Necessity of Connecting Alternator in Parallel

For high efficiency of operation, reliability, convenience and economy in maintenance and repair and possibility of additions to plant with the growth in load on the power station the alternators are put in parallel.

Circuit diagram for connecting alternators in parallel
Photo Credit - www.allaboutcircuits.com

Continuity of supply:-

The continuity of supply is one of the important requirement of any electrical apparatus. If one alternator fails, the continuity of supply can be maintained through other units. This will ensure uninterrupted supply to consumer.

Efficiency:-

The load on the power system varies during whole day, being minimum during the late night hours. Single alternator operate most efficiently only when delivering full load, units can be added or put off depending upon the load requirement. This permit the efficient operation of the power system.

Maintenance and repair:-

It is often desirable to carry out routine maintenance and repair of one or more units. For this purpose, the desired unit can be shut down and the continuity of supply can be maintained through other units.

Load Growth:-

The load demand is increasing due to the increasing use of electrical energy. The load growth can be met by adding more units without disturbing the original installation.

Star Delta Starter

If normal supply voltage is applied to the stationary motor then a very large initial current is taken by the stator for short while. This will lead to excess copper losses in the winding which will overheat the motor. This will produce line voltage drop, that in turn, will affect the operation of other electrical equipments connected to the same line and heavy starting current may damage the motor winding. In order to avoid these problem, a starter is used to start the induction motor safely.

Star-Delta Starter

The motor is designed to operate as delta connected motor under normal conditions. The motor starts as a star connected motor and the voltage per phase is ^V_L/_√3. The starting torque is reduced as it is directly proportional to square of stator voltage and there is jerk while switching from star to delta.

Start Mode

The 3 pole 6 way switch is kept in start mode. This will connect terminal R', Y' B' of stator winding to each other. This act as star point. The supply is connected to R,Y,B terminal of the stator winding. Thus in start mode the stator winding is connected to form the star.

Run Mode

The 3 pole 6 way switch is thrown in run position once the motor accelerate. This will connect the terminals in following manner:

R→B', Y→R', B→Y'

Capacitor Start Capacitor Run Induction Motor

Capacitor Run Induction Motor-Representational Image 

We know that single phase induction motor is not self starting due to the absence of rotating magnetic field at starting. In order to produce rotating field there must be some phase difference. In capacitor start capacitor run induction motor we are using two winding, one main winding and other auxiliary winding. For producing the phase difference two capacitors are used since we know that in capacitor the current leads the applied voltage by some angle. The capacitor serves to shift the phase on one of the windings so that the voltage across the winding is at 90° from the other winding, thus making the capacitor run motor a truly two-phase machine at its rated load.

Capacitor start capacitor run induction motor

Instead of one if two capacitors are used in the auxiliary winding of the motor, one of them for starting and other for running, optimum starting and running performances can be obtained. Figure shows the schematic  diagram of the motor. The capacitor Cs is cut out when the motor reaches about 75% of the synchronous speed by centrifugal device. The capacitor C and auxiliary winding A retained in the circuit for running condition. A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be two + to + (or - to -) series connected polarized electrolytic capacitors. Such AC rated electrolytic capacitors have such high losses that they can only be used for intermittent duty (1 second on, 60 seconds off) like motor starting. A capacitor for motor running must not be of electrolytic construction, but a lower loss polymer type.

Torque- Speed Characteristics of Capacitor Start Capacitor Run Induction Motor

Capacitor-start/capacitor-run motors have moderate-to-high starting torque compared to other types of single-phase motors. Starting torque generally ranges from 200%-350% of normal full-load torque.

Torque- Speed Characteristic of Capacitor Start Capacitor Run Induction Motor

Their main advantage over the capacitor-start motor is their lower starting-torque that results in lower starting current.
Because of the lower starting current, they are generally used for most single-phase applications between 3-and-10 horsepower  due to their lower starting current. Common applications include: larger single-phase compressors, pumps, grinders, conveyors, and larger single-phase air-conditioning compressors.

Divergence Theorem

It gives the facility to convert surface integral into its equivalent volume integral.

Representational Image
Photo Credit - www.wikipedia.com

Statement:- It states that the surface integral of a normal component of any vector function on a closed surface is equal to the volume integral of the divergence of vector function.
Proof:-  The total charge enclosed by volume △V is given by

........(1)

The total outward flux coming out from the volume is △V equal to the charge enclosed.

△Ψ = △Q = charge density x volume

△Ψ = △Q =ρ_v△V = ρ_v△x△y△z.......(2)

From equation (1) and (2)

........(3)

The divergence of D is given by 

........(4)

From (3) and (4) 

According to Gauss's Law

ψ = Q

Universal Motors

A universal motor is defined as a motor which may be operated on either d.c. or single phase a.c. supply at very high speed in the range of 3000 to 7000 rpm or so. It has starting torque and variable speed characteristics. Universal motor are designed for weak field to minimize commutation difficulties. The stator core is laminated and high resistance brushes are used.

Universal Motor
Photo Credit www.woodgears.ca

Construction

The construction of universal motor is similar to that of DC Machine. The field poles are mounted on stator winding. Field winding is wound on the field poles. Both the stator field poles and armature is laminated to minimize the Eddy current losses while working on a.c. supply. The armature has straight or skewed slots and the commutator and brush arrangement is resting on it. 

Universal Motor - Construction

Working

Such motors develop unidirectional torque regardless of whether they operate on A.C. or D.C. supply. The motor runs on the same principle as d.c. motor i.e., force between the main pole flux and the current carrying armature conductor. The universal motor produces the electric torque proportional to the quadrate of the supply current. Since the same current flows through the field winding and the armature, it follows that ac reversals from positive to negative, or from negative to positive, will simultaneously affect both the field flux polarity and the current direction through the armature. This means that the direction of the developed torque will remain positive, and rotation will continue in the same direction. Thus, a universal motor can run both on dc and ac. So the electric torque has the same torque direction at any current polarity and in addition at AC power. The starting torque of a universal motor is determined by the current that flows through the armature and field windings. Due to the inductive reactance of these windings the AC starting current will always be less than the DC starting current.Consequently, the starting torque on AC power will be lower than the starting torque on DC power. The characteristics of universal motor are very much similar to those of D.C. series motors, but the series motor develops less torque when operating from an A.C. supply than when working from an equivalent D.C. supply.

Torque-Speed Characteristics

Torque speed characteristics of Universal Motor

Application

1. Universal motors are used in most hand held power tools such as jigsaw, routers, drills and sanders.
2. They also find their use in household appliances like vacuum cleaner, drink and food mixers and sewing machine. 

Double Cage Induction Motor

Double Cage Induction motor Stator Construction

Construction

An induction motor with two cage rotor is used for high starting torque. The slotting arrangement for double cage induction motor is as shown in above figure. As the name indicate the double cage induction motor has two winding in rotor. The outer bars consists of rotor bars having low reactance and high resistance. On the other hand, the inner cage consists of rotor bars having high reactance and low resistance.

Working

At start the rotor frequency is high, the outer cage carries most of the current despite its high resistance. The inner cage has low reactance and is mostly ineffective. This gives high starting torque and low starting current. As the motor picks up the speed, the rotor frequency reduces and the inner cage carries most of the current. Under normal running condition, the outer cage and inner cage are in parallel giving low combined resistance and both the cages are active.

When the speed is normal frequency reduces and it is so small that the reactance of both the cages are practically negligible. Hence it has been made possible to construct a single machine which has high starting torque with reasonable starting current which maintains speed regulation and high efficiency.

Torque-Speed characteristics

Torque Speed Characteristic of Double Cage Induction Motor

Advantages

1. It has high efficiency and good speed regulation.
2. It gives higher starting torque.
3. Lower starting current and are cheaper in cost.
4. They are more robust and are explosion proof since the risk of sparking is eliminated by the absence of slip ring and brushes.

Construction and Working of Induction Motor

Construction:-

An induction motor consists of two main parts

1. Stator
2. Rotor

The stationary frame is called stator and the rotating armature is called rotor. The stator of induction motor is similar to that of the synchronous motor or generator. It is made of large number of stampings. These stampings are slotted in order to receive the stator windings. Figure below shows the construction of Induction motor.

Induction Motor

The functions of various parts is as follows:-

• Frame:- Its function is to provide mechanical support to the entire construction. The frame also contains the stator winding of Induction motor.
• Air gap:- Air gap provides the space for the rotating magnetic field between the stator and rotor.
• Fan:- The fan rotates with the rotor. Its function is to cool down the motor.
• Slip rings:- The rotor winding terminals are permanently connected to the slip rings(in slip ring type induction motor). The slip rings are continuously in contact with the brushes which are pressed against slip rings. External connections from the brushes are brought out,

Figure below shows the rotor and stator of induction motor.

Stator and Rotor construction of Induction motor

Rotor drum is provided with slots. The stator is stationary which can be star connected or delta connected to the 3Φ AC supply through a switch as shown in figure below. 

Stator winding connections

The function of stator winding is to produce a rotating magnetic field in the air gap between the stator and rotor. It should be known that the rotor is connected to any external supply. The current flows through the rotor due to principle of induction.

Working:-

The 3Φ induction motor has 3Φ winding which is supplied by 3Φ alternating voltage and 3Φ balanced current flow in the winding. The current produce magnetic flux which is constant in magnitude and rotating at synchronous speed. The rotating magnetic filed swift pass the rotor conductors which as yet are stationary. The relative velocity between the magnetic flux and stationary rotor conductors induces emf in the rotor conductors according to Faraday's Law of Electromagnetic Induction. The induced emf is directly proportional to the relative velocity between the magnetic flux and stationary rotor conductors. The frequency of induced emf is same as the supply frequency and the direction of induced emf is given by Fleming's Right Hand Rule.

Since the rotor conductor forms a closed circuit, rotor current produces which opposes the very cause producing it according to Lenz's Law. In this case, the cause which is producing the rotor current is relative velocity between the magnetic flux and stationary rotor conductors. Hence the rotor starts running in the same direction as rotating magnetic field and always try to catch up the the speed of rotating magnetic field.

Note:-For better understanding of the working of Induction motor, refer following videos.