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L100 Inverter
Appendix A
A–7
Single-phase power An AC power source consisting of Hot and Neutral wires. An Earth
Ground connection usually accompanies them. In theory, the
voltage potential on Neutral stays at or near Earth Ground, while
Hot varies sinusoidally above and below Neutral. This power source
is named Single Phase to differentiate it from three-phase power
sources. Some Hitachi inverters can accept single phase input
power, but they all output three-phase power to the motor. See also
Three-phase.
Slip The difference between the theoretical speed of a motor at no load
(determined by its inverter output waveforms) and the actual speed.
Some slip is essential in order to develop torque to the load, but too
much will cause excessive heat in the motor windings and/or cause
the motor to stall.
Squirrel Cage A “nick-name” for the appearance of the rotor frame assembly for
an AC induction motor.
Stator The windings in a motor that are stationary and coupled to the
power input of the motor. See also Rotor.
Tachometer 1. A signal generator usually attached to the motor shaft for the
purpose of providing feedback to the speed controlling device of the
motor. 2. A speed-monitoring test meter that may optically sense
shaft rotation speed and display it on a readout.
Thermal Switch An electromechanical safety device that opens to stop current flow
when the temperature at the device reaches a specific temperature
threshold. Thermal switches are sometimes installed in the motor in
order to protect the windings from heat damage. The inverter can
use thermal switch signals to trip (shut down) if the motor
overheats. See also Trip.
Thermistor A type of temperature sensor that changes its resistance according
to its temperature. The sensing range of thermistors and their
ruggedness make them ideal for motor overheating detection.
Hitachi inverters have built-in thermistor input circuits, which can
detect an overheated motor and shut off (trip) the inverter output.
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.