INDICADORES FINANCIEROS

The damper windings of synchronous machines are usually short-circuit windings, which in nonsalient-pole machines are contained in the same slots with magnetizing windings, and in salient-pole machines in particular, in the slots at the surfaces of pole shoes. There are no bars in the damper windings on the quadrature axes of salient-pole machines, and only the short-circuit rings encircle the machine. The resistances and inductances of the damper wind-ing of the rotor are thus quite different in the d- and q-directions. In a salient-pole machine constructed of solid steel, the material of the rotor core itself may suffice as a damper wind-ing. In that case, asynchronous operation resembles the operation of a solid-rotor induction machine. Figure 2.60 illustrates the damper winding of a salient-pole synchronous machine.

copper plate

copper plate damper bar

connector

d-axis q-axis

Figure 2.60 Structure of the damper winding of a six-pole salient-pole synchronous machine. The copper end plates are connected with a suitable copper connector to form a ring for the damper currents.

Sometimes real rings also connect the damper bars

Damper windings improve the performance characteristics of synchronous machines es-pecially during transients. As in asynchronous machines, thanks to damper windings, syn-chronous machines can in principle be started direct-on-line. Also a stationary asynsyn-chronous drive is in some cases a possible choice. Especially in single-phase synchronous machines and in the unbalanced load situations of three-phase machines, the function of damper wind-ings is to damp the counter-rotating fields of the air gap which otherwise cause great losses.

In particular, the function of damper windings is to damp the fluctuation of the rotation speed of a synchronous machine when rotating loads with pulsating torques, such as piston compressors.

The effective mechanisms of damper windings are relatively complicated and diverse, and therefore their mathematically accurate design is difficult. That is why damper windings are usually constructed by drawing upon empirical knowledge. However, the inductances and resistances of the selected winding can usually be evaluated with normal methods to define the time constants of the winding.

When the damper windings of salient-pole machines are placed in the slots, the slot pitch has to be selected to diverge by 10–15% from the slot pitch of the stator to avoid pulsation of the flux and noise. If the slots are skewed (usually by the amount of a single stator slot pitch), the same slot pitch can be selected both for the stator and the rotor. Damper winding comes into effect only when the bars of the winding are connected with short-circuit rings. If the pole shoes are solid, they may, similar to the solid rotor of a nonsalient-pole machine, act as a damper winding as long as the ends of the pole shoes are connected with durable short-circuit rings. In nonsalient-pole machines, an individual damper winding is seldom used; however, conductors may be mounted under slot wedges, or the slot wedges themselves are used as the bars of the damper winding.

In synchronous generators, the function of damper windings is for instance to damp counter-rotating fields. To minimize losses, the resistance is kept to a minimum in damper windings. The sectional area of the damper bars is selected to be 20–30% of the cross-sectional bar area of the armature winding. The windings are made of copper. In single-phase generators, damper bar cross-sectional areas larger than 30% of the stator copper area are employed. The frequency of the voltages induced by counter-rotating fields to the damper bars is doubled when compared with the network frequency. Therefore, it has to be con-sidered whether special actions are required with respect to the skin effect of the damper windings (e.g. utilization of Roebel bars (braided conductors) to avoid the skin effect). The cross-sectional area of the short-circuit rings is selected to be approximately 30–50% of the cross-sectional area of the damper bars per pole.

The damper bars have to damp the fluctuations of the rotation speed caused by the pulsating torque loads. They also have to guarantee a good starting torque when the machine is start-ing as an asynchronous machine. Thus, brass bars or small-diameter copper damper bars are employed to increase the rotor resistance. The cross-sectional area of copper bars is typically only 10% of the cross-sectional area of the copper of the armature winding.

In PMSMs, in axial flux machines in particular, the damper winding may be easily con-structed by placing a suitable copper or aluminium plate on the surface of the rotor, on top of the magnets. However, achieving a total conducting surface in the range of 20–30% of the stator copper surface may be somewhat difficult because the plate thickness easily increases to become too large and limits the air-gap flux density created by the magnets.

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