Standards induction motors

Since the kW developed by a 3-0 winding is 50% more than by a 2-0 winding for the same value of stator current /, the economics of this principle is employed in an induction motor for general and industrial use. As standard practice, therefore, in a multi-phase system, only 3-0 induction motors are manufactured and employed, except for household appliances and applications, where mostly single-phase motors are ttsed. 

Theory, performance and constructional features of induction motors 1/33 Relevant Standards... 

Safety Standards (enclosures) for construction and guide for selection, installation and use of rotating machines Sound level prediction for insulated rotating electrical machines Energy management guide for selection and use of three phase motors Polyphase induction motors for power generating stations... 

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. 

Service factor is mentioned nowhere in the NEMA standards for lai ge machines there is no definition of it. There is no standard for temperature rise or other characteristics at the service factor overload. In fact, the standards are being changed to state that the temperature rise tables are for motors with 1.0 service factors. Neither standard synchronous nor enclosed induction motors have included service factor for several years. 

The newest catalogs show standard induction motors designed with Class B insulation for operation in a 40 C ambient with 80°C rise by resistance at 100% load for motors with 100% service factor. Class F insulation, with the capability of operating up to a 105°C rise by resis taiice, is today frequently offered as standard for machines with a Class B rise, particularly the larger sizes. Many users specify this as a standard Previously, induction motor ratings were based on temperature rise thermometer. 

A rule of thumb that was used in the past for constant speed applications wii.s to consider the selection of a synchronous motor where the application horsepower was larger than the speed. This, of course, was only an approximation and tended to favor the selection of a synchronous motor and would be considered too severe by current standards. However, the rule can aid in the selection of the motor type by giving some insight as to when the synchronous might be chosen. For example, applications ol several hp per rpm often offer a distinct advantage of the synchronous over the induction motor. In fact, at the lowest speeds, larger sizes and highest hp/rpm ratios may be the only choice. 

Figure 14-2A. Alternating current (AC) induction motor cross-sectional view of 220/440V, 3-phase, 60-cycle, totally enclosed fan-cooled standard motor. (Used by permission Reliance Electric Co., Div. Rockwell Automation.)...

Horsepower Ratings. Standard NEMA ratings for induction motors are... 

Standard 541 Form-Wound Squirrel-Cage Induction Motors—250 Horsepower and Larger... 

Standard for Petroleum and Chemical Industry Severe Duty TEFC Squirrel Cage Induction Motors—Up To and Including 500 HP... 

IEEE 841, Standard for petroleum and chemical industry— Severe duty totally enclosed fan-cooled (TEFC) squirrel cage induction motors — Up to and including 370 kW (500 hp). )... 

Asynchronous motors (squirrel cage induction motors, the rotor-stator part of slipring induction motors) and the rotor-stator part of synchronous motors are within the scope of increased safety due to the omission of sparks and arcs assuming an appropriate construction of rotor and windings. Tire standards IEC 60079-7 and EN 50019 contain mechanical and thermal requirements that follow this aim. 

Table 6.21 tE values of cage induction motors (thermal class F, 50 cps, for temperature classes T1/T2 and T3) (by courtesy of Siemens AG, Standard Drives Division, Erlangen/Germany)... 

Q standard low voltage circuit breaker at the grid side M two-pole cage induction motor, rated power 250 kW P cable guillotine with a pneumatic drive to cut the cable under explosive hydrogen-air atmosphere... 

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