With electric drives, electrical energy is converted into mechanical
kinetic energy. Inside the electric motor, magnetic fields in the stator and
rotor interact; torque is created when they try to align themselves with each
other, creating movement. The "precise hands" of electric drives and motors are
ideal for offering high speed and precise accuracy.
In dc motors, a dc winding or a
permanent magnet generates a fixed magnetic
field in the stator. To deliver
maximum torque, coils are wound onto the rotor and connected so that the rotor
field is perpendicular to the stator field. Speed and power in dc motors can be
controlled very effectively, but the carbon brushes that are required to switch
the current in this type of motor are subject to wear and tear.
A synchronous motor features a
three-phase winding configured in a circle. Three phase-shifted currents
generate a rotating magnetic field. Since the rotor has a fixed magnetic field,
it can only develop effective torque at a synchronous speed. With modern
current converters, synchronous motors can be controlled as precisely as dc
motors, but without any wearing parts. Synchronous motors have an excellent
operating efficiency, above 90 percent, but require a complex electronic
regulation system and costly permanent magnets.
Asynchronous motors also
feature stators that generate a rotating field, but with a squirrel-cage
winding. When the rotor is not following synchronously, a current is induced
that counteracts the change in the magnetic field. Together with the magnetic
field of the stator, this generates a torque that pulls the rotor along. The
advantage is that asynchronous motors are less expensive, but because the
current flowing through the rotor generates heat, the motor has a low
efficiency level.
Hydraulic Drives
With
hydraulic drives, fluid is pressurized to move a piston inside a cylinder. A
pump supplies the volume flow. Depending on the force required to move a load,
the corresponding pressure is developed in the fluid and the pump counteracts
this pressure. In rotating drives, a
hydraulic motor delivers torque
instead of linear force.
Hydrostatic drives, which
feature adjustable pumps that push pistons or turn hydraulic motors, are
extremely efficient with years of development history. However, the speed of
the system varies when the external
forces change and there is no easy way
to maintain a position once it has been attained. Consequently, the distance
between the pump and the cylinder needs to be as short as possible.
In secondary control drives,
the motor, not the pump, is regulated. The varied torque enables the hydraulic
drive to adapt quickly to changes in force. This highly efficient technology
allows for the regulation of speed, torque and position. However, the drives
are expensive to build and the rapid adjustability is only required by users
with special machine needs, like test bed manufacturers.
Drives with valve control are
common in hydraulic systems for mobile equipment. Only a portion of the
hydraulic energy generated is applied, via a valve. The unused portion is
converted into heat. Valve control drives are extremely accurate with excellent
control but lower efficiency. This can be improved with load-sensing
technology, so that only the pressure that is needed is actually provided.
Pneumatic Drives
In
pneumatics, air is compressed and the stored energy is converted to mechanical
energy in cylinders, motors or other units. However, using compressed air in
industrial applications is only cost-effective when low forces are required.
The "nimble fingers" of pneumatics are
used when small masses need to be
moved at high speeds across short distances, like in clamping, transporting,
screw-tightening and in other industrial, trade or medical tasks.
Pneumatic drive systems include
three subsystems: air compression and processing; control (via valves); and
output drive (a cylinder or motor). These components require little maintenance
during operation and offer long service lives. Compressed air, which is readily
available, poses no fire or explosion hazards. However, producing and preparing
compressed air can be expensive and the noise of air exhaust may have to be
muffled. An advantage is that compressed air is insensitive to temperature
variations. If leaks occur, they have no effect on machine safety and do not
contaminate the surroundings. The speed and force of the actuators can be
controlled simply and continuously over a broad range, but it may be difficult
to achieve constant and uniform piston speeds.
Amy
DeFayette, is technical marketing manager for Bosch
Rexroth Corp.
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