Motors

• A motor is an electrical machine which

converts electrical energy into mechanical
energy.
• Electrical motors are frequently used as final
controlling element in position and speed
control
• systems. Basically, it consists of two parts
• Stator
• Rotor
Classification of Motors:
 3.4.1 DC Motors:
• Construction

Fig. 3.21 Construction of DC Motor

Working principle of a DC motor:

Figure 3.22 Working of DC Motor

• "Whenever a current carrying conductor is placed in a
magnetic field, it experiences a mechanical force". The
direction of this force is given by Fleming's left-hand rule
and its magnitude is given by
F = BIL
• Where, B = magnetic flux density,
• I = current and
• L = length of the conductor.
• Same magnitude and opposite forces acting on left and
right conductors causes
• torque(T=Fxr) is produce at circumference of the
conductors. Because of this twisting force
• motor stars rotating in anticlockwise direction as shown
in above figure 2.
• The direction of motor can be controlled by simply
changing the supply terminals or interchanging of south
and north poles.
 Back EMF:
• Once motor starts rotating, the conductors cuts the
magnetic flux lines causes an emf is
• induced in the armature conductors according to the
Faraday's law of electromagnetic induction and is given
by

• Where N- speed of armature

• P- number of poles
• Φ- magnetic flux lines
• A- number of parallel paths
• Z- number of conductors
• The induced emf opposes the change cause it, according
to Lenz’s Law. Here it opposes
• the supply voltage, hence it is called “Back emf”.
 Significance of Back EMF:

• Back EMF regulates flow of armature current to meet the load

• requirements. Hence dc motor acting as self-regulating machine.
• Magnitude of back emf is directly proportional to speed of the
motor. Consider the load
• on a dc motor is suddenly reduced. In this case, required torque will
be small as compared to
• the current torque. Speed of the motor will start increasing due to
the excess torque. Hence,
• being proportional to the speed, magnitude of the back emf will
also increase. With increasing
• back emf armature current will start decreasing. Torque being
proportional to the armature
• current, it will also decrease until it becomes sufficient for the load.
Thus, speed of the motor
• will regulate.
• On the other hand, if a dc motor is suddenly
loaded, the load will cause decrease in the
• speed. Due to decrease in speed, back emf
will also decrease allowing more armature
current.
• Increased armature current will increase the
torque to satisfy the load requirement. Hence,
• presence of the back emf makes a dc motor
‘self-regulating’.
 Types of DC Motors
• DC motors are usually classified of the basis of
their excitation configuration, as
• follows -
• ➢ Separately excited (field winding is fed by
external source)
• ➢ Self-excited -
• • Series wound (field winding is connected in
series with the armature)
• • Shunt wound (field winding is connected in
parallel with the armature)
• • Compound wound -
 Separately excited motor:
• The separately excited motor has separate
control of the armature and field currents
 Series-wound:
• With the series-wound motor the armature and field
coils are in series. Such a motor exerts the highest
starting torque and has the gretest no-load speed.
Reversing the polarity of the supply to the coils has
no effect on the direction of rotation of the motor
since both the field and armature currents have
been reversed
 Shunt-wound:
• With the shunt-wound motor the armature and field
coils are in parallel. It provides lowest starting
torque. It provides constant speed regardless of load.
To reverse the direction of rotation either armature
or field supply must be reversed.
 Compound-wound:
• The compound motor has two field windings, one in
series with the armature and one in parallel. The aim
is to get the best features of the series and shunt-
wound motors, such as high starting torque and
good speed regulation.
 3.4.2 AC MOTORS
• 3-Ø Induction Motor:The main body of the Induction Motor comprises of
two major parts:
• 1. Stator 2. Rotor
• Stator: The stator of an induction motor is in principle, the same as that
of a synchronous motor
• (or) generator. It is made up of a number of stampings, which are slotted
to receive the
• windings. The stator carries a 3-phase winding and is fed from a 3-phase
supply. It is wound
• for a definite number of poles; the exact number of poles being
determined by the requirements
• of speed. The number of poles is higher, lesser the speed and vice-versa.
The stator winding,
• when supplied with a 3-phase currents, produce a magnetic flux, which is
of constant
• magnitude but which revolves at synchronous speed and is given by
• Ns = 120 x f / p
• Where Ns= synchronous speed
• f = Frequency
• p = no. of poles
This revolving magnetic flux induces emf in rotor by
mutual induction.. Figure 3.23 shows construction of
stator.
• Rotor: two types of constructions are commonly used
• (i) Squirrel cage Rotor:
• (ii) Phase wound (or) slip-ring Rotor:
• Squirrel cage Rotor:
• Almost 90 percentage of induction motors are squirrel-cage
type, because this type of rotor has the simplest and most
rugged construction imaginable and is almost
indestructible.
• The Rotor consists of cylindrical laminated core with
parallel slots for carrying the rotor conductors which, it
should be noted clearly, are not wires but consists of heavy
bars of copper, aluminium or alloys. One bar is placed in
each slot; rather the bars are inserted from the end
• when semi-enclosed slots are used. The rotor bars are
brazed or electrically welded or bolted to two heavy and
stout short circuiting end-rings, thus giving us, what is
called a squirrel cage construction and is shown in figure
3.24.
Fig.3.24 Squirrel cage rotor
 Principle and working:
• Induction motor works on the principle of
electromagnetic induction. When three phase
• supply is given to the stator winding, a rotating
magnetic field of constant magnetic field is
• produced. The speed of rotating magnetic field is
synchronous speed, Ns in r.p.m.
• Ns = 120 x f / p
• Where Ns= synchronous speed
• f = Frequency
• p = no. of poles
• Now at this instant rotor is stationary and stator
flux R.M.F. is rotating. So its obvious
• that there exists a relative motion between the
R.M.F. and rotor conductors. Now the R.M.F.
• gets cut by rotor conductors as R.M.F. sweeps
over rotor conductors. Whenever a conductor
• cuts the flux, emf gets induced in it. So emf gets
induced in the rotor conductors called rotor
• induced emf. This is electromagnetic induction.
As rotor forms closed circuit, induced emf
• circulates current through rotor called rotor
current.
• Any current carrying conductor produces its own flux. So rotor
produces its flux called
• rotor flux. For assumed direction of rotor current, the direction of
rotor flux is
• clockwise/anticlockwise.
• The direction can be easily determined using right hand thumb rule.
Now there are two
• fluxes, one R.M.F. and another rotor flux. Both the fluxes interact
with each. On left of rotor
• conductor, two fluxes are in same direction hence added up to get
high flux area. On right side
• of rotor conductor, two fluxes are in opposite direction hence they
cancel each other to produce
• low flux area. So rotor conductor experiences a force from left to
right, due to interaction of
• the two fluxes. As all rotor conductor experiences a force, overall
rotor experiences a torque
• and starts rotating.
• After rotor starts rotating it try to catch-up
synchronous speed. Once rotor speed Nr
• reaches synchronous speed Ns. The relative
motion between R.M.F and rotor is zero.
Hence
• the rotor always slips by a synchronous speed.
 3.4.3 STEPPER MOTOR
• A stepper motor is an electromechanical
device which converts electrical pulses into
• discrete mechanical movements. 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.
 STEPPER MOTOR WORKING
• Stepper motors work on the principle of
electromagnetism. There is a soft iron or
• magnetic rotor shaft surrounded by the electromagnetic
stators. The rotor and stator have poles which may be
teethed or not depending upon the type of stepper.
When the stators are energized the rotor moves.
• TYPES OF STEPPER MOTOR:
By construction the step motors come into three
broad classes:
➢ Permanent Magnet Stepper
➢ Variable Reluctance Stepper
➢ Hybrid Step Motor
 PERMANENT MAGNET STEPPER:
• ➢ The rotor and stator poles are not teethed.
• ➢ The rotor has alternative north and south
poles parallel to the axis of the rotor shaft.
• ➢ When a stator is energized, it develops
electromagnetic poles.
• ➢ The magnetic rotor aligns along the
magnetic field of the stator.
Operation:
 VARIABLE RELUCTANCE STEPPER:
• It has a toothed non-magnetic soft iron rotor. When
the stator coil is energized the rotor moves to have a
minimum gap between the stator and its teeth.
Operation:
 HYBRID STEPPER:
• A hybrid stepper is a combination of both permanent
magnet and the variable reluctance. It has a
magnetic teethed rotor which better guides
magnetic flux to preferred location in the air gap.
 Operation:

If pulse is given to windings of stator, then the rotor teeths are aligned with stator teeth
as shown in above figure.
 TYPES OF WINDING AND LEAD LEAD-OUT:
• Based on winding arrangement stepper
motors are classified into two types.
• 1. Uni polar Stepper motor 2. Bipolar stepper
motor
• In unipolar stepper motor pole may have one
lead common i.e., center tapped. In bipolar
stepper motor there is a single winding per
phase. The direction of current needed to the
changed by the driving circuit so that the
driving circuit of the stepper motor becomes
complex.
Fig. 3.25 (a) Unipolar Fig. 3.25(b) bipolar
 3.2 Solenoid
• Solenoid is a insulated copper coil is wound around
some cylindrical cardboard or plastic tube such that
the length of the coils is greater than its diameter,
then it becomes like a magnet.

Fig. 3.5 Solenoid working

 3.2.1 Working
• Solenoid is an electromagnetic device made up of a coil
which produces a magnetic
• field when electric current passed through it, then it is
energized and attracts the ferrous core
• as shown in figure 3.5(b). if supply is removed the coil
is un-energized and releases the ferrous
• core as shown in figure 3.5(a). Such solenoids are used
in relaying energy from one device to
• another. such solenoids are used mainly in opening and
closing valves as shown in figure 2.
• Solenoid valves When the solenoid coil is energized,
the valve opens, allowing water to flow
• from the reservoir into the fish tank. otherwise, valve is
closed.
 3.2.2 Applications
• ➢ Robots,
• ➢ Open and close of car doors
• ➢ Dish washes
• ➢ Liquid/gas flow control (Solenoid valves)
• ➢ Solenoid engines
 3.2.3 Solenoid valves
Figure 3.6 shows Solenoid valves, which can control the
flow of liquid.

Fig. 3.6 Solenoid valves