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Appendix A
A–8
Three-phase power An AC power source with three Hot connections that have phase
offsets of 120 degrees is a 3-phase power source. Usually, Neutral
and Earth Ground wires accompany the three Hot connections.
Loads may be configured in a delta or Y configuration. A Y-
connected load such as an AC induction motor will be a balanced
load; the currents in all the Hot connections are the same. There-
fore, the Neutral connection is theoretically zero. This is why
inverters that generate 3-phase power for motors do not generally
have a Neutral connection to the motor. However, the Earth Ground
connection is important for safety reasons, and is provided.
Torque The rotational force exerted by a motor shaft. The units of measure-
ment consist of the distance (radius from shaft center axis) and
force (weight) applied at that distance. Units are usually given as
pound-feet, ounce-inches, or Newton-meters.
Transistor A solid state, three-terminal device that provides amplification of
signals and can be used for switching and control. While transistors
have a linear operating range, inverters use them as high-powered
switches. Recent developments in power semiconductors have
produced transistors capable of handling high voltages and currents,
all with high reliability. The saturation voltage has been decreasing,
resulting in less heat dissipation. Hitachi inverters use state-of-the-
art semiconductors to provide high performance and reliability in a
compact package. See also IGBT and Saturation Voltage.
Trip Event An event that causes the inverter to stop operation is called a “trip”
event (as in tripping a circuit breaker). The inverter keeps a history
log of trip events. They also require an action to clear.
Watt Loss A measure of the internal power loss of a component, the difference
between the power it consumes and what its output delivers. An
inverter’s watt loss is the input power minus the power delivered to
the motor. The watt loss is typically highest when an inverter is
delivering its maximum output. Therefore, watt loss is usually
specified for a particular output level. Inverter watt loss specifica-
tions are important when designing enclosures.
Bibliography
Title Author and Publisher
Variable Speed Drive Fundamentals, 2nd Ed. Phipps, Clarence A.
The Fairmont Press, Inc. / Prentice-Hall, Inc. 1997
ISBN 0-13-636390-3
Electronic Variable Speed Drives Brumbach, Michael E.
Delmar Publishers 1997
ISBN 0-8273-6937-9
Hitachi Inverter Technical Guide Book Published by Hitachi, Ltd. Japan 1995
Publication SIG-E002