Air gap flux

Also, torque developed is proportional to the produet of the air gap flux and the armature eurrent... 

Since the synchronous motor has an external source of excitation power it can maintain flux for a longer time during a fault. The rotor pole face construction and the field circuit help to maintain the air-gap flux and generated emf. The decay of flux during the fault is determined for the most part by the transient impedance of the synchronous motor. 

Continuously by using some form of thyristor controller which will allow feedback action in the form of closed loop control to be used to accurately regulate the speed. However, if such a scheme is used then it is the customary practice to adjust the applied frequency so as to maintain a constant air-gap flux, see 14.3.2 and 14.6. 

If an induction motor is run at a frequency below its normal operating frequency, the air-gap flux will rise if the supply voltage magnitude is kept constant. The rise in flux will cause magnetic saturation in the iron circuit of the motor and this in turn will cause a very large increase in magnetising current in the branch shown in Figures 5.1 or 15.11. 

When a running induction motor has a short-circuit applied to its terminals the air-gap flux creates an emf that drives a current into the fault. The motor is then driven by the inertia of its load. The speed may be assumed to be unchanged for the duration of the fault current, which in practice for small motors is only a few cycles at the supply frequency i.e. less than 60 milliseconds. For large motors the duration may as long as 250 milliseconds, see Reference 23. This is due to the higher X-to-R ratio in the short circuit than is the case with small motors. The impedance to the fault current consists of the transient reactance (equal to the sub-transient reactance) and the stator resistance. This will be shown below. 

In constant-horsepower designs, output torque can vary inversely with motor speed. For applications requiring constant output torque, however, the air-gap flux must be held constant over the entire speed range of the motor. 

An air-gap is required for all unipolar flux drive applieations sueh as this. One method of aehieving this is shown in Equation 3.28a (CGS system (U.S.)). 

In dc magnetic applications, an air-gap is usually required somewhere along the magnetic path of the core. In ferrite cores, the gap is placed in the center-leg of the core. The flux leaves one end of the core and flows towards the opposing end. The flux, though, repels itself and causes the flux lines to bulge out away from the centerline of the core. The presence of an air-gap creates an area... 

As Sf air gap core or armature then a small change in x will produce a large variation in fr. The self-inductance 5 of the coil in Fig. 6.14a is the total flux linked per unit current. Thus, from equation 6.18 ... 

J = diffusion flux (g/s), D = air diffusion coefficient (cm2s 1), A = cross sectional area of the sampler (cm2) and dc/dx = the concentration gradient of the compound across the stagnant air gap. 

Measurements of permeability and associated magnetic properties are usually made on toroids of uniform section when, to a close approximation, the flux density B is uniform throughout the material and lies entirely within it. In most practical applications the magnetic circuit is more complex, and variations in component section and permeability give rise to variations in flux density. Important effects arise from air gaps, which may be intentionally introduced or may be cracks or porosity. 

The stages of heat transfer in AGMD (Figure 19.7) include heat flux from the feed boundary layer to the membrane surface, vapor permeation through the membrane, and the diffusion in air gap, then condensation at the cold surface and finally heat transfer to the cooling water. 

The process parameters influencing the water vapor flux and energy efficiency are the temperature difference between hot and cold solutions, flow velocities of feed and permeate, air gap pressure, air gap width, and membrane type. 

Theoretically, there is no need for any air gap in a common mode choke, because the flux due to the line current is expected to cancel out completely. In practice, it doesn t fully, mainly due to slight differences in the individual winding arrangement (despite the equal number of turns). At a minimum, this causes the core to get dc-biased in one direction, and thereby cause an imbalance in the inductance it presents to the two lines. This would expectedly degrade the EMI performance, but in extreme cases, the core may even saturate. Note that core saturation in the filter is clearly not a catastrophic event (like the saturation of the main inductor/transformer of the converter can be), but since it is accompanied by severely worsening EMI-suppression efficacy, we need to prevent that too. Therefore, as in a forward converter transformer, a small air gap is usually present, even in a CM choke. 

The rotating held in the air gap of a synchronous machine is generally considered to be free of space harmonics, when the basic operation of the machine is being considered. In an actual machine there are space harmonics present in the air gap, more in salient pole machines than a cylindrical rotor machine, see for example References 4 and 6. It is acceptable to ignore the effects of space harmonics when considering armature reaction and the associated reactances. Therefore the flux wave produced by the rotating field winding can be assumed to be distributed sinusoidally in space around the poles of the rotor and across the air gap. 

Xm = magnetising reactance of the complete iron core, which represents the flux that passes across the air gap between the stator and the rotor. 

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