E-
2
269Plus Motor Management Relay GE Power Management
DESCRIPTION E ASYMMETRICAL STARTING CURRENT
E
A motor or a transformer is never a perfect inductor, therefore, the value of 1.73 will never be reached. The DC
offset will die away as a function of the
X
/
R
ratio (typically a few cycles). Figure 3 represents an exaggeration
of the three phase current of a motor starting.
Figure E–3: MOTOR STARTING THREE PHASE CURRENT
When is this 'asymmetrical current' a concern?
When setting instantaneous relays, care must be taken to ensure that the instantaneous element does not
operate during normal operating conditions such as a motor start. Symptoms of an instantaneous element that
is set too sensitive are nuisance or intermittent tripping of the relay during energizing of the system.
Furthermore, CTs do not react predictably when a DC current is applied. The waveforms shown in above are
not necessarily the waveforms that each of three phase CTs would output. If there is a residual connection for
ground fault detection, that element could operate when asymmetrical currents are present.
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