Motors frequency converters

In order to gain a clear picture of available drive solutions related to the actual type of conveyor, a conveyor classification system has been estabhshed. Conveyors, in general, can be classified according to several parameters such as application, length, conveying capacity, hft, kind of conveyor, and required drive system power. Looking from the electric drives system perspective, the required power and speed to turn the drive pulley and run the conveyor belt are the most relevant parameters. These parameters directly translate to the dimensioning of the motors, frequency converters, and power transmission equipment. Therefore, ABB have... 

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... 

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... 

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 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. 

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. 

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. 

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. 

With a frequency converter, stator voltage and frequency of the asynchronous motor can be varied infinitely. This transforms a standard motor into a variable speed drive system. An asynchronous motor equipped with a rotor position sensor, magnetization calculation, and the impression of the corresponding stator currents (vector regulation) has the properties of a servo drive. 

The transistors of the output-side inverter are pulse-width modulated in such a way that in every phase, a near-sinusoidal current is created after smoothing by motor inductivity. The frequency of this current can be varied to adjust the speed of the motor. The switching frequency of small to medium frequency converters is on the order of 2 to 8 kHz and adjustable. In frequency converter operation, this frequency is quite audible. The larger the motor, the smaller the switching frequency selected. In large drives, switching frequency drops to 400 Hz. 

Voltage peaks also arise from the semiconductors switching in the inverter stressing the insulation of connected motors. For this reason, motors intended for inverter operation are better insulated. Sometimes - depending on the length of the motor cable - output filters are provided between the frequency converter and the motor to protect the latter. Such measures should be undertaken when standard motors used initially at constant speed on the mains are retrofitted with an inverter for variable drive. 

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. 

If motors are operated on a frequency converter, they can be run at higher frequencies. A typical value in servomotor technology is 87 Hz. At this value, the following speeds are obtained ... 

Whereas further effects are determined by the frequency converter, this effect is determined by the asymmetry of the motor itself. The voltage levels arising here typically range below 500 mVpeak which, under normal conditions, does not cause trouble. These bearing voltages cannot be entirely eliminated, but can be reduced by precise adjustment of the air gap. One... 

Via the ground lead of the motor cable back to the frequency converter... 

Circular currents cannot be influenced by grounding measures. The size of the current depends on the size of the windings and the steepness of voltage slope in pulsed motor voltage. Thus, circular currents can be reduced by a filter between the frequency converter and the motor. This effect has practical significance for frame sizes above 280. 

It is possible to equip large extruders with fully variable drives. In this case, speed variability is often not the main intention, but rather the fact that the existing supply network does not permit a fixed-speed motor to be started, because other users on the same network would be starved by the drop in voltage. In such cases, a frequency converter offers the advantage that much less current is required for start-up. Starting currents can of course also be reduced by a starting transformer or a softstarter. 

In order to keep dust and dirt out of the inside of the motor, the internal cooling circuit is closed to the surroundings. The air in the internal cooling circuit can be circulated either by a fan driven via the motor shaft, or by auxiliary ventilators. In the first case, cooling power is speed-dependent. This cooling concept is normally used for fixed-speed motors, as their speed is constant in this case cooling power is also constant. However, motors operated on a frequency converter can also be run at relatively low speeds. To avoid any cooling... 

A frequency converter can also be used as a starting aid and also does not have to be configured for full power. However, since it often cannot be operated at motor voltage, both an inverter-transformer for voltage reduction in front of the frequency converter, as well as a motor transformer for voltage increase after the frequency converter have to be used. All three components are bridged after starting, to run at full power. 

When a frequency converter is used, it provides the torque signal. On fixed-speed motors, however, torque has to be derived from an additional current measurement. 

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 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. 

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