15-01-2013, 01:01 PM
Design fundamentals for drive systems on conveyors
Design fundamentals.pdf (Size: 564.17 KB / Downloads: 156)
Keywords
Conveyor, conveyor drive, multiple drive,
drive control system, variable voltage
variable frequency control, variable speed
drive, variable frequency drive, secondary
resistance control (SRC), squirrel cage
induction motor (SCIM), wound rotor
induction motor (WRIM), fluid coupling,
electronic soft starter, direct on line start,
breakaway torque, acceleration torque,
conveyor resistance forces, conveyor efficiency,
conveyor drive de-rating.
Synopsis
The combination of a number of factors
can result in conveyor drives being
undersized or operating in an unsuitable
manner. Some of these factors include:
The trend to reduce • drive sizes by
reducing friction coefficients
• The trend towards using squirrel
cage induction motors under variable
voltage variable frequency controls
combined without considering the
drive characteristics
• Not analysing the operation and interaction
of fluid couplings with the
motor torque curve.
• Either not allowing for break-away
resistances, or not allowing for some
components of break-away resistances
• Not allowing for load sharing in multiple
drive systems
• Not allowing for surge (transient but
sustained variations in throughput)
• Procured drive and conveyor components
being different to the design
components resulting in different resistances
and different drive characteristics
Correct conveyor drive design can be
achieved by allowing for realistic load
cases and ensuring that the drive system
selected can provide adequate torque to
overcome these resistances.
Drive control system
philosophy
A conveyor that is operating and continues
to operate safely with an alarm
condition allowing an overload to be
corrected will cause less problems
than a conveyor that is stopped as soon
as the alarm is raised. A back up trip
should occur if the alarm condition is
not resolved quickly. Two stage alarm/
trip settings give excellent control and
examples are:
• If the gearbox oil temperature reaches
a preset safe limit, it will not fail immediately
and therefore the conveyor
should not be immediately stopped.
A ten degree Celsius additional rise
should trip the drive.
• An electric motor that reaches peak
current for a few seconds will not burn
out, however if this persists and the
motor thermal rating is reached a trip
is needed. Higher end motor protection
relays (MPR) can accommodate a
detailed motor thermal limit curve to
allow for many over-current scenarios.
• The use of two stage belt wander detection/
indication is strongly preferred.
Break-away friction
Often neglected, this resistance force can
be significant, especially when analysing
relatively long, flat (horizontal) conveyors
where the resistance due to material lift
is negligible and the frictional resistances
summate to make the main resistance. It
is common industry practice to multiply
the artificial coefficient of friction (ISO
5048) by a factor of 1.1 to 1.5 to calculate
breakaway conditions.
Full chutes
Two types of philosophy apply to chute
design:
• Traditional closed chute designs
• Open profile velocity chutes (sometimes
referred to as ‘hood and spoon’
chutes or ‘soft loading’ chutes)
Many chutes are enclosed to control dust
and also to act as storage containers in
the event of an uncontrolled stop due
to power failure, or an emergency stop.
In the case where this type of chute becomes
full, or becomes blocked, it acts in
a similar fashion to a belt feeder hopper
and exerts significant resistances on the
conveyor. A conveyor which has multiple
feed chutes discharging on to it may have
a sizable proportion of its starting resistance
comprised of resistance forces due
to full chutes.
De-rating factors
Various factors need to be considered
when determining if a drive system will
be capable of overcoming the resistances
at start-up.
• Gearbox efficiency
• Fluid coupling efficiency (slip)
• Line losses from the controller to the
motor
• Losses inherent in some types of
motor - control system combinations
e.g. VVVF and electronic soft
starters
Energy considerations
There is an ever increasing requirement
to minimise energy demands plant-wide
for financial and environmental reasons.
As a result of this, the conveyor designer
is faced with the challenge of implementing
appropriate measures to achieve less
energy demanding conveyor systems.
This may be achieved through careful
and complete design considerations
coupled with good maintenance procedures
in an attempt to minimise resistance
forces within the conveyor system.
In addition to this, energy losses in
drive components (de-rating factors previously
discussed) may also be targeted
as potential energy saving areas.