Advantages of Stationary armature:

The rotating armature is suitable for only very small size synchronous machines (having rating upto 10 kW). Majority of synchronous machines have very high ratings and armature winding is designed for 6.6 kV or 11 kV or 33 kV. Synchronous machines have a rotating field stationary armature construction. This construction offers the following advantages.

Synchronous machines (a) Stationary armature wiht rotating field (b) Rotating armature with stationary field

(a) Ease of current collection

(b) Better insulation

(c) Increased armature tooth strength (d) More rigid construction

(e) Reduced armature leakage reactance

(f) Lesser number of slip-rings

(g) Lesser and inertia (h) Improved ventilation and heat dissipation

(a) Ease of current collection:

Modern synchronous machines high current machines. The armature current of a 210 MW, 11 kV, 3- phase, alternator is about 13 kA. It is easier to collect such large currents if armature is stationary. The collection of such large currents through slip rings is very difficult.

(b) Better insulation:

Modern synchronous machines have armature windings designed for 6.6 kV or 11 kV or 33 kV. It is easier to insulate such a winding if it is on stator, Moreover the shaft of every machine is electrically earthed, through metallic bearings, to the stationary frame of the machine. It is easier to insulate a low voltage dc field winding on rotor than a rotating high voltage ac winding.

(c) Increased armature tooth strength:

High armature currents require more copper and hence deep slots to accommodate the winding. When the slots of a stationary armature are made deeper, the teeth become wider and stronger. When the slots of a rotating armature are made deeper, the teeth become narrower and weaker. Strong teeth of the stationary armature construction are likely to be less damaged during fabrication and use.

(d) More rigid construction:

The high voltage, high current armature windings of modern 3-phase synchronous machines are rather complex and have many connections end interconnections. Such windings are more rigid and secure when housed on stator.

(e) Reduced armature leakage reactance:

The stationary armature construction leads to lesser leakage flux and hence reduced armature leakage reactance. This leads to improved voltage regulation.

(f) Lesser number of slip rings:

If rotating armature construction is used, the machine would require 4 slip rings, three for three phases and one for neutral. These slip rings would have to be properly insulated from shaft and also spaced suitably to avoid flashover. Rotating field structure requires only 2 slip rings insulated for low voltage dc (about 200 to 600 V, mostly 400 V).

(g) Lesser rotor weight and inertia:

Because of low voltage field winding on rotor, the amount of copper and insulation is pretty less. Therefore rotor has lesser weight and inertia. Lesser weight of rotor leads to efficient high speed operation and cheaper bearings. Due to lesser inertia the machine can be brought up to full speed, from rest, in a very small time. Since higher speeds are possible for the given dimensions power output is more.

(h) Improved ventilation and heat dissipation:

Most of the heat is generated in armature. When armature is on stator, the ventilation ducts and holes for cooling can be wider leading to better ventilation and heat dissipation. (i.e. better the cooling) In view of above advantages nearly al modern synchronous machines have rotating field stationary armature construction.

 

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