Download WEEK 3-4: DIRECT CURRENT GENERATOR

Electrical Machines I
WEEK 3-4: DIRECT CURRENT GENERATOR
II. Construction of DC machine
An electrical DC machine can convert mechanical energy into direct current electricity (DC generator) or vice
versa (DC motor) without any constructional changes. Thus, a DC generator or a DC motor can be broadly termed
as a DC machine.
Two electrical circuits present in the dc
machine:
1- Field circuit
2- Armature circuit
Rotor: rotating part of the machine
Stator: Stationary part of the machine
1- Stator:
Poles: projects inwards and
provides a path for the magnetic
flux
Poles: the end of the poles that are
close to rotor “spread out” over the
rotor surface to distribute flux evenly
over the rotor surface. We call the end
as “pole shoe”. Due to their spread out
they are often called Salient Poles.
Frame: provides physical support
THE STATOR COULD BE
LAMINATED OR MADE OF
SINGLE CAST PIECE OF METAL
Field windings: windings responsible for
magnetic flux production
Stator
2- Rotor: Rotating part of machine
Rotor of dc machine is often called “armature” as it holds the armature windings
Armature winding: carries current crossing the field,
thus creating shaft torque in a rotating machine as well
as generate an electromotive force (EMF). Some call it
“The power-producing component” of an alternator,
generator, dynamo or motor.
Commutator: built on the shaft of the rotor at one end of the core.
Made of copper bars insulated by mica. The commutator serves as a
“mechanical rectifier”. Its main function, in a DC generator, is to
collect the current generated in armature conductors and change it
from internal AC to DC output. Whereas, in case of a DC motor,
commutator helps in providing current to the armature conductors
that can produce a rotating torque in them. Since this mechanism is
called a commutator, DC machinery is also known as commutating
machinery.
Brushes: made of carbon, graphite or a mixture of both.
They have high CONDUCTIVITY and low friction
coefficient to reduce the wear but they are softer than
commutator to avoid commutator wear. They rest on
commutator segments and slide on the segments when
the commutator rotates keeping the physical contact to
collect or supply the current
Armature
THE ROTOR IS COMPOSED OF MANY LAMINATIONS
STAMPED FROM A STEEL PLATE.
II. Operation Principle of DC generator
Mechanical energy is converted to
energy
Rotating Loop in B-field
𝝋
electrical
F
Three requirements are essential
1. Conductors
2. Magnetic field
F
3. Mechanical energy
An alternating AC current is produced by
rotating a loop in a constant magnetic field
𝒊
Current on left is outward by right-hand rule
The right segment has an inward current
Faraday’s Law: If a wire moves through magnetic field, a voltage is induced in it
• A potential difference is maintained across the conductor as long as there is motion through the field
• If motion is reversed, polarity of potential difference is also reversed
𝒊
II. Operation Principle of DC generator
𝝋
𝝋
F
𝒊
𝝋
𝝋
The total induced voltage on the loop is:
𝑒𝑡𝑜𝑡𝑎𝑙 = ቊ
2𝑣𝐵𝑙 𝑢𝑛𝑑𝑒𝑟 𝑡ℎ𝑒 𝑝𝑜𝑙𝑒 𝑓𝑎𝑐𝑒
0 𝑎𝑤𝑎𝑦 𝑓𝑟𝑜𝑚 𝑡ℎ𝑒 𝑝𝑜𝑙𝑒 𝑓𝑎𝑐𝑒
Why do we need Commutator? Theory of Commutation:
Commutation : is the process of converting the ac voltage and currents
in the rotor of a dc machine to dc voltages and currents at its terminals. It
is the most critical part of the design and operation of any dc machine.
 According to Fleming’s right hand rule, the direction of induced current
changes whenever the direction of motion of the conductor changes.
 Let’s consider an armature rotating clockwise and a conductor at the left is
moving outwards. When the armature completes a half rotation,
Using “slip” rings
the direction of motion of that particular conductor will be reversed to
inwards. Hence, the direction of current in every armature conductor will be
alternating with slip rings
 Using a semicircular commutating segments (split rings), connections of the armature
conductors also gets reversed whenever the current reversal occurs.
 And therefore, the output at the fixed contacts (brushes) is always built up in
the same way resulting in unidirectional DC output current.
Using “split” rings
Why do we need Commutator? Theory of Commutation:
Adding more armature coils smooth out
induced voltage fluctuation and changes
the direct current from pulsating to
regular DC
The EMF Equation:
P = number of field poles
Ø = flux produced per pole in Wb (weber)
Z = total no. of armature conductors
a =no. of parallel paths in armature: This describes the way the machine's armature conductors
are connected relative to each other and to the number of poles. The two basic ways of
connecting these conductors are called 'lap' and 'wave‘
a=2
𝒏𝒎 = rotational speed of armature in revolutions per min (rpm)
a=P
(lap) (wave)
Induced armature voltage (emf) per conductor = −𝑵
𝒅∅
,
𝒅𝒕
Flux cut per conductor in one revolution = ∅𝑷,
Number of revolutions per second =
𝒏𝒎
,
𝟔𝟎
Hence, generated emf/conductor= ∅𝑷
and time for one revolution=
𝟏
𝟔𝟎
=
𝒇 𝒎 𝒏𝒎
𝒏𝒎
𝟔𝟎
Since, generated emf for parallel paths is equal; Hence,
𝒁
𝒂
Total induced e.m.f (Ea) = ∅𝑷
𝒏𝒎
𝟔𝟎
Lap winding is suitable when requirement is : Large DC
currents and low voltage.
Wave winding is suitable when requirement is: Low DC
current and high voltage
III. Power flow diagram
DC generators take in mechanical power and produce electric power. The efficiency of a DC machine is defined by
𝑃𝑜𝑢𝑡 = 𝑃𝑖𝑛 - total losses
𝑃𝑑𝑒𝑣 = 𝑃𝑖𝑛 - stray losses-rotational losses
= 𝑃𝑜𝑢𝑡 + copper losses
The Losses in DC Machines
The losses that occur in DC machines can be divided into four basic categories:
1. Electrical or copper losses
2. Core losses
3. Mechanical losses
4. Stray load losses
ELECTRICAL OR COPPER LOSSES. Copper losses are the losses that occur in the armature and field windings of the machine.
CORE (or MAGNETIC) LOSSES: The core losses are losses encountered in the magnetic core of the machine which include the
hysteresis losses and circulating eddy current losses.
MECHANICAL LOSSES: The mechanical losses in a dc machine are the losses associated with mechanical effects due to friction
and windage. Friction losses are caused by the friction of the bearings in the machine with the shaft and friction between brushes
and commutator, while windage losses are caused by the friction between the moving parts of the machine and the air inside the
motor's casing.
STRAY LOAD LOSSES: Stray losses are losses that cannot be placed in one of the previous categories such as losses due to
distorted flux and short circuit currents in coils.
IV. DC generator Connections:
There are five major types of dc generators, classified according to the manner in which their field flux is produced:
DC machine
Separately excited
Series
Self excited
Shunt
Permanent magnet
Compound
Cumulative
Differential
 ALL generators are driven by a mechanical force, usually called as a prime mover. A prime mover may be a diesel engine, steam turbine, or even an electric
motor.
 DC generators are quite rare in modern power systems. Even dc power systems such as those in automobiles now use ac generators plus rectifiers to
produce dc power.
Equivalent circuits
Separately excited
Shunt
Series
𝑉𝑇 = 𝐸𝐴 −𝐼𝐴 𝑅𝐴
𝐼𝐿 = 𝐼𝐴
𝑉𝐹
𝐼𝐹 =
𝑅𝐹
𝑉𝑇 = 𝐸𝐴 −𝐼𝐴 𝑅𝐴
𝐼𝐴 = 𝐼𝐹 + 𝐼𝐿
𝑉𝑇
𝐼𝐹 =
𝑅𝐹
𝑉𝑇 = 𝐸𝐴 −𝐼𝐴 (𝑅𝐴 +𝑅𝑆 )
𝐼𝐴 = 𝐼𝑆 = 𝐼𝐿
Voltage regulation (VR): 𝑉𝑅% =
𝑉𝑇𝑛𝑙 −𝑉𝑇𝑓𝑙
𝑉𝑇𝑓𝑙
× 100
where
𝑉𝑇𝑛𝑙 : No-load terminal voltage of the generator
𝑉𝑇𝑓𝑙 : Full- load terminal voltage of the generator
We study machines at steady state, meaning that all inductances
appear to be short circuited. Only resistances effects are taken into
account
V. DC generator characteristics
Generally, the following two characteristics of DC generators are taken
into considerations:
1. Open Circuit Characteristic (O.C.C.) (EA/If)
Open circuit characteristic is also known as magnetization curve. This
characteristic shows the relation between generator induced e.m.f. (EA)
and the field current (If) at a given speed.
The magnetization curve at no-load is almost similar for all type of
generators.
If Φ increases, 𝐸𝐴 = 𝐾Φ ↑ ω𝑚 increases
2. Loading characteristics (VT/IL)
If ω𝑚 increases, 𝐸𝐴 = 𝐾Φω𝑚 ↑ increases
The load characteristic curve shows the relation between the terminal voltage (VT) and load current (IL). The
terminal voltage (VT) is less than generated emf EA due to voltage drop in the armature circuit in addition to the
armature reaction effect
Armature reaction: The effect of magnetic field set up by armature current on the distribution of flux under main poles of a
generator which demagnetizes or weakens the main flux. We solve it through many techniques such as placing interpoles,
brush shifting and compensating windings
Loading characteristic curves differ according to each type of generator as follows;
Separately excited generator
𝑉𝑇 = 𝐸𝐴 −𝐼𝐴 𝑅𝐴
𝐼𝐿 = 𝐼𝐴
𝐸𝐴 = 𝐾Φω𝑚
• In this generator the field circuit is electrically separate of the armature circuit, hence the field current and in
turn the internal generated voltage (EA ) is independent of (IA ) which is meanwhile the load current.
• At no load (𝐼𝐿 =0), the terminal voltage is the internal induced voltage whose characteristic is a straight line as
it is independent of load current (𝑉𝑇 = 𝐸𝐴 ). When the load is applied (𝐼𝐿 increases):
𝐼𝐿 ⇑, ∴ 𝐼𝐴 = 𝐼𝐿 ⇑, ∴ 𝐼A 𝑅A ⇑, ∴ 𝑉𝑇 ⇓= 𝐸𝐴 − 𝐼𝐴 𝑅𝐴 ⇑
• To control the generator terminal voltage, either increasing ω𝑚 or Φ increases EA , hence increasing VT .
However, in many applications, the speed range of the prime mover is quite limited, so the terminal voltage is
most commonly controlled by changing the field current, by changing the field resistance.
• Separately excited generators operate in a stable condition with any variation in field excitation. Hence, they
are used as supply source of DC motors, whose speeds are to be controlled for various applications
Applications
Shunt generator
𝑉𝑇 = 𝐸𝐴 −𝐼𝐴 𝑅𝐴
𝐼𝐴 = 𝐼𝐹 + 𝐼𝐿
𝑉𝑇
𝐼𝐹 =
𝑅𝐹
𝐸𝐴 = 𝐾Φω𝑚
•
The voltage buildup in a DC generator depends on the presence of a residual flux in the poles of the generator. When a generator
first starts to turn, an internal voltage will be generated, which is given by
. This voltage appears at the terminals of
the generator causing a current to flow in the generator 's field coil. This field current produces a magneto motive force in the poles
which increases the flux in them. The increase in flux causes an increase (EA ), thus causing increase in VT .When VT rises, 𝐼𝐹
increases, increasing the flux more, which increases EA, ……etc.
𝐼𝐿 ⇑, ∴ 𝐼𝐴 = 𝐼𝐹 + 𝐼𝐿 ⇑, ∴ 𝐼A 𝑅A ⇑, ∴ 𝑉𝑇 ⇓= 𝐸𝐴 − 𝐼𝐴 𝑅𝐴 ⇑
BUT 𝑉𝑇 ⇓, 𝐼𝐹 ⇓, 𝜑 ⇓, 𝐸𝐴 ⇓, 𝑉𝑇 ⇓⇓
same behavior as separately excited generator
More drop compared to the separately excited
generator!
•
As with the separately excited generator, to control the generator terminal voltage in the
shunt generator, either increasing ω𝑚 or Φ increases EA , hence increasing VT . Changing
𝐼𝐹 , by changing the field resistance is the main principle to control VT.
•
The application of shunt generators are restricted for its dropping voltage characteristic.
They are used to supply power to the apparatus situated very close to its position, as
lighting, battery charge, for small power supplies
Applications
Voltage buildup in a DC generator
Series generator
𝑉𝑇 = 𝐸𝐴 −𝐼𝐴 (𝑅𝐴 +𝑅𝑆 )
𝐼𝐴 = 𝐼𝑆 = 𝐼𝐿 = 𝐼𝐹
𝐸𝐴 = 𝐾Φω𝑚
• In DC series generators field winding is connected in series with armature and load. Hence, here load current is
similar to field current. Thus, the loading characteristic curve is close to the machine magnetization curve. At no
load, there is no field current, so VT is reduced to a small level given by the residual flux in the machine.
• As the load increases, the field current rises, so EA rises rapidly, The IA (RA + Rs) drop goes up too, but at first
the increase in EA goes up more rapidly than the drop rise, so VT increases.
• After a while, the machine approaches saturation, and EA becomes almost constant. At that point, the resistive
drop is the predominant effect, and VT starts to fall.
• This machine gives constant current in the dropping portion of the characteristic curve. For this property they
can be used as constant current source and employed for applications as arc welding and for supplying field
excitation current in DC locomotives
Applications
Questions
• Explain and describe using drawings the construction of dc machine
• What is the function of the following in dc machines:
a- armature winding
b- field winding
• Explain how dc machines can work as generator
• Derive the EMF equation for dc machines
• Explain the types of connections of dc generators
• Plot the magnetization characteristics and terminal characteristics of the following generators:
a- shunt generator
b- separately excited
c- series excited
• Explain how dc shunt generators build up voltage