Armature core of a DC machine

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In the case of DC generators –

Mechanical Input is applied to the shaft/rotor (i.e, Armature) and Electrical output is collected from Armature itself.

In the case of DC Motors –

Electrical Input is applied at the Armature terminal and mechanical output at the rotor is received(i.e, Armature).

Therefore armature is the center for electromechanical conversion. Hence, it should have high permeability.

  • Si-steel (HRGO/CRGO) is used for armature cores.
  • Armature cores are made of thin laminations(0.35mm-0.5mm).
  • The core is laminated to reduce eddy current losses (eddy current didn’t have any direction and destinations).
  • Eddy current produces heat.

Core losses in a DC machine

Eddy current loss

W_e \propto {B_m}^2 . f^2 .t^2

Hysteresis loss

W_h \propto {B_m}^x . f. V

(x = 1.5 to 2.5 =Steinmetz constant)

Note: DC machine has Eddy current in the armature (most sensitive) and poles shoes. Eddy current in yoke and pole core is negligible.

Why silicon steel is used for construction?

  • Steel has high conductivity and permeability. But high conductivity is an undesirable feature. Because it will increase eddy currents. Hence Si is added to 3.5-4% extent to steel (called Stalloy)
  • The addition of Si decreases conductivity (i.e, increases resistivity).
  • But the addition of Si reduces the mechanical strength of the armature.
  • Also, Si-steel has a Steinmetz constant value of 1.6 (for steel it was higher than 1.6).
  • So Si addition Reduces Hysteresis losses too.

As we know the yoke of a DC machine comes with a time-varying flux. Hence no eddy current is associated with yoke.

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But if a DC supply is obtained from a power electronics converter, the supply contains the same ripples which will produce time-varying flux causing eddy currents in the yoke. (but this eddy currents magnitude is very less)

Due to teeth and split on the outer periphery of the rotor, flux distribution is non-uniform. That is flux is pulsating with a high frequency which creates eddy currents (called “pulsating losses”) in the pole shoes. Therefore Laminated poles are used.

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