a) If the output peak voltage is too high even when the motor cable length is shorter than the
maximum recommended cable length for the inverter capacity:
- use a motor with a high insulation rating.
- install an output circuit filter (micro surge filter).
- install a dv/dt filter, or a sine wave filter.
b) If the cable length is too long:
- use thicker cables to prevent voltage drop.
[Voltage Drop (V) = [√3 X cable resistance (mΩ/m) X cable length (m) X current (A)] / 1000]
- do not use 3-core cables.
- use a lower carrier frequency.
4.7.2
Protective Measures for the Inverter and the Motor
The inverter output voltage pulse, regardless of the actual output frequency, is identical to the
DC link voltage pulse, which has a very short rising time. When the power is transmitted
through the output cables, the output peak voltage can rise up to twice the total DC link
voltage (2.8 times the main power voltage).
If a switching device (a magnetic contactor or relay) is connected to the output side of the
inverter, high-voltage surges may result whenever a switch is made, regardless of the length of
the motor cable.
Such high-voltage surges can damage the inverter’s output components (such as the current
sensor), motor cables, and the motor itself. To protect the inverter and the motor from such
damage caused by a high-voltage surge, do not install switching devices in the output side of
the inverter. You can install an output reactor, dv/dt filter, or sine wave filter to protect the
inverter and motor from a surge voltage.
An output surge with a high switching frequency and fast rising time causes a motor shaft
current that runs through the motor bearing. It slowly corrodes the surface of the motor
bearing, eventually seizing up the motor.
To decrease the motor shaft current and protect the motor insulation, refer to <Table 1)
Maximum cable length by inverter capacity>. Install a dv/dt filter or sine wave filter if possible,
regardless of the length of the motor cable.
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