Induction motor

The stirrer stiould be provided witti a motor which will not ignite the hydrogen—eui induction motor or an air stirrer is suitable. The stirrer itself may be of glass. Monel metal or stainloss steel (c/. Fig. 11, 7, 6). 

The most popiilar form of motor speed control for adjustable-speed pumping is the voltage-controlled pulse-width-modulated (PWM) frequency synthesizer and AC squirrel-cage induction motor combination. The flexibility of apphcation of the PWM motor drive and its 90 percent- - electric efficiency along with the proven ruggedness of the traditional AC induction motor makes this combination popular. 

Alternating-Current Squirrel-Cage Induction Motors. 29-4... 

Alternating-Current Squirrel-Cage Induction Motors These motors are by far the most common constant-speed drives. They are relatively simple in design and therefore both low in cost and highly reliable. Representative prices are shown in Fig. 29-1 for various speeds and horsepowers. 

FIG. 29-1 Motor prices in dollars per horsepower for 1800 rev/min sqnirrel-cage induction motors from 3 to 10,000 hp. Dripproof and TEFC motors shown from 3 to 400 horsepower have 1.15 service factor for other motors above 250 horsepower, the service factor is 1.0. The basis of these data is July, 1994. To convert dollars per horsepower to dollars per kilowatt, multiply by 1.340 to convert horsepower to kilowatts, multiply by 0.746. 

FIG. 29-2 Typical speed versus torque curves for various NEMA-design squirrel-cage induction motors. (See Table 29-2 for an explanation of design types.)... 

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. 

FIG. 29-3 Typical speed versus torque curves for a wound-rotor induction motor with varying amounts of external secondary (rotor) resistance. Resistance values are based on resistance at 100 percent torque and zero speed = 100 percent. 

Theory, performance and constructional features of induction motors 1/5... 

In I 888 Nikola Tesla (1856-1943) at Columbus, Ohio, USA, invented the first induction motor which has become the basic prime mover to run the wheels of industry today. Below, for simplicity, we first discuss a polyphase and then a single-phase motor. 

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. 

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 why an induction motor ceases to run at synchronous speed. The rotor, however, adjusts its speed, N such that the induced e.m.f. in the rotor conductors is Just enough to produce a torque that can counter-balance the mechanical load and the rotor losses, including frictional losses. The difference in the two speeds is known as slip. S, in r.p.m. and is expressed in terms of percentage of synchronous speed, i.e. 

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 performance of an induction motor is influenced by the service conditions, when these differ from the design... 

A supply system would normally contain certain harmonic quantities, as di.scussed in Section 23.5.2. The influence of such quantities on an induction motor is also discussed in Chapter 23. To maintain a near-sinusoidal voltage waveform, it is essential that the harmonic voltage factor (HVF) of the supply voltage be contained within 0.02 for all 1-0 and 3-0 motors, other than design /V motors and within 0.03 for design N motors, where... 

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... 

This is a very useful nomogram to determine the performance of a motor with the help of only no-load and short-circuit test results. In slip-ring motors, it also helps to determine the external resistance required in the rotor circuit to control the speed of the motor and achieve the desired operating performance. Slip-ring motors are discussed in Chapter 5. The concept behind this nomogram is that the locus of the rotor and the stator currents is a circle. Consider the equivalent circuit of an induction motor as shown in Figure 1.15, where... 

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

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