In ac drives, the rectifier output is inverted to produce a variable-frequency ac voltage for the motor. Inverters are classified as voltage source inverters (VSIs) or current source inverters (CSIs). A VSI requires a constant dc (i.e., low-ripple) voltage input to the inverter stage. This is achieved with a capacitor or LC filter in the dc link. The CSI requires a constant current input; hence, a series inductor is placed in the dc link.
AC drives generally use standard squirrel cage induction motors. These motors are rugged, relatively low in cost, and require little maintenance. Synchronous motors are used where recise speed control is critical. A popular ac drive configuration uses a VSI employing PWM techniques
to synthesize an ac waveform as a train of variable-width dc pulses . The inverter uses either SCRs, gate turnoff (GTO) thyristors, or power transistors for this purpose. Currently, the VSI PWM drive offers the best energy efficiency for applications over a wide speed range for drives up through at least 500 hp. Another advantage of PWM drives is that, unlike other types of drives, it is not necessary to vary rectifier output voltage to control motor speed. This allows the rectifier thyristors to be replaced with diodes, and the thyristor control circuitry to be liminated.
Very high power drives employ SCRs and inverters. These may be 6- pulse, as shown in Fig. or like large dc drives, 12-pulse. VSI drives are limited to applications that do not require rapid
changes in speed. CSI drives have good acceleration/deceleration characteristics but require a motor with a leading power factor (synchronous or induction with capacitors) or added control circuitry to commutate the inverter thyristors. In either case, the CSI drive must be designed for use with a specific motor. Thyristors in current source
inverters must be protected against inductive voltage spikes, which increases the cost of this type of drive.
Rectification is the only step required for dc drives.Therefore, they have the advantage of relatively simple control sys-tems. Compared with ac drive systems, the dc drive offers a widerspeed range and higher starting torque. However, purchase and main-tenance costs for dc motors are high, while the cost of power electronicdevices has been dropping year after year. Thus, economic considera-tions limit use of the dc drive to applications that require the speed andtorque characteristics of the dc motor.Most dc drives use the six-pulse rectifier shown in Fig Largedrives may employ a 12-pulse rectifier. This reduces thyristor current
duties and reduces some of the larger ac current harmonics. The two largest harmonic currents for the six-pulse drive are the fifth and seventh. They are also the most troublesome in terms of system response. A 12-pulse rectifier in this application can be expected to eliminate about 90 percent of the fifth and seventh harmonics, depending on system imbalances. The disadvantages of the 12-pulse drive are that there is more cost in electronics and another transformer is generally required. The presence of a dc voltage or current in an ac power system is termed dc offset. This can occur as the result of a geomagnetic disturbance or asymmetry of electronic power converters. Incandescent light bulb life extenders, for example, may consist of diodes that reduce the rms voltage supplied to the light bulb by half-wave rectification. Direct current in ac networks can have a detrimental effect by biasing transformer cores so they saturate in normal operation. This causes additional heating and loss of transformer life. Direct current may also cause
the electrolytic erosion of grounding electrodes and other connectors.
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AC/DC drives
the electrolytic erosion of grounding electrodes and other connectors.
Voltage imbalance (also called voltage unbalance) is sometimes defined as the maximum deviation from the average of the three-phase voltages or currents, divided by the average of the three-phase voltages or currents, expressed in percent.
Imbalance is more rigorously defined in the standards6,8,11,12 using symmetrical components. The ratio of either the negative- or zerosequence component to the positive-sequence component can be used to specify the percent unbalance. The most recent standards11 specify that the negative-sequence method be used.
The primary source of voltage unbalances of less than 2 percent is single-phase loads on a three-phase circuit. Voltage unbalance can also be the result of blown fuses in one phase of a three-phase capacitor bank. Severe voltage unbalance (greater than 5 percent) can result from single-phasing conditions.
Labels:
AC/DC drives
Imbalance is more rigorously defined in the standards6,8,11,12 using symmetrical components. The ratio of either the negative- or zerosequence component to the positive-sequence component can be used to specify the percent unbalance. The most recent standards11 specify that the negative-sequence method be used.
The primary source of voltage unbalances of less than 2 percent is single-phase loads on a three-phase circuit. Voltage unbalance can also be the result of blown fuses in one phase of a three-phase capacitor bank. Severe voltage unbalance (greater than 5 percent) can result from single-phasing conditions.
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