DC MACHINES / COMMUTATING
MACHINES
DC
GENERATOR
DC MOTOR
Separately
Excited
Shunt
Self-Excited
Series
Permanent
Magnet
Compound
Separately
Excited
Shunt
Self-Excited
Series
Permanent
Magnet
Compound
DC Motor:
Armature Windings
Stator
Pole Shoe
Rotor
Rotation of shaft
(mechanical
output)
Working Principle- When current flows through the armature winding (more coils are used to
generate more torque) in the magnetic field created by current flowing (excitation current) through
field winding on the stator, the rotor rotates as well as rotating the shaft and creating mechanical
work from the DC supply given at the terminals.
Types of DC Motor:
Separately Excited DC Motor:
Usually, in other DC motors, the field coil and the armature coil both are energized from a single
source. The field of them does not need any separate excitation. But, in separately excited DC
motor, separate supply is provided for excitation of both field coil and armature coil.
Here, the field coil is energized from a separate DC voltage source and the armature coil is also
energized from another source. Armature voltage source may be variable. But independent constant
DC voltage is used for energizing the field coil.
In separately excited motor field is supplied from constant voltage so that the field current is
constant.
Eb=V-Ia Ra
Eb = PØNZ/60A
(where, P = no. of poles, Ø = flux/pole, N = speed in rpm, Z = no. of armature conductors, A = parallel
paths)
Speed of motor = Eb/flux
Field controlFlux (decreases) -> speed (increases)
Field rheostat control - Here a variable resistance is connected in series with the field coil. Thus, the
speed is controlled by means of flux variation.
Armature ControlArmature resistance control- Here, the speed is controlled by varying the source voltage to
armature. Generally, a variable resistance is provided with the armature to vary the armature
resistance.
Armature Terminal Voltage- Involving variation of variation of voltage in armature circuit.
Self-Excited DC Motor:
Series Motor:
Series motor has field coil connected in series to the armature winding. For this reason, relatively
higher current flows through the field coils. Therefore, field coils have less number of turns with the
heavier wire of larger diameter such that resistance is less.
For series DC motor I = If = Ia
If ∝φ
τa∝φIa
τa∝Ia2
If=Ia=φ
τa∝I2a
τa∝ φ2
N= Eb/ φ
N∝ 1/√τa
Ia = increase then Torque will increase by it’s square. Thus, decreasing the speed of motor. As the
load goes on increasing speed of DC series motor will drop rapidly.
Series motors are generally operated for a very small duration, about only a few seconds, just for the
purpose of starting. Because if its run for too long, the high series current might burn out the series
field coils thus leaving the motor useless.
As in this case relatively higher current flows through the circuit, heavy series field winding with
thicker diameter is used, the electromagnetic torque produced here is much higher than normal.
This high electromagnetic torque produces motor speed, strong enough to lift heavy load
overcoming its initial inertial of rest. And for this particular reason the motor becomes extremely
essential as starter motors for most industrial applications dealing in heavy mechanical load like
huge cranes or large metal chunks etc.
Shunt Motor:
In DC Shunt motor the field winding is connected in parallel with armature winding. Shunt wound DC
motor is called a constant flux motor because their field winding is directly connected across the
supply voltage which is assumed to be constant.
I total = If+Ia
If=V/Rf (V is constant, assuming that motor is running and Rf is not changing)
Therefore, If= constant
If ∝φ
Therefore, φ=constant
τa∝φIa
But, φ = constant
Therefore, τa ∝Ia
Therefore, Torque is directly proportional to armature current as flux is constant. Hence, large
current is required to start a heavy load. Thus, the shunt motor should not be started on heavy
loads.
The armature winding must be exposed to an amount of current that’s much higher than the field
windings current, as the torque is proportional to the armature current. So field winding has many
turns such that most of the current goes through armature. But the wire is kept thin as opposed to
that of series motor field winding which needs thicker wire to carry large current. So, in static
condition, DC series and shunt motor can be distinguished by this characteristic of wires.
N α Eb/Ф
Eb is back emf of armature
But, φ = constant
Therefore, N α Eb
Eb = V-IaRa
But V is constant
With an increase in the armature current with a load, the back EMF decrease very small due
to small IaRa voltage drop as armature resistance is very low. Thus, the ratio of
Eb/Φ remains almost constant, and the speed of the motor is almost constant with an
increase of armature current with loading. Therefore, the DC shunt motor is a constant
speed motor.
Compound Motor:
In DC Compound Motor, the field winding is both in series and parallel with armature winding.
We know that the compound motors are made with a combination of shunt and series field windings
with the armature winding. Hence the characteristics of dc compound motors will be combined
characteristics of shunt and series motors. But the characteristics depend upon how the two field
windings are connected.
In a cumulative compound motor, the flux produced by the series field winding assists the flux
produced by the shunt field winding. Cumulative compound motors are used where series
characteristics are required but the load is likely to be removed completely. Series winding takes
care of the heavy load, whereas the shunt winding prevents the motor from running at dangerously
high speed when the load is suddenly removed. These motors have generally employed a flywheel,
where sudden and temporary loads are applied like in rolling mills.
In differential compound motor, the series field flux opposes the shunt field flux (reduction in
resultant flux). Since in differential field motors, series flux opposes shunt flux, the total flux
decreases with increase in load. Due to this, the speed remains almost constant or even it may
increase slightly with increase in load (N ∝ Eb/ɸ). Differential compound motors are not commonly
used, but they find limited applications in experimental and research work.
Ia=Isc
I total=Ia + Ish
τa∝φIa
if Ia (load) increases τa also increases
N= Eb/ φ
In cumulative the total φ increases, while in differential the total φ decreases
With increase/decrease in φ, speed will decrease and increase respectively
Permanent Magnet DC Motor
A permanent magnet DC Motor is a DC Motor which uses permanent magnet instead of
electromagnet. As the magnetic field strength of a permanent magnet is fixed it cannot be
controlled externally, field control of this type of DC motor cannot be possible.
The permanent magnets are mounted in such a way that the N-pole and S-pole of each
magnet are alternatively faced towards armature.
Although field coil is not required in permanent magnet DC motor still it is sometimes found
that they are used along with permanent magnet. This is because if permanent magnets
lose their strength, these lost magnetic strengths can be compensated by field excitation
through these field coils. Generally, rare earth hard magnetic materials are used in these
permanent magnets. The rotor of a PMDC motor is similar to other DC motors.
The PMDC Motor circuit: (no field coils) No input power in consumed for excitation which
improve efficiency of DC motor.
V=IaRa + Eb
PMDC motor is extensively used where small DC motors are required and also very effective
control is not required, such as in automobiles starter, toys, wipers, washers, hot blowers,
air conditioners, computer disc drives and in many more.
DC Generator:
Supply: DC
supply
generated
Note: Supply is not
coming out of stator; it is
coming out of rotor
(armature winding) itself
Shaftmoved by
crank
Working Principle- In a DC generator, field coils produce an electromagnetic field (DC supply given
for the field excitation) and the armature conductors are rotated in this field. This induces induced
emf and current (Fleming’s right hand rule) in the armature winding. Finally, getting DC current
through armature.
Separately Excited DC Generator:
A separately excited DC Generator is one in which field winding is excited by a separate independent
DC source like battery. The magnitude of generated voltage depends upon the speed of rotation of
armature and the field current, i.e., greater the speed and the field current, higher is the generated
voltage. In practice, the separately excited DC generators are rarely used.
V=Eb-IaRa
Power Developed: Eb Ia
Power Delivered to the Load: Eb Ia – Ia2Ra
Self-Excited DC Generator
Self-excited DC generators have field magnets that are energized by their own supplied
current, and the field coils are connected to the armature internally. There is always some
flux in the poles because of the residual magnetism. As the armature rotates, some
current is produced, and this small current flows through the field coils with the load and
strengthening the pole flux. By increasing the pole flux, the EMF and the current
increase, and the accumulative process is continuing until the excitation is necessary.
Series DC Generator
In Series Generator, field winding is in series with armature winding so that whole armature current
would flow through the field winding as well as the load. Since the load current flows through the
field winding of the generator, so the field winding has a few turns of thick wire having low
resistance. The DC series generators are used in special applications like boosters.
Ia=If=Il
V=Eb-IaRa-If Rf
Power Developed: Eb Ia
Power Delivered to the Load: EbIa-Ia2Ra-If2Rf = EbIa-Ia2 (Ra + Rf)
Shunt DC Generator
In case of a shunt generator, the field winding is connected in parallel with the armature of the
generator so that terminal voltage of the generator is applied across it. The shunt field winding has
many turns of thin wire having high resistance so that only a fraction of armature current flows
through the shunt field winding and the rest flows through the load.
Ia= If + IL
V=Eb-IaRa
Power Developed: Eb Ia
Power Delivered to the Load: V IL= (Eb Ia-Ia2 Ra – Ish2Rsh)
Compound DC Generator
In case of a compound generator, there are two field winding on each pole – one is in series and the
other is in parallel with the armature. The DC compound generators are of two types –
Short shunt compound generator
In a short shunt generator, only shunt field winding is connected in parallel with the
armature.
IL=Ish (Rsh = Rsc)
Eb-IaRa-IL Rsc-V=0
V= Eb-IaRa-IL Rsc
or
V=Eb-IaRa-Ish Rsc
Power Developed: Eb Ia
Power Delivered to Load: V IL = Eb Ia – Ia2 Ra – IL2Rsc – Ish2Rsh
Important − In compound generator, the majority of MMF is established by the shunt field winding.
If the series field flux assists the shunt field, then the generator is called cumulatively
compounded and when they oppose each other, the generator is called differentially compounded.
Long shunt compound generator
In a long shunt generator, the shunt field winding is connected in parallel with both series field and
armature winding.
Ia = IL + Ish
V= Eb-Ia Ra-Ia Rse
Power developed =Eb Ia
Power Delivered to the Load= V IL = Eb Ia – Ia2 Ra – Ia2 Rse-Ish2 Rsh
Permanent Magnet DC Generator:
Ia
Ra
L
O
A
D
Eb-IaRa=V
Power developed = Eb Ia
Power delivered to load =Eb Ia – Ia2Ra

The Permanent Magnet DC Generator can be considered as a separately excited DC
brushed motor with a constant magnetic flux. In fact, nearly all permanent magnet direct
current (PMDC) brushed motors can be used as a permanent magnet PMDC generator, but
as they are not really designed to be generators, they do not make good wind turbine
generators.






These DC machines consist of a stator having rare earth permanent magnets such
as Neodymium or Samarium Cobalt to produce a very strong stator field flux instead
of wound coils.
The main advantage over other types of DC generator is that the permanent magnet
DC generator responds to changes in wind speed very quickly because their strong
stator field is always there and constant.
Permanent magnet DC generators are generally lighter than wound stator machines
for a given power rating and have better efficiencies because there are no field
windings and field coil losses.
Also, as the stator is provided with a permanent magnet pole system, it is resistant to
the effects of possible dirt ingress. However, if not fully sealed, the permanent
magnets will attract ferromagnetic dust and metallic swarf (also called turnings or
filings) which may cause internal damage.
The permanent magnet DC generator is a good choice for small scale wind turbine
systems as they are reliable, can operate at low rotational speeds and provide good
efficiency especially in light wind conditions as their cut-in point is fairly low.
The DC voltage generated by a permanent magnet DC machine is governed by the
following three factors:
The magnetic field developed by the stator. This depends upon the physical size of the
generator and the strength and type of the permanent magnets used.
The number of turns or loops of wire on the armature. This value is fixed by the physical
size of the generator and armature and by the size of wire conductor. The more turns
used the higher the output voltage. Likewise, the larger the wire diameter or crosssectional area the higher the current.
The rotational speed of the armature which is governed by the speed of the wind turbine
rotor blades relative to the wind velocity. For PMDC generators and motors, output
voltage is proportional to speed and is generally linear.


One of the main disadvantages of a permanent magnet DC generator, is that these
machines have commutating brushes that carry the full output current of the
generator so DC machines used as dynamos and generators require regular
maintenance as the carbon brushes used to extract the generated current quickly
wear out and produce a lot of electrically conductive carbon dust inside the machine.
Therefore, AC alternators are sometimes used.
The Permanent Magnet DC Generators is a low-speed generator that are pretty
reliable and efficient in light winds for use in “off-grid” stand alone systems to charge
batteries, or to power low voltage lighting and appliances. They generally have linear
power curves with low cut-in speeds of around 10 mph.
Induction Motor:
Air Gap
Stator Windings
Rotor
Windings
We need to give double excitation to make a DC motor to rotate. In the DC motor, we give one
supply to the stator and another to the rotor through brush arrangement. But in induction motor,
when we give three phase supply to the induction motor, rotating magnetic field/flux gets produced
as a current carrying coil produces magnetic field (windings are places 120 degree apart to create
rotation).
The flux from the stator cuts the short-circuited coil in the rotor. As the rotor coils are short-circuited
at the ends (if kept open circuited no current will flow) , according to Faraday’s law of
electromagnetic induction, the current will start flowing through the coil of the rotor.
When the current through the rotor coils flows, another flux gets generated in the rotor.
Now there are two fluxes, one is stator flux, and another is rotor flux. The rotor flux will be lagging
with respect to the stator flux. Because of that, the rotor will feel a torque which will make the rotor
to rotate in the direction of the rotating magnetic field.
Theoretically induction motor cannot run on synchronous speed. However if by some external force,
or system fault such as surge voltage, somehow if speed of rotor becomes equal to synchronous
speed there will be no torque produced and motor will stop running. Therefore, induction motors
are also called asynchronous motors because they operate at speed less than that of their
synchronous speed.
Torque slip characteristics of Three Phase Induction Motor
Torque generated by Induction Motor under running condition:
T = P / ws
T = 3 *E2 I2 cosɸ2 / (2 𝜋 𝑁𝑠)
If k1 = 3 / 2πNs
Therefore, T = k1 E2 I2 cosɸ2.
Rotor side equivalent Circuit:
𝑍𝑟 = √𝑅2 2 + (𝑠𝑋2 )2
𝑇 = 𝐾1 𝐸2 (
𝑠𝐸2 𝑅2
2
𝑅2 +
𝑠 2 𝑋2 2
) = 𝐾1 𝐸2 2 (
𝑠𝑅2
2
𝑅2 +
𝑠 2 𝑋2 2
)=
3
𝑠𝑅2
𝐸2 2 ( 2
)
2πNs
𝑅2 + 𝑠 2 𝑋2 2
The variation of slip can be obtained with the variation of speed that is when speed varies the slip
will also vary and the torque corresponding to that speed will also vary.
𝑠=
𝑁𝑠−𝑁𝑟
𝑁𝑠
3
𝑅2
2 +𝑋 2 )
2

Starting Torque / Locked Rotor Torque: When Nr=0, s=1, Tst= 2πNs 𝐸2 2 (𝑅

Nr increases and slip decreases, so torque will increase (increases till T max (break-down
troque)) (𝑇 ≅ 1/𝑠)
2

o
𝜕𝑇
o
We get,
o
Substituting
𝜕𝑠
=0
𝑠=
𝑅2
𝑋2
𝑠=
𝑅2
𝑋2
in T equation
𝑘 𝐸2 2
o Tm =
2 𝑋2
Now, Nr reaches Ns as Nr=Ns s=0 and thus T= 0
Unstable
Region
Stable
Region
Break-down Torque
Full Load Operating
Torque
Pull-up Torque
𝑠=
s=1
𝑅2
𝑋2
s=0 and T=0
s
Nr
Motoring Region
Stability means that even for a small variation in the torque the speed of the machine
must not vary significantly. This is what happens in the 0-10% slip region of the curve.
That means, slip varies almost linearly with the torque.
On the other hand, this is not the case on the other region of the curve where for a small
variation in torque the speed of the machine changes drastically (the Rectangular
Hyperbola). Hence, it’s an unstable region. And IM must always be operated in the stable
region.
Single Phase Induction Motor:
A single-phase induction motor is similar to the three-phase squirrel cage induction motor
except there is single phase two windings instead of one three phase winding in 3-phase
motors, mounted on the stator. The rotor of single-phase induction motor is the same as a
rotor of squirrel cage induction motor. Instead of rotor windings, rotor bars are used and it
is short-circuited at the end by end-rings.
Single-phase AC supply is given to the stator winding (main winding). The alternating current
flowing through the stator winding produces magnetic flux. This flux is known as the main
flux. The flux is alternating in nature and not rotating as in three phase induction motor. If we
place rotor in this field then it will not rotate but it will produce humming. But if we assume
that the rotor is rotating and it is placed in a magnetic field produced by the stator winding.
According to Faraday’s law, the current start flowing in the rotor circuit as it is a close path.
This current is known as rotor current. Due to the rotor current, the flux produced around the
rotor bars. This flux is known as rotor flux. There are two fluxes; main flux which is produced
by stator and second is the rotor flux which is produced by the rotor. Interaction between
main flux and rotor flux, the torque produced in the rotor and it starts rotating.
So, single phase IM is not self-starting. Therefore, we need to make some arrangements for selfstarting:
The single-phase induction motors are classified as:
 Split Phase Induction Motor
 Shaded Pole Induction Motor
 Capacitor Start Induction Motor
 Capacitor Start Capacitor Run Induction Motor
 Permanent Capacitor Induction Motor
Split Phase IM:
In this type of motor, an extra winding is wounded on the same core of the stator. So,
there are two windings in the stator. One winding is known as the main winding or
running winding and second winding is known as starting winding or auxiliary winding.
A centrifugal switch is connected in series with the auxiliary winding. The auxiliary
winding is highly resistive winding and the main winding is highly inductive winding.
The auxiliary winding has few turns with a small diameter. The aim of auxiliary winding
is to create a phase difference between both fluxes produced by the main winding and
rotor winding.
The connection diagram is as shown in the below figure. The current flowing through
the main winding is IM and current flowing through the auxiliary winding is IA. Both
windings are parallel and supplied by voltage V. The auxiliary winding is highly
resistive in nature. So, the current IA is almost in phase with supply voltage V. The
main winding is highly inductive in nature. So, the current IM lags behind the supply
voltage with a large angle. The total stator flux is induced by the resultant current of
these two winding.
Auxiliary winding only uses to start the motor. This winding is not useful in running
condition. When the motor reaches 75 to 80 % of synchronous speed, the centrifugal
switch opens. So, the auxiliary winding is out from the circuit. And motor runs on only
main winding.
The phase difference creates by this method is very small. Hence, the starting torque
of this motor is poor. So, this motor is used in low starting torque applications like a
fan, blower, grinder, pumps, etc.
Shaded Pole IM
This type of motor does not require auxiliary winding. This motor has stator salient pole or
projecting pole and the rotor is the same as squirrel cage induction motor. The stator poles
are constructed specially to create a rotating magnetic field.
A pole of this motor is divided into two parts; shaded part and un-shaded part. It can be created
by cutting pole into unequal distances. A copper ring is placed in the small part of the pole.
This ring is a highly inductive ring and it is known as a shaded ring or shaded band.
When an alternating supply is passed through the stator winding, an alternating flux is
induced in the stator coil. This flux induces eddy currents in the shaded ring, producing some
amount of flux in it. According to Lenz law, the flux produced due to this shaded ring will
oppose the main flux.
So, it will create a phase difference between the main flux and the induced flux in the ring by
approximately 90 degrees and this phase difference aids to rotate the rotor. By this method, a
phase difference is very less. Hence, the starting torque is very less. It is used in applications
like toy motor, fan, blower, record player, etc.
Capacitor Start Induction Motor:
This type of motor is an advanced version of the Split phase induction motor. The
disadvantage of split-phase induction is low torque production. Because in this motor,
the phase difference created is very less.
This disadvantage compensates in this motor with the help of a capacitor connected
in series with auxiliary winding. The circuit diagram of this motor is as shown in the
below figure.
The capacitor used in this motor is a dry-type/electrolytic capacitor. This is designed
to use with alternating current. But this capacitor is not used for continuous operation.
In this method also, a centrifugal switch is used which disconnects the capacitor and
auxiliary winding when the motor runs 75-80% of synchronous speed. The current
through auxiliary will lead the supply voltage by some angle. This angle is more than
the angle increased in a split-phase induction motor.
So, the starting torque of this motor is very high compared to the split-phase induction
motor. The starting torque of this motor is 300% more than the full load torque.
Due to high starting torque, this motor is used in the applications where high starting
torque is required like, a Lath machine, compressor, drilling machines, etc.
Capacitor-start, Capacitor-run single-phase induction motor
Although the use of an electrolytic capacitor increases the starting torque, the capacitor gets
damaged if operated too frequently for shorter durations or if operated for longer durations
(as they are rated for short-duty service).
Therefore, we use two capacitors parallel to each other such that during starting of the
motor both the capacitors and the armature remains in the circuit and high torque is
generated.
The first one is an oil-impregnated capacitor (Cr). It is a continuous rating
capacitor with a smaller value.
 The second one is an electrolytic capacitor (Cs). It is a short-duty capacitor
with a higher value.
When the motor picks 75% of the synchronous speed, the centrifugal switch disconnects the
starting capacitor (Cs) from the circuit. Thus, after that, only the running capacitor (Cr)
remains with the auxiliary winding. Here, the auxiliary winding remains connected in the
circuit at all times, i.e., both starting and running.

Due to a better starting and running torque, these motors are ideal for compressors,
refrigerators, and pumps. Their low noise feature makes them beneficial to use in hospitals
and studios.
Permanent Split Capacitor Motor:
This motor uses only one capacitor in series with the auxiliary winding. Here, the capacitor
remains connected in the circuit during the starting as well as running. So, there is no need
for a centrifugal switch, as discussed in the above types of single-phase induction motors.
The advantages of this motor are similar to that of a capacitor start, capacitor run induction
motor. But due to the use of only one capacitor, this motor can’t give optimum starting and
running conditions. It uses a Pyranol insulated foil paper capacitor.
They are ideal for ceiling fans, blowers, room coolers, and other domestic applications. Due
to the simple reversal of the motor, they are best for induction regulators and furnace
controls.