Page 302 - Industrial Power Engineering and Applications Handbook

Protection of electric motors 12/281
now be vulnerable to rapid burnout as the heat will be and the maximum current Zeq = 21,
localized and rapid in one part of the rotor and may The minimum heat as in equation (12.5)
damage it without appreciably raising its overall
temperature. =(I: +I: +I, XI,)
4 Since single phasing is a condition of severe unbalance, i.e =3I:
it causes varying proportions of dangerous currents
in the motor windings, as discussed below. and the minimum current Ieq = &Ir. See Figure
12.8(b).
Heat generated in a star-connected stator winding
Heat generated in a delta-connected
In the event of single phasing, in star-connected windings, stator winding
one of the phases of each positive and negative sequence
components will counter-balance each other, to produce In delta-connected windings we cannot derive the phase
zero current in the open phase (Figure 12.8(a)). The currents during single phasing by the above simple
magnitude of these components in the two healthy phases hypotheses. Now two of the phase windings are connected
are equal, i.e. I, = I", and the maximum heat of the stator in series, while the third forms the other path of the
as in equation (1 2.4) parallel circuit, as illustrated in Figure 12.9. The currents
in the immobilized phase as well as the healthy supply
=(I,? +I: +2xI, XI,) lines now vary disproportionately with load, the increase
i.e = 41; in the lone winding being more pronounced. A general
idea of the magnitude of phase and line currents so
produced is given in the curves in Figure 12.10 by
measuring these currents by creating a single-phasing
condition.

Heat generated in the rotor
Referring to equation (12.9) the rotor heat can be expressed
by
0~ (I; + 31; )
In single phasing, I, = Im, the rotor heat will be
= (I; + 31;)
/ Y or = 41;
k = 1,
(a) Winding diagram
R Y B

F1-3
+ve
Line
opened

OCR i

L
Motor winding in delta
I, = I,, lb = 0
Figure 12.8 Stator and line currents on single phasing for a Figure 12.9 Stator and line currents on single phasing for a
star-connected winding delta-connected winding