EM-III_MID-II_Key

1(a) Synchronization: The process of connecting one alternator in parallel with another alternator or with a bus bar is known

as Synchronization. The alternator carrying the load or the alternator which is already connected to bus bar is known

as Running Generator. The alternator which is to be connected in parallel with the existing alternator is known as

the Incoming Generator.

A stationary alternator should not be connected to live busbars because at standstill the induced emf is zero

which result in short circuit. The following methods are used for synchronization of alternators

1. Dark lamp method 2. Bright Lamp method

3. Two bright and one dark lamp method 4. Synchroscope

Two bright and one dark lamp method

In this method one lamp is connected between one phase while the two other lamps are cross connected

between other two phases as shown below.

The prime mover of the incoming machine is started and brought up to its rated speed. The field current of

the incoming machine is adjusted until the back emf of the incoming machine is equal to the bus voltage. The

synchronizing switch is closed when the straight connected lamp is dark and the cross connected lamps are equally

bright, this indicates correct phase sequence. If the phase sequence is not correct then the all lamps will be dark

simultaneously. The main advantages of this method are (i) This method is cheap (ii) The phase sequence is easily

determined. It suffers from the following disadvantages:

(i) The lamp filament might burn out

(ii) The flicker of the lamps does not indicate which machine has the higher frequency.

(iii) If the synchronizing switch is not closed in correct instant, this may results in high circulating current which

damages the machines.

1.b)

2) (i) Effect of change of excitation keeping mechanical steam input as constant:

Let two alternators are operating in parallel i.e each alternator supplies half of active load (KW), half of

reactive load (KVAR) and power factors of both alternators is same as load power factor as shows in figure (a). Also

each generator supplies a current of „I‟ amperes so the output current is 2I.

Figure (a) Figure (b)

Now the excitation of alternator-I is increase, so E1 > E2. The difference between emfs produces a circulating

currents i.e

Ic = (E1 - E2)/(Zs1 + Zs2)

This circulating current is added to load current produced by alternator-I and subtracted from alternator-II.

Now two alternators delivers load current I1 and I2 at power factors Cosϕ1 and Cosϕ2. This change in load current

leads to change in power factor i.e Cosϕ1 decreases and Cosϕ2 increased. The change in power factor does not affect

the active power but reactive power (KVAR) supplied by alternator-I is increase and alternator-II decreased as shown

in figure (b). The change in excitation leads to decrease in power factor, increase in KVAR power.

(ii) Effect of change of mechanical steam input keeping excitation as constant:

Let two alternators are operating in parallel i.e each alternator supplies half of active load (KW), half of

reactive load (KVAR) and power factors of both alternators is same as load power factor as shows in figure (a). Also

each generator supplies a current of „I‟ amperes so the output current is 2I.

Figure (a) Figure (c)

Now the excitation of both alternators is same (i.e E1 = E2) but the mechanical steam input (i.e speed) to the

alternator-I is increased. Since the both alternators are mechanically coupled, so the alternator-I cannot over speed

the alternator-II. With the increase in mechanical input to alternator-I, its rotor advances in its angular position than

alternator-II and supplies more KW power to the load than alternator-II as shown in figure (c). Consequently,

resultant emf (Er) will produces and setups a current Isyn which lag the Er by 90o approximately but almost in-phase

with E1. Hence the power/ph of alternator-I is increase by E1Isyn and the power/ph of alternator-II is decreased by

same amount, but the reactive power (KVAR) division of both alternator is almost same as shown in figure (b) i.e

with the increase in mechanical input power the active power (KW) increases and no change in reactive power

(KVAR), but power factor increases slightly.

3.b) Causes for occurrence of Hunting: The main causes for occurrence of Hunting in Synchronous Motor are

i). Sudden change in load ii). Sudden change in field current.

iii). Sudden variations in load torque iv). Fault in supply system

The synchronous machine has minimum load angle at NO load. On increasing the load gradually, load angle

will increase. Let us consider that load P1 is applied suddenly to unloaded machine shaft so machine will slow down

momentarily. Also load angle (δ) increases from minimum to δ1. During the first swing electrical power developed is

equal to mechanical load P1. Equilibrium is not established so rotor swings further. Load angle exceeds δ1 and

becomes δ2. Now electrical power generated is greater than the previous one. Rotor attains synchronous speed. But it

does not stay in synchronous speed and it will continue to increase beyond synchronous speed. As a result of rotor

acceleration above synchronous speed the load angle decreases. So once again no equilibrium is attained. Thus rotor

swings or oscillates about new equilibrium position. This phenomenon is known as hunting or phase swinging.

“ The phenomenon of oscillation/swings of rotor at its final steady state position is called hunting or phase

swinging”

Effects of Hunting in Synchronous Motor

The main disadvantages or effects of hunting in Syn. Motor are

i). It may lead to loss of synchronism.

ii). Produces more mechanical stresses in the rotor shaft.

iii). Increases machine losses hence efficiency is low and cause temperature rise.

iv). It cause variations in supply voltage and producing un-desirable lamp flickering.

Reduction of Hunting in Synchronous Motor

The effect of hunting in Syn. Motor can be reduced by using any one of the followings methods. These are –

i) Using Flywheels ii) Using Damper windings

4.a) Synchronous Condenser:-

When the synchronous motor is over excited, running without a load, can draws leading current and results in

increase of power factor. Hence an over excited syn. motor operating on no load condition is called synchronous

condenser. The syn. condensers can be used as phase advancers or as power factor improvement device.

Let Vph is the terminal voltage/ph, IL is the load current which lags the Vph by an angle Ф1 as shown in fig.a.

Total load current IL = I only.

Fig.(a): Without Syn. condenser Fig.(b): With Syn. condenser

Now an over excited syn. motor is connected in parallel with the load as shown in fig.b. This over excited syn.

motor can draw a leading current as leading current of Im. Now the total current is the vector sum of load current IL

and leading motor current Im i.e I= IL + Im. Here the load current IL lags the phase voltage by an angle Ф1 and the

motor current Im leads the phase voltage by an angle Ф2 as shown in vector diagram (Fig.b). From the vector diagram

the total current (I) drown from supply lags the phase voltage Vph by small angle. Therefore the power factor is

improved.

4.b)

5) Why the Syn. Motor is not self starting machine:

When 3-ph AC supply is given to stator of Syn. Motor, a 3-ph magnetic field will set up in stator core and

rotate in clock wise direction (Assume) with Syn. speed Ns = 120f/P.

With rotor position as shown in (i), suppose the poles are at that instant situated at point „a‟. The two similar

poles Ns and Nr are repel, with result that the rotor tend to run in anti-clock direction.

But half a time period later, stator poles interchanged their positions i.e at point „a‟ Ns is replaced by Ss.

Under this condition, an attraction force will develop and rotor comes to its original position. Due to continuous and

rapid rotation of stator poles, the rotor is subjected to torque which tends to move it first in one direction and then in

opposite direction. But because of more weight of the rotor, the rotor cannot responds quickly for quickly reversing

torque, with this the result is the rotor is stationary. That is why the 3-ph Syn. Motor is not self starting machine.

Now if we give initial twist to the rotor, the motor run in same direction as the initial twist.

Since the 3-Ph Syn. motor is not a self start machine, it requires an auxiliary device to make it as self start. The 3-Ph

Syn. motor can make as self start with any one of the following methods.

1) Using a DC machine i.e using a DC shunt motor

2) Using a pony motor i.e using a small induction motor

3) Using Damper windings

Using a DC Machine

In this method a DC machine is coupled to the

synchronous motor. The DC machine works like a DC

motor initially and brings the synchronous motor to

synchronous speed. Once it achieves the synchronous

speed, the DC machine works like a DC generator and

supplies DC to the rotor of the synchronous motor. This

method offers easy starting and better efficiency than the

starting of Syn. Motor suing a pony motor.

Using a pony motor:

In this method a small induction motor is coupled to

the synchronous motor to bring the rotor to synchronous

speed before switching the DC excitation. Note that the

number of poles of the induction motor should be less than

the synchronous motor otherwise it is not possible to achieve the synchronous speed of the synchronous motor. This

is because an induction motor always has a speed less than the synchronous speed and for it to become equal to the

synchronous speed of the synchronous motor, its own speed has to be increased. After the rotor of the synchronous

motor is brought to the synchronous speed, the DC excitation is switched ON. After magnetic locking of stator and

rotor poles, we simply de-couple the induction motor from the synchronous motor shaft.

Using Damper Windings

In this method, the motor is first started as an induction motor and then

starts running as a synchronous motor after achieving synchronous speed. For

this, damper windings are used. Damper windings are additional windings

consisting of copper bars placed in the slots in the pole faces. The ends of the

copper bars are short-circuited. These windings behave as the rotor of an

induction motor. When 3-Ph power is supplied to the motor, the motor

starts running as an induction motor at a speed below synchronous speed.

After some time DC supply is given to the rotor of Syn. Motor. The motor

gets pulled into synchronism after some instant and starts running as a

synchronous motor. When the motor reaches synchronous speed, there is no

induced emf in the damper windings anymore and hence they don‟t have any

effect now on the working of the motor. This is the most commonly used

technique for starting Syn. Motors.

6.a)Effects of varying excitation on armature current and power factor:

Based on the back emf cross the armature terminals of Syn. Motor, there are four types of excitations. Those are

i. If the field excitation is such that Eb < V, the motor is said to be under-excited.

ii. If the field excitation is such that Eb ≤ V, it is called normal excitation of motor.

iii. If the field excitation is such that Eb = V, it is called critical excitation of motor.

iv. If the field excitation is such that Eb > V, the motor is said to be over-excited.

Under Excitation:

The motor is said to be under-excited if the field excitation is such that Eb < Vt. Under such conditions, since

Eb < Vt, the difference between Eb and Vt is (called net voltage Er) is more. This net voltage Er produces a more

current Ia which is in lagging direction to the voltage Vt. So, the motor has a lagging power factor.

Normal Excitation:

The motor is said to be normal-excited if the field excitation is such that Eb ≤ Vt. Under such conditions, since

Eb ≤ Vt, the difference between Eb and Vt is small, so the armature current Ia also small. But armature current Ia is

almost in-phase with the voltage Vt. So, the power factor of the motor is unity.

Over Excitation:

The motor is said to be over-excited if the field excitation is such that Eb > Vt. Under such conditions, since Eb

> Vt, the difference between Eb and Vt is more. This net voltage Er produces a more current Ia which is in leading

direction to the voltage Vt. So, the motor has a leading power factor.

From the vector diagram

(i) The armature current Ia varies with excitation i.e as the excitation increases from minimum, the armature

current decreases to minimum and then increases

with increase of excitation.

(ii) The power factor of motor also varies with

excitation i.e as the excitation increases from

minimum, the power factor reaches to unity from

lagging and then decreases from unity with increase of excitation.

(iii)

The figures (c) and (d) show the V and inverted V curves of Syn. Motor. The V curves are drawn between

armature current Ia and excitation If, the inverted V curves are drawn between power factor Cosϕ and excitation If.

Form the above curves, it is clear that the armature current (Ia) is minimum at unity p.f and increases as the

power factor becomes less either leading or lagging.

6.b)

7.a) DOUBLE FIELD REVOLVING THEORY:

This theory is based on the facts that the alternating field produced the stator winding is represented as two

oppositely rotating fluxes of identical magnitudes. The magnitude of each of the flux is equal to half of the maximum

flux and rotating in opposite direction.

Let Фm is maximum flux produced by the stator winding. This flux is divided into two equal magnitudes as

forward flux Фf = Фm/2 and reverse flux Фr = Фm/2

When we apply a single phase AC supply to the stator winding of single phase induction motor, it produces its

flux of magnitude, Фm. According to the double field revolving theory, this alternating flux, Фm is divided into two

components of magnitude Фm/2. Each of these components will rotate in the opposite direction, with the synchronous

speed, Ns. Let these two components of flux as forward component of flux, Фf and the backward component of flux,

Фb. The resultant of these two components of flux at any instant of time gives the value of instantaneous stator flux at

that particular instant.

Initially, Let the forward flux (Фf) and reverse flux (Фr) are in same direction as shown in fig. 1.a. The total

flux is Ф = Фf + Фr = Фm. After a quarter cycle, the two fluxes are in opposite to each other as shown in fig. 1.b, so the

total flux is Ф = 0. Similarly, if two flux vectors are rotates to 1800 from reference with syn. speed Ns, as shown in

fig.1.c, the total flux Ф = - Фm.

Now the slip of forward flux is Sf = S = Ns

N

Ns

NN rrs

1

S

Ns

Nr 1

Similarly slip of backward flux is Sb = SSNs

N

Ns

NN rrs

2)1(11

)(

The torque produced by the motor will

depends on flux, i.e torque produced by forward flux

is Tf and torque produced by backward flux is Tb. The

torque-slip characteristics of 1-ph induction motor is

shown in fig. 1.d. Therefore the net torque produced

by the motor at starting is zero, so the 1-ph Induction

motor is not a self starting machine. To make the 1-ph

Induction Motor as self start, the stator winding is

splited into two windings.

Fig. 1.d: Torque – Speed Characteristics

7.b)Capacitor start-Capacitor run Motor or Two value Capacitor Motor:

In two value capacitor motors, two capacitors are used. One for starting and other for running purpose as

shown in fig.3.a. The starting capacitor is electrolytic type and running capacitor is oil filled capacitor.

Fig. 3(a): Circuit Diagram Fig. 3(b): Vector diagram

The running capacitor is permanently connected in series with the auxiliary winding, and the starting

capacitor connected in parallel with the running capacitor. When the motor reaches to 75% - 80% of synchronous

speed, the auxiliary winding and starting capacitor are disconnected by means of a centrifugal switch. The vector

diagram for two value capacitor start induction motor is shown in fig.3.b. The direction of rotation of this type of

induction motor may be reversed by reversing the connections of either the main or starting winding. The fig3.c

shows the torque–speed characteristics at starting & running conditions. Its power rating is upto 5KW.

This type of motors is used for loads of higher inertia requiring frequent starts. They are used in pumping equipments,

refrigeration, air compressors, air-conditioning equipment etc.

Advantages:

i) More starting torque i.e 3 to5 time the running torque.

ii) Power factor is proved with the presence of capacitor.

iii) Motor efficiency is increased.

Disadvantages:

i) Its cost is more due to presence of capacitor.

ii) It requires more maintenance hence maintenance cost is more.

iii) Damage of capacitor may damages the motor.

8.a) Stepper Motor:

A stepper motor is an electromechanical device it converts electrical power into mechanical power. It is

basically a brushless DC motor, whose rotor rotates through a fixed angular step in response to input current pulse i.e

the full rotation of the rotor is divided into equal number of steps, and rotor rotates through one step for each current

pulse. Stepper motors are used for precise speed control without closed loop feedback.

Step angle or step size = 3600/ m Nr where m = No. of stacks or phases and Nr = Rotor poles

Steps per Revolution = 3600/ Step Angle

Operation of Stepper motor:

Stepper motor doesn‟t rotate continuously, they rotate in steps. There are 4 coils with 90o angle between each

coil. The following figure shows the stepper motor diagram.

Here each coil of the phase is connected to the supply

alternatively. The table below shows the order through which coils

are energized in a 4-phase stepper motor.

T

o

make

the

motor

shaft turn, first, one pole is magnetised by supplying the

corresponding field coil. This toothed pole then aligns the rotor teeth due to magnetic attraction. Rotor teeth are

slightly offset from the next pole. At the next step, first pole is demagnetised and the second is magnetised. This

causes the rotor to rotate in a fixed angle to align with the second pole and offset with the previous pole. This was

the basic working principle of a stepper motor.

Advantages of Stepper Motor:

The rotation angle of the motor is proportional to the input pulse.

The motor has full torque at standstill.

Precise positioning and repeatability of movement since good stepper motors have an accuracy of 3 – 5%

of a step and this error is non cumulative from one step to the next.

Excellent response to starting, stopping and reversing.

Very reliable since there are no contact brushes in the motor. Therefore the life of the motor is simply

dependant on the life of the bearing.

A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input

pulses.

Applications:

Industrial Machines: Stepper motors are used in automotive gauges and machine tooling automated

production equipments.

Security: New surveillance products for the security industry.

Medical: Stepper motors are used inside medical scanners, samplers, and also found inside digital dental

photography, fluid pumps, respirators and blood analysis machinery.

Consumer Electronics: Stepper motors in cameras for automatic digital camera focus and zoom functions.

And also have business machines applications, computer peripherals applications. There are three types of stepper

motors. Those are

1) Permanent Magnet Stepper motor 2) Variable reluctance Stepper Motor 3) Hybrid Stepper Motor

Step Switch SA Switch SB Switch SC Switch SD

1 1 0 0 0

2 0 1 0 0

3 0 0 1 0

4 0 0 0 1