The d axis component of the armature MMF, F_a is denoted by F_d, and the q axis component by F_q. The component F_d is either magnetizing or demagnetizing. The component F_q results in a cross-magnetizing effect. If Ψ is the angle between the armature current I_a and the excitation voltage E_f and F_a is the amplitude of the armature MMF, then

Salient Pole Synchronous Machine Two Rection Theory

In the cylindrical rotor synchronous machine, the air gap is uniform. The pole structure of the rotor of a salient pole machine makes the air gap highly non-uniform. Consider a 2 pole, salient pole rotor rotating in the anticlockwise direction within a 2 pole stator as shown in the figure below:

The axis along the axis of the rotor is called the direct or the d axis. The axis perpendicular to the d axis is known as the quadrature or q axis. The direct axis flux path involves two small air gaps and is the path of the minimum reluctance. The path shown in the above figure by ϕ_q has two large air gaps and is the path of the maximum reluctance.

The rotor flux B_R is shown vertically upwards as shown in the figure below:

The rotor flux induces a voltage E_f in the stator. The stator armature current I_a will flow through the synchronous motor when a lagging power factor load is connected to it. This stator armature current I_a lags behind the generated voltage E_f by an angle Ψ.

The armature current produces stator magnetomotive force F_s. This MMF lags behind I_a by angle 90 degrees. The MMF F_S produces stator magnetic field B_S long the direction of Fs. The stator MMF is resolved into two components, namely the direct axis component F_d and the quadrature axis component F_q.

If,

• ϕ_d is the direct axis flux
• Φ_q is the quadrature axis flux
• R_d is the reluctance of the direct axis flux path

Therefore

As, R_d < R_q, the direct axis component of MMF F_d produces more flux than the quadrature axis component of the MMF. The fluxes of the direct and quadrature axis produce a voltage in the windings of the stator by armature reaction.

Let,

• E_ad be the direct axis component of the armature reaction voltage.
• E_aq be the quadrature axis component of the armature reaction voltage.

Since each armature reaction voltage is directly proportional to its stator current and lags behind by 90 degrees angles. Therefore, armature reaction voltages can be written as shown below:

Where,

• X_ad is the armature reaction reactance in the direct axis per phase.
• X_aq is the armature reaction reactance in the quadrature axis per phase.

The value of X_ad is always greater than X_aq. As the EMF induced by a given MMF acting on the direct axis is smaller than for the quadrature axis due to its higher reluctance.

The total voltage induced in the stator is the sum of EMF induced by the field excitation. The equations are written as follows:

The voltage E’ is equal to the sum of the terminal voltage V and the voltage drops in the resistance and leakage reactance of the armature. The equation is written as:

The armature current is divided into two components; one is the phase with the excitation voltage E_f and the other is in phase quadrature to it.

If

• I_q is the axis component of I_a in phase with E_f.
• I_d is the d axis I_a lagging E_f by 90 degrees.

Therefore,

Combining the equation (4) and (5) we get,

Combining the equation (6) and (7) we get, Let,

The reactance X_d is called the direct axis synchronous reactance, and the reactance X_q is called the quadrature axis synchronous reactance.

Combining the equations (9) (10) and (11), we get the equations shown below: The equation (12) shown above is the final voltage equation for a salient pole synchronous generator.

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For simplicity, we consider a 3 phase, 2 pole alternator shown in the figure below:

The winding of each pole is assumed to be concentrated, but the effects of armature reaction will be the same as if a distributed winding were also used. The armature reaction in a synchronous machine affects the main field flux and varies differently for different power factors.

Here armature reaction is discussed for the following three conditions, namely unity power factor, zero power factor lagging, and zero power factor leading. The power factor can be defined as the cosine of the angle between the armature phase current and the induced EMF in the armature conductor in that phase.

Armature Reaction at Unity Power Factor

The direction of rotation of the rotor is considered clockwise. By applying the right-hand rule, the direction of the induced emf in various conductors can be found. The direction of rotation of the conductors is taken anticlockwise with respect to the rotor poles.

Suppose that the alternator is supplying current at unity power factor. The phase currents I_A, I_B, and I_C will be in phase with their respective generated voltages, i.e., E_A, E_B, and E_C as shown in the figure below:

The positive direction of fluxes ϕ_A, ϕ_B, ϕ_C are shown in the figure below:

The projection of a phasor on the vertical axis gives its instantaneous value.

At t=0, the instantaneous values of currents and fluxes are given by the equation shown below:

Where the subscript m denotes the maximum values of current and flux. Thus, the flux ϕ_A is along with OA, and the fluxes ϕ_B and ϕ_C are negative and acts opposite to each other represented by OB and OC respectively as shown in the figure below. The resultant of the fluxes can be found by resolving the fluxes horizontally and vertically.

Resolving along the horizontal direction we get

Similarly, resolving along the vertical direction we get

The resultant armature reaction flux is given by

If the rotor is rotated, 30 degrees in a clockwise direction, the corresponding phasor diagram is shown below.

At the instant when ωt = 30⁰, the instantaneous values of currents and fluxes are given as:

The space diagram for fluxes at ωt = 30⁰ is shown below:

Here, ϕ_B = 0. The resultant armature reaction flux is given by the equation shown below:

The direction of the resultant flux ϕ_AR is along OD, which makes an angle with the horizontal in the clockwise direction.

Hence, it is observed that the resultant flux ϕ_AR sets up by the current in the armature remains constant in magnitude equal to 1.5 ϕ_m and it rotates at synchronous speed. When the current is in phase with the induced voltage the armature reaction flux ϕ_AR lags behind the main field by 90⁰. This is called Cross Magnetizing Flux.

Armature Reaction at Lagging Power Factor

If the alternator is loaded with an inductive load of zero power factor lagging. The phase current I_A, I_B and I_C will be lagging with their respective phase voltages E_A, E_B and E_C by 90⁰. The figure below shows the phasor diagram of armature reaction at lagging load.

At time t = 0, the instantaneous values of currents and fluxes are given by:

The space diagram of the magnetic fluxes is shown below:

The resultant flux ϕ_AR is given by the equation shown below:

The direction of the armature reaction flux is opposite to the main field flux. Therefore, it will oppose and weaken the main field flux. It is said to be demagnetized.

Armature Reaction at Leading Power Factor

If the alternator is loaded with a load of zero power factor leading. The phase currents I_A, I_B, and I_C will be leading their respective phase voltages E_A, E_B, and E_C by 90⁰. The phasor diagram is shown below:

At time t = 0, the instantaneous values of currents and fluxes are given by the equations shown below:

The direction of the flux is shown below in the phasor diagram.

The resultant flux is given by the equation shown below: The direction of the armature reaction flux is in the direction of the main field flux. It is known as magnetizing flux.

Armature Reaction Nature

The following conclusion is given below:

• The armature reaction flux is constant in magnitude and rotates at synchronous speed.
• The armature reaction is cross-magnetizing when the generator supplies a load at unity power factor.
• When the generator supplies a load, at lagging power, the armature reaction is partly demagnetizing and partly cross-magnetizing.

This is all about Armature Reaction in a Synchronous Machine.

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The load on the shaft is increased. The rotor slows down momentarily, as it required some time to take increased power from the line. In another word, it can be said that even if the rotor is rotating at synchronous speed, the rotor slips back in space because of the increase in the load. In this process, the torque angle δ becomes larger, and, as a result, the induced torque increases.

The induced torque equation is given as:

Then increased torque increases the rotor speed, and the motor again regains the synchronous speed, but with the larger torque angle. The excitation voltage E_f is proportional to ϕω, it depends upon the field current and the speed of the motor. Since the motor is moving at a synchronous speed, and the field current is also constant. Hence, the magnitude of the Voltage |E_f| remains constant. We have,

From the above equations, it is clear, that if P is increased the value of E_f sinδ and I_a cosϕ also increases.

The figure below shows the effect of an increase in load on the operation of a synchronous motor.

It is seen from the above figure that with the increase in load, the quantity jI_aX_s goes on increasing and the relation V = E_f + jI_aX_s is satisfied. The armature current is also increased. The power factor angle also changes with the change in load. It becomes less and less leading and then becomes more and more lagging as shown in the figure above.

Thus, if the load on a synchronous motor is increased the following points are considered which are given below.

• The motor continues to run at synchronous speed.
• The torque angle δ increases.
• The excitation voltage E_f remains constant.
• The armature current I_a drawn from the supply increases.
• The phase angle ϕ increases in the lagging direction.

There is a limit to the mechanical load that can be applied to a synchronous motor. As the load is increased, the torque angle δ also increases until the condition arises when the rotor is pulled out of synchronism and the motor is stopped.

Pull-out torque is defined as the maximum value of the torque which a synchronous motor can develop at rated voltage and frequency without losing synchronism. Its values vary from 1.5 to 3.5 times the full load torque.

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From the above figure, the short circuit ratio is given by the equation shown below.

Since the triangles Oab and Ode are similar. Therefore,

The direct axis synchronous reactance X_d is defined as the ratio of open-circuit voltage for a given field current to the armature short circuit current for the same field current.

For the field current equal to Oa, the direct axis synchronous reactance in ohms is given by the equation shown below:

The per-unit value of X_d is given as:

But, the base impedance is: Therefore,

From equation (1) and equation (6), we get

From equation (7) it is clear that the short circuit ratio is equal to the reciprocal of the per-unit value of the direct axis synchronous reactance.

In a saturated magnetic circuit, the value of X_d depends upon the degree of saturation.

Significance of Short Circuit Ratio (SCR)

Short Circuit Ratio is an important factor of the synchronous machine. It affects the operating characteristics, physical size, and cost of the machine. The large variation in the terminal voltage with a change in load takes place for the lower value of the short circuit ratio of a synchronous generator. To keep the terminal voltage constant, the field current (I_f) has to be varied over a wide range.

For the small value of the short circuit ratio (SCR), the synchronizing power is small. As the synchronizing power keeps the machine in synchronism, a lower value of the SCR has a low stability limit. In other words, a machine with a low SCR is less stable when operating in parallel with the other generators.

A synchronous machine with a high value of SCR had a better voltage regulation and improved steady-state stability limit, but the short circuit fault current in the armature is high. It also affects the size and cost of the machine.

The excitation voltage of the synchronous machine is given by the equation:

For the same value of Tph Excitation voltage is directly proportional to the field flux per pole.

The synchronous inductance is given as:

Therefore,

Hence, the short circuit ratio is directly proportional to the air gap reluctance or air gap length.

If the length of the air gap is increased, the SCR can be increased. With the increase in the air gap length, the field MMF is to be increased for the same value of excitation voltage (E_f). Hence, to increase the value of field MMF either field current or the number of field turns has to be increased. All this requires a greater height of field poles and, as a result, the overall diameter of the machine increases.

Thus, a conclusion is that the large value of SCR will increase the size, weight, and cost of the machine.

The typical values of the SCR for different types of machines are as follows:-

• For cylindrical rotor machines, the value of SCR lies between 0.5 to 0.9.
• In the case of the Salient-pole machine, it lies between 1 to 1.5 and
• For synchronous compensators, it is 0.4.

This is all about Short Circuit Ratio of a Synchronous Machine.

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Contents:

• Synchronisation by Synchronising lamps
• Advantages of the Dark Lamp Method
• Disadvantages of the Dark Lamp Method
• Three Bright Lamp Method
• Two Bright One Dark Lamp Method

The following methods are used for synchronization.

Synchronisation by Synchronising lamps

A set of three synchronizing lamps can be used to check the conditions for paralleling or synchronization of the incoming machine with the other machine. A dark lamp method along with a voltmeter used for synchronizing is shown below. This method is used for low-power machines.

The prime mover of the incoming machine is started and brought nearer to its rated speed. A field current of the incoming machine is adjusted in such a way so that it becomes equal to the bus voltage. The flicker of the three lamps occurs at a rate that is equal to the difference in the frequencies of the incoming machine and the bus. All the lamps will glow and off at the same time if the phases are properly connected. If this condition does not satisfy, then the phase sequence is not connected correctly.

Thus, in order to connect the machine in the correct phase sequence, two leads to the line of the incoming machine should be interchanged. The frequency of the incoming machine is adjusted until the lamp flicker at a slow rate. The flicker rate should be less than one dark period per second. After finally adjusting the incoming voltage, the synchronizing switch is closed in the middle of their dark period.

Advantages of the Dark Lamp Method

• This method is cheaper.
• The correct phase sequence is easily determined.

Disadvantages of the Dark Lamp Method

• The lamp becomes dark at about half of its rated voltage. Hence, it is possible that the synchronizing switch might be switched off even when there is a phase difference between the machine.
• The filament of the lamp might burn out.
• The flicker of the lamps does not indicate which lamp has the higher frequency.

Three Bright Lamp Method

In this method, the lamps are connected across the phases such as A₁ is connected to B₂, B₁ is connected to C_2, and C₁ is connected to A₂. If all the three lamps get bright and dark together, this means that the phase sequence is correct. The correct instant of closing the synchronizing switch is in the middle of the bright period.

Two Bright One Dark Lamp Method

In this method, one lamp is connected between corresponding phases while the two others are cross-connected between the other two phases as shown in the figure below:

Here, A₁ is connected to A₂, B₁ to C_2, and C₁ to B₂. The prime mover of the incoming machine is started and brought up to its rated speed. The excitation of the incoming machine is adjusted in such a way that the incoming machine induces the voltage E_A1, E_B2, E_C3, which is equal to the Busbar voltages V_A1, V_B1 and V_C1. The diagram is shown below.

The correct moment to close the switch is obtained at the instant when the straight connected lamp is dark, and the connected cross lamps are equally bright. If the phase sequence is incorrect, no such instant will take place, and all the lamps will be dark simultaneously.

The direction of rotation of the incoming machine is changed by interchanging the two lines of the machine. Since the dark range of the lamp extends to a considerable voltage range, a voltmeter V₁ is connected across the straight lamp. The synchronizing switch is closed when the voltmeter reading is zero.

Thus, the incoming machine is now floating on the Busbar and is ready to take up the load as a generator. If the prime mover is disconnected, it behaves as a motor. For paralleling small machines in power stations, three lamps along with the synchroscope are used. For synchronizing very large machines in power stations, the whole procedure is performed automatically by the computer.

Also See: Synchroscope Synchronizing

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• The MVA loading should not exceed the generator rating. This limit is determined by the armature of the stator heating by the armature current.
• The MW loading should not exceed the rating of the prime mover.
• The field current should not be allowed to exceed a specified value determined by the heating of the field.
• For steady-state or stable operation, the load angle δ must be less than 90 degrees. The theoretical stability limit of the stable condition occurs when δ = 90⁰.

The capability curve is based upon the phasor diagram of the synchronous machine. The phasor diagram of a cylindrical rotor alternator at lagging power factor is shown below:

For simplicity, the armature resistance and saturation are assumed to be negligible. The machine is assumed to be connected to constant voltage busbars so that the voltage V_p is constant. The length O’O (= V_p) is fixed. The axes Ox and Oy are drawn with their origin O at the tip of V_p.

From the phasor diagram,

The real power output of the generator is given as:

The reactive power output of the generator is given as:

A typical capability curve for a cylindrical rotor generator is shown below:

The curve is plotted on the S-plane, where P is the vertical axis and Q is the horizontal axis. For constant power Ia and volt-amperes S = VA, the locus is a circle with a center at O and radius OB (= 3 V_p I_a). Constant P operation lies on a line parallel to Q axis. The constant excitation locus is a circle with center O’ and radius O’B ( = 3 V_p E_f/X_s). Constant power factor lines are straight radial lines from O.

For excitation E_f equal to zero, the armature current is given as:

= short circuit current at rated voltage

= OO’

The theoretical stability limit is a straight line O’M at right angles to O’O at O’. Here δ = 90⁰. Between a and b, the operation of the alternator is limited by the maximum field current, and a circle of radius (3 V E_f/ X_s) with center O’. Between b and c, the operation is limited by the MVA limit. Here I_a is the maximum permissible armature current. Between c and d, the operation is limited by the power of the prime mover. Between d and e, the operation is limited by the practical stability limit.

The theoretical limit of stability occurs where δ = 90⁰. But there must be a safety margin between the theoretical limit and that used in practice. The practical limit is usually taken 10% less than the theoretical stability limit. The complete operating zone of the alternator is abcdkOa. The operation of the alternator within this area is safe from the standpoints of heating and stability. Once an operating point is located within this area, the desired power P, S, Q i.e., Current, power factor, and excitation are found.

Consider the figure given below.

Here an operating point F is considered, and the following information is given:

• If point F is inside the capability curve, the machine will not be overheated and will not be likely to fall out of synchronism.
• A line from F to the origin O’ of the I_f is at an angle δ from the axis.
• A line FG through F parallel to O’Oa give power equal to OG.
• A line from F to the origin O of the Q axis gives the power factor angle ϕ from the vertical axis. i.e., ∠FOG = ϕ
• The armature current I_a is given by OF.
• The VA output is given by (OF x operating voltage)
• The VAr output is given by GF x output voltage
• O’F gives the excitation E_f.

This is all about capability curve.

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The steady-state or stable operation of a synchronous motor is a condition of equilibrium. In it, the load torque is equal as well as opposite to the electromagnetic torque. The rotor of the motor runs at synchronous speed in the steady-state condition, maintain a constant value of the torque angle δ. The equilibrium gets disturbed if a sudden change occurs in the load torque. Thus, a resulting torque takes place which changes the speed of the motor. It is given by the equation shown below.

Where J is the moment of inertia

ω_M is the angular velocity of the rotor in mechanical units.

The speed of the motor slows down temporarily, and the torque angle δ is sufficiently increased. This is done to restore the torque equilibrium and the synchronous speed when there is a sudden increase in the load torque.

The electromagnetic torque is given by the equation shown below:

If the value of δ is increased, the electromagnetic torque is also increased. As a result, the motor is accelerated. As the rotor reaches the synchronous speed, the torque angle δ is larger than the required value. Here the rotor speed continues to increase beyond the synchronous speed.

As the rotor accelerates above synchronous speed, the torque angle δ decreases. At the point where the motor torque becomes equal to the load torque, the equilibrium is not restored because now the rotor speed is greater than the synchronous speed. Therefore, the rotor continues to swing backward and as a result, the torque angle goes on decreasing.

When the load angle δ becomes less than the required value, the mechanical load becomes greater than the developed power. Therefore, the motor starts to slow down. The load angle starts increasing again. Thus, the rotor starts to swing or oscillates around the synchronous speed.

The motor responds to a decreasing load torque by a temporary increase in speed and a reduction of the torque angle δ. Thus, the rotor swings and rotates around the synchronous speed. Hence, this process of rotation of the rotor speed equal to or around the synchronous speed is known as Hunting. Since, during the rotor oscillation, the phase of the phasor Ef changes about phasor V. Thus, hunting is known as Phase Swinging.

Causes of Hunting

The various causes of hunting are as follows:

• Sudden changes of load.
• Faults were occurring in the system which the generator supplies.
• Sudden change in the field current.
• Cyclic variations of the load torque.

Effect of Hunting

The various effects of hunting are as follows:-

• It can lead to loss of synchronism.
• It can cause variations of the supply voltage producing undesirable lamp flicker.
• The possibility of Resonance condition increases. If the frequency of the torque component becomes equal to that of the transient oscillations of the synchronous machine, resonance may take place.
• Large mechanical stresses may develop in the rotor shaft.
• The machine losses increase and the temperature of the machine rises.

Reduction of Hunting

The following technique given below is used to reduce the phenomenon of hunting.

• Use of damper windings
• Uses of flywheels

The prime mover is provided with a large and heavy flywheel. This increases the inertia of the prime mover and helps in maintaining the rotor speed constant.

• By designing synchronous machines with suitable synchronising power coefficients.

This is all for hunting in synchronous motors.

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The power factor of the synchronous motor can be controlled by varying the field current I_f. As we know that the armature current I_a changes with the change in the field current I_f. Let us assume that the motor is running at NO load. If the field current is increased from this small value, the armature current Ia decreases until the armature current becomes minimum. At this minimum point, the motor is operating at a unity power factor. The motor operates at a lagging power factor until it reaches up to this point of operation.

If now, the field current is increased further, the armature current increases and the motor starts operating as a leading power factor. The graph drawn between armature current and field current is known as the V curve. If this procedure is repeated for various increased loads, a family of curves is obtained.

The V curves of a synchronous motor are shown below: The point at which the unity power factor occurs is at the point where the armature current is minimum. The curve connecting the lowest points of all the V curves for various power levels is called the Unity Power Factor Compounding Curve. The compounding curves for 0.8 power factor lagging and 0.8 power factor leading are shown in the figure above by a red dotted line.

The loci of constant power factor points on the V curves are called Compounding Curves. It shows the manner in which the field current should be varied in order to maintain a constant power factor under changing load. Points on the right and left of the unity power factor corresponds to the over-excitation and leading current and under excitation and lagging current respectively.

The V curves are useful in adjusting the field current. Increasing the field current, If beyond the level for minimum armature current results in leading power factor. Similarly decreasing the field current below the minimum armature current results in a lagging power factor. It is seen that the field current for the unity power factor at full load is more than the field current for unity power factor at no load.

The figure below shows the graph between power factor and field current at the different loads.

It is clear from the above figure that, if the synchronous motor at full load is operating at unity power factor, then removal of the shaft load causes the motor to operate at a leading power factor.

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Consider asynchronous generator transferring a steady power P_a at a steady load angle δ₀. Suppose that, due to a transient disturbance, the rotor of the generator accelerates, resulting from an increase in the load angle by dδ. The operating point of the machine shifts to a new constant power line and the load on the machine increases to P_a + δP. The steady power input of the machine does not change, and the additional load which is added decreases the speed of the machine and brings it back to synchronism.

Similarly, if due to a transient disturbance, the rotor of the machine retards resulting a decrease in the load angle. The operating point of the machine shifts to a new constant power line and the load on the machine decreases to (P_a – δP). Since the input remains unchanged, the reduction in load accelerates the rotor. The machine again comes in synchronism.

The effectiveness of this correcting action depends on the change in power transfer for a given change in load angle. The measure of effectiveness is given by Synchronising Power Coefficient.

Power output per phase of the cylindrical rotor generator synchronizing torque coefficient

In many synchronous machines Xs >> R. Therefore, for a cylindrical rotor machine, neglecting saturation and stator resistance equation (3) and (5) become

For a salient pole machine

Unit of Synchronizing Power Coefficient P_syn

The synchronizing power coefficient is expressed in watts per electrical radian.

Therefore,

Since, π radians = 180⁰

1 radian = 180/π degrees

If P is the total number of pair of poles of the machine.

Synchronising Power Coefficient per mechanical radian is given by the equation shown below:

Synchronising Power Coefficient per mechanical degree is given as:

Synchronising Torque Coefficient

Synchronising Torque Coefficient gives rise to the synchronising torque coefficient at synchronous speed. That is, the Synchronizing Torque is the torque at which synchronous speed gives the synchronising power. If Ʈ_syn is the synchronising torque coefficient then the equation is given as shown below: Where,

• m is the number of phases of the machine
• ω_s = 2 π n_s
• n_s is the synchronous speed in revolution per second

Significance of Synchronous Power Coefficient

The Synchronous Power Coefficient P_syn is the measure of the stiffness between the rotor and the stator coupling. A large value of P_syn indicates that the coupling is stiff or rigid. Too rigid a coupling means and the machine will be subjected to shock, with the change of load or supply. These shocks may damage the rotor or the windings. We have,

The above two equations (17) and (18) show that P_syn is inversely proportional to the synchronous reactance. A machine with large air gaps has relatively small reactance. The synchronous machine with the larger air gap is stiffer than a machine with a smaller air gap. Since P_syn is directly proportional to E_f, an overexcited machine is stiffer than an under excited machine.

The restoring action is great when δ = 0, that is at no load. When the value of δ = ± 90⁰, the restoring action is zero. At this condition, the machine is in unstable equilibrium and at a steady-state limit of stability. Therefore, it is impossible to run a machine at the steady-state limit of stability since its ability to resist small changes is zero unless the machine provided with a special fast-acting excitation system.

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• Starting with the help of an External Prime Mover
• Starting with the help of Damper Windings

A detailed description of the methods is explained below.

Motor Starting with an External Prime Mover

In this method, an external prime mover drives the synchronous motor and brings it to synchronous speed. The synchronous machine is then synchronized with the bus bar as a synchronous generator. The prime mover is then disconnected. Once operating in parallel condition, the synchronous machine will work as a motor. Thus, the load can be connected to the synchronous motor.

Since the load is not connected to the synchronous motor before synchronizing, the starting motor has to overcome the inertia of the synchronous motor at no load. Therefore, the rating of the motor which has to be started is much smaller than the rating of the synchronous motor. Now a day, a Brushless excitation system is provided on the shafts of the large synchronous motor. These exciters are used as starting motors.

Motor Starting with Damper Windings

Damper Windings is the most widely used method to start a synchronous motor. A Damper Winding consists of heavy copper bars inserted in the slots of the pole faces of the rotor as shown in the figure below.

These copper bars are short-circuited by end rings at both ends of the rotor. Thus, these short-circuited Bars form a squirrel cage winding. When a three phase supply is connected to the stator, the synchronous motor with Damper Winding will start. It works as a three-phase induction motor. As soon as the motor approaches the synchronous speed, the DC excitation is applied to the field windings. As a result, the rotor of the motor will pull into step with the stator magnetic field.

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