45_16255_EE321_2015_1__1_1_week_9_10

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