26-05-2012, 04:55 PM
APPLICATION OF BRUSHLESS DC DRIVES IN BLOW MOLDING
APPLICATION OF BRUSHLESS DC.pdf (Size: 105.06 KB / Downloads: 97)
Introduction
The first variable speed drives were certainly mechanical and were based on
adjustable pitch diameter pulleys. Such systems are still in use but for obvious
reasons are not in general use in industrial applications today. There are four basic
types of electrically adjustable speed drives being installed in today's modern
industrial machines:
History of Brush type DC Drives
The brush DC motor was invented in 1856 by Werner Von Siemens in
Germany. Variable speed by armature voltage control was first used in the early
1930s using a system involving a constant speed AC motor driving a D.C.
generator. The generator's DC output was varied using a rheostat to vary the field
excitation and the resulting variable voltage DC was used to power the armature
circuit of another DC machine used as a motor. This system was called a Ward-
Leonard system after the two people credited with it's development. The Ward-
Leonard method of DC variable speed control continued until the late 1960s when
Electric Regulator Company brought to market a practical, general purpose, static,
solid state controller that converted the AC line directly to rectified DC using SCR
(thyristor) devices. That technology was adopted by virtually all manufacturers
and still is in use today. It is a very simple power control concept and uses the
fewest number of parts possible to produce variable speed from an electric motor.
Characteristics of DC drives
The DC motor works on the principle that speed of the shaft is a direct
function of the applied armature voltage. To a lesser extent, field control can be
used for speed control but it is not widely used except for winders and constant HP
applications and will not be discussed here. At zero volts applied to the armature,
the motor will run at zero speed, while at rated voltage (500 vdc for most industrial
motors over a few hp), the motor will run at rated speed (1750 rpm has developed
as a "standard"). The motor will produce torque based on a similar relationship
with current. The torque produced by a DC motor will vary directly with armature
current. These two simple characteristics make the DC motor continue to be the
most popular means of variable speed control in use today for constant torque
industrial applications.
Blowmolding Applications Advantages
In general, the characteristics of Brushless D.C. that are most advantageous to the
blow molding process are:
• Very precise average speed control over a very wide speed range.
• Precise instantaneous speed control due to high dynamic response
• Constant power factor means lowest possible input current.
• Small physical size of motor compared to brush type.
• No recurring motor maintenance (brush replacement)
• Feedback device (encoder) is inside the motor not outside.
• Higher efficiency overall.
The blow molding process always involves an extruder of some kind and the
variable speed drive on the extruder has to provide an output sufficient to allow the
parison to be formed in time to meet the cycle requirements. Since the final
product may require more or less volume of plastic for different shapes and
because the cycle time varies, hence the requirement for variable speed. The result
of inconsistent speed control is simply that more (or less !) material than is
necessary to make the part will be extruded.