Field winding

At lower p.f.s the field system is required to be overexcited, which may cause excessive heating of the field windings. 

In the leading mode the field system will be ineffective. When this is required the manufacturer must be consulted. In the leading mode when the machine is suitable to operate such, the voltage will improve and the machine will operate in the underexcited mode. While the field winding will now be less stressed, the leading p.f. is not healthy for the machine and the equipment connected on it because... 

Direct-current motors are classified as separately excited motors, series motors, shunt motors, and compound motors. The field winding of a separately excited motor is in a circuit that is energized by a separate dc source the field winding is not physically connected to the armature circuit (containing the armature winding). 

The damper and field windings each produce an effect on accelerating torque. The resistance of the damper winding is usually selected to produce its maximum torque during the first part of the starting period and a field-discharge resistor is selected so that the field-circuit torque peaks near pull-in speed. 

Figure 2-70 is a schematic of another AC generator called an alternator. A DC current is supplied to field windings on the rotor, which are then rotated inside the stator windings, producing the AC emf. 

Compensated Series Motor. A compensated series motor is a series motor with a compensating field winding. (The compensating field winding and the series field winding may be combined into one field winding.)... 

Compound-Wound Motor. A compound-wound motor is a direct-current motor which as two separate field windings. One, usually the predominating field, is... 

Cutaway view of the rotor of a superconducting generator. While the operating principle is the same as that of a conventional, rotating generator, the rotor is different. It is hollow and vacuum-enclosed, and instead of copper conductors, it contains a niobium-titanium Superconducting field winding held in place by radial slots. 

The map preparation is a tedious, time consuming process. It is more convenient to calculate the horizontal wind components at a point interpolating field wind data. Wayne et al. (22), Wendell (76), and McCracken et al. (36) have adopted formulas of the form... 

Atmospheric GCMs simulate the time evolution of various atmospheric fields (wind speed, temperature, surface pressure, and specific humidity), discretized over the globe, through the integration of the basic physical equations the hydrostatic equation of motion, the thermodynamic equation of state, the mass continuity equation, and a water vapor transport equation. To reproduce the... 

In separately excited DC machines (Fig. 5.3d) the excitation is obtained by means of field windings which have a supply separated from the rotor windings, and this gives a separated control of speed and torque, according to what is required in an electric vehicle. Separately excited brushed DC motors were widely... 

Turbines use the wind s forces to turn blades that power a rotor to create electricity. They can be small enough to fit onto a roof and big enough to stand alone on top of a hill or in a field. Wind speed increases with height, so it s worth checking out the wind speeds of your site before going any further. In urban areas in particular, turbines might not pick up enough wind to be viable. 

The stator, also called the armature, carries the three-phase AC winding. The rotor, also called the field, carries the DC excitation or field winding. The field winding therefore rotates at the shaft speed and sets up the main magnetic flux in the machine. 

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. 

A power amplifier to amplify the error signal and to provide sufficient power to energise the field winding of the exciter. 

The power amplifier raises the signal level from a few volts and few milliamps to tens of volts and tens of amps that are required by the field winding of the exciter. The amplification is shown as the gain Ga that is typically in the range 200 to 500 per unit (see example below). 

The time constant Te of the exciter is mainly related to its field winding. 

Induction motors can be represented by the 2-axis theory, by using the derivations for synchronous machines but deleting the field winding. In this case some of the reactances become zero, and the field resistance is infinity. Hence, the derived reactances X ... 

E is the internal emf produced by the field winding on the rotor. Hence, for any given value of power P supplied by the generator there will be a wide range of Eg and rotor angle Sg values. 

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