Glossary - Drives Terminology - GE Fuji Electric AF-300ES Instructions Manual

Section 11: Glossary - Drives Terminology
The following are standard definitions of terms that are used
when discussing adjustable frequency drives:
+BUS: +Bus is the portion of the DC bus that is at a positive
potential.
-BUS: -Bus it the portion of the DC bus that is at a negative
potential.
AC contactor: An alternating-current (AC) contactor is de-
signed for the specific purpose of establishing or interrupt-
ing an AC power circuit.
Adjustable Speed: The concept of varying the speed of a
motor, either manually or automatically. The desired oper-
ating speed (set speed) is relatively constant regardless of
load.
Adjustable Speed Drive (Electrical): The adjustable speed
drive is comprised of the motor, drive controller and
operator's controls (either manual or automatic).
Ambient Temperature: Ambient temperature is the tempera-
ture of air, water or a surrounding medium where equip-
ment is operated or stored.
Axis: A principal direction along which movement to the
tool or workpiece occurs. The term axis also refers to one
of the reference lines of a coordinate system.
Bandwidth: Generally, frequency range of system input over
which the system will respond satisfactorily to a command.
Base Speed: Base speed is the manufacturer's nameplate
rating where the motor will develop rated Hp at rated load
and voltage. With DC drives, it is commonly the point where
full armature voltage is applied with full rated field excita-
tion. With AC systems, it is commonly the point where
60Hz is applied to the induction motor.
Bias: The steady state deviation of a controlled variable from
a fixed setpoint.
Braking: Braking provides a means of stopping an AC or
DC motor and can be accomplished in several ways:
A. Dynamic Braking (AC Drives) - Dynamic braking is ac-
complished by continuing to excite the motor from the drive.
This causes a regenerative current to the drive's DC inter-
mediate bus circuit. The dynamic brake resistors are then
placed across the DC bus to dissipate the power returned.
The brake resistor is usually switched by a transistor or other
power switch controlled by the drive.
B. Regenerative Braking - Similar to dynamic braking, but
is accomplished electronically. The generated power is re-
turned to the line through the power converter. It may also
be just dissipated as losses in the converter (within its limi-
tations).
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C. Squirrel-cage Motor Dynamic Braking - "DC Injection Brak-
ing" (This form is not the same as that noted for AC drives.)
It is another form of braking which uses a control circuit that
applies a dc voltage across the ac motor's stator windings
at a set frequency point for a set duration of time. This type
of braking results in a low amount of braking torque until the
motor reaches a speed well below 50%. At a speed below
7 Hz the available braking torque increases sharply.
D. Motor Mounted or Separately Mounted Brake is a posi-
tive action, mechanical, friction device. Normal configura-
tion is such that when the power is removed, the brake set
is set. This can be used as a holding brake. (Note: A sepa-
rately Mounted Brake is one which is located on some part
of the mechanical drive train other than the motor.)
Breakaway Torque: The torque to start a machine from stand-
still. It is always greater than the torque needed to maintain
motion.
Breakdown Torque: The AC motor maximum torque which is
developed with rated voltage applied at rated frequency.
Bridge Rectifier: A full-wave rectifier that conducts current in
only one direction. AC applied to the input results in ap-
proximate DC at the output.
Bridge Rectifier (Diode): A diode rectifier is a non-controlled
full wave rectifier that produces a constant rectified DC volt-
age.
Closed Loop: Closed loop refers to a regulator circuit in which
the actual value of the controlled variable (e.g. speed) is
sensed and a signal proportional to this value (feedback sig-
nal) is compared with a signal proportional to the desired
value (reference signal). The difference between these sig-
nals (error signal) causes the actual value to change in the
direction that will reduce the difference in signals to zero.
Cogging: A condition in which a motor does not rotate
smoothly but "steps" or "jerks" from one position to another
during shaft revolution. Cogging is most pronounced at
low motor speeds and can cause objectionable vibrations
in the driven machine.
Commutation (Inverter): The process by which forward cur-
rent is interrupted or transferred from one switching device
to the other. In most circuits where power is supplied from
an AC source, turn-on control is adequate and turn-off oc-
curs naturally when the AC cycle causes the polarity across
a given device to reverse.
Constant Horsepower Range: A range of motor operation
where motor speed is greater than base rating of the motor,
in the case of the ac motor operation usually above 60Hz
where the voltage remains constant as the frequency is in-
creased.
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