Control frequency converters

The pressure should be regulated as close as possible to the vacuum pump. Ideally it should be regulated IN the vacuum pump. Vacuum pumps that do not use pump fluid can be set to a speed range of 1 5 using a frequency converter, provided that the drive motor is monitored for overheating. A control sequence consisting of pressure transmitter, controller, frequency converter and motor allows automatic responses to different recipes or quality fluctuations in the output products. 

Some induction heating furnaces must operate at frequencies higher than the supply frequency. Formerly, rotating motor alternator frequency converters were used. Now the avadabdity of high speed, high power sdicon controlled rectifiers for use in frequency converters has made rotary converters obsolete. Modem units operate at higher efficiency, cost less, require less factory space, and coordinate readdy with process controls (2). 

The above methods provide speed variation in steps, as in squirrel cage motors or in two machines or more, as in frequency converters, and cannot be u.sed for a process line, which requires frequent precise speed controls. Until a few years ago there was no other option with all such applications and they had to be fitted with d.c. motors only. D.C. motors possess the remarkable ability of precise speed control through their separate armature and field controls. In d.c. motors the speed... 

The method of control of the ZVS QR converter is inverse from the ZCS. At light loads, the control frequency is high, thus performing many off-times. When the load is heavy, the number of off-times decreases. So its control relationship is... 

The current-mode controlled forward converter has one additional consideration there is a double pole at one-half the operating switching frequency. The compensation bandwidth normally does not go this high, but it may cause problems if the closed-loop gain is not sufficiently low enough to attenuate its effects. Its influence on the control-to-output characteristic can be seen in Figure B-14. 

The next task is to determine the plaeement of the eompensating zero and pole within the error amplifier. The zero is plaeed at the lowest frequency manifestation of the filter pole. Since for the voltage-mode controlled flyback converter, and the current-mode controlled flyback and forward converters, this pole s frequency changes in response to the equivalent load resistance. The lightest expected load results in the lowest output filter pole frequency. The error amplifier s high frequency compensating pole is placed at the lowest anticipated zero frequency in the control-to-output curve cause by the ESR of the capacitor. In short ... 

Sensors TS 1-2-4 regulate the batteries for heating and cooling in a se quence to achieve the required temperatures (Fig. 9.56). Regulating valves for heat recovery are controlled by a frequency converter RCl for the pump motor. When a greater output is required from the heating battery, the pump motor speed increases before the valve MV2 opens. If the extract temperature is lower than the outdoor temperature, the speed of the pump motor increases before valve MVl opens. To avoid ice formation at low outdoor temperatures, the sensor TS7 operates on a lower limit, depending on the demands of the battery in the exhaust. 

The speed of the fans is controlled by sensors PSl and PS2 and frequency converters RCl and RC2 (Fig. 9.58). The sensors measure the difference in pressure between duct and atmosphere outside and maintain constant pressure in the ducts. 

For controlled-potential coulometry the voltage drop over a standard resistor is measured as a function of time by means of a voltage-to-frequency converter the output signal consists of a time-variant and integrally increasing number of counts (e.g., 10 counts mV-1), which by means of an operational amplifier-capacitor yields the current-time curve and integral158. 

Recently there has been a growing emphasis on the use of transient methods to study the mechanism and kinetics of catalytic reactions (16, 17, 18). These transient studies gained new impetus with the introduction of computer-controlled catalytic converters for automobile emission control (19) in this large-scale catalytic process the composition of the feedstream is oscillated as a result of a feedback control scheme, and the frequency response characteristics of the catalyst appear to play an important role (20). Preliminary studies (e.g., 15) indicate that the transient response of these catalysts is dominated by the relaxation of surface events, and thus it is necessary to use fast-response, surface-sensitive techniques in order to understand the catalyst s behavior under transient conditions. 

Modern frequency converters now make it possible to handle smaller automation tasks via the motor drive alone. Here, the controller intended for controlling the inverter assumes monitoring and control tasks that it can communicate externally via the available (or expandable) I/O. Moreover, it is also possible to connect various bus systems. Sometimes the functioning of small programmable logic controls can be integrated into the inverter. Parameters and control function programming can be entered via a small user interface. It may be even more convenient to connect to a laptop with an RS 232 interface. 

As a rule, frequency converter cabinets have small operating devices with the aid of which operating conditions can be monitored and parameters altered. There are hundreds of adjustable parameters, 20 to 30 of which usually have to be altered per application. Bus systems are used to exchange reference and actual values with system control. 

To monitor the transformer, the following monitoring devices are installed that are evaluated in part in the frequency converter, in part in the user control. 

The pump-probe pulses are obtained by splitting a femtosecond pulse into two equal pulses for one-color experiments, or by frequency converting a part of the output to the ultraviolet region for bichromatic measurements. The relative time delay of the two pulses is adjusted by a computer-controlled stepping motor. Petek and coworkers have developed interferometric time-resolved 2PPE spectroscopy in which the delay time of the pulses is controlled by a piezo stage with a resolution of 50 attoseconds [14]. This set-up made it possible to probe decoherence times of electronic excitations at solid surfaces. 

The BEWAG battery was connected to a 30-kV distribution line via four parallelled converters. When providing frequency control, the converters were programmed to limit power flow to 8.5 MW. For spinning reserve, the power limit was increased to supply 17 MW [16]. 

Converter 2. The stator winding are connected in parallel so the motor current demanded at the starting process is around 570Arms (285 x 2) including the overload. The converter 2 is able to provide 600A at OHz and even more as the frequency increases so the full power (full speed-full torque) operation including the overload of 150% is ensured. This mode is selected when a failure occurs in the Converter 1. The CPU-1 controls the converter 2, the motor, its field excitation as well as the water cooling unit. 

The hoist drive solution presented in this work, which is based on the Ingedrive MVlOO medium voltage AG frequency converters, contains two completely independent Active Front End Rectifier and Inverter sets under the well-known AC-DC-AC topology. The entire drive system solution allows the operation at full speed and full load (then full hoist performance, with no limitation) even in the case that one frequency converter (AG-DC-AC set) is unavailable. The solution presents some extra benefits comparing to the original hoist system solution that was based on two slip ring rotor wound induction motors in which the speed control... 

The ore hoisting equipment is a top mounted fiction hoist (Koepe winder) with a single skip and counterweight. The motor is air cooled and the heat excess from the motor during wintertime is used for heating up the headframe. The brake units are operated by 2 hydraulic systems. Main transformer, frequency converter and operator control system will be located in a separate building at ground level near the headframe. 

Internal circular grinding machines are usually equipped with high frequency grinding spindles (three-phase asynchronous motors) and frequency converters for continuously variable speed control. As a measure of the mechanical power emitted by the grinding spindle, one can use the electrical output power delivered by the converter, which can easily be acquired by conventional measurement techniques. The required control variable - one speaks in this case of constant-power grinding - is therefore almost free of charge. 

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