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Standard Drives Buyers Guide
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Glossary– Drives Terminology
Hysteresis Loss:
The resistance offered by materials to becoming
magnetized results in energy being expended and corresponding
loss. Hysteresis loss in a magnetic circuit is the energy expended
to magnetize and demagnetize the core.
IGBT (Insulated Gate Bipolar Transistor):
Semiconductor devices
used for power circuits that are capable of high power output
that operate at carrier frequencies of 20 kHz or more. IGBTs have
a faster rate of rise resulting in high di/dt that occurs when the
device is turned off.
Induction Motor:
An alternating current motor in which the pri-
mary winding on one member (usually the stator) is connected to
the power source. A secondary winding on the other member
(usually the rotor) carries the induced current. For a squirrel cage
induction motor, there is no physical electrical connection to the
secondary winding; its current is induced.
Inertia:
A measure of a body’s resistance to changes in velocity,
whether the body is at rest or moving at a constant velocity. The
velocity can be either linear or rotational. The moment of inertia
(WK
2
) is the product of the weight (W) of an object and the square
of the radius of gyration (K
2
). The radius of gyration is a measure
of how the mass of the object is distributed about the axis of
rotation. WK
2
is usually expressed in units of lb/ft
2
.
Instability:
A situation where the output of a system does not
track the input. For closed loop systems, the control system error
is either increasing or oscillating.
Intermittent Duty (INT):
A motor that never reaches equilibrium
temperature (equilibrium), but is permitted to cool down between
operations. For example, a crane, hoist, or machine tool motor is
often rated for 15 or 30 minute duty.
Interposing Relay:
A relay that accepts control signals of one
logic level in order to provide isolated contact signals in a circuit
operating a different logic level.
Inverter:
A term commonly used for an ac adjustable frequency
drive. An inverter is also used to describe a particular section of
an ac drive. This section uses the dc voltage from a previous cir-
cuit stage (intermediate dc circuit) to produce an ac current or
voltage having the desired frequency.
Isolation Transformer:
A transformer that electrically separates
the drive from the ac power line. An isolation transformer pro-
vides the following advantages:
1.
Guards against inadvertent grounding of plant power
lines through grounds in the drive.
2.
Enhances protection of semiconductors from line
voltage transients.
3.
Reduces disturbances from other solid-state control
equipment, such as drives without isolation transformers,
time clock systems, electronic counters, etc.
Jogging:
Means of accomplishing momentary motor movement
by repetitive closure of a circuit using a single push button or
contact element.
Kinetic Energy:
The energy of motion possessed by a body.
Linear Acceleration/Deceleration:
A circuit that controls the rate
at which the motor is allowed to accelerate to a set speed or
decelerate to zero speed. On most drives, this circuit is adjustable
and can be set to accommodate a particular application.
Linearity:
A measure of how closely a characteristic follows a
straight line function.
Locked-Rotor Current:
Steady-state current taken from the line
with the rotor at standstill (at rated voltage and frequency). This is
the current when starting the motor and load.
Locked-Rotor Torque:
The minimum torque that a motor will
develop at rest for all angular positions of the rotor (with rated
voltage applied at rated frequency).
Mechanical Safe Speed:
Defined by the operating limits of the
driven equipment being controlled by the adjustable frequency
drive which should not be exceeded. The manufacturer’s specifi-
cation is to be followed carefully.
Megger Test:
A test used to measure an insulation system’s
resistance. This is usually measured in megohms and tested by
passing a high voltage at low current through the motor windings
and measuring the resistance of the various insulation systems.
NEC:
The National Electric Code contains recommendations of
the National Fire Protection Association and is revised every three
years. City or state regulations may differ from code regulations
and take precedence over NEC rules.
Negative Feedback:
A condition where feedback is subtractive to
the input reference signal. Negative feedback forms the basis for
automatic systems.
NEMA:
The National Electrical Manufacturers Association is a
non-profit organization organized and supported by manufactur-
ers of electrical equipment and supplies. Some of the standards
NEMA specifies are: horsepower ratings, speeds, frame sizes and
dimensions, torques, and enclosures.
Open Loop:
A control system that lacks feedback.
Operating/Service Deviation:
A means of specifying the speed
regulating performance of a drive controller, generally in percent
of base speed. Operating deviation defines speed change due to
load change and typically assumes:
1.
A change from one steady-state load value to another
(not transient).
2.
A 95% maximum load change.
Service deviation defines speed change due to changes in ambi-
ent conditions greater than typical variations (noted in basic
adjustable frequency drive specifications).
Overload Capacity:
The ability of the drive to withstand currents
beyond the system’s continuous rating. It is normally specified as
a percentage of full-load current for a specified time period.
Overload capacity is defined by NEMA as 150% of rated full-load
current for one minute.
Overshoot:
The amount that a controlled variable exceeds
desired value after a change of input.
PCC (Point of Common Coupling):
The electrical connecting point
or interface between the utility distribution system and the user’s
electrical distribution system. The selection of the PCC within the
system is often done by the utility.