How does a Servo Motor work? How to use it?

Here is the document about the working of a servo motor based on the data collected  from multiple websites listed in the reading list below. Either you may download the pdf document provided or refer the reading list. 

Download Link (Servo Motor Working.pdf)


Reading list:

1. http://electrical4u.com/servo-motor-servo-mechanism-theory-and-working-principle/
2. https://www.servocity.com/html/how_do_servos_work_.html
3. http://en.wikipedia.org/wiki/Servomechanism
4. http://en.wikipedia.org/wiki/Pulse-position_modulation
5. http://arduino.cc/en/Reference/Servo

EE2251 - Electrical Machines-I Previous year QP Anna Univ.

Electrical Machines - I Previous year Anna Univ Question Papers for EEE students.

1. EE2251 - Electrical MachinesI_May-June_2012.pdf , Size: 835 KB
2. EE2251 - Electrical MachinesI_May-June_2013.pdf , Size: 478 KB
3. EE2251 - Electrical MachinesI_Nov-Dec_2012.pdf , Size: 477 KB

EE3205 - Electrical Machines and Drives Handwritten Notes for Mechatronics Students

EE3205 - Electrical Machines and Drives for Mechatronics Students by Mr. R. SivaBalaKrishnan

Download Link (Unit 1-4)

Video on Basic Electrical Instruments_Working Principles (playlist)

Basic video on operation of some electrical machines and concepts

1. woodencasing
2. springcontrol
3. singlephasemotor
4. pvcwiring
5. inductionwattmeter
6. fluidfriction
7. dynamometer
8. dcmotor
9. cleatwiring
10. alternator
11. airfriction
12. 3phasemotor
13. emf

Braking of DC Machines and Speed control of Induction machines

Connection diagram and Characteristics curves

.DOC File

.PDF File

Permanent Magnet Moving Coil (PMMC) Instrument

The permanent magnet moving coil instrument is the most accurate type for D.C. Measurements. The working principle of these instruments is the same as that of the d’Arsonval type of galvanometers, the difference being that a direct reading instrument is provided with a pointer and a scale

(Fig) Permanent magnet moving coil instrument

Construction of PMMC Instruments

•     The constructional features of this instrument are shown in Fig.
•     The moving coil is wound with many turns of enameled or silk covered copper wire.
•     The coil is mounted on rectangular aluminum former, which is pivoted on jeweled bearings.
•     The coils move freely in the field of a permanent magnet.
•     Most voltmeter coils are wound on metal frames to provide the required electro-magnetic damping.
•     Most ammeter coils, however, are wound on non-magnetic formers, because coil turns are effectively shorted by the ammeter shunt.
•     The coil itself, therefore, provides electro magnetic damping.

Magnet Systems

•     Old style magnet system consisted of relatively long U shaped permanent magnets having soft iron pole pieces.
•     Owing to development of materials like Alcomax and Alnico, which have a high co-ercive force, it is possible to use smaller magnet lengths and high field intensities.
•     The flux densities used in PMIMC instruments vary from 0.1 Wb/m to 1 Wb/m.

Control

•     When the coil is supported between two jewel bearings two phosphor bronze hairsprings provide the control torque.
•     These springs also serve to lead current in and out of the coil. The control torque is provided by the ribbon suspension as shown.
•     This method is comparatively new and is claimed to be advantageous as it eliminates bearing friction.

Damping

•     Damping torque is produced by movement of the aluminium former moving in the magnetic field of the permanent magnet.

Pointer and Scale

•     The pointer is carried by the spindle and moves over a graduated scale.
•     The pointer is of lightweight construction and, apart from those used in some inexpensive instruments has the section over the scale twisted to form a fine blade.

•      This helps to reduce parallax errors in the reading of the scale. When the coil is supported between two jewel bearings two phosphor bronze hairsprings provide the control torque.
•     These springs also serve to lead current in and out of the coil.

Torque Equation.

The torque equation of a moving coil instrument is given by

As the deflection is directly proportional to the current passing through the meter (K and G being constants) we get a uniform (linear) scale for the instrument.

 Errors in PMMC Instruments

The main sources of errors in moving coil instruments are due to

•     Weakening of permanent magnets due to ageing at temperature effects.
•     Weakening of springs due to ageing and temperature effects.
•     Change of resistance of the moving coil with temperature.

Advantages and Disadvantages of PMMC Instruments

The main advantages of PMMC instruments are

•     The scale is uniformly divided.
•     The power consumption is very low
•     The torque-weight ratio is high which gives a high accuracy. The accuracy is of the order of generally 2 percent of full-scale deflection.
•     Using different values for shunts and multipliers may use a single instrument for many different current and voltage ranges.
•     Since the operating forces are large on account of large flux densities, which may be as high as 0.5 Wb/m, the errors due to stray magnetic fields are small.
•     Self-shielding magnets make the core magnet mechanism particularly useful in aircraft and aerospace applications.

The chief disadvantages are

•     These instruments are useful only for D.C. The torque reverses if the current reverses. If the instrument is connected to a.c., the pointer cannot follow the rapid reversals and the deflection corresponds to mean torque, which is zero. Hence these instruments cannot be used for a.c.
•     The cost of these instruments is higher than that of moving iron instruments.

Faraday's Law of Electromagnetic Induction

Faraday's Law of electromagnetic induction is the basic principle behind the operation of Transformer, Induction motor, DC motor, DC generator and several other special machines.

It states that, the electromotive force (EMF) produced in a conductor is proportional to rate of change of magnetic flux linked with the conductor.

In other words,

For a transformer, Induction motor:
"Whenever a conductor is placed in a varying magnetic field, an emf is induced in the conductor."

i.e. in transformer and induction motor, varying magnetic flux is produced by exciting a coil with a time varying (AC) supply which links with the secondary (in transformer) or with the rotor conductors (in induction motor).

For a Generator:
"Whenever a conductor moves in a magnetic field such that it cuts the magnetic field, an emf is induced in the conductor."

Here in generator, flux linked with the conductors is varied by means of a mechanical force. i.e. the conductors are moved into and away from the magnetic field which means that when conductor moves towards a magnetic pole(field), flux linkage increases and when it moves away from a pole, flux linkage decreases thus a varying flux linkage is achieved mechanically.

For a motor (Converse):
"Whenever a current carrying conductor is placed in a magnetic field, it experiences a force."

Here, assume there is magnet (stator) and a current carrying conductor wound on a core(rotor). Current carrying conductor will behave as an electromagnet with it own poles. Now, the interaction of the stator and rotor poles produces a force (attractive or repulsive) which is directed in a rotational path to produce rotating motion of the motor.

Principle, Construction, EMF equation and methods of excitation of DC Machines

Principles of d.c. machines

D.C. machines are the electro mechanical energy converters which work from a d.c. source and generate mechanical power or convert mechanical power into a d.c. power.

Construction of d.c. machines

A D.C. machine consists mainly of two part the stationary part called stator and the rotating part called rotor. The stator consists of main poles used to produce magnetic flux ,commutating poles or interpoles in between the main poles to avoid sparking at the commutator but in the case of small machines sometimes the interpoles are avoided and finally the frame or yoke which forms the supporting structure of the machine. The rotor consist of an armature a cylindrical metallic body or core with slots in it to place armature windings or bars,a commutator and brush gears The magnetic flux path in a motor or generator is show below and it is called the magnetic structure of generator or motor. The major parts can be identified as, 1. Frame 2. Yoke 3. Poles Institute of Technology Madras 4. Armature 5. Commutator and brush gear 6. Commutating poles 7. Compensating winding 8. Other mechanical parts .

Frame: Frame is the stationary part of a machine on which the main poles and commutator poles are bolted and it forms the supporting structure by connecting the frame to the bed plate. The ring shaped body portion of the frame which makes the magnetic path for the magnetic fluxes from the main poles and interpoles is called Yoke.

Why we use cast steel instead of cast iron for the construction of Yoke?

In early days Yoke was made up of cast iron but now it is replaced by cast steel.This is because cast iron is saturated by a flux density of 0.8 Wb/sq.m where as saturation with cast iron steel is about 1.5 Wb/sq.m.So for the same magnetic flux density the cross section area needed for cast steel is less than cast iron hence the weight of the machine too.If we use cast iron there may be chances of blow holes in it while casting.so now rolled steels are developed and these have consistent magnetic and mechanical properties.

End Shields or Bearings: If the armature diameter does not exceed 35 to 45 cm then in addition to poles end shields or frame head with bearing are attached to the frame. If the armature diameter is greater than 1m pedestral type bearings are mounted on the machine bed plate outside the frame. These bearings could be ball or roller type but generally plain pedestral bearings are employed. If the diameter of the armature is large a brush holder yoke is generally fixed to the frame.

Main poles:Solid poles of fabricated steel with separate/integral pole shoes are fastened to the frame by means of bolts. Pole shoes are generally laminated. Sometimes pole body and pole shoe are formed from the same laminations. The pole shoes are shaped so as to have a slightly increased air gap at the tips. Inter-poles are small additional poles located in between the main poles. These can be solid, or laminated just as the main poles. These are also fastened to the yoke by bolts. Sometimes the yoke may be slotted to receive these poles. The inter poles could be of tapered section or of uniform cross section. These are also called as commutating poles or com poles. The width of the tip of the com pole can be about a rotor slot pitch.

Armature The armature is where the moving conductors are located. The armature is constructed by stacking laminated sheets of silicon steel. Thickness of these lamination is kept low to reduce eddy current losses. As the laminations carry alternating flux the choice of suitable material, insulation coating on the laminations, stacking it etc are to be done more carefully. The core is divided into packets to facilitate ventilation. The winding cannot be placed on the surface of the rotor due to the mechanical forces coming on the same. Open parallel sided equally spaced slots are normally punched in the rotor laminations. These slots house the armature winding. Large sized machines employ a spider on which the laminations are stacked in segments. End plates are suitably shaped so as to serve as ’Winding supporters’. Armature construction process must ensure provision of sufficient axial and radial ducts to facilitate easy removal of heat from the armature winding.

Field windings: In the case of wound field machines (as against permanent magnet excited machines) the field winding takes the form of a concentric coil wound around the main poles. These carry the excitation current and produce the main field in the machine. Thus the poles are created electromagnetically. Two types of windings are generally employed. In shunt winding large number of turns of small section copper conductor isof Technology Madras used. The resistance of such winding would be an order of magnitude larger than the armature winding resistance. In the case of series winding a few turns of heavy cross section conductor is used. The resistance of such windings is low and is comparable to armature resistance. Some machines may have both the windings on the poles. The total ampere turns required to establish the necessary flux under the poles is calculated from the magnetic circuit calculations. The total mmf required is divided equally between north and south poles as the poles are produced in pairs. The mmf required to be shared between shunt and series windings are apportioned as per the design requirements. As these work on the same magnetic system they are in the form of concentric coils. Mmf ’per pole’ is normally used in these calculations. Armature winding As mentioned earlier, if the armature coils are wound on the surface of the armature, such construction becomes mechanically weak. The conductors may fly away when the armature starts rotating. Hence the armature windings are in general pre-formed, taped and lowered into the open slots on the armature. In the case of small machines, they can be hand wound. The coils are prevented from flying out due to the centrifugal forces by means of bands of steel wire on the surface of the rotor in small groves cut into it. In the case of large machines slot wedges are additionally used to restrain the coils from flying away. The end portion of the windings are taped at the free end and bound to the winding carrier ring of the armature at the commutator end. The armature must be dynamically balanced to reduce the centrifugal forces at the operating speeds. Compensating winding One may find a bar winding housed in the slots on the pole shoes. This is mostly found in d.c. machines of very large rating. Such winding is called compensating winding. In smaller machines, they may be absent.

Commutator: Commutator is the key element which made the d.c. machine of the present day possible. It consists of copper segments tightly fastened together with mica/micanite insulating separators on an insulated base. The whole commutator forms a rigid and solid assembly of insulated copper strips and can rotate at high speeds. Each com- mutator segment is provided with a ’riser’ where the ends of the armature coils get connected. The surface of the commutator is machined and surface is made concentric with the shaft and the current collecting brushes rest on the same. Under-cutting the mica insulators that are between these commutator segments has to be done periodi- cally to avoid fouling of the surface of the commutator by mica when the commutator gets worn out. Some details of the construction of the commutator are seen in Fig. 8.

Brush and brush holders: Brushes rest on the surface of the commutator. Normally electro-graphite is used as brush material. The actual composition of the brush depends on the peripheral speed of the commutator and the working voltage. The hardness of the graphite brush is selected to be lower than that of the commutator. When the brush wears out the graphite works as a solid lubricant reducing frictional coefficient. More number of relatively smaller width brushes are preferred in place of large broad brushes. The brush holders provide slots for the brushes to be placed. The connection Brush holder with a Brush and Positioning of the brush on the commutator from the brush is taken out by means of flexible pigtail. The brushes are kept pressed on the commutator with the help of springs. This is to ensure proper contact between the brushes and the commutator even under high speeds of operation. Jumping of brushes must be avoided to ensure arc free current collection and to keep the brushcontact drop low. Other mechanical parts End covers, fan and shaft bearings form other important me- chanical parts. End covers are completely solid or have opening for ventilation. They support the bearings which are on the shaft. Proper machining is to be ensured for easy assembly. Fans can be external or internal. In most machines the fan is on the non-commutator end sucking the air from the commutator end and throwing the same out. Adequate quantity of hot air removal has to be ensured.

Bearings: Small machines employ ball bearings at both ends. For larger machines roller bearings are used especially at the driving end. The bearings are mounted press-fit on the shaft. They are housed inside the end shield in such a manner that it is not necessary to remove the bearings from the shaft for dismantling.

Generator E.M.F Equation:

Let,=

Φ = flux/pole in weber

Z = total number of armture conductors = No.of slots x No.of conductors/slot

P = No.of generator poles

A = No.of parallel paths in armature

N = armature rotation in revolutions per minute (r.p.m)

E = e.m.f induced in any parallel path in armature Generated e.m.f

Eg = e.m.f generated in any one of the parallel paths i.e E.

Average e.m.f geneated /conductor = dΦ/dt volt (n=1).

Now, flux cut/conductor in one revolution dΦ = ΦP Wb

No.of revolutions/second = N/60

Time for one revolution, dt = 60/N second

Hence, according to Faraday's Laws of Electroagnetic Induction, E.M.F generated/conductor is For a simplex wave-wound generator

No.of parallel paths = 2

No.of conductors (in series) in one path = Z/2

E.M.F. generated/path is For a simplex lap-wound generator

No.of parallel paths = P

No.of conductors (in series) in one path = Z/P

E.M.F.generated/path In general generated e.m.f where A = 2 - for simplex wave-winding A = P - for simplex lap-winding

METHODS OF EXCITATION:

·         Various methods of excitation of the field windings are shown in Fig.

Figure shows Field-circuit connections of dc machines: (a) separate excitation, (b) series, (c) shunt, (d) compound.

Consider first dc generators.

·         Separately-excited generators.

·          Self-excited generators: series generators, shunt generators, compound generators.

·         With self-excited generators, residual magnetism must be present in the machine iron to get the self-excitation process started.

·         N.B.: long- and short-shunt, cumulatively and differentially compound.

·         Typical steady-state volt-ampere characteristics are shown in Fig.7.5, constant-speed operation being assumed.

·         The relation between the steady-state generated emf Ea and the armature terminal voltage Va is Va=Ea−IaRa            (7.10)

Figure Volt-ampere characteristics of dc generators. Any of the methods of excitation used for generators can also be used for motors.

·         Typical steady-state dc-motor speed-torque characteristics are shown in Fig.7.6, in which it is assumed that the motor terminals are supplied from a constant-voltage source.

·         In a motor the relation between the emf Ea generated in the armature and and the armature terminal voltage Va is

Va=Ea+IaRa               (7.11)

·         The application advantages of dc machines lie in the variety of performance characteristics offered by the possibilities of shunt, series, and compound excitation.

EC2201 – Electrical Engineering 2 Mark Questions and Answers UNIT -I DC MACHINES

EE 2201 – Electrical Engineering

2 Mark Questions and Answers

UNIT -I DC MACHINES

1.      State the basic parts of a DC machine.

Stationary Parts: Frame, Main pole, field coils, interpoles, interpole winding Rotating Parts: Armature core, Armature winding, Commutator, Shaft.

2.      Name the various parts of a DC machine that control the magnetic circuit.

Poles, Air-gap, Armature core, Yoke.

3.      What is prime mover?

The basic source of mechanical power which drives the armature of the generator is called prime mover.

4.      How is voltage generated in rotating machines?

In rotating machines voltage is generated in windings or group of coils by rotating them through a magnetic field or by mechanically rotating a magnetic field past the winding or by designing the magnetic circuit so that the reluctance varies with rotation of the rotor.

5.      Write down the emf equation for d.c generator.

E = (ΦNZ / 60)(P/A) V Where, P= number of poles Z= Total number of conductors A= number of parallel paths Φ= flux per pole N= speed in rpm

6.      Why is Commutator employed in d.c machines? Or what is the function of a commutator in a DC generator?

·         Conduct electricity between armature and fixed brushes

·         Converts alternating emf into unidirectional emf and vice versa

7.      How will you change the direction of rotation of a d.c motor?

Either the direction of the main field or the direction of current through the armature conductors is to be reserved.

8.      What is back emf in d.c motors?

As the motor armature rotates, the system of conductor come across alternate North and South Pole magnetic fields causing an emf induced in the conductors. The direction of the emf induced in the conductors is in the direction opposite to the current .As this emf always opposes the flow of current in motor operation it is called back emf.

9.      Under what condition the mechanical power developed in a dc motor will be maximum?

Condition for mechanical power developed to be maximum is Eb=Va/2 or Ia= Va / 2Ra

10.  What is the function of a no-voltage release coil provided in a dc motor starter?

As long as the supply voltage is on healthy condition the current through the NVR coil produce enough magnetic force of attraction and retain the starter handle in the ON position against spring force. When the supply voltage fails or becomes lower than a prescribed value the electromagnet may not have enough force and the handle will come back to OFF position due to spring force automatically. Thus, a no-voltage or under voltage protections is given to the motor.

11.  Define critical field resistance in dc shunt generator.

Critical field resistance is defined as the resistance of the field circuit which will cause the shunt generator just to build up its emf at a specified field.

12.  Why is the emf not zero when the field current is reduced to zero in a dc generator?

Even after the field current/magnetizing force is reduced to zero the machine is left out with some flux as residue. Emf due to this residual flux is available when field current is zero.

13.  Define the term „critical speed‟ in dc shunt generator.

Critical sped is defined as the speed at which the generator is to be driven to cause self-excited generator to Build up its emf for the given field circuit resistance.

14.  On what occasions dc generators may not have residual flux?

·         The generator may be put for its first operation after its construction.

·         In previous operation the generator would have been fully demagnetized.

15.  What are the conditions to be fulfilled for a dc shunt generator to build up emf?

·         The generator should have residual flux

·         The field winding should be connected in such a manner that the flux set up by the field winding should be in the same direction as that of residual flux

·         The field circuit resistance should be less than critical field resistance

·         Load circuit resistance should be above its critical load resistance

16.  What are the types of DC starters?

1. Two point starters 2. Three point starters 3. Four point starters

17.  What are the major categories of losses in a DC machine?

Magnetic losses, Electrical losses, Mechanical losses

18.  Name the different types of DC motors.

Shunt motor, Series motor, cumulative compound motor, differential compound Motor.

19.  Name any four applications of DC series motors.

Electric traction, Food mixies, Hoist work, Drilling machine

20.  Why starters are used for DC motors? Or Why a starter is necessary for a DC motor? Starters are used in DC motors to limit the starting current within about 2 to 3 times the rated current by adding resistance in series with the armature circuit. Apart from starting resistances starters are invariably fitted with protective devices such as No-voltage protection.

21.  Why are carbon brushes preferred for dc machines?

The high contact resistance carbon brushes help the current in the coil undergoing commutation to attain its full value in the reverse direction at the end of commutation. The carbon brushes also lubricate and give less wear and tear on commutator surface.

22.  Name any two applications of DC series generator.

Booster, electric welding, Constant current source, Constant illumination

23.  What is the basic principle of a dc generator?

Basic principle of a dc generator is Faraday’s law of electromagnetic induction. i.e. whenever a conductor is moved in a magnetic field, dynamically induced emf is produced in that conductor.

24.  What is the purpose of yoke in a dc machine? Or The outer frame of a DC machine serves double purpose. What are they?

·         It acts as a protecting cover for the whole machine and provides mechanical support for the machine.

·         It carries the magnetic flux produced by the poles. The flux per pole divides at the yoke so that; the yoke carries only half the flux produced by each pole.

25.  What are the causes of failure of dc shunt generator to exite?

·         The residual magnetism may not be present in the poles.

·         The field winding may not be properly connected with armature.

·         Under no load condition, the shunt field resistance should be greater than the critical field resistance.

·         Under loaded condition, the shunt field resistance should be less than the critical field resistance.

26.  Why a dc shunt motor is also called a constant flux motor or constant speed motor?

In shunt motor, flux produced by field winding is directly proportional to the field current i.e. (Φ α Ish). Here, the input voltage is constant and so the flux is also constant. Therefore, DC shunt motor is also called a constant flux motor or constant speed motor.

27.  Why series motor cannot be started without any load?

In dc series motor, flux is directly proportional to armature current. i.e. (Φ α Ia). Under no load condition, the armature current is very low and flux also be less. By using the formula N α (1/ Φ), here Φ is less; the motor speed will be very high. Due to this motor will be damaged. Hence dc series motor should always be started with some load on the shaft.

28.  What is the function of starters in DC motor?

·         To limit the starting current.

·         To protect against low voltage and over load condition.

29.  List the important parts of a DC starter.

Starting resistance, Handle, over load relay, No voltage relay

30.  What are the drawbacks of brake test on DC machines?

·         The brake test can be used for small motors only, because in case of large motors, it is difficult to dissipate the large amount of heat generated at the brake.

·         This method cannot be used for determining internal losses.

·         The output of the motor cannot be measured directly.