Rotor windings

To obtain this rotor current, the required rotor circuit resistance can be calculated as below for the various configurations of the rotor windings and the resistance units. It may be noted that, when the resistance configuration is not the same as that of the rotor, it must... 

Excessive vibrations according to international codes can cause mechtinieal failure in the insulation by loosening wedges, overhangs, blocks and other supports that hold the stator and the rotor windings or rotor bars in their slots. Vibrations also tend to harden and embrittle copper windings and may eventually break them when they become loose (see also Sections 1 1.4.6 and I 1.4.7). 

In slip-ring motors the rotor winding resistance will be measured at the point of connection of the rotor winding to the slip-rings, so that the slip-ring resistance is eliminated from the measurement of the true rotor winding resistance. 

Semiconductor control modules gate the thyristors, which switch cm rent to the motor field at the optimum motor speed and precise phase angle. This assures synchronizing with minimum system disturbance. On pull-out, the discharge resistor is reapplied and excitation is removed k> provide protection to the rotor winding, shaft, and external electrical system. The control resynchronizes the motor after the cause of pull-out i.n removed, if sufficient torque is available. The field is automatically applied if the motor synchronizes on reluctance torque. The control is calibrated at the factory and no field adjustment is required. The opti-... 

The device of Figure 2-70 can also operate as a motor if a DC current is applied to the rotor windings as in the alternator and an AC current is imposed on the stator windings. As the current to the stator flows in one direction, the torque developed on the rotor causes it to turn until the rotor and stator fields are aligned (5 = 0°). If, at that instant, the stator current switches direction, then mechanical momentum will carry the rotor past the point of field alignment, and the opposite direction of the stator field will cause a torque in the same direction and continue the rotation. 

Repulsion-Start Induction Motor. A repulsion-start induction motor is a single-pliasc motor having the same windings as a repulsion motor, but at a predetermined speed the rotor winding is short circuited or otherwise connected to give the equivalent of a squirrel-cage winding. This type of motor starts as a repulsion motor but operates as an induction motor w ith constant-speed characteristics. 

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... 

The working principle of the induction machines is based on the fact that rotor currents are induced by the stator currents, and this is the main cause of losses in the rotor windings, which makes these types of machines less efficient than other brushless machines [11]. However, asynchronous machines are widely used and their mass production makes them reliable and well developed, for that reason their cost is quite low. 

The DC rotor winding is supplied with an external DC voltage by means of a brush-ring system, similar to the brush-collector system for the DC electric machine. The excitation of the rotor winding can also be obtained with a DC generator, known as exciter, located on the rotating shaft, or other excitation systems using AC exciter and solid state rectifiers. 

Before thyristors and power transistors were introduced for AC to DC and AC to DC to AC converter systems, there were a number of special designs of AC motors that gave better performance than standard squirrel-cage motors. These motors required connections to the rotor windings. They had better speed control, superior torque versus speed characteristics and some methods were energy efficient. However, they were more complicated and hence more expensive. 

Special motors that have connections made to their rotor windings. 

The simplest method of achieving the effect is to insert extra resistance into the rotor circuit. The rotor of the induction motor has to be specially wound so the winding can be split into three sections. Each section is connected to shaft-mounted slip-rings. The conductors of the rotor winding are carefully insulated from the iron core and from each other. The extra resistance is an external static unit mounted near to the motor. 

It is reasonable to regard the rotor windings as damper windings and use the notation of sub-transient reactances. Hence the following derived reactances and time constants are appropriate to induction machines. 

Application of a three-phase short circuit to the terminals of an unloaded induction motor is not a practical factory test, especially for a large high-voltage motor, because the motor can only be excited at its stator windings from the power supply. A three-phase short circuit at or near the stator terminals can occur in practice e.g. damaged supply cable, damage in the cable terminal box. The parameters of the stator and rotor windings can be obtained from other factory tests. However, the derived reactance can be defined in the same manner as those for the synchronous machine, but with... 

Where R2 is the rotor winding resistance and (1 — s)R2/s is the equivalent rotor resistance of the mechanical load. 

The primary events of the fault tree may be further decomposed. For example, the failure of the pump motor Ml might be caused by a failure of its stator or rotor windings, cables or such like. This would make sense if the motor itself were the object of the fault tree analysis. In practice the degree of decomposition (degree of detail) is determined by the boundaries (deUmitation) of the reliability data for describing component behaviour, which are needed for quantifying a fault tree. 

The inverse bond graph is obtained from the direct bond graph (Fig. 4.11) by replacing each of the two effort sources representing the voltage source and the external moment by a flow source-effort sensor, SS, as depicted in Fig. 4.24. The source-sensor elements lead to differential causality at the ports of the two I elements accounting for the self-inductance La of the rotor winding and the mechanical inertia Jm of rotor and load. That is, the inverse model has no states. Hence, the denominator of all transfer functions of the inverse model is a constant. 

---