Frequency electric motors requirements

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. 

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 rotary atomizer machines are high-speed machines traditionally built with a step-up gear to increase the speed from the 3000 or 3600 rpm of the standard two-pole electric motors to 10,000 to 20,000 rpm normally required to achieve sufficiently fine atomization. Newer designs feature high-speed electric motors with frequency control of the atomizer speed. Table 12-41 gives the main operational parame-... 

Most electronic equipment shares the television set s need for a number of differing voltages for the operation of individual components. This alone may be sufficient justification for the inclusion of a direct current to alternating current converter in fuel cell power systems. In addition, alternating current electric motors are more suitable in most applications. They tend to operate at a rotational speed controlled by the frequency of the current. If completely unloaded they speed up to this fixed velocity and accelerate no further. Many types of direct current motors, if operated unloaded, will continue to accelerate until they fail. A belt driven fan operated by an alternating current motor is undamaged by the failure of the belt. A direct current motor will require a special safety circuit to shut it down in case of belt failure. If the belt and the safety circuit both fail, the motor will speed up until it destroys itself. 

AC electric drives require more sophisticated converters when they are supplied with DC sources, because electric machines requires periodic voltage and current waves with a variable frequency depending on the load requirements. In Fig. 5.8, the scheme of an example of three-phase induction motor driven by a pulse-width-modulated inverter is reported. In this scheme a three-phase bridge connection with six power modules is shown to form the so-called inverter. Each power module can be composed by a number of power switches connected in parallel to carry higher currents. Across each power switch (IGBT) a parallel diode is connected to provide a return path for the phase current when the power module is switched off. 

U-tube = a tube bent into the shape of the letter U. Often used as radiant tubes or in a heat exchanger. Also a type of manometer, variable frequency drive = VFD = an energy-saving way to control speed or input by controlling electric motor speed (rpm)—applied to fans, blowers, exhausters, compressors for air and fuel, and to load pumps and conveyors, velocity pressure, vp = the pressure drop necessary to accelerate a fluid (gas or liquid) to a certain velocity. When a fluid is in motion at some velocity, the velocity pressure is the pressure rise that was required to raise it to that velocity. (Compare with static and total pressure.)... 

Special consideration is required for heavy loads such as crushers, high capacity electric furnaces, big pumps, or blowers which may cause flucmations in the incoming supply grid and disturb other nearby industries or power consumers. Hence, arrangements must be provided in the equipments to start on a minimum load and then slowly increase the load. Some of the methods for this could be provision of variable-frequency drive motors, throttling inlel/exit valves, controlled feed rates of solid lumps to crushing machines, etc. 

In alternating current, the movement of electrons alternates direction at a set sinusoidal wave frequency. In North America this frequency is 60 hertz, which means that electrons reverse the direction of their movement in the conductor 120 times each second, effectively oscillating back and forth through the wires. Because of this, the vector direction of the magnetic held around those conductors also reverses at the same rate. This oscillation requires that the phase of the cycle be factored into design and operating principles of electric motors and other machines that use alternating current. 

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. 

This is a very good motor for direct connection to certain loads, particularly where constant speed is required. NEMA defines it as a synchronous machine which transforms electrical power from an alternating-current system into mechanical power. It usually has direct-current field excitation by a separately driven direct-current generator or one directly connected to the motor. This motor remains synchronous with the supply frequency and is not affected by the load. Proper application requires consideration of the following ... 

DC motor drives have always offered high torque at all speeds and exact control of motion speed. AC induction motors have reliably converted electricity into rotary power for many years, and recently adjustable-frequency controls add variable-speed capability. While AC motors were originally relegated to relatively simple tasks, such as varying the flow rates of fans or pumps, advances in both motor and control technologies have allowed their use in higher performance operations. They are reliable sources of fixed-speed and variable-speed rotating power. Electric drives with appropriate closed-loop control operate only when required. However, to avoid unsuccessful apphca-tions, it is important to properly match the load, motor, and controller. 

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... 

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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