Excitation direct -current field

This is a very good motor for direct connection to certain loads, particularly where constant speed is required. NEMA defines it as a synchronous machine which transforms electrical power from an alternating-current system into mechanical power. It usually has direct-current field excitation by a separately driven direct-current generator or one directly connected to the motor. This motor remains synchronous with the supply frequency and is not affected by the load. Proper application requires consideration of the following ... 

In contrast to a direct injection of dc or ac currents in the sample to be tested, the induction of eddy currents by an external excitation coil generates a locally limited current distribution. Since no electrical connection to the sample is required, eddy current NDE is easier to use from a practical point of view, however, the choice of the optimum measurement parameters, like e.g. the excitation frequency, is more critical. Furthermore, the calculation of the current flow in the sample from the measured field distribution tends to be more difficult than in case of a direct current injection. A homogenous field distribution produced by e.g. direct current injection or a sheet inducer [1] allows one to estimate more easily the defect geometry. However, for the detection of technically relevant cracks, these methods do not seem to be easily applicable and sensitive enough, especially in the case of deep lying and small cracks. 

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). 

In synchronous motors, the excitation is supplied by a separate direct current source, either as a separate motor-generator (M-G) set or as an exciter mounted directly on the motor shaft. The current can be made to lead to various degrees by varying the magnitude of the field strength. 

Straight Shunt-Wound Motor. A straight shunt-wound motor is a direct-current motor in which.the field circuit is connected either in parallel with the armature circuit or to a separate source of excitation voltage. The shunt field is the only winding supplying f ield excitation. 

Permanent Magnet Motor. A permanent magnet motor is a direct-current motor in which the field excitation is suppled by permanent magnets. 

In a brushless system an a.c. exciter with a rotating armature and stationary field system is provided. The voltage applied to the stationary field system is varied, thus changing the output of the rotating armature. This output is rectified via shaft-mounted diodes to produce a direct current (D.C.) supply that is connected to the main generator field. 

Flames and plasmas can be used as atomisation/excitation sources in OES. Electrically generated plasmas produce flame-like atomisers with significantly higher temperatures and less reactive chemical environments compared with flames. The plasmas are energised with high-frequency electromagnetic fields (radiofrequency or microwave energy) or with direct current. By far the most common plasma used in combination with OES for analytical purposes is the inductively coupled plasma (ICP). 

Figure 166 The time dependence of device luminance (a) for a three layer LED (b) operated at a field ca. lMV/cm as driven by the direct current (DC), alternating current (AC) and pulsed current (PC) modes. The frequency of the pulsed excitation was 1kHz for both the AC and DC modes. After Ref. 586. Copyright 2000 Jpn. JAP, with permission.

Inductively coupled plasma (ICP) reactors (Fig. 20a) are particularly attractive because their design is relatively simpler and they are easily scaleable to large diameter substrates [84, 85]. In ICPs, the plasma is excited in a cylindrical chamber (r, 2,0) by a solenoidal or planar (stovetop-type) coil powered at radio frequencies, for example 13.56 MHz. The coil current induces a time-varying magnetic field which in turn induces an azimuthal (in the 0-direction) electric field that couples power to the plasma, i.e., heats the plasma electrons. For common excitation frequencies (less than the plasma frequency), the electromagnetic fields are absorbed by the plasma within the skin depth. For typical conditions, fields penetrate a few cm into the plasma. The power is deposited non-uniformly in the shape of a toroid (see also Fig. 

We have given perhaps undue attention to the mobile carrier mechanism because at one time it was assumed that the Na and K transport in excitable cell membranes occurred precisely via this mechanism. In 1965, Chandler and Meves undertook an experiment to assess the aforementioned specifics of the high-frequency conductance. A nerve fiber was placed in a solution containing no Na or K ions. This precluded direct current through the membrane. However, if there had been any mobile charged carriers in the membrane, the authors would have detected current on application of a variable field. The authors did not observe a detectable current under these conditions, from which it could be deduced that the transport systems of excitable membrane are structured as ion channels whose conductance is controlled by electric field. 

In the previous chapter we studied surface waves on passive structures as for example finite FSSs. They were excited by an incident plane wave. We observed that a finite FSS in addition to the Floquet currents excited directly by the incident plane wave conld also snpport surface waves. These would radiate and thereby lead to an increase in the scattered field that is, the RCS could be larger than expected. The scattered field associated with the surface waves could be significantly rednced by resistively loading one or more columns at the edges of the finite FSS. This approach would leave the Floquet currents in the rest of the FSS unaffected that is, the transmission and reflection properties of the FSS were basically left intact. 

The principal function of the excitation system is to furnish power in the form of direct current and voltage to the generator field, creating the magnetic field.The excitation system also comprises control and protective equipment which regulates the generator electrical output. In today s complex power system transmission design, the performance and protection fea-... 

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. 

---