CLASS NOTES ON
ELECTRICAL AND ELECTRONICS ENGINEERING
FOR
4
thSEMESTER OF
MECHANICAL ENGINEERING (B.E PROGRAMME)
DEPARTMENT OF ELECTRICAL ENGINEERING
Prepared By: Prof R. V. Patel Page 2 of 16
Stepper Motor
Stepper motor is a special type of electric motor that moves in precisely defined increments of rotor position (Steps).
The size of the increment is measured in degrees and can vary depending on the application. Due to precise control, stepper motors are commonly used in medical, satellites, robotic and
control applications.
There are several features common to all stepper motors that make them ideally suited for these types of applications. They are as under
High accuracy: Operate under open loop Reliability: Stepper motors are brushless.
Load in dependent: Stepper motors rotate at a set speed under different load, provided the rated torque is maintained.
Holding torque: For each and every step, the motor holds its position without brakes. Stepper motor requires sequencers and driver to operate.
Sequencer generates sequence for switching which determines the direction of rotation and mode of operation.
Driver is required to change the flux direction in the phase windings. Sequencer Driver Stepper
Circuit Motor Power
Supply
Figure 2.1 Block diagram of stepper motor system Types Of Stepper Motors
It can be classified into several types according to machine structure and principle of operation as explained by Kenjo (1984).
Basically there are three types
1. Variable Reluctance Motor (VRM)
2. Permanent Magnet Stepper Motor (PMSM) 3. Hybrid Stepper Motor (HSM)
1. Variable Reluctance Motor
It 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.
Both the stator and rotor materials must have high permeability and be capable of allowing high magnetic flux to pass through even if a low magneto motive force is applied.
When the rotor teeth are directly lined up with the stator poles, the rotor is in a position of minimum reluctance to the magnetic flux.
A rotor step takes place when one stator phase is de-energized and the next phase in sequence is energized, thus creating a new position of minimum reluctance for the rotor as explained by Kenjo (1984).
Figure 2.2 Cross-section of variable reluctance motor 2. Permanent Magnet Stepper Motor
A stepper motor using a permanent magnet in the rotor is called a PMSM. The rotor no longer has teeth as with the VRM.
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 VRM type.
An elementary PM motor is shown in Figure 2.3 which employs a cylindrical permanent magnet as the rotor and possesses four poles in its stator.
Two overlapping windings are wound as one winding on poles 1 and 3 and these two windings are separated from each other at terminals to keep them as independent windings.
The same is true for poles 2 and 4.
The terminals marked “ Ca” or “ Cb” denotes “common” to be connected to the positive terminal of the power supply as explained by Kenjo (1984).
When the windings are excited in the sequence A - B - A1 - B1 --- the rotor will be driven in a clockwise direction.
The step length is 900 in this machine.
If the number of stator teeth and magnetic poles on the rotor are both doubled, a two-phase motor with a step length of 450 will be realized.
Figure 2.3 Cross-section of permanent magnet stepper motor 3. Hybrid Stepper Motor
length and high torque in spite of motor size.
Standard HSM have 50 rotor teeth and rotate at 1.8 degree per step. Figures 2.4 & 2.5 show a cross section and cut view of two phase HSM.
The windings are placed on the stator poles and a PM is mounted on the rotor. The important feature of the HSM is its rotor structure.
A cylindrical or disk-shaped magnet lies in the rotor core. Stator and rotor end-caps are toothed. The coil in pole 1 and pole 3 is connected in series consisting of phase A and poles 2 and 4
are for phase B.
If stator phase A is excited pole 1 acquires north polarity while pole 2 acquires south polarity.
Pole 1 attracts the rotor’s South Pole while pole 3 aligns with the rotor’s North Pole.
Figure 2.4 Cross-section of HSM
Figure 2.5 Cut view of HSM
When the excitation is shifted from phase A to phase B, in which case the stator pole 2 becomes north pole and stator pole 4 becomes south pole, it would cause the rotor to turn 900 in the clockwise direction.
Again phase A is excited with pole 1 as south pole and pole 3 as north pole causing the rotor to move 900 in the clockwise direction.
If excitation is removed from phase A and phase B is excited, then pole 2 produces South Pole and pole 4 produces North Pole resulting in rotor movement of 900 in the clockwise direction.
rotor movement.
The step length for a HSM and angle through which the rotor moves for each step pulse is known as step angle and is calculated by
Step length = 90o/Nr (2.1) Step angle is calculated using the formula,
Where,
θ = Step angle in degrees Ns = Number of stator teeth Nr= Number of rotor teeth m = Number of phases Brushless DC motor
A brushless DC motor (known as BLDC) is a permanent magnet synchronous electric motor which is driven by direct current (DC) electricity.
Conventional brushed type DC motor, wherein the brushes make the mechanical contact with commutator on the rotor so as to form an electric path between a DC electric source and rotor armature windings.
BLDC motor employs electrical commutation with permanent magnet rotor and a stator with a sequence of coils.
The armature coils are switched electronically by transistors or silicon controlled rectifiers at the correct rotor position in such a way that armature field is in space quadrature with the rotor field poles.
Hence the force acting on the rotor causes it to rotate. Construction of BLDC Motor
BLDC motors can be constructed in different physical configurations.
Depending on the stator windings, these can be configured as single-phase, two-phase, or three-phase motors.
However, three-phase BLDC motors with permanent magnet rotor are most commonly used. There can be two types of BLDC motor on the basis of construction : (i) inner rotor design & (ii)
outer rotor design.
i) Inner Rotor Design
In an inner rotor design, the rotor is located in the centre of the motor and the stator winding surround the rotor.
As the rotor is located in the core, rotor magnets do not insulate heat inside and heat get dissipated easily.
ii) Outer Rotor Design
In outer rotor design, the rotor surrounds the winding which is located in the core of the motor. The magnets in the rotor trap the heat of the motor inside and do not allow to dissipate from the
motor.
Such type of designed motor operates at lower rated current and has low cogging torque.
Working Principle
Stator windings of a BLDC motor are connected to a control circuit (an integrated switching circuit).
The control circuit energizes proper winding at proper time, in a pattern which rotates around the stator.
The rotor magnet tries to align with the energized electromagnet of the stator, and as soon as it aligns, the next electromagnet is energized.
Thus the rotor keeps running.
Advantages of BLDC Motor
BLDC motor has several advantages over conventional DC motors and some of these are It has no mechanical commutator and associated problems
High efficiency due to the use of permanent magnet rotor
High speed of operation even in loaded and unloaded conditions due to the absence of brushes that limits the speed
Long life as no inspection and maintenance is required for commutator system Higher dynamic response due to low inertia and carrying windings in the stator Less electromagnetic interference
Quite operation (or low noise) due to absence of brushes Disadvantages of Brushless Motor
These motors are costly
Electronic controller required control this motor is expensive
Not much availability of many integrated electronic control solutions, especially for tiny BLDC motors
Requires complex drive circuitry Need of additional sensors
Applications of Brushless DC Motors (BLDC)
Brushless DC Motors (BLDC) are used for a wide variety of application requirements such as varying loads, constant loads and positioning applications in the fields of industrial control, automotive, aviation, automation systems, health care equipments, etc. Some specific applications of BLDC motors are
1) Computer hard drives and DVD/CD players
2) Electric vehicles, hybrid vehicles, and electric bicycles
3) Industrial robots, CNC machine tools, and simple belt driven systems 4) Washing machines, compressors and dryers
5) Fans, pumps and blowers Linear Induction Motor
Linear Induction Motor (LIM) is an asynchronous motor, working on the same principle an Induction Motor works, but is designed to produce the rectilinear motion, hence the word Linear Induction Motor.
LIM is an advanced version of rotary induction motor which gives a linear translational motion instead of the rotational motion.
Construction
The basic construction of a linear induction motor is similar to a three phase induction motor. If we cut the stator of a polyphase induction motor and lay on a flat surface, it forms the primary
of the linear induction motor system.
Similarly, after cutting the rotor of the induction motor and making it flat, we get the secondary of the system.
In this type of motor, the stator and rotor are called primary and secondary respectively.
The secondary of the linear induction motor consists of a flat Aluminium conductor with a ferromagnetic core.
For understanding the construction of Linear Induction Motor, we will first take a look at the construction of Induction Motor as shown in figure below.
This is what which makes the Primary of a LIM. So Primary of Linear Induction Motor is flat and three phase winding is wound on it.
Now if we make the Rotor of Induction Motor flat then it will be nothing but a sheet of flat Aluminium which is called the Secondary of Linear Induction Motor (LIM) as shown in figure below.
Working of Linear Induction Motor:
If the primary of the Linear Induction Motor is connected to the three phase supply, a flux is produced which will travel across the length of the primary.
Because of the travelling magnetic flux, a current will be generated in the conductor which is made of the aluminium material in the secondary of Linear Induction Motor.
This current, which is induced in the LIM secondary interacts with the travelling flux and produces a linear force from i(dL×B).
If secondary is fixed and the primary is free to move, the force will move the primary in the direction of the travelling wave.
Application of Linear Induction Motor 1. Automatic sliding doors in electric trains.
2. Mechanical handling equipment, such as propulsion of a train of tubs along a certain route. 3. Metallic conveyor belts.
4. Pumping of liquid metal, material handling in cranes, etc. Universal Motor
A universal motor is a special type of motor which is designed to run on either DC or single phase AC supply.
These motors are generally series wound (armature and field winding are in series), and hence produce high starting torque.
Most of the universal motors are designed to operate at higher speeds, exceeding 3500 RPM. They run at lower speed on AC supply than they run on DC supply of same voltage, due to the
reactance voltage drop which is present in AC and not in DC.
There are two basic types of universal motor : (i)compensated type and (ii) uncompensated type.
Construction Of Universal Motor
Construction of a universal motor is very similar to the construction of a DC machine. It consists of a stator on which field poles are mounted.
Field coils are wound on the field poles.
The whole magnetic path (stator field circuit and also armature) is laminated.
Lamination is necessary to minimize the eddy currents which induce while operating on AC. The rotary armature is of wound type having straight or skewed slots and commutator with
Working Of Universal Motor
When the universal motor is fed with a DC supply, it works as a DC series motor. When current flows in the field winding, it produces an electromagnetic field. The same current also flows from the armature conductors.
“When a current carrying conductor is placed in an electromagnetic field, it experiences a mechanical force. Due to this mechanical force, or torque, the rotor starts to rotate.”
The direction of this force is given by Fleming's left hand rule. When fed with AC supply, it still produces unidirectional torque.
Because, armature winding and field winding are connected in series, they are in same phase. As polarity of AC changes periodically, the direction of current in armature and field winding reverses at the same time.
Thus, direction of magnetic field and the direction of armature current reverses in such a way that the direction of force experienced by armature conductors remains same.
Speed torque characteristics; the speed torque characteristics of universal motor are similar
to that of dc series motor.
Applications Of Universal Motor
Universal motors find their use in various home appliances like vacuum cleaners, drink and food mixers, domestic sewing machine etc.
The higher rating universal motors are used in portable drills, blenders etc.
Induction Motor
A IM is an electrical machine which converts electrical power into mechanical power. The motor which works on the principle of electromagnetic induction is known as the
The electromagnetic induction is the phenomenon in which the electromotive force induces across the electrical conductor when it is placed in a rotating magnetic field.
A single phase induction motor is very similar to a 3-phase squirrel cage induction motor. Unlike a 3-phase induction motor, a single-phase induction motor is not self starting but requires
some starting means.
Construction
The construction parts on of single phase induction motor consist of main two parts:
stationary stator and revolving rotor. The stator separate from rotor by small air gap have ranges from 0.4 mm to 4 mm depends to size of motor.
(A) Stator
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.
Greater the number of poles, lesser the speed and vice versa.
The stator windings, when supplied with 3-phase currents, produce a magnetic flux, which is of constant magnitude but which revolves (or rotates) at synchronous speed (given by Ns = 120 f/P). This revolving magnetic flux induces an e.m.f in the rotor by mutual induction. (B) Rotor
(i) Squirrel-cage rotor: Motors employing this type of rotor are known as squirrel-cage induction motors.
(ii) Phase-wound or wound rotor: Motors employing this type of rotor are variously known as ‘phase-wound’ motors or ‘wound’ motors or as ‘slip-ring’ motors.
Squirrel-Cage Rotor
In the squirrel-cage rotor, the rotor winding consists of single copper or aluminium bars placed in the slots and short-circuited by end-rings on both sides of the rotor.
Most of single phase induction motors have Squirrel-Cage rotor. One or 2 fans are attached to the shaft in the sides of rotor to cool the circuit.
Wound Rotor
The ends of the star-connected rotor winding are brought to three slip rings on the shaft so that a connection can be made to it for starting or speed control.
It is usually for large 3 phase induction motors.
Rotor has a winding the same as stator and the end of each phase is connected to a slip ring. Compared to squirrel cage rotors, wound rotor motors are expensive and require
maintenance of the slip rings and brushes, so it is not so common in industry applications.
(C) Frame.
Made of close-grained alloy cast iron. (D) Stator and Rotor Core.
Built from high-quality low-loss silicon steel laminations and flash-enamelled on both sides. (E) Stator and Rotor Windings.
Have moisture proof tropical insulation embodying mica and high quality varnishes.
Are carefully spaced for most effective air circulation and are rigidly braced to withstand centrifugal forces and any short-circuit stresses.
(F) Air-gap.
The stator rabbets and bore are machined carefully to ensure uniformity of air-gap. (G) Shafts and Bearings.
Ball and roller bearings are used to suit heavy duty, toruble-free running and for enhanced service life.
(H) Fans.
Light aluminium fans are used for adequate circulation of cooling air and are securely keyed onto the rotor shaft.
(I) Slip-rings and Slip-ring Enclosures.
Slip-rings are made of high quality phosphor-bronze and are of moulded construction. WORKING OF SINGLE-PHASE INDUCTION MOTOR:
A single phase induction motor is inherently not self-staring can be shown easily.
Consider a single phase induction motor whose rotor is at rest. Let a single phase a.c. source be connected to the stator winding (it is assumed that there is no starting winding).
Let the stator bewound for two poles.
When power supply for the stator is switched on, an alternating current flows through the stator winding.
This sets up an alternating flux.
This flux crosses the air gap and links with the rotor conductors.
By electromagnetic induction e.m.f.’s are induced in the rotor conductors. Since the rotor forms a closed circuit, currents are induced in the rotor bars.
Due to interaction between the rotor induced currents and the stator flux, a torque is produced. It is readily seen that if all rotor conductors in the upper half come under a stator N pole, all
rotor conductors in the lower half come under a stator S pole.
The two equal and opposite torques cancel out, with the result that the net driving torque is zero.
Hence the rotor remains stationary.
Thus the single phase motor fails to develop starting torque.
This argument holds good irrespective of the number of stator poles and the polarity of the stator winding.
The net torque acting on the rotor at standstill is zero.
If, however, the rotor is in motion in any direction when supply for the stator is switched on, it can be shown that the rotor develops more torque in that direction.
The net torque then, would have non-zero value, and under its impact the rotor would speed up in its direction.
The analysis of the single phase motor can be made on the basis of two theories: i. Double revolving field theory ii. Cross field theory.
Types of Single-Phase Motors
Single-phase motors are generally built in the fractional-horsepower range and may be classified into the following four basic types:
1. Single-phase induction motors (i) split-phase type (ii) capacitor start type
(iii) capacitor start capacitor run type (v) shaded-pole type
2. A.C. series motor or universal motor 3. Repulsion motors
(i) Repulsion-start induction-run motor (ii) Repulsion-induction motor
4. Synchronous motors (i) Reluctance motor (ii) Hysteresis motor (i) Split-phase induction motors
The stator of a split-phase induction motor is provided with an auxiliary or starting winding S in addition to the main or running winding M.
The starting winding is located 90° electrical from the main winding and the picture of split phase induction motor [See Fig3.12 (i))] and operates only during the brief period when the motor starts up.
The two windings are so resigned that the starting winding S has a high resistance and
relatively small reactance while the main winding M has relatively low resistance and large reactance to be as inductance (the current delay with voltage) to make shifting current as shown in the schematic connections in Figure 3.12 (ii)).
Consequently, the currents flowing in the two windings have reasonable phase difference c (25°
Figure 3.12 Split-phase induction motors. Operation
When the two stator windings are energized from a single-phase supply, the main winding carries current Im while the starting winding carries current Is.
Since main winding is made highly inductive while the starting winding highly resistive, the currents Im and Is have a reasonable phase angle a (25° to 30°) between them.
Consequently, a weak revolving field approximating to that of a 2-phase machine is produced which starts the motor.
When the motor reaches about 80% of synchronous speed, the centrifugal switch opens the circuit of the starting winding.
The motor then operates as a single-phase induction motor and continues to accelerate till it reaches the normal speed.
The normal speed of the motor is below the synchronous speed and depends upon the load on the motor.
Shaded-pole induction motors
A picture of shaded pole induction motor are shows in Figure 3.15 (i). A typical shaded-pole motor with a cage rotor is shown in Figure 3.15 (ii).
This is a single phase induction motor, with main winding in the stator. A small portion of each pole is covered with a short-circuited, single-turn copper coil called the shading coil.
The sinusoidal varying flux created by ac (single-phase) excitation of the main winding induces in the shading coil.
As a result, induced currents flow in the shading coil producing their own flux in the shaded portion of the pole as shown in Figure 3.15 (iii) and lags the flux φ m ′ of the remaining pole by the angle α .
The two sinusoidal varying fluxes φ m ′ and φ sp ′ are displaced in space as well as have a time phase difference (α ), thereby producing forward and backward rotating fields, which produce a net torque.
It may be noted that the motor is self-starting unlike a single-phase single-winding motor. It is seen from the phasor diagram (Figure 3.15 (iii) that the net flux in the shaded portion of
the pole (φ sp ) lags the flux (φ m′ ) in the unshaded portion of the pole resulting in a net torque, which causes the rotor to rotate from the unshaded to the shaded portion of the pole. The motor thus has a definite direction of rotation, which cannot be reversed. Atypical torque
3.15 Shaded-pole induction motors. Operation
The operation of the motor can be understood by referring to Figure (3.16) which shows one pole of the motor with a shading coil.
(i) During the portion OA of the alternating-current cycle [See Figure (3.16)], the flux begins to increase and an EMF is induced in the shading coil.
The resulting current in the shading coil will be in such a direction so as to oppose the change in flux. Thus the flux in the shaded portion of the pole is weakened while that in the unshaded portion is strengthened as shown in Figure (3.16 (ii)).
(ii) During the portion AB of the alternating-current cycle, the flux has reached almost maximum value and is not changing.
Consequently, the flux distribution across the pole is uniform [See Figure (3.16 (iii))] since no current is flowing in the shading coil.
As the flux decreases (portion BC of the alternating current cycle), current is induced in the shading coil so as to oppose the decrease in current.
Thus the flux in the shaded portion of the pole is strengthened while that in the unshaded portion is weakened as shown in Figure (3.16 (iv)).
The effect of the shading coil is to cause the field flux to shift across the pole face from the unshaded to the shaded portion.
This shifting flux is like a rotating weak field moving in the direction from unshaded portion to the shaded portion of the pole.
(iv)The rotor is of the squirrel-cage type and is under the influence of this moving field. Consequently, a small starting torque is developed.
As soon as this torque starts to revolve the rotor, additional torque is produced by single-phase induction-motor action.
The motor accelerates to a speed slightly below the synchronous speed and runs as a single-phase induction motor.
