motor basic

موضوع: motor basic الخميس فبراير 19, 2009 7:56 pm

A stepper motor is an electromechanical
device which converts electrical pulses into
discrete mechanical movements. The shaft
or spindle of a stepper motor rotates in
discrete step increments when electrical
command pulses are applied to it in the
proper sequence. The motors rotation has
several direct relationships to these applied
input pulses. The sequence of the applied
pulses is directly related to the direction of
motor shafts rotation. The speed of the
motor shafts rotation is directly related to
the frequency of the input pulses and the
length of rotation is directly related to the
number of input pulses applied.
Stepper Motor Advantages
and Disadvantages
Advantages
1. The rotation angle of the motor is
proportional to the input pulse.
2. The motor has full torque at standstill
(if the windings are energized)
3. Precise positioning and repeatability
of movement since good
stepper motors have an accuracy of
3 – 5% of a step and this error is
non cumulative from one step to
the next.
4. Excellent response to starting/
stopping/reversing.
5. Very reliable since there are no contact
brushes in the motor.
Therefore the life of the motor is
simply dependant on the life of the
bearing.
6. The motors response to digital
input pulses provides open-loop
control, making the motor simpler
and less costly to control.
7. It is possible to achieve very low
speed synchronous rotation with a
load that is directly coupled to the
shaft.
8. A wide range of rotational speeds
can be realized as the speed is
proportional to the frequency of the
input pulses.
Disadvantages
1. Resonances can occur if not
properly controlled.
2. Not easy to operate at extremely
high speeds.
Open Loop Operation
One of the most significant advantages
of a stepper motor is its ability to be
accurately controlled in an open loop
system. Open loop control means no
feedback information about position is
needed. This type of control
eliminates the need for expensive
sensing and feedback devices such as
optical encoders. Your position is
known simply by keeping track of the
input step pulses.
Stepper Motor Types
There are three basic stepper motor
types. They are :
• Variable-reluctance
• Permanent-magnet
• Hybrid
Variable-reluctance (VR)
This type of stepper motor has been
around for a long time. It is probably
the easiest to understand from a
structural point of view. Figure 1
shows a cross section of a typical V.R.
stepper motor. This type of motor
consists of a soft iron multi-toothed
rotor and a wound stator. When the
stator windings are energized with DC
current the poles become magnetized.
Rotation occurs when the rotor teeth
are attracted to the energized stator
poles.
Permanent Magnet (PM)
Often referred to as a “tin can” or
“canstock” motor the permanent
magnet step motor is a low cost and
low resolution type motor with typical
step angles of 7.5° to 15°. (48 – 24
steps/revolution) PM motors as the
Figure 1. Cross-section of a variablereluctance
(VR) motor.

Industrial Circuits Application Note [size=9]name implies have permanent
magnets added to the motor structure.
The rotor no longer has teeth as with
the VR motor. Instead the rotor is
magnetized with alternating north
and south poles situated in a straight
line parallel to the rotor shaft. These
magnetized rotor poles provide an
increased magnetic flux intensity and
because of this the PM motor exhibits
improved torque characteristics when
compared with the VR type.
Hybrid (HB)
The hybrid stepper motor is more
expensive than the PM stepper motor
but provides better performance with
respect to step resolution, torque and
speed. Typical step angles for the HB
stepper motor range from 3.6° to 0.9°
(100 – 400 steps per revolution). The
hybrid stepper motor combines the
best features of both the PM and VR
type stepper motors. The rotor is
multi-toothed like the VR motor and
contains an axially magnetized concentric
magnet around its shaft. The
teeth on the rotor provide an even
better path which helps guide the
magnetic flux to preferred locations in
the airgap. This further increases the
detent, holding and dynamic torque
characteristics of the motor when compared
with both the VR and PM
types.
The two most commonly used types
of stepper motors are the permanent
magnet and the hybrid types. If a
designer is not sure which type will
best fit his applications requirements
he should first evaluate the PM type as
it is normally several times less expensive.
If not then the hybrid motor may
be the right choice.
There also excist some special
stepper motor designs. One is the disc
magnet motor. Here the rotor is
designed sa a disc with rare earth
magnets, See fig. 5 . This motor type
has some advantages such as very low
inertia and a optimized magnetic flow
path with no coupling between the
two stator windings. These qualities
are essential in some applications.
[b]Size and Power
In addition to being classified by their
step angle stepper motors are also
classified according to frame sizes
which correspond to the diameter of
the body of the motor. For instance a
size 11 stepper motor has a body diameter
of approximately 1.1 inches.
Likewise a size 23 stepper motor has a
body diameter of 2.3 inches (58 mm),
etc. The body length may however,
vary from motor to motor within the
same frame size classification. As a
general rule the available torque output
from a motor of a particular frame
size will increase with increased body
length.
Power levels for IC-driven stepper
motors typically range from below a
watt for very small motors up to 10 –
20 watts for larger motors. The maximum
power dissipation level or
thermal limits of the motor are seldom
clearly stated in the motor manufacturers
data. To determine this we
must apply the relationship PÊ=V ´ÊI.
For example, a size 23 step motor may
be rated at 6V and 1A per phase.
Therefore, with two phases energized
the motor has a rated power dissipation
of 12 watts. It is normal practice
to rate a stepper motor at the power
dissipation level where the motor case
rises 65°C above the ambient in still
air. Therefore, if the motor can be
mounted to a heatsink it is often
possible to increase the allowable
power dissipation level. This is
important as the motor is designed to
be and should be used at its maximum
power dissipation ,to be efficient from
a size/output power/cost point of view.
When to Use a Stepper
Motor
A stepper motor can be a good choice
whenever controlled movement is
required. They can be used to advantage
in applications where you need to
control rotation angle, speed, position
and synchronism. Because of the inherent
advantages listed previously,
stepper motors have found their place
in many different applications. Some
of these include printers, plotters,
highend office equipment, hard disk
drives, medical equipment, fax
machines, automotive and many more.
The Rotating Magnetic Field
When a phase winding of a stepper
motor is energized with current a
magnetic flux is developed in the
stator. The direction of this flux is
determined by the “Right Hand
Rule” which states:
“If the coil is grasped in the right
hand with the fingers pointing in the
direction of the current in the winding
(the thumb is extended at a 90° angle
to the fingers), then the thumb will
point in the direction of the magnetic
field.”
Figure 5 shows the magnetic flux
path developed when phase B is energized
with winding current in the
direction shown. The rotor then aligns
itself so that the flux opposition is
minimized. In this case the motor
would rotate clockwise so that its
south pole aligns with the north pole
of the stator B at position 2 and its
north pole aligns with the south pole
of stator B at position 6. To get the
motor to rotate we can now see that
we must provide a sequence of
energizing the stator windings in such
a fashion that provides a rotating
magnetic flux field which the rotor
follows due to magnetic attraction.
Torque Generation
The torque produced by a stepper
motor depends on several factors.
• The step rate
• The drive current in the windings
• The drive design or type
In a stepper motor a torque is developed
when the magnetic fluxes of the
rotor and stator are displaced from
each other. The stator is made up of a
high permeability magnetic material.
The presence of this high permeability
material causes the magnetic flux to
be confined for the most part to the
paths defined by the stator structure
in the same fashion that currents are
confined to the conductors of an electronic
circuit. This serves to concentrate

the flux at the stator poles. The[size=9]torque output produced by the motor
is proportional to the intensity of the
magnetic flux generated when the
winding is energized.
The basic relationship which
defines the intensity of the magnetic
flux is defined by:
H = (N ´ i) ¸ l where:
N = The number of winding turns
i = current
H = Magnetic field intensity
l = Magnetic flux path length
This relationship shows that the
magnetic flux intensity and consequently
the torque is proportional to
the number of winding turns and the
current and inversely proportional to
the length of the magnetic flux path.
From this basic relationship one can
see that the same frame size stepper
motor could have very different torque
output capabilities simply by changing
the winding parameters. More
detailed information on how the
winding parameters affect the output
capability of the motor can be found
in the application note entitled “Drive
Circuit Basics”.
Phases, Poles and Stepping
Angles
Usually stepper motors have two
phases, but three- and five-phase
motors also exist.
A bipolar motor with two phases
has one winding/phase and a unipolar
motor has one winding, with a center
tap per phase. Sometimes the unipolar
stepper motor is referred to as a “fourphase
motor”, even though it only has
two phases.
Motors that have two separate
windings per phase also exist—these
can be driven in either bipolar or
unipolar mode.
A pole can be defined as one of the
regions in a magnetized body where
the magnetic flux density is concentrated.
Both the rotor and the
stator of a step motor have poles.
Figure 2 contains a simplified picture
of a two-phase stepper motor having 2
poles (or 1 pole pairs) for each phase
on the stator, and 2 poles (one pole
pair) on the rotor. In reality several
more poles are added to both the rotor
and stator structure in order to
increase the number of steps per
revolution of the motor, or in other
words to provide a smaller basic (full
step) stepping angle. The permanent
magnet stepper motor contains an
equal number of rotor and stator pole
pairs. Typically the PM motor has 12
pole pairs. The stator has 12 pole pairs
per phase. The hybrid type stepper
motor has a rotor with teeth. The
rotor is split into two parts, separated
by a permanant magnet—making half
of the teeth south poles and half north
poles.The number of pole pairs is
equal to the number of teeth on one of
the rotor halves. The stator of a hybrid
motor also has teeth to build up a
higher number of equivalent poles
(smaller pole pitch, number of
equivalent poles = 360/teeth pitch)
compared to the main poles, on which
the winding coils are wound. Usually
4 main poles are used for 3.6 hybrids
and 8 for 1.8- and 0.9-degree types.
It is the relationship between the
number of rotor poles and the equivalent
stator poles, and the number the
number of phases that determines the
full-step angle of a stepper motor.
Step angle=360 ¸ (NPh ´ Ph)=360/N
NPh = Number of equivalent poles per
phase = number of rotor poles
Ph = Number of phases
N = Total number of poles for all
phases together
If the rotor and stator tooth pitch is
unequal, a more-complicated relationship
exists.
Stepping Modes
[size=9]The following are the most common
drive modes.
• Wave Drive (1 phase on)
• Full Step Drive (2 phases on)
• Half Step Drive (1 & 2 phases on)
• Microstepping (Continuously
varying motor currents)
For the following discussions please
refer to the figure 6.
In Wave Drive only one winding is
energized at any given time. The
stator is energized according to the

موضوع: رد: motor basic الجمعة فبراير 20, 2009 1:37 am

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