Electrical Machines I

Electrical Machines I
Week 9-10: Commutation and Armature reaction
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.
The total induced voltage on the loop is:
𝑒𝑡𝑜𝑡𝑎𝑙 = ቊ
2𝑣𝐵𝑙 𝑢𝑛𝑑𝑒𝑟 𝑡ℎ𝑒 𝑝𝑜𝑙𝑒 𝑓𝑎𝑐𝑒
0 𝑎𝑤𝑎𝑦 𝑓𝑟𝑜𝑚 𝑡ℎ𝑒 𝑝𝑜𝑙𝑒 𝑓𝑎𝑐𝑒
Commutation: Generator action
 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, the direction of
motion of that particular conductor will be reversed to
Using slip rings
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
Commutation:
Adding more armature coils smooth out induced voltage fluctuation and changes the direct current
from pulsating to regular DC
Two coils in armature
Are there any problems
with commutation???!!!
OF COURSE YES
Four coils in armature
Problems with commutation in DC machines
1. Armature reaction (neutral plane shift + flux weakening):
 If the magnetic field windings of a DC machine are connected to the power source and the rotor is
rotated by prime miver, a voltage will be induced in the conductors of the rotor.
 This voltage is rectified and can be supplied to external loads. However, if a load is connected, a
current will flow through the armature winding.
 Armature current produces its own magnetic field that distorts the original magnetic field from
the machine’s poles. This distortion of the machine’s flux as the load increases is called armature
reaction and can cause two problems:
1) neutral-plane shift: The magnetic neutral plane is the plane within the
machine where the velocity of the rotor wires is exactly parallel to the
magnetic flux lines, so that the 𝑒𝑖𝑛𝑑 in the conductors in the plane is
exactly zero. ‫هو المحور اللي عنده الجهد المولد في الملف يصل زيرو‬
Problems with commutation in DC machines
1) neutral-plane shift:
This
rotor
field
will
magnetic
affect
the
original magnetic field
from
A two-pole DC machine: initially,
the
pole
flux
is
the
poles.
In
some places under
uniformly
the poles, both fields
distributed and the magnetic
will sum together, in
neutral plane is vertical.
other places, they will
Location of brush must
shift
The effect of the air gap on
subtract from each
other
the pole flux. Fringing effect
When
the
load
is
connected
(generator action), a current – flowing
through the rotor – will generate a
magnetic
field
from
the
windings.‫المجال المولد من تيار الروتور‬
rotor
Therefore,
the
net
magnetic field will not be
uniform and the neutral
plane will be shifted.
Problems with commutation in DC machines
1) neutral-plane shift:
In general, the neutral plane shifts in the
direction of motion for
generator and
opposite to the direction of motion for a
motor.
The amount of the shift depends on
the amount of rotor current and
hence on the load of the machine.
I still don’t get it,
what's the big deal
if the neutral plane
shift?
Problems with commutation in DC machines
 A simple 4-loop DC machine has four complete loops buried in slots curved in the laminated steel of its rotor.
 The pole faces are curved to make a uniform air-gap and uniform flux density everywhere under the faces.
Under
south pole face
b
a
Under
north pole face
Loops 1 and 3 are under pole
Loop 1 stretches between commutator segments a and b, loop 2 stretches between
segments d and c…
Brushes are away
and disconnecting
any two
commutator
segments
Commutation: t = 00
At a certain time instance, when t = 00, the 1, 2, 3’, and 4’
ends of the loops are under the north pole face and the 1’, 2’,
3, and 4 ends of the loops are under the south pole face. The
voltage in each of 1, 2, 3’, and 4’ ends is given by:
𝑒𝑖𝑛𝑑 = 𝑣 × 𝐵 × 𝑙 = 𝑣𝐵𝑙 ⇒ 𝒑𝒐𝒔𝒊𝒕𝒊𝒗𝒆 𝒐𝒖𝒕 𝒐𝒇 𝒕𝒉𝒆 𝒑𝒂𝒈𝒆
The voltage in each of 1’, 2’, 3, and 4 ends is
𝑒𝑖𝑛𝑑 = 𝑣 × 𝐵 × 𝑙 = 𝑣𝐵𝑙 ⇒ 𝒑𝒐𝒔𝒊𝒕𝒊𝒗𝒆 𝒊𝒏𝒕𝒐 𝒕𝒉𝒆 𝒑𝒂𝒈𝒆
If the induced voltage on any side of a loop is 𝑒 = 𝑣𝐵𝑙, then the
total voltage at the brushes of the machine is:
𝐸 = 4 𝑒 ⇒ t = 0°
Under
Under
north pole face south pole face
Commutation: t = 450
If the machine keeps rotating, at t = 450, loops 1 and 3 have rotated into the gap between poles, so the voltage
across each of them is zero.
At the same time, the brushes short circuits the commutator segments ab and cd.
gap between poles then no
voltage is induced
This is ok since the voltage
across loops 1 and 3 is
zero and only loops 2 and 4
are under the pole faces.
𝐸 = 2 𝑒 ⇒ t = 450
Brushes are in contact
and connecting segments
ab and cd together
Loops 1 and 3 are in the gap
Problems with commutation in DC machines
1) neutral-plane shift:
Reduces the brush lifetime,
The commutator must short out the commutator segments right at the moment
when the voltage across them is zero. The neutral-plane shift may cause the
brushes short out commutator segments with a non-zero voltage across them.
This leads to arcing and sparkling at the brushes!
Arcing!
pitting the commutator
segments and greatly
increases maintenance cost
Theory of Commutation
 Ideally,
the
process
of
commutation
should
be
1
Coil B=+𝑰𝒄
Coil A=-𝑰𝒄
instantaneous, as indicated, This can, however, be
achieved only if the brush width and the commutator
segments are infinitesimally small.
 In practice, not only do the brush and the commutator have
2
Coil B=0
finite width but the coil also has a finite inductance.
Therefore, it takes some time for the current reversal to take
3
place
Coil B=-𝑰𝒄
At position 2, coil B is undergoing
commutation and the current through
each brush is still 𝟐𝑰𝒄 . The induced emf in
that coil is NOT equal to zero due to the
armature reaction flux.
𝑰𝒄
𝑰𝒄
Theory of Commutation
 For a commutation process to be perfect, the reversal of current from its value in one direction to an equal
value in the other direction must take place during the time interval 𝑡𝑐
 When the current reverses its direction during commutation in a straight-line fashion the commutation
process is said to be linear
The coil undergoing
Reasons for under
commutation is the coil
leakage inductance.
Where did it come from?
commutation experiences emf as
well as ac current, as a result an
inductance is formed, known as
leakage inductance.
Ideal commutation
Problems with commutation in DC machines
2) Flux weakening.
 Most machines operate at flux densities near
the saturation point.
 At the locations on the pole surfaces where the
rotor mmf adds to the pole mmf, only a small
increase in flux occurs (due to saturation).
 However, at the locations on the pole surfaces
where the rotor mmf subtracts from the pole
mmf, there is a large decrease in flux.
 Therefore, the total average flux under the
entire pole face decreases.
𝐵1
𝐵0
𝐵2
Problems with commutation in DC machines
 In generators, flux weakening reduces the voltage
supplied by a generator.
 In motors, flux weakening leads to increase of the
motor speed. Increase of speed may increase the
load, which, in turns, results in more flux weakening.
Some shunt DC motors may reach runaway
conditions
this
way… (flux
and
speed
are
inversely proportional in motor)
Ideally at this instant (neutral zone) the emf is zero, but due
to armature reaction, there is a flux at this point so there
exists an emf
Observe a considerable decrease in the region
where two mmfs are subtracted
Solutions to the problems with commutation
1- Brush shifting Approach:
If the neutral plane of the machine shifts, why not shift the
brushes with it in order to stop sparking? Looks like a good
idea but there are several problems associated!!
1- The neutral pane shifts for each load and shift direction reverses from
motor to generators action.
2- More flux weakening occurs!
𝑭𝒏𝒆𝒕
Brush shifting
Approach is
obsolete. Only
used in very
small machines
𝑭𝒑𝒐𝒍𝒆
Brush in
vertical plane
𝑭𝒓𝒐𝒕𝒐𝒓
𝑭𝒏𝒆𝒕
𝑭𝒑
𝑭𝒓𝒐𝒕𝒐𝒓
𝑭𝒑𝒐𝒍𝒆
Brush shifted
plane
𝑭𝒓
Solutions to the problems with commutation
2- Commutating poles or interpoles
 To avoid sparkling at the brushes while the machine’s load changes, instead of adjusting the brushes’ position (by
human interference).
 If the voltage in the wires undergoing commutation can be made zero, then there will be no sparking
problem!
 it is possible to introduce small poles (commutating poles or interpoles) between the main poles. Such poles are
located directly over the conductors being commutated and provide the flux that can exactly cancel the
voltage in the coil undergoing commutation.
The interpole creates flux which will create an emf that
cancels out the induced emf in the coil undergoing
commutation
• since they are so small that only affect few conductors being
commutated. Flux weakening is unaffected as the interpoles
effect does not extend that far. Machine operation is not
changes.
Solutions to the problems with commutation
 Interpole windings are connected in series with the rotor
windings. As the load increases and the rotor current
increases, the magnitude of neutral-plane shift increase
increasing
the
voltage
in
the
conductors
undergoing
How does
interpoles
cancels all
voltages for all
load values??
commutation.
 However, the interpole flux increases too producing a larger voltage in the conductors that opposes the
voltage due to neutral-plane shift. Therefore, both voltages cancel each other over a wide range of
loads. This approach works for both DC motors and generators.
 The interpoles must be of the same polarity as the next upcoming main pole in a generator
 The interpoles must be of the same polarity as the previous main pole in a motor.
 The use of interpoles is very common because they correct the sparkling problems of DC machines at a
low cost. However, since interpoles do nothing with the flux distribution under the pole faces, fluxweakening problem still persists.
Solutions to the problems with commutation
3- Compensating windings:
To solve the problem of BOTH neutral plane shift and
flux weakening – use compensating winding
2.The
flux weakening windings
problem can be very severe for large DC motors with high loading. Therefore,
Compensating
compensating windings can be placed in slots carved in the faces of the poles parallel to the rotor
conductors. These windings are connected in series with the rotor windings, so when the load
changes in the rotor, the current in the compensating winding changes too…
Pole
flux
Pole flux in machine
Rotor and comp. fluxes
(equal and opposite)
The net flux
Solutions to the problems with commutation
 The mmf due to the compensating windings is equal and
opposite to the mmf of the rotor. These two mmfs cancel
each other, such that the flux in the machine is unchanged.
The main disadvantage of compensating windings is that
they are expensive since they must be machined into the
faces of the poles. Also, any motor with compensative
windings must have interpoles to cancel L di/dt effects
which occurs in the commutator segments being shorted
out by the brushes due to current reversal.
Solutions to the problems with commutation
A stator of a sixpole DC machine
with interpoles and
compensating
windings.
pole
interpole
Questions:
 Explain with diagrams how dc voltage and currents are formed in dc machine
 Explain what is meant by armature reaction, its effects and how can you reduce its
effects
 What causes commutation problems and how can you solve it
 Explain the difference in using interpoles and compensating windings in dc machines