Solid state power converters are employed for obtaining the appropriate form of electrical energy such as direct current or adjustable-frequency alternating current (required to operate most electronic circuits and motor drives) from fixed-frequency alternating current.
According to functions, the static power converters may be classified in the four basic categories namely ac/dc converters, ac/ac converters, dc/dc converters and dc/ac converters.
The inverters (dc/ac converters) may be further categorised as per their source nature as voltage source and current source inverters. A voltage source inverter (VSI) is fed by a constant voltage source system while a current source inverter (CSI) is supplied from a constant current source. A voltage source inverter is one in which dc source has small or negligible impedance. Because of low internal impedance, the terminal voltage of a VSI remains almost constant with variations in load.
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It is, therefore, equally suitable to single motor and multimotor drives. On the other hand, CSI is supplied with a controlled current from a dc source of high impedance. Because of large impedance, the terminal voltage of a CSI varies substantially with a change in load. CSIs are not suitable for multimotor drives because a change in load on any motor affects other motors.
The static power converters may also be categorised into two categories viz. transistor converters and thyristor converters. The transistor converters, which do not require additional commutation circuitry as needed in thyristor converters, have almost completely replaced thyristor converters below 75 kW. The availability of low-cost high-power BJTs has also made possible the use of pulse width modulation (PWM) for improving the motor current waveform.
According to the method of commutation the thyristor converters can mainly be categorised in two types viz. naturally commutated and force commutated converters. In case of ac circuits, ac line voltage is available across the device. When the current in the SCR passes through a natural zero, the device is turned off. This process is known as natural commutation process and the converters based on this principle are known as line commutated or naturally commutated converters. AC/DC converters, cycloconverters, and ac/ac voltage regulators using SCRs are of the naturally commutated type.
In case of dc circuits, since the supply voltage does not pass through the zero point, some external source is required to commutate the device. This process is known as forced commutation process and the inverters based on this principle are known as force commutated inverters. As the device is to be commutated forcefully, such inverters need complicated commutation circuitries.
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Converters may also be grouped into one-quadrant, two- quadrant and four-quadrant converters. A semi-converter is a one- quadrant converter and it has one polarity of output voltage and current. A full converter is a two-quadrant converter and polarity of its output voltage can be either positive or negative. However, its output current has one polarity only. A dual converter is a four-quadrant converter and both its output voltage and current can be either positive or negative.
Matching Power Electronic Converter and Motor:
The motor has to be selected to match the load requirements. The power electronic converter has to match the motor.
Some important considerations in matching power electronic converter and motor are as under:
1. Current Rating:
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The torque developed by an electric motor depends on the current drawn by it. During most of the operating duration a motor is required to deliver its rated torque, and therefore, the current drawn is also equal to its rated current. But a motor is capable of developing much higher torque (as compared to its rated torque) provided the duration of the torque is small as compared to thermal constant of the motor. Since torque depends on the current, a motor for developing a large torque requires a large current which has to be supplied by the converter.
However, the overcurrent capability of electronic devices is very small. A large current leads to high junction temperature which may destroy the electronic converter. The thermal time constants of the power electronic devices are very small in comparison to those of motors. The current of the converter must be selected properly on the basis of average, rms and peak currents of the motor.
2. Voltage Rating:
Every motor develops a back emf eb that opposes the applied voltage v. The rate of change of torque depends on the rate of change of current which, in turn, can be expressed as-
where di/dt is the rate of change of current and L is the inductance of the motor. For a reasonably large value of di/dt, the output voltage v of converter should be much larger than eb. Since eb depends on the motor speed, it is necessary that the voltage rating of the converter is selected on the basis of maximum speed of the motor.
3. Switching Frequency and Motor Inductance:
The steady-state ripple content in the motor current depends on motor inductance L. Large ripple content leads to large motor losses. So the steady-state ripple content in the motor current should be small. This is possible if inductance L is large. But large inductance means that the motor cannot respond quickly to the change in load demand. To meet these opposing requirements, the switching frequency of converter should be high (so that the steady-state ripple content is small). However, a high switching frequency leads to large power losses in the converter. Therefore, a proper selection of switching frequency and inductance is essential.
DC Motor Control through Converters:
DC motors are widely used in adjustable-speed drives and position control applications. Their speeds below base speed can be controlled by armature voltage control while speeds above base speed are obtained by field flux control. DC motors are preferred over ac motors because these are inherently most suitable for smooth, efficient, wide-range speed control and quick reversals.
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Phase-controlled converters provide an adjustable dc output voltage from a fixed ac input voltage. DC choppers also provide adjustable dc output voltage from a fixed dc input voltage. When available supply is ac, an ac-dc converter (phase-controlled converter) is ideally suited to convert the constant ac voltage into a variable dc voltage. 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. The power conversion efficiency of these converters is more than 95% because of low losses in thyristors. Thus, dc motors can be controlled conveniently and efficiently through phase-controlled converters.
The dc motors used in conjunction with power converters are dc separately excited motors and dc series motors. The power converters are used in speed control of fractional kW dc motors as well as in large sized motors used in variable-speed reversing drives for rolling mills with motor ratings as large as several MWs.
Three Phase Drives:
For large sized (rating exceeding 15 kW) motors 3-phase drive is employed. The output voltage of a 3-phase converter has less ripple contents than that of single phase converter. So the filtering requirements for smoothing out the armature current are less and the current is mostly continuous. Three-phase converters could be half-wave, semi-converter, full-converter and dual converter.
The 3-phase half-wave converter drives are only of theoretical importance and are generally not employed in industrial applications. This is because of presence of dc component in supply currents. Semi- and full-converters are widely used. A dual converter is employed in case of reversible drives with a power rating up to several MW.
i. Three-Phase Semiconverter Drives:
It is a one quadrant drive and is employed for motors of rating up to about 100 kW. The field converter may be 3-phase or single-phase. The armature voltage Va is given as-
where Vmax is the peak value of line-to-neutral voltage and αa is the firing angle of the converter in the armature circuit.
The circuit diagram for a separately excited dc motor supplied from 3-phase ac supply through a three-phase semiconverter is shown in Fig. 3.13.
ii. Three-Phase Full Converter Drives:
Three-phase full converter drive is a two quadrant drive and is employed up to 1,500 kW motors. The converter in the field circuit may be single-phase or 3-phase. When 3-phase full converters are used for both armature and field, the armature and field voltages are given as-
iii. Three-Phase Dual Converter Drives:
3-phase dual converter consists of two converters and can be operated in all the four quadrants. Either converter 1 is operated to supply armature voltage Va or converter 2 is operated to supply armature voltage –Va. It is used for motors of rating up to 1,500 kW. The armature voltage supplied by converter 1 is given as-
where αa1 is the firing angle for converter 1.
For converter 2, the voltage Va is given as-
If the field is also supplied by a 3-phase full converter,
