Stator, motor

Core-Loss Limits. In the United States, flat-roUed, electrical steel is available in the following classes (12) nonoriented, fiiUy processed nonoriented, semiprocessed nonoriented, fiiU-hard and grain-oriented, fiiUy processed. Loss limits are quoted at 1.5 T (1.5 x lO" G). The loss limits at 1.7 T (1.7 X ICf G) of the fourth class and of the high induction grades are shown in Table 2. Typical appHcations include use for transformers, generators, stators, motors, ballasts, and relays. 

Statistical treatment of random errors, 971-983 Stator, motor, 53 Stern-Volnier equation, 408,409 Stokes... 

A mud motor (Fig. 3.17) is a positive displacement hydraulic motor, driven by the circulated drilling fluid. A continuous seal is formed between the body ( stator ) and the... 

The nonoriented steels are subdivided into low, intermediate, and high siUcon steels. The first contain about 0.5—1.5% siUcon, used mainly in rotors and stators of motors and generators. Steels containing ca 1% siUcon are used for reactors, relays, and small intermittent-duty transformers. 

The typic medium-sized squirrel-cage motor is designed to operate at 2 to 3 percent shp (97 to 98 percent of synchronous speed). The synchronous speed is determined by the power-system frequency and the stator-winding configuration. If the stator is wound to produce one north and one south magnetic pole, it is a two-pole motor there is always an even number of poles (2, 4, 6, 8, etc.). The synchronous speed is... 

Another concept is brushless excitation, in which an ac generator (exciter) is direc tfy coupled to or mounted on the motor shaft. The ac exciter has a stator field and an ac rotor armature which is directly connected to a static controllable rectifier on the motor rotor (or a shaft-mounted drum). Static control elements (to sense synchronizing speed, phase angle, etc.) are also rotor-mounted, as is the field discharge resistor. Changing the exciter field adjusts the motor field current without the necessity of brushes or slip rings. Brushless excitation is suitable for use in hazardous atmospheres, where conventional brush-type motors must have protective brush and slip-ring enclosures. 

Squirrel-cage induc tion motors are inherently single-speed machines, but multispeed operation can be obtained by reconnecting the stator windings of motors designed for this purpose. 

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. 

Saturable reactors, which are adjustable by a small dc signal, have also been used for both primary (stator) and secondary (rotor) control. In the primary they control motor voltage and therefore torque. In combination with fixed secondary resistors and feedback from a tachometer, this system can be used for precise speed and torque control of cranes, hoists, etc. Even reversing can be accomplished by using two saturable reactors in each of two (of three) phases. Other combinations of fixed or saturable reac tors in the primaiy and/or secondaiy, all combined with secondary resistors, provide a wide range of capabiUties and flexibihty for the wound-rotor motor. 

Direct-current motor fields are on the stator. The rotor is the armature. The magnetic field does not rotate like the field in ac machines. Current in the armature reacts with the stator field to produce torque. 

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. 

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. 

Note For all practical purposes the stator performance data are only a replica of rotor data for torque and current. The performance of a motor is the performance of its rotor circuit and its design. 

Since the motor now operates at a higher slip, the slip losses as well as the stator losses will increase. A circle diagram (Figure 1.16) illustrates this. 

Magnetizing losses, however, as the name implies, are a phenomenon in electromagnetic circuits only. They are absent in a non-magnetic circuit. A motor is made of steel laminates and the housing is also of steel, hence these losses. Some manufacturers, however, use aluminium die-cast stator frames in smaller sizes, where such losses will be less (the bulk of the losses being in the laminations). 

Since the resistive loss would vary in a square proportion of the current, the motor will overheat on lower voltages (drawing higher currents). At higher voltages, while the stator current may decrease, the core losses will be higher. 

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... 

For a lower range of motors, say up to a frame size of 355, the silicon steel normally used for stator and rotor core laminations is universally 0.5-0.65 mm thick and possesses a high content of silicon for achieving better electromagnetic properties. The average content of silicon in such sheets is of the order of 1.3-0.8% and a core loss of roughly 2.3-3.6 W/kg, determined al a flux density of I W[ym and a frequency of 50 Hz. For medium-sized motors, in frames 400-710, silicon steel with a still better content of silicon, of the order of 1.3-1.8% having lower losses of the order of 2.3-1.8 W/kg is prefeired, with a thickness of lamination of 0.5-0.35 mm. 

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... 

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