CH- 4 MOTORS Marks: 08 (12)
Motor (Definition): A motor is a machine which converts electrical energy into mechanical rotational force (torque).
Type: Ac motors, DC motors
1. AC Motors: These motors uses Alternating current (AC) to rotate the shaft. These motors are simple in construction and cheap. They usually have constant speed characteristics and low starting torque.
2. DC Motors: These motors uses Direct current (DC) to rotate the shaft. These motors are some what complicated in construction hence costly. The DC motor can have variable speed and torque that can be controlled.
Generator: A generator is a machine which converts mechanical rotational force (torque) into an electrical energy.
Types: AC generators, DC generators.
DC motor:
Working principle of DC motor:
When 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 which is perpendicular to both, the direction of current and the direction of magnetic filed.
Magnitude of this force is given by,
F = B x I x L Newton
Where, F – mechanical force
B – Magnetic flux density
I – Current flowing through conductor
L – Length of the conductor
Basic construction of a DC motor: -
Fig. shows the basic construction of DC motor.
The essential parts of DC motor are,
a) Yoke
b) Field Poles
c) Field winding
d) Armature core
e) Armature winding
f) Commutator
g) Brushes
h) Bearings
(a)Yoke (field frame or body):-
It is a cylindrical in shape. It performs two important functions.
It provides mechanical support to poles & also acts as a protecting cover for machine.
It forms a part of the magnetic circuit. The magnetic flux completes its path through yoke.
(b) Field Poles: - These poles are projected from the yoke on the inner side of the cylinder. They hold the field coils of insulated copper wire through which magnetic current flows and carry the magnetic flux to and from the armature.
(c) Field Windings: - Field windings are supported by the field poles. When current flows in field coils, magnetic field is setup & armature rotates in it.
The function of field system is to produce uniform magnetic field to rotate armature.
(d) Armature Core: - The armature core is cylindrical in shape. This is the rotating part held at the centre of the motor frame. It carries conductor along the length round its surface.
(e) Armature Winding: - These winding runs lengthwise on the armature surface. When current passes through these conductors lying in the magnetic field, torque is produced, this rotates the armature.
(f) Commutator: - It is of cylindrical in structure which forms electrical contact between the revolving armature conductors and the external circuit.
(g) Brushes: - These are stationary portion that make a rubbing contact with the moving surface of commutator.
(h) Bearings: - The bearings support the armature in proper alignment and it also helps to reduce the friction of shaft.
Terms used in DC motor:
Torque: It is the term which expresses the turning or twisting movement of force about axis.
Torque = Force x radius at which force acts.
Speed: Total revolutions taken by an armature per minute is called speed.
Back emf/counter emf: When armature of motor rotates, an emf is induced in conductors as they cut the lines of force. The induced emf is in opposition to the applied voltage. It is called back emf.
Types of DC Motors:
1. Series Motor:
In series motor, the field winding is connected in series with the armature winding.
This has very high starting torque, as field winding also get high current.
As load changes, the speed shows large variation.
If load is reduced too much, the speed may increase to a dangerous level.
Applications: The series motor is used,
Where large torque is required for accelerating heavy masses quickly and load is directly coupled to motor. For example, in elevators and electric traction, railway engines, cranes, fans, air compressors etc.
Where the load is subjected to fluctuations and the speed is automatically required to reduce at higher torques and vice versa.
2. Shunt Motor:
In shunt motor, the field winding is connected in shunt (in parallel) with the armature winding.
It has good starting torque and speed can be maintained constant for all loads.
Applications: The shunt motor is used,
Where the speed is required to remain almost constant from no load to full load e. g. lathe, drilling, boring grinding machines, printing press, spinning and weaving machines etc.
Where the load has to be driven at a number of speeds and any one of which is required to remain nearly constant.
3. Compound Motor:
In compound motor, there are two sets of field windings on each pole, one is in series and the other is in parallel with the armature winding.
This motor shows characteristics in between those of series and shunt motor.
It has high starting torque and upper speed level can be limited.
Applications: Hence these are used where load suddenly comes and goes e. g. punching machine, cutting, stamping machines pumps, coal cutting machines etc.
Necessity of speed control
In Tape Recorder: To pull the tape at constant speed in playback and record mode. Speed of motor is five times during forward/rewind operation than the speed during play/record operation.
In film projectors: To synchronize the reproduced motion with natural one.
In Medical Electronics: Like in ECG, EEG machines to pull the strip chart paper at constant speed to get correct record.
In lifts, the motor has moderate speed. If the speed is very high it may affect blood pressure of passenger for tall building.
Electric cranes and trolleys are to be controlled by controlling speed of DC motor.
Speed of electric trains is very important to avoid accidents.
In CD/DVD Player: Most of the electronic instruments uses DC source and requires accurate speed control otherwise recording on disc and reading from disc varies. Hence a DC shunt type of motor is commonly used.
For performing various operations on the lathe machine and punching machines as well rolling machines, it is necessary to control the speed of DC motor for proper operation.
Equivalent circuit of a DC shunt motor:
To analyze the speed of DC shunt motor we have to consider the electrical equivalent circuit of a DC shunt motor.
Fig. Equivalent circuit of a DC shunt motor
Vdc=DC voltage
Ra =Armature resistance
La = Armature inductance
Eb=Back e.m.f.
Eb= K.Ø.N
Where K=constant, Ø=magnetic field, N=speed of armature
The counter emf or back emf generated in the armature winding has a polarity opposite to that of the applied voltage. It is represented by K.Ø.N.
The armature current ‘Ia’ depends on the difference (Vdc – Eb) and armature resistance ‘Ra’.
Analysis (for speed N):
Applying KVL for armature loop, we get,
IaRa +Eb =Vdc
IaRa + K.Ø.N =Vdc
KØN=Vdc – IaRa
Factors affecting the speed of the motor:
The formula for speed of DC motor is-
N=speed of armature
Vdc=DC voltage
Ia = Armature current
Ra =Armature resistance
Where K=constant, Ø=magnetic field,
The factors affecting DC motor are –
When applied voltage increases, the speed of DC motor increases
When armature current Ia increases speed of motor decreases
When magnetic field i.e. field current increases speed of motor decreases
Speed Control of a DC Motor:
Speed Control of DC Motor: (using transistor)
Working:
Voltage across resistor R2. It is given by,
From the above equation as R2 increases VR2 also increases, which increases forward bias of transistor and thus the speed of motor increases.
Similarly as R2 decreases VR2 also decreases which decreases forward bias of transistor and thus the speed of motor decreases.
When the value of R2 becomes zero Ohm, VR2 also becomes zero and transistor gets zero base bias & therefore speed of motor will be zero.
The power transistor 2N3055 is specially used in this circuit since it dissipates maximum heat.
A diode (1N4007) is connected across the motor to avoid the effect of back emf generated by the armature coil of the motor on the transistor.
Alternative circuit:
Speed Control of DC Motor: (using SCR)
Applications of DC motor:
1. Audio system: In audio tape recorder DC motor is used for moving audio cassette. For play and record functions comparatively low speed is necessary. For fast forward and rewind operations five times high speed is necessary. A synchronous motor is used in tape recorder transport mechanism. Speed of synchronous motor depends on supply frequency. Therefore speed remains constant even when there are changes in supply voltage or load.
In audio compact disk player, DC motor is used to rotate the disk. In this system motor speed are different for forward, rewind, play and record of disk.
2. Video system: In video system DC motor is required in drive system to control video head cylinder. There are two head gaps on drum. Tape is pressed against one or other gap while moving. Servo controlled motor provides precise speed to tape and drum containing heads. If there are changes in speed of motor, they are converted into control voltage which is supplied to the motor driver.
Different speeds of motor are required for read and write VCD and DVD.
3. Camera: In digital camera DC motor is required to adjust position of lens.
4. Computer peripherals: In CD drive of computer motor is required to rotate CD at different speeds for read and write purpose. In computer printer stepper motor is used. In CPU, DC motor is used in air cooling fan.
In robotics, electric motors are used robot wheels, legs, arm fingers, sensors etc. In industry, robots are for handling various processes for precise and better quality production. Choosing ideal motor for perfect robot is always tough task.
Applications of Servo motor:
In robotics
Servo stabilizers
Machine tools
Process controllers
Air craft control system
Applications of AC motor:
Fan blowers
Centrifugal pumps
Home refrigerators
Drilling machine
Table fans
Water pumps
Air conditioning (AC)
Small fans
Toys
Hair dryer
Questions
Marks wise Questions: 1M (2), 3M (2), 4M (1)
1. Fill in the blanks-
The part of motor which rotates is called as———. [M-15]
(a) Yoke (b) Armature (c) Stator (d) Brush
————— is a constant speed motor
(a)DC series motor (b) Single phase AC motor
(c) DC shunt motor (d) Three phase AC motor
The speed of DC motor can be increased by ——————.
(a) Increasing the field current (b) Decreasing the field current
(c) Decreasing the armature voltage (d) Keeping armature voltage constant.
To control the speed of DC motor in electronic devices generally vary ______
(a) Armature current (b) Field current (c) Applied voltage (d) Coil resistance
-----------motor has high starting torque.
(a) A.C. series (b) D.C. series (c) Induction
Motor converts _______. [O-09,M-14]
(a) Mechanical energy into electric energy
(b) Electrical energy into mechanical force.
(c) Electrical energy into torque.
(d) Force in to displacement.
DC motor is used in ----------
(i) Mixer (ii) Wasing Machine (iii) Camera [M-17]
2. Match the following-
A B
(a) Series motor (1) Constant speed/Drill machine
(b) Shunt motor [M-16] (2) High starting torque/Train/Elevators
(c) Compound motor (3) Speed control of DC motor
(d) SCR [M-11, M-13] (4) Punching machine
3. True or false-
As armature current increases, speed of motor also increases.
Speed of motor increases with increases with increase in supply voltage.
DC motor converts electrical energy into mechanical energy.
Rotating part of motor is called armature/rotor.
Stationary part of a D.C. motor is called armature. [O-11]
Series DC motor has high starting toque. [M-11]
Shunt DC motor has low starting speed/torque. [M-13]
Compound motor has both series and field winding.
An electric motor is a machine which converts electric energy into mechanical energy.
4. Answer the following-
What is motor? What are the three types of motors? [M-11, M-13]
Explain necessity to control speed of motor? [M-11, O-11, M-13]
Why speed control of D.C. motor circuit is necessary in audio system [M-12/ m-17]
State working principle of DC motor. [O-11, M-12]
Draw a simple circuit diagram using transistor to control the speed of DC motor and explain its working. [M-09, O-09, M-10, M-11, O-11, M-12, M-13, M-17]
Draw equivalent circuit of DC motor. State the expression for speed of DC motor. [M-09, O-09, M-10]
What is working principle of DC motor? State its different types with one application of each.
Write the formula for speed of DC motor. State on which factors speed depends
[M-10, M-11, O-11, M-13,M-14]
Explain factors affecting speed of D.C. motor and its characteristics. [M-12, M-17]
What are the important parts of DC motor? Explain them. [O-09]
Explain the applications of motor in audio and video system. [O-09, M-16]
Explain the terms torque, speed and back emf in DC motor. [M-10]
State types of DC motors. Draw their circuits and explain each. Write applications of each.
What is electric motor? State its types. [M-09]
Write applications of each (i) AC motor (ii) DC motor (iii) Servo motor. [O-09]
Draw internal structure of DC motor and write function of each part. [O-09, M-17]
Draw and explain construction of DC motor. [M-11, O-11, M-13,M-15]
Explain any three applications of DC motor. [M-10]
Enlist applications of DC motor. [M-11, M-13,M-14,M-15]
Write short note on DC shunt motor. [M-10]
Explain applications of DC motor in computer peripherals, camera and audio-video system.
State types of DC motors. Draw their circuits and explain each. [M-14]
Explain back emf in DC motor. [M-16]
Explain construction and working of DC motor. [M-16]
Draw equivalent of DC moror. [M-17]
