0195136047.pdf

596 ROTATING MACHINES

(a)

If

Ia

+

−

+−Vf

Vt

Rf Ra

Ea

(b)

If

Ia

+

−

+

+

−

+

−

Vf −

Vt

Rf Ra

Ea

Figure 13.4.3(a)Circuit representation of a dc generator under steady-state conditions,Vf=RfIfand

Vt=Ea−IaRa.(b)Circuit representation of a dc motor under steady-state conditions,Vf=RfIfand

Vt=Ea+IaRa.

are in the opposite direction. Thus, the magnetic field of the armature currents is stationary in

space in spite of the rotation of the armature.

The process of reversal of currents in the coil is known ascommutation. The current changes

from+Ito−Iin timet. Ideally, the current in the coils being commutated should reverse

linearly with time, as shown in Figure 13.4.4. Serious departure fromlinear commutationresults

in sparking at the brushes. Means for achieving sparkless commutation are touched upon later.

As shown in Figure 13.4.4, with linear commutation, the waveform of the current in any coil as

a function of time is trapezoidal.

Interpoles and Compensating Windings

The most generally used method for aiding commutation is by providing the machine withinter-

poles, also known ascommutating poles, or simply ascompoles. These are small, narrow auxiliary

poles located between the main poles and centered on the interpolar gap. The commutating

(interpole) winding is connected in series with the armature.

The demagnetizing effect of the armature mmf under pole faces can be compensated for, or

neutralized, by providing acompensating(pole-face)winding,arranged in slots in the pole face

in series with the armature, having a polarity opposite to that of the adjoining armature winding

and having the same axis as that of the armature. Because they are costly, pole-face windings

Commutation

Coil current

−I

+I

t

∆t ∆t

Figure 13.4.4Waveform of current in an armature coil undergoing linear commutation.