Standard_Drives_Guide

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Standard Drives Buyers Guide 121
Prices and data subject
to change without notice
PLC (Programmable Logic Controller): Solid-state control logic
for machines and processes where a sequence of operations can
be changed easily with programming (software).
Plugging: A type of motor braking provided by reversing either
line voltage polarity or phase sequence so that the motor devel-
ops a counter torque which exerts a retarding force to brake the
motor.
Position Transducer: An electronic device (ex. encoder or
resolver) that measures actual position and converts this meas-
urement into a feedback signal convenient for transmission. This
signal may then be used as an input to a programmable con-
troller which controls the parameters of the positioning system.
Positive Feedback: A condition where the feedback is additive to
the input signal. This generally results in an unstable system.
Power: Work done per unit of time. Measured in horsepower or
watts: 1 hp = 33,000 ft/lbs = 746 watts.
Power Factor: A measurement of the time phase difference
between the voltage and current in an ac circuit. It is represented
by the cosine of the angle of this phase difference. Power factor is
the ratio of real power (kW) to total kVA, or the ratio of actual
power (W) to apparent power (VA).
Displacement Power Factor - The displacement component
of power factor. The ratio of the active power of the funda-
mental wave, in watts, to the apparent power of the
fundamental wave, in volt-amperes.
Total Power Factor - The ratio of the total power input, in
watts, to the total volt-ampere input to the converter. The
power factor is determined at the ac line terminals of the
converter.
Distortion Factor - The ratio of the root-mean square of
the harmonic content to the root-mean-square value of
the fundamental quantity, expressed as a percent of
the fundamental.
Preset Speed: One or more fixed speeds at which the drive will
operate.
Pull-Up Torque: The torque required to accelerate the load from
standstill to full speed (where breakdown torque occurs),
expressed in percent of running torque. It is the torque required
not only to overcome friction, windage, and product loading, but
also to overcome the inertia of the machine. The torque required
by a machine may not be constant after the machine has started
to turn. This load type is characteristic of fans, centrifugal pumps,
and certain machine tools.
PWM (Pulse Width Modulated): A type of ac adjustable frequen-
cy drive that accomplishes frequency and voltage control at the
output section (inverter) of the drive. The drive’s output voltage is
always a constant amplitude and by “chopping” (pulse width
modulating) the average, voltage is controlled.
Reactance: Any force that opposes changes in current or voltage.
The inertia of electrons causes them to oppose sudden changes
in current flow or voltage.
Rectifier: A device that transforms alternating current to direct
current.
Regeneration: The characteristic of a motor to act as a generator
when the rotor synchronous frequency is greater than the applied
frequency.
Regenerative Braking: The technique of slowing or stopping a
drive by regeneration.
Regenerative Control: A regenerative drive contains the inherent
capability and/or power semiconductors to control the flow of
power to and from the motor.
Regulation: The ability of a control system to hold a speed once
it has been set. Regulation is given in percentages of either base
speed or set speed. Regulation is rated upon two separate sets
of conditions:
A. Load regulation (speed regulation) is the percentage of
speed change with a defined change in load. Assuming all
other parameters to be constant.
B. Line regulation is the percentage of speed change with
a given line voltage change, assuming all other parameters
to be constant.
Resolution: The smallest distinguishable increment into which a
quantity can be divided (ex. position or shaft speed). It is also the
degree to which nearly equal values of a quantity can be discrim-
inated. For encoders, it is the number of unique electrically
identified positions occurring in 360 degrees of input shaft
rotation.
SCR (Silicon Control Rectifier): A solid-state device that has an
anode, a cathode, and gate which controls when it allows
conduction.
Service Factor: When used on a motor nameplate, a number
that indicates how much above the nameplate rating a motor
can be loaded without causing serious degradation (ex. A motor
with 1.15 S.F. can produce 15% greater torque than one with 1.0
S.F.). When used in applying motors or gearmotors, it is a figure of
merit which is used to adjust measured loads in an attempt to
compensate for conditions which are difficult to measure or
define.
Set Speed: The desired operating speed.
Shock Load: The load seen by a clutch, brake, or motor in a
system which transmits high peak loads. This type of load is
present in crushers, separators, grinders, conveyors, winches,
and cranes.
Slewing: An incremental motion of the motor shaft or machine
table from one position to another at maximum speed without
losing position control.
Slip: The difference between rotating magnetic field speed (syn-
chronous speed) and rotor speed of ac induction motors. Usually
expressed as a percentage of synchronous speed.
Speed Range: The speed minimum and maximum at which a
motor must operate under constant or variable torque load con-
ditions. A 10:1 speed range for a motor with top speed of 1800
rpm means the motor must operate as low as 180 rpm and still
remain within regulation specifications. Controllers are capable of
wider controllable speed ranges than motors because there is no
thermal limitation, only electrical. Controllable speed range of a
motor is limited by the ability to deliver 100% torque below base
speed without additional cooling.
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