Induction motor:Classification of A.C. Motors

Classification of A.C. Motors

With the almost universal adoption of a.c. system of distribution of electric energy for light and power, the field of application of a.c. motors has widened considerably during recent years. As a result, motor manufactures have tried, over the last few decades, to perfect various types of a.c. motors suitable for all classes of industrial drives and for both single and three-phase a.c. supply. This has given rise to bewildering multiplicity of types whose proper classification often offers considerable difficulty. Different a.c. motors may, however, be classified and divided into various groups from the following different points of view :

1. AS REGARDS THEIR PRINCIPLE OF OPERATION

Induction Motor : General Principle

As a general rule, conversion of electrical power into mechanical power takes place in the rotating part of an electric motor. In d.c. motors, the electric power is conducted directly to the armature (i.e. rotating part) through brushes and commutator (Art. 29.1). Hence, in this sense, a d.c. motor can be called a conduction motor. However, in a.c. motors, the rotor does not receive electric power by conduction but by induction in exactly the same way as the secondary of a 2-winding transformer receives its power from

the primary. That is why such motors are known as induction motors. In fact, an induction motor can be treated as a rotating transformer i.e. one in which primary winding is stationary but the secondary is free to rotate (Art. 34.47).

Of all the a.c. motors, the polyphase induction motor is the one which is extensively used for various kinds of industrial drives. It has the following main advantages and also some dis-advantages: Advantages:

1. It has very simple and extremely rugged, almost unbreakable construction (especially squirrel- cage type).

2. Its cost is low and it is very reliable.

3. It has sufficiently high efficiency. In normal running condition, no brushes are needed, hence frictional losses are reduced. It has a reasonably good power factor.

4. It requires minimum of maintenance.

5. It starts up from rest and needs no extra starting motor and has not to be synchronised. Its starting arrangement is simple especially for squirrel-cage type motor.

Disadvantages:

1. Its speed cannot be varied without sacrificing some of its efficiency.

2. Just like a d.c. shunt motor, its speed decreases with increase in load.

3. Its starting torque is somewhat inferior to that of a d.c. shunt motor.

Construction

An induction motor consists essentially of two main parts :

(a) a stator and            (b) a rotor.

(a) Stator

The stator of an induction motor is, in principle, the same as that of a synchronous motor or generator. It is made up of a number of stampings, which are slotted to receive the windings [Fig.34.2 (a)]. The stator carries a 3-phase winding [Fig.34.2 (b)] and is fed from a 3-phase supply. It is wound for a definite number of poles*, the exact number of poles being determined by the requirements of speed. Greater the number of poles, lesser the speed and vice versa. It will be shown in Art. 34.6 that the stator windings, when supplied with 3-phase currents, produce a magnetic flux, which is of constant magnitude but which revolves (or rotates) at synchronous speed (given by Ns = 120 f/P). This revolving magnetic flux induces an e.m.f. in the rotor by mutual induction.

(b) Rotor

(i) Squirrel-cage rotor : Motors employing this type of rotor are known as squirrel-cage induction motors.

(ii) Phase-wound or wound rotor : Motors employing this type of rotor are variously known as ‘phase-wound’ motors or ‘wound’ motors or as ‘slip-ring’ motors.

Squirrel-cage Rotor

Almost 90 per cent of induction motors are squirrel-cage type, because this type of rotor has the simplest and most rugged construction imaginable and is almost indestructible. The rotor consists of a cylindrical laminated core with parallel slots for carrying the rotor conductors which, it should be

noted clearly, are not wires but consist of heavy bars of copper, aluminium or alloys. One bar is placed in each slot, rather the bars are inserted from the end when semi-closed slots are used. The rotor bars are brazed or electrically welded or bolted to two heavy and stout short-circuiting end-rings, thus giving us, what is so picturesquely called, a squirrel-case construction (Fig. 34.3).

It should be noted that the rotor bars are permanently short-circuited on themselves, hence it is not possible to add any external resistance in series with the rotor circuit for starting purposes.

The rotor slots are usually not quite parallel to the shaft but are purposely given a slight skew (Fig. 34.4). This is useful in two ways :

(i) it helps to make the motor run quietly by reducing the magnetic hum and

(ii) it helps in reducing the locking tendency of the rotor i.e. the tendency of the rotor teeth to remain under the stator teeth due to direct magnetic attraction between the two.*

In small motors, another method of construction is used. It consists of placing the entire rotor core in a mould and casting all the bars and end-rings in one piece. The metal commonly used is an aluminium alloy.

Another form of rotor consists of a solid cylinder of steel without any conductors or slots at all. The motor operation depends upon the production of eddy currents in the steel rotor

Phase-wound Rotor

This type of rotor is provided with 3-phase, double-layer, distributed winding consisting of coils as used in alternators. The rotor is wound for as many poles as the number of stator poles and is always wound 3-phase even when the stator is wound two-phase.

The three phases are starred internally. The other three winding terminals are brought out and connected to three insulated slip-rings mounted on the shaft with brushes resting on them [Fig.

(b)]. These three brushes are further externally connected to a 3-phase star-connected rheostat [Fig. 34.5 (c)]. This makes possible the introduction of additional resistance in the rotor circuit during the starting period for increasing the starting torque of the motor, as shown in Fig.

(a) (Ex.34.7 and 34.10) and for changing its

speed-torque/current characteristics. When running under normal conditions, the slip-rings are automatically short-circuited by means of a metal collar, which is pushed along the shaft and connects all the rings together. Next, the brushes are automatically lifted from the slip-rings to reduce the frictional losses and the wear and tear. Hence, it is seen that under normal running conditions, the wound rotor is short-circuited on itself just like the squirrel-case rotor.

Fig. 34.6 (b) shows the longitudinal section of a slip-ring motor, whose structural details are as under :

1. Frame. Made of close-grained alloy cast iron.

2. Stator and Rotor Core. Built from high-quality low-loss silicon steel laminations and flash-enamelled on both sides.

3. Stator and Rotor Windings. Have moisture proof tropical insulation embodying mica and high quality varnishes. Are carefully spaced for most effective air circulation and are rigidly braced to withstand centrifugal forces and any short-circuit stresses.

4. Air-gap. The stator rabbets and bore are machined carefully to ensure uniformity of air-gap.

5. Shafts and Bearings. Ball and roller bearings are used to suit heavy duty, toruble-free running and for enhanced service life.

6. Fans. Light aluminium fans are used for adequate circulation of cooling air and are securely keyed onto the rotor shaft.

7. Slip-rings and Slip-ring Enclosures. Slip-rings are made of high quality phosphor-bronze and are of moulded construction.

Fig. 34.6 (c) shows the disassembled view of an induction motor with squirrel-cage rotor. According to the labelled notation (a) represents stator (b) rotor (c) bearing shields (d) fan (e) venti- lation grill and (f) terminal box.

Similarly, Fig. 34.6 (d) shows the disassembled view of a slip-ring motor where (a) represents stator (b) rotor (c) bearing shields (d) fan (e) ventilation grill (f) terminal box (g) slip-rings (h) brushes and brush holders.