24-07-2012, 02:43 PM
SERVO CONTROL FACTS
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TYPES OF MOTORS
The direct current (DC) motor is one of the first machines devised to convert electrical energy to
mechanical power. Its origin can be traced to machines conceived and tested by Michael Faraday,
the experimenter who formulated the fundamental concepts of electromagnetism. These concepts
basically state that if a conductor, or wire, carrying current is placed in a magnetic field, a force will
act upon it. The magnitude of this
force is a function of strength of the
magnetic field, the amount of current
passing through the conductor and
the orientation of the magnet and
conductor. The direction in which
this force will act is dependent on the
direction of current and direction of
the magnetic field.
Electric motor design is based on the
placement of conductors (wires) in a
magnetic field. A winding has many
conductors, or turns of wire, and the
contribution of each individual turn
adds to the intensity of the interaction.
The force developed from a
winding is dependent on the current
passing through the winding and the magnetic field strength. If more current is passed through the
winding, then more force (torque) is obtained. In effect, two magnetic fields interacting cause
movement: the magnetic field from the rotor and the magnetic field from the stators attract each
other. This becomes the basis of both AC and DC motor design.
AC MOTORS
Most of the world's motor business is addressed by AC motors. AC motors are relatively constant
speed devices. The speed of an AC motor is determined by the frequency of the voltage applied
(and the number of magnetic poles). There are basically two types of AC motors: induction and
synchronous.
INDUCTION MOTOR.
If the induction motor is viewed as a type of transformer, it becomes
easy to understand. By applying a voltage onto the primary of the transformer winding, a current
flow results and induces current in the secondary winding. The primary is the stator assembly and
the secondary is the rotor assembly. One magnetic field is set up in the stator and a second magnetic
field is induced in the rotor. The interaction of these two magnetic fields results in motion. The
speed of the magnetic field going around the stator will determine the speed of the rotor. The rotor
will try to follow the stator's magnetic field, but will "slip" when a load is attached. Therefore
induction motors always rotate slower than the stator's rotating field.
Typical construction of an induction motor consists of 1) a stator with laminations and turns of copper
wire and 2) a rotor, constructed of steel laminations with large slots on the periphery, stacked
together to form a "squirrel cage" rotor. Rotor slots are filled with conductive material (copper or
aluminum) and are short-circuited upon themselves by the conductive end pieces. This "one" piece
casting usually includes integral fan blades to circulate air for cooling purposes.
SYNCHRONOUS MOTOR.
The synchronous motor is basically the same as the induction
motor but with slightly different rotor construction. The rotor construction enables this type of
motor to rotate at the same speed (in synchronization) as the stator field. There are basically two
types of synchronous motors: self excited ( as the induction motor) and directly excited (as with permanent
magnets).
The self excited motor (may be called reluctance synchronous) includes a rotor with notches, or
teeth, on the periphery. The number of notches corresponds to the number of poles in the stator.
Oftentimes the notches or teeth are termed salient poles. These salient poles create an easy path for
the magnetic flux field, thus allowing the rotor to "lock in" and run at the same speed as the
rotating field.
DC MOTORS
Most of the world's adjustable speed business is addressed by DC motors. DC motor speeds can
easily be varied, therefore they are utilized in applications where speed control, servo control,
and/or positioning needs exist. The stator field is produced by either a field winding, or by permanent
magnets. This is a stationary field (as opposed to the AC stator field which is rotating). The
second field, the rotor field, is set up by passing current through a commutator and into the rotor
assembly. The rotor field rotates in an effort to align itself with the stator field, but at the appropriate
time (due to the commutator) the rotor field is switched. In this method then, the rotor field
never catches up to the stator field. Rotational speed (i.e. how fast the rotor turns) is dependent on
the strength of the rotor field. In other words, the more voltage on the motor, the faster the rotor
will turn.
OPEN LOOP/CLOSED LOOP
In a system. the controller is the device which activates motion by providing a command to do
something, i.e. start or change speed/position. This command is amplified and applied onto the
motor. Thus motion commences . . . but how is this known?
There are several assumptions which have been made. The first assumption is that power is
applied onto the motor and the second is that the motor shaft is free to rotate. If there is nothing
wrong with the system, the assumptions are fine – and indeed motion commences and the motor
rotates.
If for some reason, either the signal or power
does not get to the motor, or the motor is
somehow prevented from rotating, the
assumptions are poor and there would be no
motion.
Systems that assume motion has taken place
(or is in the process of taking place) are
termed "open loop". An open loop drive is
one in which the signal goes "in one direction
only". . . from the control to the motor.
There is no signal returning from the
motor/load to inform the control that
action/motion has occurred.