Helix deltaT Dynamic Analysis  Starting Conveyors
Helix deltaT version 6 has a powerful capability to
design conveyors which are curved in the horizontal and vertical plane.
Helix deltaT allows the designer to control the
starting of a conveyor by means of
 Torque Speed Control  Starting
 Speed Time Control  Starting
 Constant Torque Brake  Stopping
 Speed Time Curve Control  Stopping
All of the above controls can be programmed in by
entering data in the
Starters Database
and the using these curves to control the conveyor during the dynamic analysis.
Torque Speed Control
Torque control means that the Torque, expressed as a %
of Full Load Torque is the controlled parameter at the Drive. This means that
the Driving Peripheral force on the drive pulley is controlled and the magnitude
of the force depends on the actual pulley speed at each time step expressed as a
% of Full Load Speed.
Typical Starting Torque Curve for an Induction motor
The deltaT program allows you to model each Drive's
Starting Torque vs Speed characteristics. The method used is a tabular
description of the % of Full Load Torque vs the % of Full load speed. All you
need to do is enter the Torque % at the relevant % Speed values and the program
will draw the curves for you and then use regression methods to get the actual
values of Torque to apply during the calculation process.
Speed Control and Regenerative Drives
The Full Load Speed and Torque and is reached when 100% Speed is reached. Note
that if the conveyor tries to run at speeds above the Full load speed of the
motor, the available torque from the drive drops off rapidly, and the Conveyor
load will then tend to bring to reduce the speed. Above the asynchronous speed
the torque is reversed ie it becomes negative and the motor acts as brake.
This overspeed braking effect is very important in Conveyor operation and
Dynamic Analysis, as it helps to control the drive speed and keep it from
overspeeding. For regenerative conveyors, the motor operates in the band above
the 100% FL speed range and thus acts as a brake.
Load Torque vs Drive Torque
During the Dynamic Analysis calculations, the Torque
supplied by the drive is applied to the drive pulley. When the Drive starts, the
Starting Torque is applied and as the drive pulley accelerates, the Torque %
along the curve is progressively applied until the pulley reaches 100% of Full
Load speed. If the drive pulley is pushed over the full load speed by a Tension
wave, the Torque reduces to below the Load Torque and the drive slows down.
Eventually equilibrium is reached where the Drive Torque equals the Load Torque.
Load Sharing between Drives
If two or more induction motors are installed on a
conveyor drive (or multiple drive pulleys) the motors will almost certainly have
slightly different torque speed characteristics. If we examine two motors'
torque speed curves close to the full load speed we may have something like the
curve shown below:
The above equilibrium explains why Squirrel Cage
electric motors automatically load share. If one Drive takes less than its fair
share of load, the other drives takes more share. This causes the second drive
to slow down, and as it slows down, the first drive will automatically take more
load.
Wound Rotor or Slip Ring Motor
Slip ring motors or wound rotor motors are a variation
on the standard cage induction motors. The slip ring motor has a set of windings
on the rotor which are not short circuited, but are terminated to a set of slip
rings for connection to external resistors and contactors. The slip ring motor
enables the starting characteristics of the motor to be totally controlled and
modified to suit the load. As the motor accelerates, the value of the rotor
resistance can be reduced altering the start torque curve in a manner such that
the maximum torque is gradually moved towards synchronous speed. This results in
a step controlled starting torque from zero speed to full speed at a relatively
low starting current. The sliprings and brush assemblies need regular
maintenance which is a cost not applicable to the standard cage motor.
Typical Torque Speed Curve for a Slip Ring Wound Rotor
Motor
The above graph shows the motor performance when the
rotor resistance is varied. The resistors can be switched on fixed time steps or
on reaching a % speed setting. Torque is drawn as a PU (Per Unit) basis above
graph and is shown and input as % of full load torque in Helix deltaT.
Wound Rotor Motor Speed Torque Curve Input into
DeltaT
Note the speed curve is input for speeds above 100% to
simulate the negative torque the motor will develop if pushed above 100% speed.
DeltaT applies the calculated torque at each time step to the Drive pulley
according to the relationship shown in the Torque speed Curve. This means that
the program can model any Torque Speed relationship you wish.
WR Switched Resistances for an Empty Conveyor
For an empty conveyor the torque speed may look
something like the one below. Because the load is say only 25% to 30% of the
motor FLT the conveyor will accelerate to a large degree on the first or second
resistance step with the remaining acceleration occurring at closely spaced
intervals until the normal run resistance is finally connected. We still need to
show the negative torque curve beyond the 100% speed mark as this controls the
conveyor belt speed, both in practice on actual conveyors and in the Helix
Dynamic analysis calculations.
Fluid Coupling Torque Control
A Fluid Coupling is a device consisting of an impeller
and a runner where the impeller is driven by the motor and torque is transmitted
to the runner by fluid between the impeller and runner. This allows the motor to
start freely and as fluid is drawn into the impeller / runner interface, the
torque on the output shaft of the fluid coupling increase gradually until it is
sufficient to move the conveyor and accelerate it. To use a Fluid Coupling Start
in Helix deltaT, merely enter the output shaft Torque Speed curve for the fluid
coupling as a dataset in the Torque Speed curve table. The program will then use
whatever shape of curve you specify.
Cross sectional drawing of a soft start fluid coupling
and some typical Torque speed curves  Drawing and Graphs Courtesy Voith
Transmissions.
Fluid Coupling curve entered into Helix deltaT
Speed Time Control
The second method of starting control is known as
Speed Control or a Velocity Ramp control. This method of control does not
specify the amount of Torque applied to the Drive pulley. It specifies a pulley
Speed at each time step during acceleration and sufficient Torque is applied in
order to maintain the specified speed. This method of starting is usually
provided by electronic solid state Variable Speed Drives which control the motor
speed accurately to with fractions of a percent of Full Load Speed.
A typical linear Velocity Ramp
In the above starting speed ramp the speed increases linearly with time with a
dwell time of 5 seconds when speed reaches 5% of speed. In this case the starter
type is selected as Speed Time and the % Speed and Time is seconds are input
into the starter database..
S curve Acceleration Ramps
Notably A. Harrison and L. Nordell have proposed various 'S' curve acceleration
ramps. Both of these starting methods can be simulated in deltaT. Refer to the
papers on these subjects in the References section for more details.
Cycloidal Front S curve  Harrison Model
This form of S curve was first proposed by Dr Alex
Harrison and it is called a cycloidal front characteristic.
The cycloidal front curve is derived from
The maximum acceleration is a = V/2 when t = T/2
S curve  Nordell Model
This form of S curve was first proposed by Nordell. It
takes the form
This S curve is obtained as follows:
Nordell's model has a higher acceleration than Harrison's but a lower Jerk
(first derivative of acceleration)
In deltaT, you are free to use any Velocity ramp you
wish  merely type in the speed time values and the program will do the rest.
You can also derive your own relationships using a spreadsheet program such as
Excel and then paste the values into deltaT.
Hermite Cubic Spline Curve Starter
This shape of starting ramp allows a very soft start
of the conveyor especially if a long time is used and it is also useful for
conveyors with head and tail or tripper drives which are far apart. It allows a
delay time to be built into the curve so that acceleration of the tail drives
can be matched to the head drives. It also allows a dwell period to be added
which allows the drive to start and ramp up to say 5% speed and it then holds
the speed at this level for the dwell time before accelerating along the spline
curve
The curve below has 20 second delay time, it then
ramps up to 5% speed, holds it at this speed for a 20 second dwell time and then
accelerates along a cubic spline S curve, reaching full speed after 245 seconds.
Refer to Hermite Cubic Spline
link for more details.
Aborted Start Torque Speed Curve
You can model an aborted start by truncating the Drive Torque vs Speed curve.
For example, if the start is aborted at 85% of Full load Speed the following
(simplified) Torque speed curve could be used to model the conveyor.
With the above curve, the drive will accelerate and at a speed equal to 85% of
the Full Load Speed of the conveyor the drive torque will be cut to zero. By
observing the tension graphs the belt tensions can be determined for the aborted
start.
Braking
The Helix deltaT program allows you to program a
Speed Time graph to apply to a braking stop. The principle is the same as the
Speed time starting method except that it is applied when the conveyor is
stopping. The full speed of the conveyor is taken as 100% speed and the brake
pulley will follow the Speed Time you curve you input down to zero % speed. A
sample is shown below.
VVVF and Variable Speed Starters
Variable Voltage Variable Frequency starters (VVVF)
are basically electronic controllers which can control induction motor speed and
torque by varying the electrical supply to the motor. They are also called
Variable Speed Drives or VSD's. These starter can be programmed so that they
will start a conveyor motor and force it follow a Speed Time curve such as the
ones detailed above. They have speed loop feedback from the motor and control
the motor speed to follow the programmed ramp by varying the torque the motor is
developing. If the motor speed is falling behind the curve the torque is
increased, if it is getting ahead of the curve the torque is decreased. This
forces the motor to follow the programmed Speed Time curve. It is interesting to
note the even though we program it to Speed Time parameters, it is still
actually a torque control start.
These VVVF Drives can also be used to control the
stopping of a conveyor by ramping down the torque in a controlled manner to
follow a Speed Time curve such as the one shown under the Braking heading above.
To to program this type of stopping use a Brake or Drive pulley with an S curve
as shown above.
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Copyright © Helix Technologies  27 July
2016
