Induction motors large

Theory, performance and constructional features of induction motors 1/25 Table 1.12 Normal systems of cooling for totally enclosed large machines... 

Secco, M.. Bressani, M. and Razza, F., Medium Motor and Generator Plant, Ansaldo Component i SPA. Italy. Progress and development trends in large induction motor stator winding insulation. 

Consider a process plant having a connected load of 15 000 kW and a running load of 12 500 h.p. at almost 0.65 p.f. lagging. Let a few large induction motors aggregating 2000 h.p. be replaced by as many oversized synchronous machines, with the purpose of improving the system p.f. in addition to performing the motor s duties. 

After replacing these large induction motors with as many oversized synchronous motors, while the active load at 9325 kW remains the same, the reactive load of induction motors at 785 kVAr will be eliminated and instead a leading reactive load of 5415 kVAr will be added. The net compensation therefore will be... 

Central Board oflmgation and Power. Re-switching transients and their effects on the design and operation of large induction motor drives for power station auxiliaries. New Delhi. India. March (1995). 

Interrupting an induction motor immediately after a switch on, when the current is large and highly inductive. 

If a large induction motor is switched on such a system it is possible that its rotor may lock up at the sub-synchronous speed and keep running at higher slips. This situation is also undesirable, as it would cause higher slip losses in addition to higher stator current and overvoltage across the series capacitors. 

Table 1 from the GPSA Engineering Data Book compares standard and high efficiency motors. Table 2 from GPSA compares synchronous and induction motors. Table 3 from Evans shows the effect of a large range of speeds on efficiency. 

For many years it was common practice to give standard open motors a 115% service factor rating that is, the motor would operate at a safe temperature at 15% overload. This has changed for large motors, which are closely tailored to specific applications. Large motors, as used here, include synchronous motors and all induction motors with 16 poles or more (450 rpm at 60Hz). 

New catalogs for large induction motors ai e based on standard motors with Class B insulation of 80°C rise by resistance, 1.0 service factor. Previously, they were 60°C rise by thermometer, 1.15 service factor. 

Induction motors with a 15% service factor are still available. Large open motors (except splash-proof) are available for an addition of 5% to the base price, with a specified temperature rise of 90°C for Class B insulation by resistance at the overload horsepower. This means the net price will be approximately the same. At nameplate... 

Service factor is mentioned nowhere in the NEMA standards for large machines. There is no standard for temperature rise or other characteristics at the service factor overload. In fact, the standards explicitly state that the temperature rise tables are for motors with I.O service factor. Neither standard synchronous nor enclosed large induction motors have included service factor for several years. 

Figure 14-2D. Large horizontal induction motor. Motors can meet API 541 requirements and ISO 9001 Certification for petroleum and chemical industry applications. (Used by permission Bui. M DD 3226-3. Siemens Corporation, Motors and Drives Division.)...

Figure 14-7. Internal details of large induction motor. (Used by permission Reference Handbook, TECO-Westinghouse Motor Co.)...

The induction motor is usually used in industrial service however, many important large horsepower applications still exist for the synchronous motor. 

Effect of Altitude on Operation of Large 200-2,000 hp Induction Motors (for Altitudes Greater Than 3,300 ft)... 

Use of wound-rotor induction motors has been largely in continuous-duty constant-speed supplications where particularly high starting torques and low starting currents are required simultaneously, such as in reciprocating pumps and compressors. These motors are also used where only alternating current is available to drive machines that require speed adjustment, such as types of fans and conveyors. 

Power-Factor Correction. The induction motors used for oil-well pumping have high starting torques with relatively low power factors. Also, the average load on these motors is fairly low. Therefore, it is advisable to consider the installation of capacitors to avoid paying the penalty imposed by most power companies for low-power factor. They will be installed at the individual motors and switched with them, if voltage drop in the distribution system is to be corrected as well as power factor. Otherwise they may be installed in large banks at the distribution center, if it is more economical to do so. 

Synchronous motors are made in speeds from 1800 (two-pole) to 150 rpm (48-pole). They operate at constant speed without slip, an important characteristic in some applications. Their efficiencies are 1-2.5% higher than that of induction motors, the higher value at the lower speeds. They are the obvious choice to drive large low speed reciprocating compressors requiring speeds below 600 rpm. They are not suitable when severe fluctuations in torque are encountered. Direct current excitation must be provided, and the costs of control equipment are higher than for the induction types. Consequently, synchronous motors are not used under 50 HP or so. 

The most usual situation that leads to low power factors is the use of inductive motors that, as the name implies, can introduce very large inductive reactance in the line. The electric load introduced by an inductive motor can be represented as a resistance and an inductance in series. This combination will have an intensity vector that will be delayed with respect to the voltage. 

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