AC motors

Three-phase induction motors operating at a fixed speed are used industrially. By using permanent magnets in the induction motors, it is possible to attain reduction in size and weight and improved efficiency. 

MODERN ASPECTS OF RARE EARTHS AND THEIR COMPLEXES 

The major limitations for extensive use of NdFeB alloys in motors are (i) cost and (ii) restricted operating temperature range. Improved alloys with linear B vs H behaviour up to 150°C would satisfy the requirements. 

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In the preceding discussion of multispeed ac motors note that only induction motors are considered. These have no discrete physical rotor poles, so that only the stator-pole configuration need be modified to change speed. To operate multispeed, a synchronous motor would require a distinct rotor structure for each speed. Thus multispeed is practical only for squirrel-cage induction motors. 

Adequate single-phase protection is provided on low-voltage ac motor starters by three overload relays, which are now standard. Rotor heating is not particularly a problem on smaller motors which have more thermal capacity, but it is important to protect the stator windings of these machines against burnout. 

FIG. 29-8 Typical high-voltage ac motor starter illiistrating several protective schemes fuses, overload relays, ground-fault relays, and differential relays with the associated current transformer that act as fault-current sensors. In practice, the differential protection current transformers are located at the motor, hut the relays are part of the starter. 

If a greater reduction in line current is required for starting ac motors than is possible with reac tor starting, autotrausformers may be used. Because of transformer action, the reduction in motor-starting torque is directly proportional to the reduction in line current. Table 29-3 compares reactor and autotrausformer starting with respect to... 

Humphries, J.T., Electronic AC Motors and Controls. Merrill Publishing, Columbia, OH. (19 ). 

Lythall, R.T., AC Motor Control (on earth fault protection and thermistor protection). 

Low oil pressure switehes are provided on the pumps and diseharge header ahead of the eoolers and filters, sometimes after the eooler and filters, and always at the end of the line where the redueed oil pressure feeds the various users. A signal from any of these should start the motor-driven pump and all alarms should be aetivated in the eontrol room. The emergeney oil pump ean be driven with an AC motor but from a power souree that is different to the standby pump. When de power is available, DC eleetrie motors ean also be used. Proeess gas or air-driven turbines and quiek-start steam turbines are often used to drive the emergeney pumps. 

The following equations are useful in determining the eurrent, voltage, horsepower, torque, and power factors for three phase AC motors ... 

Most induction ac motors are fixed-speed. However, a large number of motor applications would benefit if the motor speed could be adjusted to match process requirements. Motor speed controls are the devices which, when properly applied, can tap most of the potential energy savings in motor systems. Motor speed controls are particularly attractive in applications where there is variable fluid flow. In many centrifugal pump, fan, and compressor applications mechanical power grows roughly with the cube of the fluid flow. To move 80 percent of the nominal flow only half of the power is required. Centrifugal loads are therefore excellent candidates for motor speed control. Other loads that may benefit from the use of motor speed controls include conveyers, traction drives, winders, machine tools and robotics. 

Electrical management, or power conditioning, of fuel cell output is often essential because the fuel cell voltage is always dc and may not be at a suitable level. For stationai y applications, an inverter is needed for conversion to ac, while in cases where dc voltage is acceptable, a dc-dc converter maybe needed to adjust to the load voltage. In electric vehicles, for example, a combination of dc-dc conversion followed by inversion may be necessary to interface the fuel cell stack to a, 100 V ac motor. 

Fixed speed. Advise synchronous speed desired (i.e., 3,600, 1,800, etc.) for AC motors. For DC motors, advise desired basic speed at full, load, and maximum speed by field control. 

Inverter-AC Motor Drives. An adjustable-frequency control of AC motors provide efficient operation with the use of brushless, high-performance induction, and synchronous motors. A typical system is shown in Figure. 3-14. Such a system consists of a rectifier (which provides DC power from the AC line) and an inverter (which converts the DC power to acljustable-frequency AC power for the motor). Inverter cost per kilowatt is about twice that of controller rectifiers thus the power convertor for an AC drive can approach three times the cost of a DC drive. 

The power convertor must provide the AC motor with low-harmonic voltage waveform and simultaneously allow the amplitude to be adjusted. This avoids magnetic saturation of the motor as the frequency is adjusted. For constant torque, from maximum speed to base speed, the voltage is adjusted proportional to frequency. Above base speed, the motor is usually operated at constant horsepower. In this region the voltage is held constant and the flux density declines. Also, the convei tor must limit the starting current, ensure operation at favorable slip, and provide a path fitr reverse power flow during motor slowdown. 

Figure 3-14. Typical invester AC motor drive consisting of rectifier-DC link, adjustable-frequency inverter, and induction of synchronous motor [10].

PrImary-Voltage-Control-AC Motor Driver. Induction motor torque at any slip s is proportional to primary V. Rotor-power dissipation is equal to s times the air-gap power. These two relationships define the boundary of operation of an induction motor with primary voltage control of speed. As the speed is reduced (s increased) at constant torque, the air-gap power remains fixed, but the power divides between rotor circuit dissipation and mechanical shaft power. 

Second part Belt-type intralox flat top 900, first and second part driven by one AC motor... 

Most extruders manufactured today are built using AC vector motor systems instead of the older DC motor systems. AC motors operate at about 5% higher efficiencies, require less maintenance, and are lower in cost than a DC motor. Typical efficiencies for AC and DC motors rated at 83 kW are provided in Table 10.3. It is... 

There are two general types of steam turbines extraction and condensing. The most common turbine with which the process operator comes into contact is used to spare an electric-motor-driven centrifugal pump. The three-phase, AC motors used in the United States and South America are either 1800 or 3600 rpm. The motors used in Europe are 1500 or 3000 rpm. Small steam turbines (20 to 500 hp) used to drive centrifugal pumps, are rated for the same speed as the electric motors that are used in that particular service. 

If the compressor is suction-volume-limited, we say that it is speed-limited. If the compressor is driven by an AC motor, its speed is determined by the motor speed and any connecting gears. If the compressor is driven by a turbine, then the rated speed of the turbine limits the compressor speed. 

Answer Nothing. An AC motor is a fixed-speed machine. It will continue to spin at 3600 rpm. 

Applications. The kinds of motors that are being used successfully with particular kinds of. chemical process equipment are identified in Table 4.1. As many as five kinds of AC motors are shown in some instances. The choice may be influenced by economic considerations or local experience or personal preference. In this area, the process engineer is well advised to enlist help from electrical experts. A checklist of basic data that a supplier of a motor must know is in Table 4.2. The kind of enclosure may be specified on the last line, operating conditions. 

In a different type of RUPS system, during loss of normal power the AC motor is supplied from a battery bank by means of an inverter (Figure 2.19). The batteries are kept charged by the normal power source. The motor is powered from the batteries until the batteries become depleted. In some applications, standby generators are used to supply the battery bank in case of loss of normal power. Other combinations are used to provide uninterrupted power to critical loads, but we will not attempt to review all the available technologies. It is sufficient to point out that low-frequency disturbances are effectively mitigated using one of the means mentioned in this section. 

FIGURE 2.18 Rotary uninterruptible power source (RUPS) system using a diesel engine, AC motor, and AC generator to supply uninterrupted power to critical loads. 

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. 

Solid-State AC Motor Controls Selection and Application, Sylvester Campbell... 

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