The flexibility of electric drives makes them an ideal choice for
unwinding or rewinding duties but there are a number of important points
to note before deciding which electric drive system is right for a
centre-driven coiling application. Peter Worland reports
Speed control systems are only practical on web or filament unwinding and
rewinding applications where direct feed back of material tension from a
dancing roll mechanism is available. Load cell tension feedback is not
normally practical for speed controlled winders as no material storage is
provided to allow for control system span. Torque control systems can be
used with or without direct tension feedback, the latter being derived
from either a dancer mechanism or load cell. Torque control solutions are
normally adopted when the material to be wound is non-extensible; paper,
steel and non-ferrous metals are common examples.
However, care should be exercised where no tension feedback is to be
used. Low-tension applications, where tension powers are comparable to
transmission losses, should not be attempted without some form of direct
tension feedback. Where tension powers are high compared to transmission
losses, simple predictive torque control systems with no overriding
tension feedback are satisfactory. Extensible materials such as certain
types of plastic and polyester films, or machine configurations where
non-extensible material is drawn from a catenary (looping pit), should
normally be approached with a speed control solution in mind. Some
materials require the tension to be reduced as the rewound diameter
increases. This so-called taper tension can be implemented on open or
closed loop torque controlled systems using load cell measurement, quite
simply by modifying the system tension set point as the diameter
increases.
Field weakening
The decision concerning the use of field weakening depends to a large
extent on the power requirements of the application. Modern dc motors can
normally only be operated over 3:1 or 3.5:1 range of diameter by field
weakening; any larger ratio must be provided by the constant torque range
of the motor and the converter oversized accordingly. Some older motors
encountered on refits may have 4:1 or even 5:1 by field weakening.
Constant power solutions using field weakening on ac systems are also
feasible.
However, a limit of 2.5:1 by field weakening is suggested as the absolute
maximum constant torque, with over-sizing of the converter for diameter
ranges above this limit. During line speed changes energy must be
supplied to, or removed from the rotating masses of the rewind mechanism.
The amount of energy transferred depends upon the inertia of the total
system and the rate of change of speed. An estimate of acceleration
torque referred to tension torque at various diameters will give an
indication of the degree of tension disturbance to be expected during
speed changes. Systems with low inertia or slow rates of acceleration,
where the acceleration torque is small compared to the tension torque
will need very little or no compensation.
High speed winders with rapid acceleration and high inertia, where
acceleration torque can be equal to, or greater than the tension torque
will obviously need precise inertia compensation. Deriving rate of change
signals for use in the inertia compensation calculation can be
troublesome. The most satisfactory system is to use an S-Ramp to set the
acceleration characteristic of the line speed controller. The S-Ramp
should be configured to provide not only a speed reference value but also
an acceleration rate signal, which can be used in conjunction with the
inertia values to produce a torque feed forward for the winder drive to
compensate for inertia effects. Transmission systems should be as loss
free as possible. T
here are two types: basic stiction loss,