Stator losses

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. 

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. 

V = Loss or active component supplying the hysteresis and eddy current losses to the stator core. 

Let us consider the simple equivalent motor circuit diagram as shown earlier in Figure 1.15. The no-load component of the current, / , that feeds the no-load losses of the machine contains a magnetizing component, produces the required magnetic field, (p, , in the stator and the rotor circuits, and develops the rotor torque so that... 

A very important feature of solid-state technology is energy conservation in the process of speed control. The slip losses that appear in the rotor circuit are now totally eliminated. With the application of this technology, we can change the characteristics of the motor so that the voltage and frequency are set at values just sufficient to meet the speed and power requirements of the load. The power drawn from the mains is completely utilized in doing useful work rather than appearing as stator losses, rotor slip losses or external resistance losses of the rotor circuit. 

Calculate the no-load stator copper loss watts. 

Subtract the no-load stator copper loss from the stator input power at no-load, i.e. (P ( - i R ). This is the net no-load loss made up of core, friction, windage and stray losses. 

Add the net no-load loss and stator copper loss on load and subtract their sum from the stator input power measured on load. The remainder is the power input to the rotor. 

As a check, add together the net no-load loss, the stator copper loss on load, the rotor copper loss on load and stray loss to give the total losses (total fixed loss plus load loss) ... 

When the torque is determined by the above method, the voltage during the test should be so adjusted that the locked rotor current is approximately equal to the full load current. After the locked rotor test, the resistance of the stator windings should be measured and may also be considered for calculating the I R losses. 

The unbalanced voltage will produce an additional rotor current at nearly twice the supply frequency. For example, for a 2% slip, i.e. a slip of 1 Hz, the negative sequence stator current, due to an unbalanced supply voltage, will induce a rotor current at a frequency of (2/- 1) = 99 Hz for a 50 Hz system. These high-frequency currents will produce significant skin effects in the rotor bars and cause high eddy current and hysteresis losses (Section 1.6.2(A-iv)). Total rotor heat may be represented by... 

In all the above conditions, the rotor would heatup much more rapidly than the stator due to its low thermal time constant (t), and its smaller volume compared to that of the stator, on the one hand, and high-frequency eddy current losses at high slips, due to the skin effect, on the other. True motor protection will therefore require separate protection of the rotor. Since it is not possible to monitor the rotor s temperature, its protection is provided through the stator only. Separate protection is therefore recommended through the stator against these conditions for large LT and all HT motors. 

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. 

The aeeurate ealeulation and proper evaluation of losses within a eentrifugal eompressor is as important as the ealeulation of the blade-loading parameters. If the proper parameters are not eontrolled, effieieney deereases. The evaluation of various losses is a eombination of experimental results and theory. The losses are divided into two groups (1) losses eneountered in the rotor, and (2) losses eneountered in the stator. 

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