Rating induction motors

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 general-purpose alternating-current motor is an induction motor, rated 200 horsepower and less, which incorporates all of the following ... 

AC traction motor - Cage induction motor - Rated continuous power 45kW Speed, max. 2300 min 1 Frequency 0-78 cps Voltage 0-180 V Torque, max. ... 

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... 

Table 5.23 shows a comparison of the units of measure in the International System (metric) and the American customary system, as related to induction motor rating. This table also shows the induction motor kilowatt output equation for both systems of measurement and the conversion from one system to the other. 

The magnetic field rotates at a synchronous speed, so it should also rotate the rotor. But this is not so in an induction motor. During start-up, the rate of cutting of llux is the maximum and so is the induced e.m.f. in the rotor circuit. It diminishes with motor speed due to the reduced relative speed between the rotor and the stator flux. At a synchronous speed, there is no linkage of flux and thus no induced e.m.f. in the rotor circuit, consequently the torque developed is zero. 

This is a vital relationship, which reveals that during start-up and until such speed, the reactance of the motor windings / 2> the rotor current will also remain almost the same as the starting current and will fall only at near the rated speed. (Refer to the current curves in Figures 1.5(a) and (b)). The initial inrush current in a squirrel cage induction motor is very high. In a slip-ring motor, however, it can be controlled to a desired level. (Refer to Section 5.2.1.)... 

The declared efficiency and power factor of a motor are affected by its loading. Irrespective of the load, no-load losses as well as the reactive component of the motor remain constant. The useful stator current, i.e. the phase current minus the no-load current of a normal induction motor, has a power factor as high as 0.9-0.95. But because of the magnetizing current, the p.f. of the motor does not generally exceed 0.8-0.85 at full load. Thus, at loads lower than rated, the magnetizing current remaining the same, the power factor of the motor decreases sharply. The efficiency, however, remains practically constant for up to nearly 70% of load in view of the fact that maximum efficiency occurs at a load when copper losses (f R) are equal to the no-load losses. Table 1.9 shows an approximate variation in the power factor and efficiency with the load. From the various tests conducted on different types and sizes of motors, it has been established that the... 

Recommended capacitor ratings for direct switching with induction motors, to improve power factor to 0.95 or better at... 

Induction motor mechanical considerations. If the motor is plaeed between the eompressor and the expander its shaft ends must be eapable of full torque transmission. Unit torque requirements are generally well above those required by the motor rating. The result is speeially designed motors. 

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). 

Power-factor can be rated at unity, leading, or even lagging. The synchronous motor can supply corrective kvar to counteract lagging power factor caused by induction motors or other inductive loads. 

Figure 14-10 compares the efficiencies of the synchronous and induction motors. For a synchronous motor designed with an 0.8 power factor, the motor delivers a leading magnetizing kva component equal to 60% of the motor kva rating. The power factor of an induction motor is always... 

Figure 14-10. Synchronous and induction motor efficiencies Full load efficiencies of high-speed 0.8 power factor synchronous motors in the ratings shown are 1-2% lower than unity power factor motors. (Used by permission E-M Synchronizer, 200-SYN-33. Dresser-Rand Company.)...

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

Polyphase, Squirrel-Cage Induction Motors, General Purpose Ratings, 1105, Allis Chalmers Mfg. Co., Milwaukee, WI, May (1957). 

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