GE Multilin MM3 Motor Manager 3 Instruction Manual page 208

ASYMMETRICAL STARTING CURRENT
10–2
Depending on when the voltage is applied, the RMS current may vary by as much as 1.73
times.
2
1
0
–1
–2
Figure 10-2: Maximum Current when Voltage Applied at Zero Crossing
The asymmetrical RMS current is defined by:
I
RMSassym
Squaring both sides gives:
2
I
RMSassym
Which results in:
Where I is the current when voltage is applied at a maximum – or the symmetrical
RMS
current.
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).
The following figure represents an exaggeration of the three phase current of a motor
starting.
I
1
I
2
I
3
Figure 10-3: Motor Start Three-phase Current
When is this 'asymmetrical current' a concern?
I
max
I
max
voltage
2 2
DC AC
= +
2
2
⋅
2 I I
= +
RMS RMS
2
2
( ⋅ )
2 I I
= +
RMS RMS
2
( ⋅ )
3 I
=
RMS
I 3 I ⋅
=
RMSassym RMS
MM3 MOTOR MANAGER 3 – INSTRUCTION MANUAL
CHAPTER 10: MISCELLANEOUS
current
Eq. 10-1
(EQ 10.2)
(EQ 10.3)
time
time
time