2.1.4. Inrush Current Limiting
A large current can occur on the motor supply lines when power is first applied to the drive. This
current is due to charging of the internal motor supply capacitance and can cause the circuit
breakers to open unexpectedly.
Inrush limiting devices, such as Negative Temperature Coefficient (NTC) devices, can be installed
externally to the drive to reduce this current. NTC devices can be inserted into each motor AC supply
line. Ideally these devices are bypassed using a contactor shortly after power has been applied and
the capacitors are fully charged. This helps to keep the NTC device cool so that it is effective on
future power up events and also provides maximum stiffness of the motor supply voltage.
Alternately, high surge power resistors can be used to further reduce the inrush current levels.
The figure that follows shows a typical configuration of an inrush limiting circuit implemented
external to the drive.
Figure 2-3:
Inrush Limiting Circuit External to Drive
Inrush Limiting Device (R)
The inrush current is given by the equation that follows, assuming that 3 limiters are used, one per
phase.
I
in
= V
in
· (√2/√3) / R
Where:
I
in
= peak inrush current A
V
in
= line-line input voltage (RMS)
R
= resistance of the limiting device in Ohms
If only two limiters are used, the inrush current increases by a factor of 3x in the worst case. See the
examples that follow for calculating the inrush current while using different limiting devices.
iXC6e/XC6e Hardware Manual
2.1.4. Inrush Current Limiting
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