A−7
Setpoint (SP)
The
setpoint
is the desired value of a process variable of interest. See
also
Process Variable (PV)
and
PID Loop
.
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
T
hree-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
.
achometer
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.
hermal 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
.
hermistor
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.
hree-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. Therefore, 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.
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