Basics-DC machines, induction machines, and synchronous machines

1. INTRODUCTION
DC machines are an example of rotating machines. Rotating machines are used for electro-mechanical energy conversion i.e., the Generator converts Mechanical energy to electrical energy and the Motor converts electrical energy to mechanical energy.
When Energy is converted from electrical energy to mechanical energy or vice versa, then it is first converted to magnetic energy and then to mechanical energy and hence magnetic energy acts as an intermediate stage between electrical and mechanical energy.

DC machine is a highly versatile energy conversation device. It can meet the demands of loads requiring high starting torque and better running performance. The main part of DC machines is Commutator along with starter & Rotor.

CONSTRUCTION OF DC MACHINES

DC machines have two parts stator & rotor. The field winding of DC machines is wound on the stator and armature winding is wound on the rotor. Different parts of the stator and rotor have been explained below-

Yoke

Cast Iron is used to make small D.C machine yoke and fabricated steel for large DC machine. It provides mechanical support to all machine. It provides the path for flux ϕ and carries half of it.

Field winding

The copper winding wound around the pole core for establishing the main field flux in the machine.

Interpoles

It lies in between the main poles of the DC machine. The shape is so designed wider at the bottom and tapered on the outer side, to avoid magnetic saturation. The interpole winding is connected in series with the armature

Compensating winding

It is placed in the slots cut in pole faces of a DC machine. It is also connected in series with the armature winding.

Brushes

It is housed in box type brush holders fixed to the stator end cover. They are made of carbon in small DC machine and copper graphite for low-voltage, high current DC machine.

Armature winding

This is also made of copper. This is a large number of insulated coils, each having one or more turns placed in the rotor slots. It is connected in series or parallel depending on the type of winding.

Commutator

The ends of armature winding are connected to commutator which is connected to brushes which are then connected to supply or load. It performs two important functions.

(i) Convert ac quantity to DC quantity and vice-versa.

(ii) Keep rotor or armature MMF stationary in space.

POLYPHASE INDUCTION MOTOR

Polyphase induction motors are the widely used AC motors due to their low cost, simple and rugged construction, high reliability, high efficiency, reasonably good

power factor and simple starting arrangement. It differs from either type of electric motors in that there is no electrical connection from the rotor winding at any source of supply. It receives current and voltage by induction action and therefore, it is called an induction motor. A magnetic field is induced in the stator by rotating the magnetic field.

1.1. Rotating Magnetic Field:

If a 3-phase supply (120° mutual displacement with respect to time) is applied to a 3-phase winding (Star or Delta), then a magnetic field of constant magnitude of 1.5ϕ_m and with standard constant speed known as synchronous speed (N_s) and with definite direction depending on the phase sequence of supply is induced.

Synchronous speed

Ns=120f/p

Where, f = supply frequency

P = number of poles

CONSTRUCTION

The induction motor essentially consists of two parts:

Stationary part called Stator
Revolving part, called Rotor

2.1. Stator

It consists of 3-phase winding which is mutually displaced by an angle of 120⁰.

The stator can be represented in the figure below:

The stator of a 3-phase induction motor is like the stator of a synchronous generator.

The synchronous generator generates 3-ϕ phase voltage at 3-ϕ winding but in the case of induction motor, we apply 3-ϕ phase voltage at 3-ϕ winding.

# The mutual phase displacement produces by slot caused slot angle.

∝ = 180°/(slot/pole)

For 2 pole rotor:

∝ = 180°/6 = 30°

PRINCIPLE OF OPERATION

The basic principle of operation of an induction machine is mutual induction.

It is a singly excited motor (voltage is applied to the stator only) but voltage is induced in the rotor winding through mutual induction.

When a 3-ϕ supply is given across the stator it produces an RMF rotating at N_s (Synchronous speed). It sweeps pass the rotor and cut the rotor conductor which will induce emf. As the rotor is essentially closed, it results in current and torque and makes the rotor to rotate in the direction of RMF according to Lenz’s law. The rotation of the rotor is in such a way to oppose the relative speed between RMF and the rotor. Actually, the rotor wants to catch the RMF and to rotate at synchronous speed. But it could not catch it and runs at a speed N slightly less than N_s. As in slips Back N_s by a slip speed (N_s – N).

SYNCHRONOUS GENERATOR (ALTERNATOR) & ITS CONSTRUCTION DETAILS

The construction of an alternator consists of field poles placed on the rotating fixture of the machine. An alternator is made up of two main parts: a rotor and a stator. The rotor rotates in the stator, and the field poles get projected onto the rotor body of the alternator. The armature conductors are housed on the stator.

An alternator is basically a type of AC generator. The field poles are made to rotate at synchronous speed N_s = 120 f/P for effective power generation. Where, f signifies the alternating current frequency and the P represents the number of poles.

In most practical construction of alternator, it is installed with a stationary armature winding and a rotating field unlike in the case of DC generator where the arrangement is exactly opposite. This modification is made to cope with the very high power of the order of few 100 Megawatts produced in an AC generator contrary to that of a DC generator.

To accommodate such high power the conductor weighs and dimensions naturally must be increased for optimum performance. For this reason, is it beneficial to replace these high-power armature winding by low power field windings, which is also consequently of much lighter weight, thus reducing the centrifugal force required to turn the rotor and permitting higher speed limits.

A synchronous machine essentially consists of two parts:

Armature (rotor)
Field magnet system

Small AC generators and of low voltage rating are commonly made of rotating armature. In such generators, the required magnetic field is produced by DC electromagnet placed on the stationary member called stator and the current generated is collected by means of brushes and slip ring on the revolving member called the rotor.

Practically all large rating generators are made of revolving field. In such generators revolving field structure or rotor has slip rings and brushes for supply of excitation current from an outside DC source and the stationary armature, which is made of thin silicon sheet steel laminations securely clamped and held in place of steel frame, accommodates coils or winding in the slots. The slots are provided on the stator core and of mainly two types viz. open type or semi-closed type. Totally closed type slots are never used.

Types of Rotor

Salient pole type:

The term salient means protruding or projecting. The salient pole type of rotor is generally used for slow speed machines having large diameters and relatively small axial lengths. The poles, in this case, are made of thick laminated steel sections riveted together and attached to a rotor with the help of a joint.

It is mostly responsible for the generation of very high electrical power. To enable that, the mechanical input given to the machine in terms of rotating torque must also be very high. This high torque value results in oscillation or hunting effect of the alternator or synchronous generator. To prevent these oscillations from going beyond bounds the damper winding is provided in the pole faces.

The damper windings are basically copper bars short-circuited at both ends are placed in the holes made in the pole axis. When the alternator is driven at a steady speed, the relative velocity of the damping winding with respect to the main field will be zero. But as soon as it departs from the synchronous speed there will be relative motion between the damper winding and the main field which is always rotating at synchronous speed. This relative difference will induce the current in them which will exert a torque on the field poles in such a way as to bring the alternator back to synchronous speed operation.

The salient feature of pole field structure has the following special feature:

They have a large horizontal diameter compared to a shorter axial length.
The pole shoes cover only about ²/₃rd of pole pitch.
Poles are laminated to reduce eddy current loss.
The salient pole type motor is generally used for low-speed operations of around 100 to 400 rpm, and they are used in power stations with hydraulic turbines or diesel engines.

Salient pole alternators driven by water turbines are called hydro-alternators or hydro generators. These rotors are used almost entirely for slow and moderate speed alternator and cannot employed in high speed generators. Such rotors have large diameter and small axial length.

Non-salient or smooth cylindrical type

The cylindrical rotor is generally used for very high-speed operation and employed in steam turbine driven alternators like Turbogenerators. The machines are built in several ratings from 10 MVA to over 1500 MVA. The cylindrical rotor type machine has a uniform length in all directions, giving a cylindrical shape to the rotor thus providing uniform flux cutting in all directions. The rotor, in this case, consists of a smooth solid steel cylinder, having several slots along its outer periphery for hosting the field coils.

These rotors are used in very high-speed alternators. Such rotors have a small diameter and long axial length.

The frequency of the induced emf is given by,

f= PN/120

where P = number of poles

N = speed of the rotor in rpm

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