Variable frequency

Fokker Bond Tester. An ultrasonic inspection technique commonly used for aircraft structures is based on ultrasonic spectroscopy [2]. Commercially available instruments (bond testers) used for this test operate on the principle of mechanical resonance in a multi-layer structure. A piezoelectric probe shown in Figure 3b, excited by a variable frequency sine signal is placed on the surface of the inspected structure. A frequency spectrum in the range of some tens of kHz to several MHz is acquired by the instrument, see Figure 3a. 

The variable frequencies of suites V and VIII on one side, and VI and VIII on the other, correspond to oscillations resulting from the coupling of the v(C-X) vibration with the cjf mode in the case of 2- or 5-substituted derivatives and with the mode in the case of 4-substituted derivatives. For 2,5-disubstituted thiazoles the ojf, vibration is only slightly different from that of thiazole itself and the 5 oscillation is coupled with both v(C(2iX) and vfC(5,X or Y) modes, giving rise to three frequencies, two of which are higher and classified in suites V and V, the third, being lower, is assigned to suite VIII. 

It is possible that for the methyl derivatives of thiazole the variable frequencies of the series V and VI correspond to the modes resulting from a coupling of the V(c-ch,) vibration with the vibration in the case of 2- or 5-substituted derivatives and with the vibration in the case of 4-substituted derivatives (99). 

Variable Air Flow Fans. Variable air flow fans are needed ia the process iadustry for steam or vapor condensing or other temperature critical duties. These also produce significant power saviags. Variable air flow is accompHshed by (/) variable speed motors (most commonly variable frequency drives (VFDs) (2) variable pitch fan hubs (J) two-speed motors (4) selectively turning off fans ia multiple fan iastaHations or (5) variable exit louvers or dampers. Of these methods, VFDs and variable pitch fans are the most efficient. Variable louvers, which throttle the airflow, are the least efficient. The various means of controlling air flow are summarized ia Table 3. 

Rietz disintegrators are normally supplied in rotor diameters from 10 to 60 cm (4 to 24 in), with rotational speeds to produce hammer tip speeds in ranges of 300 to 6700 m/min (1000 to 22,000 ft/min) and power ranges from 0.4 to 1.50 kW (V2 to 200 hp). Higher speeds and higher power are available. AC variable-frequency drives can eliminate belts and provide easier variation of speed. Models are available... 

A modification of this basic drive system uses solid-state rectifiers and thyristors to convert the wound-rotor, variable-frequency slip power first to direct current and then to hne-frequency power (60 Hz in the United States). This in turn is fed back to the power system as useful energy. 

Thc.se systems were evolved to provide a variable -frequency supply source to feed directly the stator terminals of the a.e. motor or its rotor through the slip-rings, The motors had to be invariably a combination of two or more slip-ring motors to receive the rotor frequency voltage from the other machine or feed back the rotor frequency voltage to another machine. The easiest method was to have a variable-frequency supply source, which was not possible, unless the supply source itself was captive and specified for this drive alone or a combination of these drives on the same bus. 

This is also known as variable frequency control. Consider the following equations from Chapter 1 ... 

Triangular voltage waveform of fixed amplitude Variable frequency and modulated voltage output (V/f) as desired. 

In addition to the above inverter systems there is one more system, called a cyclo converter system. These drives tire employed for very large motors, when IGBTs in such ratings are a limitation. It converts the fixed a.c. supply frequency to a variable frequency, generally lower than rated, directly and without rectifying it to a d.c, source. They are basically frequency converters. This system is more complex and expensive and has only... 

Constant off-time Zero voltage Variable frequency... 

Variable Frequency Control. At light loads, fixed frequency control methods loose efficiency because of the fixed switching losses. Some controllers will... 

Quasi-resonant converters are a separate class of switching power supplies that tune the ac power waveforms to reduce or eliminate the switching loss within the supply. This is done by placing resonant tank circuits within the ac current paths to create pseudo-sinusoidal voltage or current waveforms. Because the tank circuits have one resonant frequency, the method of control needs to be modified to a variable frequency control where the resonant period is fixed and the control varies the period of the non-resonant period. The quasi-resonant converters usually operate in the 300 kHz to 2 MHz frequency range. 

The control-to-output characteristic curves for a current-mode controlled flyback-mode converter, even though it is operating in variable frequency, are of a single-pole nature. So a single pole-zero method of compensation should be used. The placement of the filter pole, ESR zero, and dc gain are... 

This converter is intended to function as a bulk power supply for a distributed system. It has only one +28VDC output at 10 A. This is going to be a classic ZVS quasi-resonant half-bridge converter that is, variable frequency, voltagemode controlled with averaging overcurrent protection. It is representative of the designs using the available control ICs on the market today. 

This eompensation is intended for voltage-mode eontrolled forward eonverters whieh exliibit a seeond order output filter pole eharaeteristie. This would also inelude a quasi-resonant forward-mode eonverter that uses variable frequency, voltage-mode control. The T-C filter has a severe 180 degree phase lag and a -40dB/decade gain rolloff. To get any sort of wide bandwidth from the supply at all, this type of compensation must be used. 

I will be using a seeond order, eommon-mode filter. The diffieulty in eonsider-ing an input eondueted EMI for this power faetor eorreetion eireuit is its variable frequency of operation. The lowest instantaneous frequency of operation occurs at the crests of the sinusoid voltage waveform. This is where the core requires the longest time to completely discharge the core. The estimated frequency of operation has been 50 kHz, so I will use this as an assumed minimum frequency. 

Variable frequency motor drives are becoming more popular because of the ability to provide capacity control. 

A relatively new innovation for use in electric motor compressor drives is the variable frequency power source. Fundamentally, the power source converts an existing three-phase source into DC then uses an inverter to convert back to a variable frequency supply. Thyristors or transistors are used to switch the output at the required frequency. 

Tlie electric motor is basically a standard, single-speed, single-voltage motor. When receiving a variable frequency, the motor will operate at a variable speed. The motor may be either an induction or synchronous motor. The decision as to which type to apply is essentially similar to that for a single-speed application. 

While the motor is basically standard, there are several items that may be considered when the variable frequency system and motor are purchased-... 

The merits of a variable-speed motor would appear to be obvious, as many compressors in the past have benefited from the variable speed available in a steam turbine. A compressor may be adjusted as required to meet the process needs. The advent of the variable-frequency drive returns some of the benefits to the process operator that were lost when the more favor able electric energy caused motors to replace steam turbines. 

While cost will probably decrease with increased usage, this is a factor to be evaluated before a decision to use the variable frequency is made. Because of the harmonics, the output torque contains some unsteady torsional components. These can be handled by evaluating the compressor train torsional response. This will be further covered in a later chapter. 

Application, relative to size, is not too much of a problem as the variable frequency drive has been used to 40,000 hp. High horsepower drives have been in service in Europe for a significant period of time [4J. In the U.S., the applications have been somewhat more modest in size but, as the popularity grows, the size will doubtless follow. 

Variable-frequency drive technology is constantly improving in step with the advances in power electronic device technology and with the associated microprocessor controls. The following list of desirable fea tures is offered ... 

Scholey, D., Induction Motors for Variable Frequency Power Supplies. IEEE Transactions, July 1982, pp. 368-372,... 

Electric Motor Variable Frequency Drive No. of Pulaas " Motor Una Fraquancy Steady... 

Molecular weight, effect on centrifugal sizing, 159 Mollier charts, 27 Monitoring system, 356 Motor, 146 enclosure, 260 equations, 267 insulation, 257 locked rotor torque, 270 selection, 270 service factor, 262 starting characteristics, 270 starting time, 273, 274 synchronous vs induction, 265 variable frequency drives, 27/, 280 voltage, 258 Motors... 

This is an important and useful motor drive arrangement for many process and nonprocess applications. For a variable-frequency controller, see reference 94. 

The most recent development in the starting of squirrel-cage induction motors is the introduction of the electronic soft-start. This principle has been derived from variable-frequency speed controllers using switched Thyristor or power transistor bridges. The supply sine wave is chopped so that a reduced voltage and frequency is applied to the motor.These are gradually increased so that the motor speed rises in a controlled manner, with the starting current limited to any chosen value. 

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