GE 269Plus

GE Power Management 269Plus Motor Management Relay E- 1
E ASYMMETRICAL STARTING CURRENT DESCRIPTION
E
APPENDIX EE ASYMMETRICAL STARTING CURRENT E.1 DESCRIPTION
It is commonly known that current lags voltage by 90° when a voltage is applied to a purely inductive load. As
can be seen from the figure below, if the AC voltage is applied at a peak, the current will rise from 0 A to its
peak, 90° later in time. It may also be seen that during the time voltage completes a positive or negative half-
cycle, current has made the transition from one peak to another.
Figure E–1: VOLTAGE & CURRENT ON AN INDUCTIVE LOAD
Thus, as shown below, if voltage is applied at a zero crossing, current will make the transition from minimum
peak to maximum peak. Current of course, cannot instantaneously be at its minimum value, it must begin at
zero.
Thus it rises from zero to a value that is equal to 2 times the peak value (2 ×
I
max
).
Figure E–2: APPLYING VOLTAGE AT A ZERO CROSSING ON AN INDUCTIVE LOAD
Depending on when the voltage is applied, the RMS current may vary by as much as 1.73 times.
where
I
RMS
is current when voltage is applied at a maximum, or the symmetrical current.
I
RMSasymm
DC
2
AC
2
+ 2 I
RMS
2
I
RMS
2
+==
I
RMSasymm
2
⇒ 2 I
RMS
2
()
2
I
RMS
2
+ 3 I
RMS
2
==
I
RMSasymm
⇒ 3 I
RMS
=