Poly phase

 If more than one phase is called poly phase  .like two phase ,three phase ,four phase ,six phase etc.....

TWO PHASE : The alternator producing two-phase supply has two windings kept in such a way that phase difference between them is 90°. In such a supply the voltage between phase ad neutral is 200 V, then the voltage between two phases will be 200 × √2 .

THREE PHASE:3-phase alternator consists of 3 windings placed in such a way that the phase difference between them is 120°. If the voltage between phase and neutral is 250 volt, then the voltage between any two phases will be equal 250 × √3 .

SINGLE PHASE : The simple alternator with it's single coil in field is a single phase alternator.

TWO-PHASE: If two different coil is added , having the same number of turns, but with its plane perpendicular to that of the first, the e.m.f. generated is of the same frequency and amplitudes as in the first coil, but owing to their relative displacement the maximum value occurs in one when that in other is passing through zero and vice versa. The two e.m.f.s are, therefore, in quadrature and are represented by two sine waves displaced by the quadrature and are represented by two sine waves displaced by the quarter period or by two vectors of equal length displaced by 90° instantaneous value of two e.m.f.s will be : 

Such an arrangement constitutes a simple two-phase alternator and be used either for supplying two single-phase circuits or to feed a two-phase system supplying other apparatus having winding arranged in a similar manner.

THREE-PHASE. If a third similar coil is added and the three arranged wis their planes mutually inclined at 120°, the e.m.f.s are represented by threes waves of equal frequency and amplitude, but displaced in phase by 1/3 of period, or by three vectors of equal length mutually inclined at 120° .

eA = Em * sin omega*t, eB = Em * sin(omega*t + 120 deg) , eC = Em * sin(omega*t + 240)

Such an arrangement is essentially a three-phase alternator and may b used to supply three separate single-phase circuits or to feed a three-phase system supplying other apparatus having winding arranged in a similar manner.

The value of current in each winding (or phase) and its phase relationshi to the e.m.f depend upon the characteristics of the circuit of which it for a part. If these are the same for all these phases (as is usually the case in the three phase machine )

The connection of three phase machine are two types : 

A) star 🌟 connection

B) Delta connection

Star. In each 3-phase alternator six terminals come out from 3 coils placed at 120° apart from each other. If any three ends starting or finishing are connected at one point and we get supply from other three points, the connections are called star. That junction point is called star point or neutral point .

Delta . If the six ends are so connected that end of first coil (assumed) is connected to start of the second coil and end of second coil is connected to start of the third coil and end of third coil is connected with start of first coil, the connections are called delta connected. The supply is taken from those connected points .

                TERMS AND DEFINITIONS

PHASE VOLTAGE : between one of the phases and the neutral terminal is known as phase voltage and is represented by Vph .

PHASE CURRENT: The current flowing through any of the phase windings is called phase current and is denoted by Iph Line Voltage. The voltage between any two phases of the supply system is called line voltage and is denoted by VL-

LINE CURRENT :  The current flowing between any two phases of the windings is termed as line current and is denoted by IL

BALANCED LOAD. If in a three-phase system, the power factors and the phase (or line) currents of the three phases are equal, then that load is called balanced load

UNBALANCED LOAD : If the three phases have different power factors and the phase current, then the load is an unbalanced load. Phase Power. The power measured between a phase and the neutral terminal is known as phase power = VI cos 0. 

TOTAL POWER. In a three-phase system, the total power measured between the three phases is called total power = √3 VLlL COS thita .

Comparison between Star and Delta connected systems.

STAR 🌟 SYSTEM : the phase voltage of a 3 phase star system is 1/√3 time line voltage .so if alternator is connected in star then less number of turns are to be provided per phase and there will be less difficulty insulating them.

It is possible to earth the neutral and to use relays and protective devices for the protection of alternators, and it is possible to get balanced voltages across each phase of an unbalanced load.  

It is the only system which can be used for both lighting and power purposes. 

DELTA CONNECTION.  There is no 3rd harmonic e.m.f.s. in a delta connection; so it gives results more near to sine waves.

it is mostly used for running 3-phase induction motors.

This connection is most suitable for rotary convertors. Measurement of Power. The following are the methods of measurement

Different methods for measuring three phase power:

 There are three methods for measuring  phase power:

 1. Single Watt Meter Method

2. Two Watt Meter Method

 3. Three Watt Meter Method.

SINGLE WATT METER METHOD: This system of method is used for measuring power for balanced load only. One watt meter is connected with phase and anathorneutral is connected in single phase. Total power is equal to the reading of one watt meter multiplied by three i.e.

TWO WATT METER METHOD: In this system two watt meters are connected with two different phases and their pressure coils are connected with third phase . The reading of the watt meters indicate the total power but its reading differs with the nature of load . 

THREE WATT METER METHOD : in this method 3 single phase watt meter are connected with meters and  connected each phase .

SOME IMPORTANT QUESTIONS AND ANSWER : 

1. What are the advantages of a star connection?

Ans=We can take the neutral from star point and less insulation is required per phase

for high voltage generators

2. What do you mean by polyphase?

Ans=Two phase and more than two phases are called polyphase.

3. If in three-phase supply there is 200 V supply between phase and neutral, what will be the supply voltage between phase to phase?

Ans=200×√3

4. Draw the connection diagram of mesh connection.

Ans=For Mesh or Delta connection

5. If in two-phase supply there is 200 V supply between phase and neutral, what

will be the supply voltage between phase to phase ?

Ans= 200×√2 v

6. Make a diagram to connect two wattmeters to measure the 3-phase power in unbalanced load.

Ans=

7. Why we only induce/produce 3-phase

supply?

Ans=(i) Because it is economical. (ii) We can get one phase, two phase, three phase, six phase supply from three phase supply.

8. In what connection we get neutral (Star-or-Delta) ?

Ans=star connection

9. Can we induce six phase supply?

Ans=yes but it is not economical

19. What are the ways to connect 6 wires of an alternator?

Ans= two ways ,star and Delta connection