barcode generator in vb.net free download Figure 515 Current and harmonic spectrum for PWM-type ASD in Software

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Figure 515 Current and harmonic spectrum for PWM-type ASD
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The input to the PWM drive is generally designed like a three-phase version of the switch-mode power supply in computers The rectifier feeds directly from the ac bus to a large capacitor on the dc bus With little intentional inductance, the capacitor is charged in very short pulses, creating the distinctive rabbit ear ac-side current waveform with very high distortion Whereas the switch-mode power supplies are generally for very small loads, PWM drives are now being applied for loads up to 500 horsepower (hp) This is a justifiable cause for concern from power engineers
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DC drives Rectification is the only step required for dc drives
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Therefore, they have the advantage of relatively simple control systems Compared with ac drive systems, the dc drive offers a wider speed range and higher starting torque However, purchase and maintenance costs for dc motors are high, while the cost of power electronic devices has been dropping year after year Thus, economic considerations limit use of the dc drive to applications that require the speed and torque characteristics of the dc motor Most dc drives use the six-pulse rectifier shown in Fig 516 Large drives may employ a 12-pulse rectifier This reduces thyristor current
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Downloaded from Digital Engineering Library @ McGraw-Hill (wwwdigitalengineeringlibrarycom) Copyright 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website
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Fundamentals of Harmonics 192 Five
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6-pulse thyristor rectifier v(t)
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field may be supplied from same or separate power supply
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Figure 516 Six-pulse dc ASD
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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
AC drives 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 (ie, 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 precise 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 (see Fig 517) 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 eliminated
Downloaded from Digital Engineering Library @ McGraw-Hill (wwwdigitalengineeringlibrarycom) Copyright 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website
Fundamentals of Harmonics Fundamentals of Harmonics 193
diode rectifier
transistor inverter
v(t)
Va Vb Vc
AC MOTOR
Figure 517 PWM ASD
Very high power drives employ SCRs and inverters These may be 6pulse, as shown in Fig 518, or like large dc drives, 12-pulse VSI drives (Fig 518a) are limited to applications that do not require rapid changes in speed CSI drives (Fig 518b) 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
(a) VSI v(t)
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