Methods of limiting the effects of armature reaction –

The cross-magnetizing effect of armature m.m.f. can be minimised at the design and construction stage of a D.C. machine. Various methods of mitigating the effects of armature
reaction are discussed below :
(a) High-reluctance pole tips – If the
reluctance of the pole tips is increased, then the magnitude of armature cross-flux is reduced and the distortion of the resultant flux density wave is minimised. The
reluctance of the pole tips can be augmented by using chamfered or eccentric pole shoes. A machine fitted with chamfered or eccentric
pole face has short air-gap length at the pole centre and longer air gap lengths under the pole tips, i.e. the proflle of the pole shoe is not
concentric with the armature core as shown in Fig. 4.19 (a).
Another method reluctance to cross-flux is to assemble alternatively the pole laminations depicted in Fig. 4.19 (b). That is, if the first lamination has the pole tip to the left, the second lamination has its pole tip to the right, the third lamination pole tip to the left and so on, until the required pole depth is developed. Since the iron area under the pole tips is almost halved, the
reluctance under the pole tips is considerably increased.
The two constructional techniques mentioned above reduce the main field flux to some extent. In order to maintain it constant, the main field m.m.f. must be raised accordingly. But the influence of increased pole-tip reluctance is more pronounced on the cross-flux than on the main field flux.
In D.C. machines, the short air gap at the pole centre and longer air gaps at the pole tips are kept only to limit the effect of cross-magnetizing armature m.m.t. on the main pole
flux. The distribution of the flux density wave along the air-gap periphery need not be a sine wave in D.C. machines. But in synchronous machines, the air gap at the pole centre is short and at the pole tips it is larger from the view point of obtaining sine wave for the flux density wave. In synchronous machines of the salient-pole type, the non-uniform air gap under the pole faces has nothing to do with the armature reaction.

(b) Reduction in armature flux – Another constructional technique of reducing the armature cross-flux is to create more reluctance in the path of armature flux without reducing
the main field flux noticeably. This is achieved by using field-pole laminations having several rectangular holes punched in them. One such lamination having four holes or slots is shown in Fig. 4.20 (a). It is seen from Fig. 4.20 (b) that reluctance offered to armature flux is increased due to four air-gap openings introduced in the path of cross-flux. As a result, armature cross-flux is reduced considerably, whereas the main-field flux remains almost uneffected.

Combination of the constructional features described in Figs. 4.19 and 4.20 may be used most effectively in reducing the armature cross flux.
(c) Strong main-field flux – During the design of a D.C. machine, it should be ensured that the main field m.m.f. is sufficiently strong in comparison with full-load armature m.m.f. Greater the ratio of main field m.m.f. to full-load armature m.m.f, less is the distortion produced by armature cross flux and predominant would be the control of field m.m.f. over the air-gap flux. Actually, this ratio depends on the type of duty cycle the D.C. machine has to perform.
(d) Interpoles – The effect of armature reaction in the interpolar zone can be overcome by interpoles, placed in between the main poles. The magnetic axis of interpole winding is in line with the quadrature axis. Interpole winding is connected in series with armature so that
interpole m.m.f is able to neutralize the effect of armature m.m.f. in the interpolar zone at all levels of load current not exceeding the safe limit.

(e) compensating winding – The effect of armature reaction under the pole shoes can be limited by using compensating winding. This winding is embedded in slots cut in the pole is a D.C. machines this is the best but the most expensive method.

Armature reaction in d.c machines –

The armature m.m.f. produces two undesirable effects on the main field flux and these are :
(1) net reduction in the main field flux
(2) distortion of the main field flux

Effects of armature reaction in d.c machine

(1) iron losses – These losses depend on the maximum value of flux density in teeth and in the pole shoes. The armature reaction, by distorting the main field flux waverorm, increases the flux density considerably over its corresponding no-load value. As a result ; iron losses, particularly in teeth, are much greater on load than on no-load. In addition, high degree of saturation in teeth forces the flux to stray into the core-end plates, end covers etc. This all leads to more eddy-current and hysteresis losses. Roughly, iron losses at full-load is taken to be 1.5 times its value at no-load.
(2) Commutation – At no-load, zero-crossing of the flux density wave is along the GNA such as point C, C’. Under loaded conditions of the D.C. machine, zero-crossing of the flux
density wave is shifted by an angle theta, which depends on the magnitude of armature current.
For good commutation, the coils short-cireuited by the brushes should have zero e.m.f. induced In them. The brushes are usually placed along the GNA. Since zero-crossing of the flux density wave is shifted from GNA or q-axis, the coils undergoing commutation do not have zero e.m.f. induced in them. The induced e.m.f. in the commutated coils delays the l
reversal ot armature current in the short-circuited coils ; this may result in detrimental sparking, or poor commutation, at the brushes.
(3) Sparking – Under heavy load, the flux density waveform is distorted considerably. If the two coil-sides of a coil are under the maximum flux density points, a much greater voltage (infinity Blv) may be generated in this coil. If this rotational voltage between adjacent commutator segments exceeds 30 or 40 V, a spark may occur between these adjacent segments.
Sometimes, this spark may spread around the commutator in the form of a ring fire.
(4) Cost of field winding – Demagnetizing effect of cross-magnetizing armature m.m.f.
is to reduce the total flux per pole from its no-load value due to magnetic saturation. In a generator, the magnitude of the e.m.f. generated in the armature decreases with increase in load. In a motor, electromagnetic torque is decreased as the flux per pole is reduced under
load. In order to compensate for this reduction in total flux, the field m.m.f. must be augmented. This is possible (i) by increasing the number of turns in the field winding or (ii) by using a thick wire for field winding. Either of these schemes entails more copper and, therefore, more cost of the field winding.

Machine Ratings –

A name-plate fixed to the outside frame of an electrical machine records the data pertaining to
its rating. A machine rating species the voltage, current, speed, excitation, pf, efliciency, power output etc. under which it can operate satistactorily. Here satisfactory operation implies that temperature rise of the machine above ambient (or surrounding) temperature does not exceed a specified temperature when machine operates in accordance with the data on the name-plate. For all types of A.C. motors and D.C. machines, output power rating is given in kW (kilowatt). Older practice was to specifty the power output of A.C, and D.C, motors in horse-power (1 h.p. = 746 watts). For A.C. generators, rating is in kVA or MVA.
Electrical machines are rated on the basis of their temperature rise resulting from the power losses in iron and conductor. The temperature rise mentioned on the name-plate is the temperature difference between the hottest part of the winding under specified conditions of load, speed, voltage, excitation, cooling and the ambient temperature. For reliable and satisfactory operation of an electrical machine, it should be ensured that its temperature rise remains within specified limits. The temperature rise not only affects the insulation of an electrical machine but also its mechanical parts ; however, the extent of damage is more detrimental to insulating materials than to the mechanical parts.
Deterioration of insulation depends on the temperature as well as the time. It has been found that time to failure for organic insulation is reduced to half for every 8 to 10° C rise in machine temperature.

Losses and Efficiency of Electrical Machines –

In electrical machines, the power input (mechanical or electrical) is always more than the power output. The difference between power input and power output, under steady state coniditions, Is called power loss in watts. Thus, in accordance with the law of conservation of
power (or energy),

power input = power output + power loss
power loss = power input— power output
Power loss in a machine does not perform any useful work, it leads only to heating of the electrical machine.
A consideration of the power losses in electrical machines is essential for the following
three reasons:
(i) Losses influence the operating cost of electrical machines. For example, a machine with lower efficiency has more losses and therefore increased operating cost.
(ii) Losses cause heating of the machine and therefore its temperature rise. Greater the
loss, more is the temperature rise and therefore, faster is the deterioration of the machine
insulation. Temperature rise determines the machine rating through its effect on the life of
winding insulation. It can, therefore, be stated that temperature rise, and hence the losses determine the rating, or safe power output, of electrical machines.
(iii) Voltage drop IR is associated with ohmic loss whereas current component, like core loss current, pertains to the iron loss in electrical machines. Obviously, this suggests that losses associated with voltage drops or current-components must be appropriately taken into
account in the equivalent circuit of a machine so that electrical-machine analysis can be carried out as desired.

Pole-Pitch – The peripheral distance between two adjacent poles, is called pole-pitch. Pole-pitch is always expressed in electrical degrees, rather than in mechanical degrees, it can therefore, be inferred that pole pitch is always equal to 180 electrical degrees or π electrical radians.

Synchronous machines

In synchronous machines, the armature winding either exports A.C. power (synchro-nous generator) or imports A.C. power (synchronous motor), whereas the field winding is always energised from a D.C. source. In other words, the synchronous machines are doubly excited energy-conversion devices. The generation of e.m.f, in general, depends on the relative motion between field flux and armature winding. In view of this, an A.C. generator, alternator or synchronous generator may have either rotating field poles and stationary armature, or rotating armature and stationary field poles. Nevertheless, synchronous machines are in variably constructed with high-power armature winding on the stator and low-power field winding on the rotor; though small synchronous machines with the reverse arrangement may also be built.

The advantages of providing the field winding on rotor and armature winding on the stator are given below:

(1) More Efficient. With armature winding on the stator and field winding on the rotor, only two slip rings are required in a synchronous machine. There are, therefore, reduced slip ring losses and a more efficient synchronous machine.

(2) Better Insulation. Stationary armature windings can be insulated satisfactorily for higher voltages, allowing the construction of high-voltage, say 33 kV, synchronous machines.

(3) Eficient Cooling. Stationary armature winding can be cooled more efficiently, thus l permitting the construction of large synchronous machines, say 1000 MW or above.

(4) More Output. Low-power field winding on the rotor gives a lighter rotor and,therefore, low centrifugal forces. In view of this, higher rotor speeds are permissible, thus
increasing the synchronous machine output for given dimensions.

(5) Lesser Rotor Weight and Inertia. Field winding on the rotor requires less amount of copper and insulation.This reduces overall weight of rotor and its inertia. Reduced rotor
weight allows the use of low-priced bearings and also their longer life because of minimal wear and tear.

(6) Rigid and Convenient Construction. Three-phase armature winding, capable of handling high voltage and high current, can be more easily braced against electromagnetic forces when it is placed in stator slots In addition, flexible water tube connection for water cooling can be installed more conveniently on stator than on the rotor. This all results in a rigid and convenient construction of a synchronous machine.

(7) More Armature Tooth Strength. High-current synchronous machines require more armature copper for each slot. Greater amount of copper can be accommodated by making the slots deeper so that wider and stronger teeth are prepared for the armature. Armature on stator would have wider and stronger teeth whereas the armature on rotor would lead to narrower and weaker teeth. Strong teeth also results in less noise due to vibration and are less likely to be damaged during fabrication and use. Therefore, armature winding must be provided on the stator and field winding on the rotor.

Synchronous machies are of two types depending upon the geometrical structure of the rotor,

(a) Salient-Pole type rotor

(B) Cylindrical-rotor, round rotor or non-salient Pole type rotor.

(a) Salient-Pole type rotor – Is Salient means standing out, Sticking out or Projecting and Four salient poles are shown on the rotor. The field winding on the salient poles is a concentrated winding.

(B) Cylindrical-rotor, round rotor or non-salient Pole type rotor – In This rotor the field winding is a distributed winding housed in the rotor slots. The air-gap is uniform throughout, neglecting the slot-openings.

Induction motor has two types of Rotar

(1) Squirrel cage rotor

(2) Wound-rotor ya Slip Ring

SQUIRREL CAGE ROTOR – The rotor winding consists of an insulated conductors, in the form of copper or aluminium bars embedded in the semi closed slots. These solids bar short circuited at both ends by end-rings of the same material for good electrical connection the bars are rivetedd, brazed or welded with the two end-rings in smaller size, below 40 kw the assembled rotor core is placed in a mould and the molten conducting material usually aluminium is forced into the slots. Thus the rotor bars end-rings and the cooling fan are cast in the operation. without the rotor core the route bars and end-rings look like the cage of a squirrel hence the name squirrel cage induction motor. Note that the rotor bars form a uniformly distributed winding in the rotor slots. As the rotor bars are short-circuited by two end-rings, No external resistance can be inserted in the rotor circuit of a squirrel cage induction motor.

Wound rotar ya slip ring –

In the wound-rotor type, the rotor slots accommodate an insulated winding similar to that used on the stator.The rotor winding is uniformly distributed and is usually connected in star. The three leads from the star connection are then connected to three slip rings or collector rings mounted on but insulated from the shaft, Carbon brushes pressing on the slip rings allow, external resistors to be inserted in series with the rotor winding for speed and starting-torque control. Actually, the wound-rotor type of induction motor costs more and requires increased maintenance; it is therefore only used where,

(1) the driven load requires
speed control or

(2) high starting torque is required.

Since the rotor is wound with polyphase windings and carries slip rings, it is called wound-rotor or slip-ring induction motor.
In both the types, the rotor slots are not parallel to the shaft axis, i.e, the rotor slots are skewed for obtaining a quieter and smoother operation of the induction motor.