DC Motor
• A DC motor is any of a class of electrical machines that converts direct
current electrical power into mechanical power. The most common types
rely on the forces produced by magnetic fields. Nearly all types of DC
motors have some internal mechanism, either electromechanical or
electronic, to periodically change the direction of current flow in part of
the motor. Most types produce rotary motion; a linear motor directly
produces force and motion in a straight line.
• These motors can further be classified into brushed DC motor and
brushless DC motors.
Brush type DC motor
• A typical brushed motor consists of an
armature coil, slip rings divided into two
parts, a pair of brushes and horse shoes
electromagnet as shown in Fig. 4.1.4. A
simple DC motor has two field poles namely
a north pole and a south pole. The magnetic
lines of force extend across the opening
between the poles from north to south. The
coil is wound around a soft iron core and is
placed in between the magnet poles. These
electromagnets receive electricity from an
outside power source. The coil ends are
connected to split rings. The carbon brushes
are in contact with the split rings. The
brushes are connected to a DC source. Here
the split rings rotate with the coil while the
brushes remain stationary.
• The working is based on the principle that
when a current-carrying conductor is placed
in a magnetic field, it experiences a
mechanical force whose direction is given by
Fleming's left-hand rule.
Brushless DC motor (BDLC)
• The rotor and stator of a BLDC motor are shown in the Fig. The Rotor of a BLDC is
a permanent magnet; the stator has a winding arrangement. The internal winding
of the stator is illustrated in the Fig. The stator has 3 coils, named A, B and C. The
coil arrangement in a BLDC is shown here, with different color for different coils.
Out of these 3 coils, only one coil is illustrated in the Fig. for simplicity. By applying
DC power to the coil, the coil will energize and become an electromagnet.
• The operation of a BLDC is based on the simple force interaction between the
permanent magnet and the electromagnet. In this condition, when the coil A is
energized, the opposite poles of the rotor and stator are attracted to each other
(The attractive force is shown in green arrow). As a result the rotor poles move
near to the energized stator. The rotor moves towards the energized coil, due to
the attractive force
• As the rotor nears coil A, coil B is energized. As the rotor nears coil B, coil C is
energized. After that, coil A is energized with the opposite polarity. In a BLDC, as
the rotor nears the energized coil, the next coils is energized; this will make the
rotor continuously rotate. This process is repeated, and the rotor continues to
rotate.
Advantages of brushless DC motor:
More precise due to computer control
More efficient
No sparking due to absence of brushes
Less electrical noise
No brushes to wear out
Electromagnets are situated on the stator hence easy to cool
Motor can operate at speeds above 10,000 rpm under loaded and unloaded
conditions
Responsiveness and quick acceleration due to low rotor inertia
Disadvantages of brushless DC motor:
Higher initial cost
Complex due to presence of computer controller
Brushless DC motor also requires additional system wiring in order to power the
electronic commutation circuitry
AC Motor
• An AC motor is an electric motor driven by an alternating current (AC).
The AC motor commonly consists of two basic parts, an outside
stationary stator having coils supplied with alternating current to
produce a rotating magnetic field, and an inside rotor attached to the
output shaft producing a second rotating magnetic field. The rotor
magnetic field may be produced by permanent magnets, reluctance
saliency, or DC or AC electrical windings.
• The working principle of AC motor is . Consider the rotor to be a permanent
magnet. Current flowing through conductors energizes the magnets and
develops N and S poles. The strength of electromagnets depends on current.
First half cycle current flows in one direction and in the second half cycle it
flows in opposite direction. As AC voltage changes the poles alternate.
• When an AC motor is in steady-state rotation (motion), the magnetic fields of
the rotor and stator rotate (move) with little or no slippage (near synchrony).
The magnetic forces (repulsive and attractive) between the rotor and stator
poles create average torque, capable of driving a load at rated speed. The
speed of the stator rotating magnetic field and the speed of the rotor rotating
magnetic field , relative to the speed of the mechanical shaft , must maintain
synchronism for average torque production by satisfying the synchronous
speed relation. Otherwise, asynchronously rotating magnetic fields would
produce pulsating or non-average torque.
Stepper motor
• A stepper motor or step motor or stepping motor is a brushless DC
electric motor that divides a full rotation into a number of equal steps. The
motor's position can then be commanded to move and hold at one of
these steps without any feedback sensor (an open-loop controller), as long
as the motor is carefully sized to the application in respect to torque and
speed.
• Types of stepper motors:
• Permanent Magnet
o Employ permanent magnet
o Low speed, relatively high torque
• Variable Reluctance
o Does not have permanent magnet
o Low torque
Variable Reluctance STEPPER Motor
• Figure 4.2.1 shows the construction of
Variable Reluctance motor. The
cylindrical rotor is made of soft steel
and has four poles as shown in
Fig.4.2.1. It has four rotor teeth, 90⁰
apart and six stator poles, 60⁰ apart.
Electromagnetic field is produced by
activating the stator coils in sequence.
It attracts the metal rotor. When the
windings are energized in a
reoccurring sequence of 2, 3, 1, and so
on, the motor will rotate in a 30⁰ step
angle. In the non-energized condition,
there is no magnetic flux in the air gap,
as the stator is an electromagnet and
the rotor is a piece of soft iron; hence,
there is no detent torque. This type of
stepper motor is called a variable
reluctance stepper.
Permanent magnet (PM) stepper motor
• In this type of motor, the rotor is a
permanent magnet. Unlike the
other stepping motors, the PM
motor rotor has no teeth and is
designed to be magnetized at a
right angle to its axis. Figure 4.2.2
shows a simple, 90⁰ PM motor with
four phases (A-D). Applying current
to each phase in sequence will
cause the rotor to rotate by
adjusting to the changing magnetic
fields. Although it operates at
fairly low speed, the PM motor has
a relatively high torque
characteristic. These are low cost
motors with typical step angle
ranging between 7.5⁰ to 15⁰.
Hybrid stepper motor
• Hybrid stepping motors combine a permanent
magnet and a rotor with metal teeth to provide
features of the variable reluctance and permanent
magnet motors together. The number of rotor
pole pairs is equal to the number of teeth on one
of the rotor’s parts. The hybrid motor stator has
teeth creating more poles than the main poles
windings
• Rotation of a hybrid stepping motor is produced in
the similar fashion as a permanent magnet
stepping motor, by energizing individual windings
in a positive or negative direction. When a winding
is energized, north and south poles are created,
depending on the polarity of the current flowing.
These generated poles attract the permanent poles
of the rotor and also the finer metal teeth present
on rotor. The rotor moves one step to align the
offset magnetized rotor teeth to the corresponding
energized windings. Hybrid motors are more
expensive than motors with permanent magnets,
but they use smaller steps, have greater torque
and maximum speed.
Step angle of a stepper motor is given by,
Step angle= 360°Number of poles
Advantages of stepper motors
• Low cost
• Ruggedness
• Simplicity of construction
• Low maintenance
• Less likely to stall or slip
• Will work in any environment
• Excellent start-stop and reversing responses
Disadvantages of stepper motors
• Low torque capacity compared to DC motors
• Limited speed
• During overloading, the synchronization will be broken. Vibration and
noise occur when running at high speed.
Servomotors V/s. stepper motors
• A servomotor consumes power as it rotates to the commanded position but then the
servomotor rests. Stepper motors run warm to the touch because they continue to consume
power to lock in and hold the commanded position.
• Servomotors are generally used as a high-performance alternative to the stepper motor.
Stepper motors have some inherent ability to control position, as they have built-in output
steps. This often allows them to be used as an open-loop position control, without any
feedback encoder, as their drive signal specifies the number of steps of movement to rotate,
but for this the controller needs to 'know' the position of the stepper motor on power up.
Therefore, on first power up, the controller will have to activate the stepper motor and turn
it to a known position. This can be observed when switching on an inkjet printer; the
controller will move the ink jet carrier to the extreme left and right to establish the end
positions. A servomotor will immediately turn to whatever angle the controller
instructs it to, regardless of the initial position at power up.
• The lack of feedback of a stepper motor limits its performance, as the stepper motor can
only drive a load that is well within its capacity, otherwise missed steps under load may lead
to positioning errors and the system may have to be restarted or recalibrated. The encoder
and controller of a servomotor are an additional cost, but they optimise the performance of
the overall system (for all of speed, power and accuracy) relative to the capacity of the basic
motor. With larger systems, where a powerful motor represents an increasing proportion of
the system cost, servomotors have the advantage.
Harmonic Drive
• Harmonic Drive is a strain wave gear which can improve certain
characteristics compared to traditional gearing systems. They are
typically used in industrial motion control, machine tool, printing
machine,robotics and aerospace,for gear reduction but may also be used
to increase rotational speed, or for differential gearing.
Advantages
• The advantages include: no backlash, high compactness and light weight,
high gear ratios, reconfigurable ratios within a standard housing, good
resolution and excellent repeatability (linear representation) when
repositioning inertial loads, high torque capability, and coaxial input and
output shafts.
• High gear reduction ratios are possible in a small volume (a ratio from
30:1 up to 320:1 is possible in the same space in which planetary
gears typically only produce a 10:1 ratio).
• The strain wave gearing theory is based on elastic dynamics and utilizes the
flexibility of metal. The mechanism has three basic components: a wave
generator (C), a flex spline (B), and a circular spline (A). More complex
versions have a fourth component normally used to shorten the overall length or
to increase the gear reduction within a smaller diameter, but still follow the same
basic principles.
• The wave generator is made up of two separate parts: an elliptical disk
called a wave generator plug and an outer ball bearing. The gear plug is
inserted into the bearing, giving the bearing an elliptical shape as well.
• The flex spline is shaped like a shallow cup. The sides of the spline are very thin,
but the bottom is relatively rigid. This results in significant flexibility of the
walls at the open end due to the thin wall, and in the closed side being quite rigid
and able to be tightly secured (to a shaft, for example). Teeth are positioned
radially around the outside of the flex spline. The flex spline fits tightly over the
wave generator, so that when the wave generator plug is rotated, the flex spline
deforms to the shape of a rotating ellipse and does not slip over the outer
elliptical ring of the ball bearing. The ball bearing lets the flex spine rotate
independently to the wave generator's shaft.
• The circular spline is a rigid circular ring with teeth on the inside. The flex
spline and wave generator are placed inside the circular spline, meshing the
teeth of the flex spline and the circular spline. Because the flex spline is
deformed into an elliptical shape, its teeth only actually mesh with the teeth of
the circular spline in two regions on opposite sides of the flex spline (located on
the major axis of the ellipse).
• Assume that the wave generator is the input rotation. As the wave generator plug
rotates, the flex spline teeth which are meshed with those of the circular spline
slowly change position. The major axis of the flex spline's ellipse rotates with
wave generator, so the points where the teeth mesh revolve around the center
point at the same rate as the wave generator's shaft. The key to the design of the
strain wave gear is that there are fewer teeth (often for example two fewer) on
the flex spline than there are on the circular spline. This means that for every full
rotation of the wave generator, the flex spline would be required to rotate a slight
amount (two teeth in this example) backward relative to the circular spline. Thus
the rotation action of the wave generator results in a much slower rotation of the
flex spline in the opposite direction.
• For a strain wave gearing mechanism, the gearing reduction ratio can be
calculated from the number of teeth on each gear:
