Page 303 - Industrial Power Engineering and Applications Handbook

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connection (see also Section 4.2.1). Generally, in
LT motors the stator and in HT motors the rotor
are more vulnerable to a fast burnout in the event
of single phasing.
2 Delta-connected stators
(a) In delta-connected windings, the lone winding X,
(Figure 12.9) for motor loads 50% and above
carries a current higher than the rated full load
current and also higher than that of the rotor, and
becomes more vulnerable to damage compared
to the rotor. This difference is more significant at
loads closer to the rated load (Figure 12.10).
(b) A study of Figure 12.10 will suggest that in the
event of single phasing, protection should be such
that it traverses the replica of the heating curve of
1 phase X .
(c) It also suggests that the heat generated in the rotor
circuit, due to voltage unbalance or single phasing,
is less than the maximum stator heat. The factor
of 6 considered in equation (1 2.6) is adequate to
take account of it.
(d) For loads of less than 50% of the rated motor
current, this protection is not required as the current
in phase X will not exceed 100% of the full load
current during a single phasing.

Corollary
% Motor current under healthy condition - Since an inter-turn fault also causes unbalance, it is
protected automatically when a negative sequence
protection is provided depending upon its sensitivity and
Figure 12.10 Current magnitudes in different phases of a the setting.
delta-connected stator winding and rotor during single phasing 3 Rotor circuit
(a) In a rotor circuit the rotor current and the heating
effects, due to single phasing, remain the same
for both star- or delta-connected stators due to
i.e. four times the normal heat of the rotor, and the rotor the same (2f- S) = 100 Hz rotor currents on a 50
current = 21, Hz system.
If single phasing occurs at 50% of the full load, (b) A stator thermal withstand curve cannot be
the rotor current will be 100%. See curve 4 of Figure considered a true reflection of the rotor thermal
12.10. conditions. In a delta-connected motor (mostly
LT motors), the stator would heat-up more rapidly
Conclusion than the rotor, and normally protection of the stator
may also be regarded as protection for the rotor.
1 Star-connected stators But This is not so in the following cases:
(a) The theoretical heats of the motor as derived earlier Prolonged starting time
for star-connected stators and rotors are almost Stalling or a locked rotor condition
the same. But even if single phasing does nut Frequent starts, and
lead to a stalled or a locked rotor condition, it All HT motors wound in star.
may cause the motor to operate at a much higher In all the above conditions, the rotor would heat-
slip due to a lower torque, T,. The rotor is therefore up much more rapidly than the stator due to its
subjected to a faster temperature rise compared low thermal time constant (r), and its smaller
to the stator due to excessive eddy current losses volume compared to that of the stator, on the one
and its smaller volume compared to that of the hand, and high-frequency eddy current losses at
stator. In single phasing, therefore, although the high slips, due to the skin effect, on the other.
stator and rotor current curves may appear similar, True motor protection will therefore require
their relative heats will be substantially different. separate protection of the rotor. Since it is not
The rotor in such motors is thus more critical and possible to monitor the rotor’s temperature, its
must be protected specifically. protection is provided through the stator only.
(b) Three-phase LT motors are built in delta connection Separate protection is therefore recommended
except for very small sizes, say, up to 1 or 2 h.p., through the stator against these conditions for large
but all HT motors are generally wound in star LT and all HT motors.