stepper motors

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Transcript stepper motors

Syafruddin Hasan
STEPPER MOTOR
Stepper motors are device that rotate by discrete incremental
steps
The primary advantage:
 they can be controlled almost directly by digital circuitry

Although current amplifiers must be placed between the digital control
source and the stator windings of a stepper motor, no complex digital-toanalog conversion circuitry is required for controlling motor speed.
 They are typically more efficient and develop significantly more torque than
the synchronous servomotor.
 their ability to operate reliably in an open loop control mode, eliminating the
need for complex feedback circuitry.
Three basic (main) types of stepper motors:
1. Permanent magnet (PM) stepper motor
2. Variable reluctance stepper motors
3. Hybrid stepper motors, which is a combination of the
previous two.
The number of phase windings present in the stator determines the degree by
which the motor steps.
For the most commonly used industrial stepper motors, this step angle
(designated θs) may be as low as 7.5o or as high as 30o.
A stepper motor with a small step angle is capable of a higher resolution of
motion than a motor with larger step angle.
The number of steps required for a given stepper motor to complete one
revolution is its stepping rate (designated Rs).
360 o
s 
Rs
DC Stepping Motor Operation
An important principle that applies to the operation of dc stepping motors is
the basic law of magnetism:
like magnetic poles repel and unlike magnetic poles attract
permanent magnet (PM) rotor is placed
between two-series connected stator coils,
The direction of rotation in this case is
unpredictable.
A stepper motor may contain several sets of phase windings. However, for
the sake of simplicity, only two pairs are shown here that mounted opposite
each other on stator (L1-L3 and L2-L4 pairs).
S1 and S3 closed
S2 and S4 closed
Adding two more stator coils to this simple motor, would make the direction
of rotation predictable.
With the stator polarities indicated, the rotor would align it self midway
between the two pairs of stator coils.
The direction of rotation can now be
predicted and is determined by the
polarities of the stator coil sets. Adding
more stator coil pairs to a motor of this
type improves its rotation and makes
the stepping action very accurate.
Since the PM stepper motor shown here has 8 stator windings, the stepping
rate is also 8. This is because there are 8 possible positions for the rotor
within a single revolution. Thus the stepping angle is determined as:
360 o
s 
 45o
8
Pulse train required for operation of 4-phase PM stepper motor
The rotor will require 12 pulses of
electricity to move the 12 steps to make
one complete revolution. Another way to
say this is that the rotor will move
precisely 30o for each pulse of electricity
that the motor receives.
When no power is applied to the motor,
the residual magnetism in the rotor
magnet will cause the rotor to detent or
align one set of its magnetic poles with
the magnetic poles of one of the stator
magnets. This means that the rotor will
have 12 possible detent positions.
Typical stepper motor of the six pole
rotor and four pole stator
When power is applied, it is directed to only one of the stator pair of windings, which
will cause that winding pair to become a magnet. One of the coils for the pair will
become the north-pole, and the other will become the south-pole. When this occurs,
the stator coil that is the north-pole will attract the closest rotor tooth that has the
opposite polarity, and the stator coil that is the south-pole will attract the closest rotor
tooth that has the opposite polarity. When current is flowing through these poles, the
rotor will now have a much stronger attraction to the stator winding, and the increased
torque is called holding torque.
Movement of the stepper motor rotor as
current is pulsed to the sataor
Switching Sequence for Full-Step Motors
Half-Step Switching Sequence
The main feature of this switching sequence is that we can double the
resolution of the stepper motor by causing the rotor to move half the distance
it does when the full-step sequence is used