Planning the electrical installation 37
Protecting the motor insulation and bearings
The output of the drive comprises – regardless of the output frequency – pulses of
approximately 1.35 times the equivalent mains network voltage with a very short rise
time. This is true for all drives employing modern IGBT inverter technology.
The voltage of the pulses can be almost double at the motor terminals, depending
on the attenuation and reflection properties of the motor cable and the terminals.
This in turn can cause additional stress on the motor and motor cable insulation.
Modern variable speed drives with their fast rising voltage pulses and high switching
frequencies can generate current pulses that flow through the motor bearings,
which can gradually erode the bearing races and rolling elements.
The stress on motor insulation can be avoided by using optional ABB d
u
/d
t
filters.
The d
u
/d
t
filters also reduce bearing currents.
To avoid damage to motor bearings, the select and install the cables according to
the instructions in the hardware manual. In addition, insulated N-end (non-driven
end) bearings and output filters from ABB must be used according to the following
table.
Two types of filters are used individually or in combination:
• Optional d
u
/d
t
filter (protects motor insulation system and reduces bearing
currents).
• Common mode filter (mainly reduces bearing currents).
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