Electrical Machines
D.C. Motors
Introduction
D. C. motors are seldom used in ordinary applications because all electric supply companies furnish alternating current However, for special applications such as in steel mills, mines and electric trains, it is advantageous to convert alternating current into direct current in order to use d.c. motors. The reason is that speed/torque characteristics of d.c. motors are much more superior to that of a.c. motors. Therefore, it is not surprising to note that for industrial drives, d.c. motors are as popular as 3-phase induction motors. Like d.c. generators, d.c. motors are also of three types viz., series-wound, shunt-wound and compoundwound. The use of a particular motor depends upon the mechanical load it has to drive.
D.C. Motor Principle
A machine that converts d.c. power into mechanical power is known as a d.c. motor. Its operation is based on the principle that when a current carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force. The direction of this force is given by Fleming’s left hand rule and magnitude is given by;
F = BIl newtons
Basically, there is no constructional difference between a d.c. motor and a d.c. generator. The same d.c. machine can be run as a generator or motor.
D. C. motors are seldom used in ordinary applications because all electric supply companies furnish alternating current However, for special applications such as in steel mills, mines and electric trains, it is advantageous to convert alternating current into direct current in order to use d.c. motors. The reason is that speed/torque characteristics of d.c. motors are much more superior to that of a.c. motors. Therefore, it is not surprising to note that for industrial drives, d.c. motors are as popular as 3-phase induction motors. Like d.c. generators, d.c. motors are also of three types viz., series-wound, shunt-wound and compoundwound. The use of a particular motor depends upon the mechanical load it has to drive.
D.C. Motor Principle
A machine that converts d.c. power into mechanical power is known as a d.c. motor. Its operation is based on the principle that when a current carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force. The direction of this force is given by Fleming’s left hand rule and magnitude is given by;
F = BIl newtons
Basically, there is no constructional difference between a d.c. motor and a d.c. generator. The same d.c. machine can be run as a generator or motor.
Working of D.C. Motor
Consider a part of a multipolar d.c. motor as shown in Fig. (4.1). When the terminals of the motor are connected to an external source of d.c. supply:
(i) the field magnets are excited developing alternate N and S poles;
(ii) the armature conductors carry ^currents. All conductors under N-pole carry currents in one direction while all the conductors under S-pole carry currents in the opposite direction.
Consider a part of a multipolar d.c. motor as shown in Fig. (4.1). When the terminals of the motor are connected to an external source of d.c. supply:
(i) the field magnets are excited developing alternate N and S poles;
(ii) the armature conductors carry ^currents. All conductors under N-pole carry currents in one direction while all the conductors under S-pole carry currents in the opposite direction.
Suppose the conductors under N-pole
carry currents into the plane of the paper and those under S-pole carry
currents out of the plane of the paper as shown in Fig.(4.1). Since each
armature conductor is carrying current and is placed in the magnetic
field, mechanical force acts on it. Referring to Fig. (4.1) and applying
Fleming’s left hand rule, it is clear that force on each conductor is
tending to rotate the armature in anticlockwise direction. All these
forces add together to produce a driving torque which sets the armature
rotating. When the conductor moves from one side of a brush to the
other, the current in that conductor is reversed and at the same time it
comes under the influence of next pole which is of opposite polarity.
Consequently, the direction of force on the conductor remains the same.
Types of D.C. Motors
Like generators, there are three types
of d.c. motors characterized by the connections of field winding in
relation to the armature viz.:
(i) Shunt-wound motor in which the field winding is connected in parallel with the armature [See Fig. 4.4]. The current through the shunt field winding is not the same as the armature current. Shunt field windings are designed to produce the necessary m.m.f. by means of a relatively large number of turns of wire having high resistance. Therefore, shunt field current is relatively small compared with the armature current.
(i) Shunt-wound motor in which the field winding is connected in parallel with the armature [See Fig. 4.4]. The current through the shunt field winding is not the same as the armature current. Shunt field windings are designed to produce the necessary m.m.f. by means of a relatively large number of turns of wire having high resistance. Therefore, shunt field current is relatively small compared with the armature current.
(ii) Series-wound motor
in which the field winding is connected in series with the armature
[See Fig. 4.5]. Therefore, series field winding carries the armature
current. Since the current passing through a series field winding is the
same as the armature current, series field windings must be designed
with much fewer turns than shunt field windings for the same m.m.f.
Therefore, a series field winding has a relatively small number of turns
of thick wire and, therefore, will possess a low resistance.
(iii) Compound-wound motor
which has two field windings; one connected in parallel with the
armature and the other in series with it. There are two types of
compound motor connections (like generators). When the shunt field
winding is directly connected across the armature terminals [See Fig.
4.6], it is called short-shunt connection. When the shunt winding is so
connected that it shunts the series combination of armature and series
field [See Fig. 4.7], it is called long-shunt connection
The compound machines (generators or
motors) are always designed so that the flux produced by shunt field
winding is considerably larger than the flux produced by the series
field winding. Therefore, shunt field in compound machines is the basic
dominant factor in the production of the magnetic field in the machine.
Losses in a D.C. Motor
The losses occurring in a d.c. motor are
the same as in a d.c. generator (i) copper losses (ii) Iron losses or
magnetic losses (iii) mechanical losses As in a generator, these losses
cause (a) an increase of machine temperature and (b) reduction in the
efficiency of the d.c. motor.
The following points may be noted:
The following points may be noted:
(i)
Apart from armature Cu loss, field Cu loss and brush contact loss, Cu
losses also occur in interpoles (commutating poles) and compensating
windings. Since these windings carry armature current (Ia),
Loss in interpole winding = Ia 2× Resistance of interpole winding
Loss in compensating winding = Ia 2× Resistance of compensating winding
(ii) Since d.c. machines (generators or motors) are generally operated at constant flux density and constant speed, the iron losses are nearly constant.
(iii) The mechanical losses (i.e. friction and windage) vary as the cube of the speed of rotation of the d.c. machine (generator or motor). Since d.c. machines are generally operated at constant speed, mechanical losses are
considered to be constant.
Loss in compensating winding = Ia 2× Resistance of compensating winding
(ii) Since d.c. machines (generators or motors) are generally operated at constant flux density and constant speed, the iron losses are nearly constant.
(iii) The mechanical losses (i.e. friction and windage) vary as the cube of the speed of rotation of the d.c. machine (generator or motor). Since d.c. machines are generally operated at constant speed, mechanical losses are
considered to be constant.
Commutation in D.C. Motors
Since the armature of a motor is the
same as that of a generator, the current from the supply line must
divide and pass through the paths of the armature windings.
In order to produce unidirectional force
(or torque) on the armature conductors of a motor, the conductors under
any pole must carry the current in the same direction at all times.
This is illustrated in Fig. (4.10). In this case, the current flows away
from the observer in the conductors under the N-pole and towards the
observer in the conductors under the S-pole. Therefore, when a conductor
moves from the influence of N-pole to that of S-pole, the direction of
current in the conductor must be reversed. This is termed as
commutation. The function of the commutator and the brush gear in a d.c.
motor is to cause the reversal of current in a conductor as it moves
from one side of a brush to the other. For good commutation, the
following points may be noted:
(i) If a motor does not have commutating poles (compoles), the brushes
must be given a negative lead i.e., they must be shifted from G.N.A.
against the direction of rotation of, the motor.
(ii) By using interpoles, a d.c. motor can be operated with fixed brush
positions for all conditions of load. For a d.c. motor, the commutating
poles must have the same polarity as the main poles directly back of
them. This is the opposite of the corresponding relation in a d.c.
generator.
Note. A d.c. machine may be used as a motor or a generator without changing the commutating poles connections. When the operation of a d.c. machine changes from generator to motor, the direction of the armature current reverses. Since commutating poles winding carries armature current, the polarity of commutating pole reverses automatically to the correct polarity.
must be given a negative lead i.e., they must be shifted from G.N.A.
against the direction of rotation of, the motor.
(ii) By using interpoles, a d.c. motor can be operated with fixed brush
positions for all conditions of load. For a d.c. motor, the commutating
poles must have the same polarity as the main poles directly back of
them. This is the opposite of the corresponding relation in a d.c.
generator.
Note. A d.c. machine may be used as a motor or a generator without changing the commutating poles connections. When the operation of a d.c. machine changes from generator to motor, the direction of the armature current reverses. Since commutating poles winding carries armature current, the polarity of commutating pole reverses automatically to the correct polarity.
Armature Reaction in D.C. Motors
As in a d.c. generator, armature
reaction also occurs in a d.c. motor. This is expected because when
current flows through the armature conductors of a d.c. motor, it
produces flux (armature flux) which lets on the flux produced by the
main poles. For a motor with the same polarity and direction of rotation as is for generator, the direction of armature reaction field is reversed.
(i) In a generator, the armature current flows in the direction of the induced e.m.f. (i.e. generated e.m.f. Eg) whereas in a motor, the armature current flows against the induced e.m.f. (i.e. back e.m.f. Eg). Therefore, it should be expected that for the same direction of rotation and field polarity, the armature flux of the motor will be in the opposite direction to that of the generator. Hence instead of the main flux being distorted in the direction of rotation as in a generator, it is distorted opposite to the direction of rotation. We can conclude that:
Armature reaction in a d.c. generator weakens the jinx at leading pole tips and strengthens the flux at trailing pole tips while the armature reaction in a d. c. motor produces the opposite effect.
(ii) In case of a d.c. generator, with brushes along G.N.A. and no commutating poles used, the brushes must be shifted in the direction of rotation (forward lead) for satisfactory commutation. However, in case of a d.c. motor, the brushes are given a negative lead i.e., they are shifted against the direction of rotation.
(i) In a generator, the armature current flows in the direction of the induced e.m.f. (i.e. generated e.m.f. Eg) whereas in a motor, the armature current flows against the induced e.m.f. (i.e. back e.m.f. Eg). Therefore, it should be expected that for the same direction of rotation and field polarity, the armature flux of the motor will be in the opposite direction to that of the generator. Hence instead of the main flux being distorted in the direction of rotation as in a generator, it is distorted opposite to the direction of rotation. We can conclude that:
Armature reaction in a d.c. generator weakens the jinx at leading pole tips and strengthens the flux at trailing pole tips while the armature reaction in a d. c. motor produces the opposite effect.
(ii) In case of a d.c. generator, with brushes along G.N.A. and no commutating poles used, the brushes must be shifted in the direction of rotation (forward lead) for satisfactory commutation. However, in case of a d.c. motor, the brushes are given a negative lead i.e., they are shifted against the direction of rotation.
With no commutating poles used, the brushes are given a forward lead in a d.c. generator and backward lead in a d.c. motor.
(iii) By using commutating poles (compoles), a d.c. machine can be operated with fixed brush positions for all conditions of load. Since commutating poles windings carry the armature current, then, when a machine changes from generator to motor (with consequent reversal of current), the polarities of commutating poles must be of opposite sign.
(iii) By using commutating poles (compoles), a d.c. machine can be operated with fixed brush positions for all conditions of load. Since commutating poles windings carry the armature current, then, when a machine changes from generator to motor (with consequent reversal of current), the polarities of commutating poles must be of opposite sign.
Therefore, in a d.c. motor, the commutating poles must have the same
polarity as the main poles directly back of them. This is the opposite of
the corresponding relation in a d.c. generator.
polarity as the main poles directly back of them. This is the opposite of
the corresponding relation in a d.c. generator.
Torque and Speed of a D.C. Motor
For any motor, the torque and speed are
very important factors. When the torque increases, the speed of a motor
increases and vice-versa. We have seen that for a d.c. motor;
N = K (V- IaRa)/ Ф = K Eb/ Ф…………………………………………….(i)
Ta α ФIa…………………………………………………………………………(ii)
If the flux decreases, from Eq.(i), the
motor speed increases but from Eq.(ii) the motor torque decreases. This
is not possible because the increase in motor speed must be the result
of increased torque. Indeed, it is so in this case. When the flux
decreases slightly, the armature
current increases to a large value. As a result, in spite of the
weakened field, the torque is momentarily increased to a high value and
will exceed considerably the value corresponding to the load. The
surplus torque available causes the motor to accelerate and back e.m.f
(Ea=PФZN/60A)to rise. Steady conditions of speed will ultimately be achieved when back e.m.f. has risen to such a value that armature current[Ia = (V- Ea)/ Ra]develops torque just sufficient to drive the load.
Illustration
Let us illustrate the above point with a numerical example. Suppose a 400 V
shunt motor is running at 600 r.p.m., taking an armature current of 50 A. The armature resistance is 0.28 Ω. Let us see the effect of sudden reduction of flux by 5% on the motor.
Let us illustrate the above point with a numerical example. Suppose a 400 V
shunt motor is running at 600 r.p.m., taking an armature current of 50 A. The armature resistance is 0.28 Ω. Let us see the effect of sudden reduction of flux by 5% on the motor.
Initially (prior to weakening of field), we have,
Ea = V-IaRa= 400 – 50 × 0.28 = 386 volts
We know that Eb α Ф N. If the flux is reduced suddenly, Eb α Ф because inertia
of heavy armature prevents any rapid change in speed. It follows that when the flux is reduced by 5%, the generated e.m.f. must follow suit. Thus at the instant of reduction of flux, E’b = 0.95 × 386 = 366.7 volts.
of heavy armature prevents any rapid change in speed. It follows that when the flux is reduced by 5%, the generated e.m.f. must follow suit. Thus at the instant of reduction of flux, E’b = 0.95 × 386 = 366.7 volts.
Instantaneous armature current is
I’a=(V- E’b)/ Ra =(400-366.7)/0.28=118.9A
Note that a sudden reduction of 5% in
the flux has caused the armature current to increase about 2.5 times the
initial value. This will result in the production of high value of
torque. However, soon the steady conditions will prevail. This will
depend on the system inertia; the more rapidly the motor can alter the
speed, the sooner the e.m.f. rises and the armature current falls.
Speed of a D.C. Motor
Eb = V-IaRa
But Eb=PФZN/60A
PФZN/60A = V- IaRa
Or N = (V- IaRa)/ Ф × 60A/ PZ
Or N = K (V- IaRa)/ Ф
But V- IaRa = Ea
Therefore N= K Eb/ Ф
Or N α Eb/ Ф
Therefore, in a d.c. motor, speed is directly proportional to back e.m.f. Eb and inversely proportional to flux per pole Ф.
Speed Relations
If a d.c. motor has initial values of speed, flux per pole and back e.m.f. as N1 ,Ф1 and Eb1 respectively and the corresponding final values are N2 ,Ф2 and Eb2 then,
N1 α Eb1/ Ф1 and N2 α Eb2/ Ф2
Therefore N2/ N1 = (Eb2/ Eb1) ×( Ф1 / Ф2)
(i) For a shunt motor, flux practically remains constant so that Ф1 = Ф2.
therefore N2/ N1 = Eb2/ Eb1
(ii) For a series motor, Ф α Ia prior to saturation.
therefore N2/ N1 = (Eb2/ Eb1) × (Ia1/Ia2)
where Ia1 = initial armature current
Ia2 = final armature current
Ia2 = final armature current
Speed Regulation
The speed regulation of a motor is the
change in speed from full-load to no-load and is expressed as a
percentage of the speed at full-load i.e.
% Speed regulation = [( N.L. speed - F.L.speed)/F.L.speed ] × 100
=[(No -N)/N] × 100
where No = No – load .speed
N = Full – load speed
N = Full – load speed
Shaft Torque (Tsh)
The torque which is available at the motor shaft for doing useful work is known as shaft torque. It is represented by Tsh. Fig. (4.9) illustrates the concept of shaft torque. The total or gross torque Ta
developed in the armature of a motor is not available at the shaft
because a part of it is lost in overcoming the iron and frictional
losses in the motor. Therefore, shaft torque Tsh is somewhat less than
the armature torque Ta. The difference Ta – Tsh is called lost torque.
For example, if the iron and frictional losses in a motor are 1600 W and the
motor runs at 800 r.p.m., then,
Ta - Tsh =9.55 × 1600 /800 =19.1 N-m
As stated above, it is the shaft torque Tsh that produces the useful output. If the speed of the motor is N r.p.m., then,
Output in watts= 2πN Tsh/60
or Tsh =Output in watts /(2πN /60 ) N-m
or Tsh = 9.55 ×Output in watts /N N-m
Brake Horse Power (B.H.P.)
W.D./revolution = force x distance moved in 1 revolution
F × 2π r = 2π×Tsh J
W.D./minute = 2π N Tsh J
W.D./ sec= 2πNTsh J/60 jS-1 or watt=2πNTsh J/(60 ×746) H.P.
Useful output power =2πNTsh J/(60 ×746) H.P.
or B.H.P. =2πNTsh J/(60 ×746)
Armature Torque of D.C. Motor
Torque is the turning moment of a force about an axis and is measured by the
product of force (F) and radius (r) at right angle to which the force acts i.e.
D.C. Motors torque
T = F × r
product of force (F) and radius (r) at right angle to which the force acts i.e.
D.C. Motors torque
T = F × r
In a d.c. motor, each conductor is acted
upon by a circumferential force F at a distance r, the radius of the
armature (Fig. 4.8). Therefore, each conductor exerts a torque, tending
to rotate the armature. The sum of the torques due to all armature
conductors is known as gross or armature torque (Ta).
Let in a d.c. motor
r = average radius of armature in m
l = effective length of each conductor in m
r = average radius of armature in m
l = effective length of each conductor in m
Z = total number of armature conductors
A = number of parallel paths
i = current in each conductor = Ia/A
B = average flux density in Wb/m2
Φ = flux per pole in Wb
P = number of poles
B = average flux density in Wb/m2
Φ = flux per pole in Wb
P = number of poles
Force on each conductor, F = B i l newtons
Torque due to one conductor = F × r newton- metre
Total armature torque, Ta = Z F r newton-metre
= Z B i l r
Total armature torque, Ta = Z F r newton-metre
= Z B i l r
Now i = Ia/A, B = Φ/a where a is the x-sectional area of flux path per pole at
radius r. Clearly, a = 2πr l /P.
radius r. Clearly, a = 2πr l /P.
Ta = Z × (Ф/a)×( Ia/A)×l×r
Ta = Z × (ФP/2πr l)×( Ia/A)×l×r = Z Ф IaP/(2πA) N-m
or Ta = 0.159Z Ф Ia(P/A) N-m……………………………………(i)
so Ta
Ta α Ф Ia
Hence torque in a d.c. motor is directly proportional to flux per pole and
armature current.
armature current.
(i) For a shunt motor, flux Φ is practically constant.
Ta α Ia
(ii) For a series motor, flux Φ is directly proportional to armature current Ia
provided magnetic saturation does not take place.
provided magnetic saturation does not take place.
Ta α Ia2
up to magnetic saturation
Alternative expression for Ta
Eb = PФZN/60A
(60×Eb) /N= PФZ/A
From Eq.(i), we get the expression of Ta as:
Ta =0.159×(60× Eb/N)× Ia
Ta =9.55×( EbIa/N)
Voltage & Power Equation of D.C. Motor
Let in a d.c. motor (See Fig. 4.3),
V = applied voltage
Eb = back e.m.f.
Ra = armature resistance
Ia = armature current
Eb = back e.m.f.
Ra = armature resistance
Ia = armature current
Since back e.m.f. Eb acts in opposition to the applied voltage V, the net voltage across the armature circuit is V- Eb. The
armature current Ia is given by;
armature current Ia is given by;
Ia = (V – Eb)/ Ra
or V = Eb + IaRa ……………………………..(i)
This is known as voltage equation of the d.c. motor.
Power Equation
If Eq.(i) above is multiplied by Ia throughout, we get,
VIa = EbIa +I2aRa
VIa= electric power supplied to armature (armature input)
EbIa = power developed by armature (armature output)
I2aRa = electric power wasted in armature (armature Cu loss)
EbIa = power developed by armature (armature output)
I2aRa = electric power wasted in armature (armature Cu loss)
Thus out of the armature input, a small portion (about 5%) is wasted as a I2aRa and the remaining portion EbIa is converted into mechanical power within the armature.
Condition For Maximum Power
The mechanical power developed by the motor is Pm= EbIa
Now Pm=VIa -I2aRa
Since, V and Ra are fixed, power developed by the motor depends upon armature current. For maximum power, dPm/dIa should be zero.
dPm/dIa = V – 2IaRa
or IaRa = V/2
Now, V = Eb + IaRa =Eb + V/2
therefore Eb= V/2
Hence mechanical power developed by the motor is maximum when back e.m.f. is equal to half the applied voltage.
Limitations
In practice, we never aim at achieving maximum power due to the following reasons:
(i) The armature current under this condition is very large—much excess of rated current of the machine.
(ii) Half of the input power is wasted in the armature circuit. In fact, if we take into account other losses (iron and mechanical), the efficiency will be well below 50%.
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.
Let us check the parts in detail
The field coils are supported by the pole shoe and it spread out the flux in the airgap and owing to its large cross-section it reduces the magnetic reluctance.
The general appearance of commutator when completed and commutator and armature assembly is shown below.
Even for soft brushes there is wear but it takes place gradually.Though the mica segments between the commutator doesn’t wear easily but it reduces the effective contact with the segments and it may result in sparking and damage to the commutator.So to reduce this the micas ‘undercut’ a level below the commutator surface using a narrow milling cutter.
In practice, we never aim at achieving maximum power due to the following reasons:
(i) The armature current under this condition is very large—much excess of rated current of the machine.
(ii) Half of the input power is wasted in the armature circuit. In fact, if we take into account other losses (iron and mechanical), the efficiency will be well below 50%.
Construction of D.C. Machines
A D.C. machine consists mainly of two part the stationary part called stator and the rotating part called stator.
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.
Let us check the parts in detail
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.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. Field Poles
In early stages or say in the case of small machines the poles were cast integral to the yoke.But nowadays we use completely laminated pole or solid steel poles with laminated pole shoes.Why we use laminated field poles?
We use laminated field poles because the surface of armature is not uniform since it has got notches or slots.And so when it rotates through the field produced by the pole shoes there may be pulsations in the field and this varying field could produce an eddy current inside the field pole.So to avoid this we use laminated field poles.How laminated field poles reduce eddy current ?
The laminated field poles allows only the eddy current to pass through the length of lamination not through the entire body of pole ie from lamination to another laminations.These lamination are held together by means of a rivet.The outerside of the laminations are curved to fit closely to the inner frame.The field coils are supported by the pole shoe and it spread out the flux in the airgap and owing to its large cross-section it reduces the magnetic reluctance.
How the pole shoe is attached to the yoke?
Generally two methods are employed for attaching pole shoes to the yoke.In the case of small pole shoes,poles is drilled and tapped to receive pole bolts but for large size a circular or rectangular pole bar is fitted or passed through the pole.This pole bar is drilled and tapped and the pole bolts passing through laminations is screwed int o the tapped bar.
Commutating poles /Interpoles
Interpoles or commutating poles are similar to that of main poles and has got a core which terminates with in a poleshoe.it is constructed in various shapes and the coils are mounted on the core.It is usually spaced in between two two main poles and bolted to yoke.Interpoles are usually made of solid steel but for the machines with varying loads sheet steel is used for construction.Interpoles are used to reduce spark while commutation.
Armature
The armature consists of a core and
winding.Due to the good magnetic properties of iron it is used as the
armature core.Iron is also a good conductor of electricity so the
rotation of iron core in the magnetic field could produce a current in
the core and this current called eddy current cause the wastage of
energy as heat.To reduce eddy current the core is made of thin
laminations.These laminations are made up of low loss silicon steel of
of 0.4 to 0.5 mm thick and insulated with varnish.
In small machines armature laminations
are fitted to the shaft and clamped tightly between the flanges.These
flanges also acts as a support for the armature winding.One end of
flange rests against a shoulder on the shaft,the laminations are fitted
and then end is pressed on the shaft and retained by a key.
The core(except small size is divided
into number of packets by radial ventilation spacers.The spacers are
usually ‘I’ sections welded to thick steel laminations and arranged to
pass centrally down each tooth.
Armature Lamination for small machine
For small machines the punching is made on the same piece which is built directly on the shaft and ventilation holes are provided to pass air into ventilating ducts.Armature lamination for medium size machine
Medium size machines are machines having
more than four poles,these machines are built on a spider.The spider
may be fabricated and the lamination about 100cm are punched in one
piece and keyed directly to spider.
Armature lamination for large machine
For large type of machines segmental
lamination is employed since thin lamination may get distorted or become
wavy when assembled together.Hence instead of being cut into one piece
it is cut into a number of segments or sections which forms a complete
ring.A complete circular ring could be made of 4,6 or even 8 segmental
sections or laminations.Usually two key ways are notched in each
segments and are dove tailed or wedge shaped to make the laminations
self locking in position.
Armature Windings
The armature windings are placed or
housed in the slots grooved on the surface of armature and and are so
spaced such that one coils side or say a conductor of coil comes under
north pole and the other coilside or second conductor comes under south
pole.
Generally in D.C. machines two layer windings with diamond shaped coil is used.The coils are usually former wound.
How the coils are kept in position?
In small machines armature coils are
held i position by means of band of steel wire which is wound under
tension along core length.In large machine wedges of fiber or wood is
used to keep the coils in position ie in the slots and use the wire band
to overhang the coil.
Factors affecting armature winding
Armature winding plays an important role
in the conversion of energy fro one form to another in electrical
machines.The factors which should be given care while designing a
winding is that the weight and material should be optimum or best for
the high efficiency of machine.It should have necessary
mechanical,electrical and thermal strength to meet the machine
requirements and a life span about 16 to 20 years.It should be able to
maintain the current collection at the commutator side without sparking.
Equilizer Rings
Equalizer connections or equilizer rings
are located under the over hang on the commutator side.Equilizer rings
could be also accommodated in the other end of armature.Equilizer rings
are used to get uniform voltage.
Commutator
A commutator converts alternating voltage into direct voltage.A commutator is a cylindrical structure built up of segments made of hard drawn copper.These segments separated from each other and from frame by means of mica strips.These segments are connected to the winding by means of risers .The risers have air spaces between one another or that the air is drawn across the commutator there by keeping the commutator cool.The components of a commutator is shown below
The general appearance of commutator when completed and commutator and armature assembly is shown below.
Brush Gear
We use brush gear to collect or feed current from a rotating commutator.A brush gear consists of following parts.- Brushes
- Brush Holders
- Brush studs or Brush holder arm
- Brush Rocker
- Current collecting busbars
Brushes
The brushes used for machines are divided into five classes according to the material with which it is made they are- Metal Graphite brush
- Carbon Graphite brush
- Graphite brush
- Electro – Graphite brush
- Copper brush
Even for soft brushes there is wear but it takes place gradually.Though the mica segments between the commutator doesn’t wear easily but it reduces the effective contact with the segments and it may result in sparking and damage to the commutator.So to reduce this the micas ‘undercut’ a level below the commutator surface using a narrow milling cutter.
Brush Holders & Brush Rockers
Usually we use box type brush holders in all D.C. machines.At the outer end of arm a brush box is provided which is open at top and the bottom is attached.The brush is pressed to the commutator by means of a clock spring and this pressure could be adjusted by a level arrangement in the spring.The brush is connected to a flexible conductor called pigtail.The flexible conductor may be attached to the brush by a screw or may be soldered.Brush boxes are generally made of bronze casting or sheet brass.For small machines working on low voltage commutation conditions are easy and galvanized steel boxes are employed.There are individual and multiple brush holders available on the market.In multiple brush holders a number of single brush holders are built into one long assembly.
Brush Rockers
Brush holder are connected or fixed to brush rockers with bars.The brush rockers arranged concentrically round the commutator.Cast iron is is usually used for brush rockers.
Armature Shaft Bearings
For small machines roller bearings are used at both ends. For large machines roller bearings are used at driving end and ball bearings at non driving or commutative end.Sometimes pedastal bearings are also used for large machines.The bearings are housed in the endshield.The figure below shows a pedestal bearing.
Important terms regarding armature winding
Armature Windings
The meaning of the following terms used in connection with armature winding should be clearly kept in mind.
Pole-pitch
It may be variously defined as :
i) The periphery of the armature divided by the number of poles of the generator i.e. the distance between two adjacent poles.
ii) It is equal to the number of
armature conductors (or armature slots) per pole. If there are 48
conductors and 4 poles, the pole pitch is 48/4 = 12.
Conductor
The length of a wire lying in the
magnetic field and in which an e.m.f. is induced, is called a conductor
(or inductor) as. for example, length AB or CO in Fig. 26.21
Coil and Winding Element
With reference lo Fig. 26.21. the two
conductors AB and CD along with their end connections constitute one
coil of the armature winding. The coil may be single-turn coil( Fig.
26.21) or multi-turn coil (Fig. 26.22). A single-turn coil will have two
conductors. But a multi-turn coil may have many conductors per coil
side. In Fig. 26.22, for example, each coil side has 3 conductors. The
group of wires or conductors constituting a coil side of a multi-turn
coil is wrapped with a tape as a unit (Fig. 26.23) and is placed in the
armature slot. It may be noted that .since the beginning and the end of
each coil must be connected to a commutator bar. there are as many
commutator bars as coils for both the lap and wave windings The side of a
coil (1 -turn or multiturn ) is called a winding element. Obviously,
the number of winding elements is twice the number of coils.
Coil-span or Coil-pitch (YS)
It is the distance measured in terms of
armature slots (or armature conductors) between two sides of a coil. It
is. in fact, the periphery of the armature spanned by the two sides of
the coil.
If the pole span or coil pitch is equal
to the pole pilch (as in the case of coil A in Fig. 26.24 where
pole-pitch of 4 has been assumed), then winding is called full-pitched.
It means that coil span is 180 electrical degrees. In this case, the
coil sides lie under opposite poles, hence the induced e.m.fs. in them
are additive. Therefore, maximum e.m.f. is induced in the coil as a
whole, it being the sum of the e.m.f.s induced in the two coil sides.
For example, if there arc 36 slots and 4 poles, then coil span is 36/4 =
9 slots. If number of slots is 35. then Ys = 35/4 = 8 because it is customary to drop fractions.
If the coil span is less than the pole
pitch (as in coil B where coil pitch is 3/4th of the pole pitch), then
the winding is fractional-pitched. In this case, there is a phase
difference between the e.m.fs. in the two sides of the coil. Hence, the
total e.m.f. round the coil which is the vector sum of e.m.fs. in the
two coil sides, is less in this case as compared to that in the first
case
Pitch of a Winding (Y)
In general, it may be defined as the
distance round the armature between two successive conductors which are
directly connected together. Or, it is the distance between the
beginnings of two consecutive turns.
Y = YB – YF ………… for lap winding
= YB + YF —— for wave winding
In practice, coil-pitches as low as
eight-tenths of a pole pitch are employed without much serious reduction
in the e.m.f. Fractional-pitched windings are purposely used to effect
substantial saving in the copper of the end connections and for
improving commutation.
Back Pitch (YB)
The distance, measured in terms of the
armature conductors, which a coil advances on the back of the armature
is called back pitch and is denoted by YB ,
As seen from Fig. 26.28. element I is connected on the back of the armature to clement 8. Hence. YB = (8 – 1) = 7.
Front Pitch (YF)
The number of armature conductors or
elements spanned by a coil on the front (or commutator end of an
armature) is called the front pitch and is designated by YF
Again in Fig. 26.28, element 8 is connected to clement 3 on the front
of the armature, the connections being made at the commutator segment.
Hence. YF = 8-3 = 5.
Alternatively, the front pitch may be
defined as the distance (in terms of armature conductors) between the
second conductor of one coil and the first conductor of the next coil
which are connected together at the front ie. commutator end of the
armature. Both front and back pitches for lap and wave-winding are shown
in Fig. 26.25 and 26.26.
Resultant Pitch (YR)
It is the distance between the beginning
of one coil and the beginning of the next coil to which it is connected
(Fig. 26.25 and 26.26).
As a matter of precaution, it should be
kept in mind that all these pitches, though normally stated in terms of
armature conductors, are also sometimes given in terms of armature slots
or commutator bars because commutator is, after all, an image of the
winding
Commutator Pitch (YG)
It is the distance (measured in
commutator bars or segments) between the segments to which the two ends
of a coil arc connected. From Fig. 26.25 and 26.26 it is clear that for
lap winding. YG is the difference of YB and Yf whereas for wavewinding it is the sum of YB and Yf Obviously, commutator pitch is equal to the number of bars between coil leads. In general. Yc
equals the ‘plex’ of the lap-wound armature. Hence, it is equal to 1,
2,3,4 etc. for simplex-, duplex, triplex-and quadruplex etc.
lap-windings.
Back or Counter E.M.F.
When the armature of a d.c. motor
rotates under the influence of the driving torque, the armature
conductors move through the magnetic field and hence e.m.f. is induced
in them as in a generator The induced e.m.f. acts in opposite direction
to the applied voltage V(Lenz’s law) and in known as back or counter
e.m.f. Eb. The back e.m.f. Eb (= P Φ ZN/60 A) is always less
than the applied voltage V, although this difference is small when the
motor is running under normal conditions.
Consider a shunt wound motor shown in
Fig. (4.2). When d.c. voltage V is applied across the motor terminals,
the field magnets are excited and armature conductors are supplied with
current. Therefore, driving torque acts on the armature which begins to
rotate. As the armature rotates, back e.m.f. Eb is induced
which opposes the applied voltage V. The applied voltage V has to force
current through the armature against the back e.m.f. Eb .
The electric work done in overcoming and causing the current to flow
against Eb is converted into mechanical energy developed in the
armature. It follows, therefore, that energy conversion in a d.c. motor
is only possible due to the production of back e.m.f. Eb.
Net voltage across armature circuit = V – Eb
If Ra is the armature circuit resistance, then, Ia = (V – Eb)/ Ra
Since V and Ra are usually fixed, the value of Eb will determine the current drawn by the motor. If the speed of the motor is high, then back e.m.f. Eb (= P Φ ZN/60 A) is large and hence the motor will draw less armature current and viceversa.
Significance of Back E.M.F.
The presence of back e.m.f. makes the
d.c. motor a self-regulating machine i.e., it makes the motor to draw as
much armature current as is just sufficient to develop the torque
required by the load.
Armature current,Ia = (V – Eb)/ Ra
(i)
When the motor is running on no load, small torque is required to
overcome the friction and windage losses. Therefore, the armature
current Ia is small and the back e.m.f. is nearly equal to the applied voltage.
(ii) If the motor is suddenly loaded, the first effect is to cause the armature to slow down. Therefore, the speed at which the armature conductors move through the field is reduced and hence the back e.m.f. Eb falls. The decreased back e.m.f. allows a larger current to flow through the armature and larger current means increased driving torque. Thus, the driving torque increases as the motor slows down. The motor will stop slowing down when the armature current is just sufficient to produce the increased torque required by the load.
(iii) If the load on the motor is decreased, the driving torque is momentarily in excess of the requirement so that armature is accelerated. As the armature speed increases, the back e.m.f. Eb also increases and causes the armature current Ia to decrease. The motor will stop accelerating when the armature current is just sufficient to produce the reduced torque required by the load.
(ii) If the motor is suddenly loaded, the first effect is to cause the armature to slow down. Therefore, the speed at which the armature conductors move through the field is reduced and hence the back e.m.f. Eb falls. The decreased back e.m.f. allows a larger current to flow through the armature and larger current means increased driving torque. Thus, the driving torque increases as the motor slows down. The motor will stop slowing down when the armature current is just sufficient to produce the increased torque required by the load.
(iii) If the load on the motor is decreased, the driving torque is momentarily in excess of the requirement so that armature is accelerated. As the armature speed increases, the back e.m.f. Eb also increases and causes the armature current Ia to decrease. The motor will stop accelerating when the armature current is just sufficient to produce the reduced torque required by the load.
It follows, therefore, that back e.m.f.
in a d.c. motor regulates the flow of armature current i.e., it
automatically changes the armature current to meet the load requirement
Compound Generators in Parallel
Under-compounded generators also operate
satisfactorily in parallel but overcompounded generators will not
operate satisfactorily unless their series fields are paralleled. This
is achieved by connecting two negative brushes together as shown in Fig.
(3.16) (i). The conductor used to connect these brushes is generally
called equalizer bar. Suppose that an attempt is made to operate the two
generators in Fig. (3.16) (ii) in parallel without an equalizer bar.
If, for any reason, the current supplied by generator 1 increases
slightly, the current in its series field will increase and raise the
generated voltage. This will cause generator 1 to take more load. Since
total load supplied to the system is constant, the current in generator 2
must decrease and as a result its series field is weakened. Since this
effect is cumulative, the generator 1 will take the entire load and
drive generator 2 as a motor. Under such conditions, the current in the
two machines will be in the direction shown in Fig. (3.16) (ii). After
machine 2 changes from a generator to a motor, the current in the shunt
field will remain in the same direction, but the current in the armature
and series field will reverse. Thus the magnetizing action, of the
series field opposes that of the shunt field. As the current taken by
the machine 2 increases, the demagnetizing action of series field
becomes greater and the resultant field becomes weaker. The resultant
field will finally become zero and at that time machine 2 will
shortcircuit machine 1, opening the breaker of either or both machines.
When the equalizer bar is used, a
stabilizing action exist? and neither machine tends to take all the
load. To consider this, suppose that current delivered by generator 1
increases [See Fig. 3.16 (i)]. The increased current will not only pass
through the series field of generator 1 but also through the equalizer
bar and series field of generator 2. Therefore, the voltage of both the
machines increases and the generator 2 will take a part of the load.
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