Frequency converter

The efficiency of an induction furnace installation is determined by the ratio of the load usehil power, P, to the input power P, drawn from the utihty. Losses that must be considered include those in the power converter (transformer, capacitors, frequency converter, etc), transmission lines, cod electrical losses, and thermal loss from the furnace. Figure 1 illustrates the relationships for an induction furnace operating at a constant load temperature with variable input power. Thermal losses are constant, cod losses are a constant percentage of the cod input power, and the usehd out power varies linearly once the fixed losses are satisfied. 

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). 

Voltage source inverter (VSl) using IGBTs 6/125 Current source inverter (CSl) 6/126 Cyclo converters (frequency converters) 6/127 The regenerative schemes 6/127... 

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... 

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... 

Variable AC voltage and frequency converter (IGBT or thyristor)... 

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. 

The speed of an electric motor can be changed by altering the frequency of the electric current. This is because the ratio is the same as 60 or 50 f/p (f = the frequency of the current, p = the number of poles in the stator). Frequency converters are built of electronic components, frequently combined with microprocessors. They provide good motor protection and are superior to the traditional bimetal protection. The characteristic curve for a pump and fan motor is also quadratic, making lower demands to the frequency converters When the frequency of the electrical current is changed in the frequency converter, the main AC supply is transformed into DC. The DC is then treated... 

Frequency converters constructed for use with three-phase motors less than 4 kW can therefore be delivered with a single-phase supply from the electrical mains. For greater power requirements, it is most common to use a three-phase power supply. Through switches and potentiometers or the programming module of the frequency converter, the important parameters of electrical motors can be adjusted as needed. 

The advantage of using frequency converters is that the possibility exists to use a ramp function when starting and stopping the motor (soft start). By using this function, it is possible to avoid starting both fans at full speed with closed dampers it also reduces stresses on the fan transmission (belts) at the start. 

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. 

There is a shortest reasonable exposure time that is related to the frequency of the heartbeat quartz and the adjustment of the voltage-to-frequency converters. Below the reasonable exposure interval quantization errors become a problem and the measured value will be chosen from a small number of possible steps. 

In a radioactive environment standard turbomolecular pumps can be used without hazard at dose rates of from 10 to 10 rad. If higher dose rates are encountered, then certain materials in the pump can be modified in order to withstand the greater loading. The electronic frequency converters in such cases are to be set up outside the radioactive areas since the semiconductors used inside them can tolerate a dose rate of only about 10 rad. The use of motor-driven frequency converters which can withstand up to 10 rad represents another option. 

Turbomolecular pumps and frequency converters are nearly maintenance-free. In the case of oil-lubricated pumps it is necessary to replace the bearing lubricant at certain intervals (between 1500 and 2500 hours in operation, depending on the type). This is not required in the case of grease-lubricated pumps (lifetime lubrication). If it should become necessary to clean the pump s turbine unit, then this can easily be done by the customer, observing the procedures described in the operating instructions. 

This information was obtained in private conversations with engineers directly involved with the project and involved with the frequency converter both before and after its modification. The converter exhibited COP = 1.05-1.15, prior to modification to prevent it. 

Class B corresponds to the standard. Class F is generally used in motors operated with a frequency converter. Class H is used in special cases involving high ambient temperatures. 

Power electronic devices, such as frequency converters, have become quite reliable and have lifetimes of 7 to 10 years. The main lifetime-limiting factor is the drying out of the electrolytics. After 12 to 15 years, the danger increases that, in case of breakdowns, the installed electronic components are no longer available. 

The energy for driving all high-capacity electric motors is obtained from the AC network. AC motors can be supplied directly from the AC network, as long as the motor is to be operated at nominal speed. If variable speed is specified, AC motors are equipped with a frequency converter. In DC motors, power converters are used. 

Asynchronous motors can operate directly on the AC network current. The resulting speed is constant and dependent on the number of pole pairs and network frequency. However, an extruder drive should at least have variable speed. For this, a frequency converter is used. 

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