Installation and Wiring
25
Refer to the following drawing for the path of leakage current:
R
S
T
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Inverter
Wire-to-wire distributed capacitance
Earth distributed
capacitance of the
Earth distributed
capacitance of
Motor
Earth Leakage Current
Leakage currents may leak into not only the inverter system, but also into other devices via the earth
wire. These leakage currents may cause misoperations of the residual current circuit-breaker, relay or
other devices. The higher the carrier frequency of the inverter is, the larger the leakage current is; the
longer the motor cable is, the larger the leakage current is.
Control methods include:
z
Reduction of carrier frequency (although this method may increase motor noise);
z
Reduction of the motor cable length as much as possible;
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Adoption of residual current circuit-breakers specifically designed for the inverter system and
other systems to minimize higher harmonics and surge leakage current.
Wire-to-Wire Leakage Current
Leakage currents which leak into the wire-to-wire distributed capacitance on the output side of the
inverter generate higher harmonics which may cause misoperations of external thermal relays,
especially a small-capacity inverter. If the wire is very long (50m or above), there will be a substantial
increase of leakage currents which may cause misoperations of external thermal relays. It is
recommended that a temperature transducer be used to monitor motor temperature directly, or the
inverter function of motor overload protection be adopted to substitute an external thermal relay.
Control methods include:
Reduction of carrier frequency and installation of a reactor are on the output side.
Summary of Contents for SB200 Series
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