SYNCHRONOUS MOTOR QUESTION AND ANSWER

Q: Why synchronous motor is not self starting?

Synchronous motor is a doubly excited machine i.e two electrical inputs are provided to it. It’s stator winding which consists of a 3 phase winding is provided with 3 phase supply and rotor is provided with DC supply. The 3 phase stator winding carrying 3 phase currents produces 3 phase rotating magnetic flux. The rotor carrying DC supply also produces a constant flux. At a particular instant rotor and stator poles might be of same polarity (N-N or S-S) causing repulsive force on rotor and the very next second it will be N-S causing attractive force. But due to inertia of the rotor, it is unable to rotate in any direction due to attractive or repulsive force and remain in standstill condition. Hence it is not self starting.

Q: What are the methods of starting synchronous motor?

1.    Pony motor starting( using ac or dc motor)

2.    Starting as squirrel cage Induction motor or using damper winding

3.    Starting as slip ring Induction motor or synchronous motor

4.    Using dc machine coupled to it.

Q: What is pony motor starting?

In this method the rotor is made to rotate by some external device called pony motors. Once the rotor attains synchronous speed, the DC excitation to the rotor is switched on. Once synchronism is established the pony motor is decoupled. The synchronous motor continuously runs at synchronous speed.

Q: Why damper windings are used in synchronous motor? Or How synchronous motor is started as squirrel cage motor?

The damper winding consists of short circuited copper bars embedded in the the face of the rotor poles. When an ac supply is provided to stator of a 3-phase synchronous motor, stator winding produces rotating magnetic field. Due to the damper winding present in the rotor winding of the synchronous motor, emf is induced in damper winding and hence currents starts to flow. Hence torque acts on rotor. Damper windings in synchronous motor will carryout the same task of induction motor rotor windings. Therefore due to damper windings synchronous motor starts as induction motor and continue to accelerate. When the motor attains about 95% of the synchronous speed, the rotor windings is connected to exciter terminals and the rotor is magnetically locked by the rotating magnetic field of stator and it runs as a synchronous motor. Now as the rotor rotates at synchronous speed the relative motion between rotating magnetic field and damper winding is zero. Hence there will be no emf induced in damper winding.

Q: What is the function of damper winding?

To provide starting torque

To prevent or minimize hunting

What is hunting?

The sudden changes of load on synchronous motor set up oscillations in the rotor. Such oscillation of rotor about its new equilibrium position is called hunting.

Q: What are the effects of hunting?

Loses synchronism

Develops mechanical stress on rotor shaft

Increase machine losses

Increase temperature of machine

Q: What is pull in torque?

The torque which is required to pull the motor into synchronism when it is running as induction motor is called pull in torque.

Q: What is pull out torque?

The maximum torque developed by the motor without pulling out of synchronism is called pull out torque. Its value varies from 1.5 to 3.5 times full load torque.

Q. What is the effect on speed if the load is increased on a 3 phase synchronous motor?

The speed of operation remains constant from no load to maximum load in

the motor operating at constant frequency bus bars.

Q.Why a synchronous motor is a constant speed motor?

When load increase in synchronous motor the load angle
δ also increases  but speed remains constant. The further increase in load cause further increase in load angle. When load angle reaches 90 degree electrical then the motor comes out of synchronism. Hence the motor rotates at synchronous speed otherwise it comes out of synchronism.

Q: What is normal excitation?

The excitation for which power factor of the synchronous motor is lagging and the back emf is equal to supply voltage.

Q: What is under excitation?

The excitation for which power factor of the synchronous motor is lagging and the back emf is less than supply voltage.

Q: What is over excitation?

The excitation for which power factor of the synchronous motor is leading and the back emf is greater than supply voltage.

Q: What is critical excitation?

The excitation for which power factor of the synchronous motor is unity and the back emf is equal to supply voltage. 

Q. What is synchronous condenser?

An over-excited synchronous motor under no load , used for the improvement of power factor is called as synchronous condenser because, like a capacitor it takes a leading current.

Q. Distinguish between synchronous phase modifier and synchronous condenser

A synchronous motor used to change the power factor or power factor in

the supply lines is called synchronous phase modifier.

A synchronous motor operated at no load with over excitation condition to

draw large leading reactive current and power is called a synchronous

condenser.

Q. How the synchronous motor can be used as s synchronous condenser?

Synchronous motor is operated on over excitation so as to draw leading

reactive current and power from the supply lines. This compensates the

lagging current and power requirement of the load making the system

power factor to become unity. The motor does the job of capacitors and

hence called as synchronous condenser. 

Q. What are V and inverted V curves of synchronous motor ?

The variation of magnitude of line current with respect to the field current

is called V curve .

The variation of power factor with respect to the field current is called inverted V curve.

Q Write the applications of synchronous motor.

a. Used for power factor improvement in sub-stations and in industries.

 b. As synchronous condenser

c. Used for constant speed applications

Q. State the characteristic features of synchronous motor.

a. the motor is not inherently self starting

b. The speed of operation is always in synchronous with the supply frequency

irrespective of load conditions

c. The motor is capable of operating at any power factor.

Q. In what way synchronous motor is different from other motors?

All dc and ac motors work on the same principle. Synchronous motor operates

due to magnetic locking taking place between stator and rotor magnetic fields. 

Q A synchronous motor starts as usual but fails to develop its full torque. What could it be due to?

a. Exciter voltage may be too low.

b. Field spool may be reversed

c. There may be either open-circuit or short-circuit in the field.

Q What could be the reasons if a 3-phase synchronous motor fails to start?

It is usually due to the following reasons

a. Voltage may be too low.

b. Too much starting load.

c. Open circuit in one phase or short circuit.

d. Field excitation may be excessive

Power system Analysis

Q: Write the most important mode of operation of power system and mention the major problems encountered with it.

 Symmetrical steady state is the most important mode of operation of power system. Three major problems are encountered in this mode of operation. They are,

 1) Load flow problem

 2)  Optimal load scheduling problem

3) Systems control problem

Q: Why power flow analysis is made?

Power flow analysis is performed to calculate the magnitude and phase angle of voltages at the buses and also the active power and reactive voltamperes flow for the given terminal or bus conditions.

The variables associated with each bus or node are,

a. Magnitude of voltage |V|

 b. Phase angle of voltage δ

 c. Active power, P

d. Reactive voltamperes, Q

Q: What is power flow study or load flow study?

 The study of various methods of solution to power system network is referred to as load study. The solution provides the voltages at various buses, power flowing in Various lines and line losses.

Q: What are the information that are obtained from a load flow study?

The information obtained from a load flow study are magnitude and phase angles of bus voltages, real and reactive power flowing in each line and line losses. The load flow solution also gives the initial conditions of the system when the transient behavior of the system is to be studied.

Q: What is the need for load flow study?

The load flow study of a power system is essential to decide the best operation of existing system and for planning the future expansion of the system. It is also essential for designing a new power system

Q: What are the works involved in a load flow study?

 The following has to be performed for a load flow study.

a. Representation of the system by single line diagram.

 b. Formation of impedance diagram using the information in single line diagram.

 c. Formulation of network equations

 d. Solution of network equations.

Q: What are the different types of buses in a power system?

 The buses of a power system can be classified into three types based on the quantities being specified for the buses, which are as follows:

 a. Load bus or PQ bus (P and Q are specified)

 b. Generator bus or voltage controlled bus or PV bus (P and V are specified)

c. Slack bus or swing bus or reference bus (|V| and δ are specified)

 Q:Define voltage controlled bus(generator bus/PV bus).

A bus is called voltage controlled bus if the magnitude of voltage |V| and real power (P) are specified for it. In a voltage controlled bus, the magnitude of the voltage is not allowed to change. Voltage controlled bus is also called as Generator bus and PV bus.

Q: What is PQ bus (load bus)?

 A bus is called PQ bus or load bus when real and reactive components of power are specified for the bus. In a load bus, the voltage is allowed to vary within permissible limits.

Q:What is swing bus(slack bus/reference bus)?

 A bus is called swing bus when the magnitude and phase of bus voltage are specified for it. The swing bus is the reference bus for load flow solution and it is required for accounting for the line losses. Usually one of the generator bus is selected as the swing bus.

Q: What is the need for slack bus?

The slack bus is needed to account for transmission line losses. In a power system, the total power generated will be equal to sum of power consumed by loads and losses. In a power system, only the generated power and load power are specified for the buses. The slack bus is assumed to generate the power required for losses. Since the losses are unknown, the real and reactive power are not specified for slack bus. They are estimated through the solution of line flow equations.

Q:Define one line diagram.

 A simplified diagram by omiting the completed circuit through the neutral and by indicating the components of the power system by standard symbols rather than by their equivalent circuits.

Q:What is meant by impedance diagram.

The equivalent circuit of all the components of the power system are drawn and they are interconnected is called impedance diagram

Q:What is meant by reactance diagram.

Omitting all static loads ,all resistance. The magnetizing components of each transformer and the capacitance of the transmission line are reduced from the impedance diagram is called reactance diagram

Q:.Define per unit value.

Per unit of any quantity is defined as the ratio of the quantity ti its base value is expressed as a decimal

 Q:Define base current.

 Ratio of base MVA to base KV

Q:What is the need for short circuit study.

To determine the current interrupting capacity of the circuit breakers so that the faulted equipments can be isolated. To establish the relay requirements and settings to detect the fault and cause the circuit breaker to operate when the current flowing through it exceeds the max. value.

 Q:Define stability study. Stability studies are performed in order to ensure that the system remains stable following a severe fault or disturbance

Q:What is the need for base values?

The components of power system may operate at different voltage and power levels. It will be convenient for analysis of power system if the voltage, power, current ratings of the components of the power system is expressed with referance to a common value called base value.

Q:What is impedance and reactance diagram?

The impedance diagram is the equivalent circuit of power system in which the various components of power system are represented by their approximate equivalent circuits. The impedance diagram is used for load flow studies. The reactance diagram is the simplified equivalent circuit of the power system in which the various components are represented by their reactance. The reactance ddiagram can be obtained from impedance diagram if all the resistive components are neglected.

Q: What are the approximations made in impedance diagram?

The neutral reactances are neglected. The shunt branches in equivalent circuit of induction motor are neglected

 Q:What are the approximations made in reactance diagram?

 The neutral reactances are neglected. The resistances are neglected. All static loads and induction motors are neglected.

Q: What is a bus?

The meeting point of various components in a power system is called a bus. The bus is a conductor made of copper having negligible resistance. The buses are considered as a points of constant voltage in a system

 Q:What is bus admittance matrix? The matrix consisting of the self and mutual admittances of the network of a power system is called bus admittance matrix.

Q:Name the diagonal and off diagonal elements of bus admittance matrix.

 The diagonal elements of bus admittance matrix are called self admittances of the buses and off diagonal elements are called mutual admittances of the buses.

Q: What is bus imbedance matrix?

The matrix consisting of driving point impedances and transfer impedances of the network of a power system is called bus impedance matrix.

Q: Name the diagonal elements and off diagonal elements of bus impedance matrix. The diagonal elements of bus impedance matrix are called driving point impedances of the buses and off diagonal elements of bus impedance matrix are called transfer impedances of the buses.

Q:What are the methods available for forming bus impedance matrix. 1. Form the bus impedance matrix and then take its inverse to get bus impedance matrix. 2. Directly form the bus impedance matrix from the reactance diagram. This method utilizes the techniques of modifications of existing bus impedance matrix due to addition of new bus.

Q: Write the four ways of adding an impedance to an existing system so as to modify bus impedance matrix. 1. Adding a branch of impedanceZb from a new bus p to the referance bus. 2. Adding a branch of impedance Zb from a new busp to an existing bus. 3. Adding a branch of impedance Zb from an existing busq to the referance bus. 4. Adding a branch of impedance Zb between two existing buses h and q.

Q.What are symmetrical components?

An unbalanced system of N related vectors can be resolved into N systems of balanced vectors. The N sets of balanced vectors are called symmetrical components.

Q Write the symmetrical components of three phase system.

 1.positive sequence components 3. negative sequence components. 4. zero sequence components.

Q What are positive sequence components?

The positive sequence components of a three phase unbalanced vectors consists of three vectors of equal magnatude, displased from each other by 120 in phase and having the same phase sequence as the original vectors.

 Q What are negative sequence components? The negative sequence components of a three phase unbalanced vectors consists of three vectors of equal magnitude displaced from each other by 120 degree in phase and having the phase sequence opposite to that of the original vectors.

Q What are zero sequence components? The zero sequence components of a three phase unbalanced vectors consists of 3 vectors of equal magnitude and with zero phase displacement from each other.

 Q What are sequence impedance and sequence networks? The sequence impedances are the impedances offered by the devices for the like sequrnce component of the current. The single phase equivalent circuit of a power system consists of impedances to current of any one sequence is called sequence network

Q What is meant by positive, negative and zero sequence impedance. The impedance of the circuit element for positive, negative and zero sequence component currents are called positive, negative and zero sequence impedances resp.

Q What is meant by positive, negative and zero sequence reactance. The reactance diagram of a power system, when formed using positive, negative and zero sequence reactances are called positive, negative and zero sequence reactance diagram resp.

Q What is load flow or power flow study?

The study of various methods of solution to power system network is reffered to as load flow study. The solution provides the voltages at various buses, power flowing in various lines and line losses.

Q What are the information that are obtained from a load flow study. The information obtained from a load flow study are magnatude and phase of bus voltages,real and reactive power flowing in each line and the line losses. T he load flow solution also gives the initial conditions of the system when the transient behaviour of the system to be studied.

Q.What is the need for load flow study. The load flow study of a power system is essential to decide the best operation existing system and for planning the future expansion of the system. It is also essential for designing the power system.

Q. What are the quantities associated with each bus ina system? Each bus in a power system is associated with four quantities and they are real power, reactive power, magnitude of voltage, and phase angle of voltage.

Q What are the different types of buses. Load bus,generator bus,slack bus.

 Q Define voltage controlled bus?

A bus is called voltage controlled bus if the magnitude of volte\age and real power are specified for it. In a voltage controlled bus the magnitude of the voltage is not allowed to change.

Q What is PQ bus?

 A bus is called PQ bus when real and reactive components of power are specified for the bus. In a load bus the voltage is allowed to vary within permissible limits. 46. What is swing bus? Abus is called swing bus when the magnitude and phase of the bus voltage are specified for it. The swing bus is the referance bus for load flow solution and it is required for accounting line losses. Usually one of the generator bus is selected as swing bus.

Q What is the need for slack bus

 The slack bus is needed to account for transmission line losses. In a power system the total power generated will be equal to sum of power consumed by loads and losses. In a power system only the generated power and load power are specified for buses. The slack bus is assumed to generate the power required for losses. Since the losses are unknown the real and reactive power are not specified for slack bus.

Q What are the operating constraints imposed in the load flow studies? The operating constraints imposed in load flow studies are reactive power limits for generator buses and allowable change in magnitude of voltagefor load buses.

 Q What are the iterative methods mainly used for solution of load flow study. The Guass sedal method and Newton Raphson method are the two iterative methods.

 Q.Discuss the effect of acceleration factor in load flow study. The acceleration factor is a real quantity and it modify the magnitude of voltage alone.

Q What is meant by flat voltage start. In iterative methods of load flow solution, the initial voltages of all buses exept slack bus are assumed as 1+j0 pu.This is reffered as flat voltage profile.

Q.When the generator buses are treated as load bus. If the reactive power constraints of a generator bus violates the specified limits then the generator is treated as load bus.

 Q What will be the reactive power and bus voltage whan the generator bus is treated as load bus.

 When the generator bus is treated as load bus, the reactive power of the bus is equated to the limit it has violated, and the previous iteration value of bus voltage is used for calculating current iteration value.

 Q What are the advantages of Guass sedal methoa? Calculations are simple and so the programming task is lesses. The memory requirement is less Useful for small systems 55 What are the disadvantages of Guass seidal method? Requires large no. of iterations to reach converge Not suitable for large systems Convergence time increases with size of the system

Q. How approximations is performed in Newton Raphson method? In N-R method, the set of nonlinear simultaneous equations are approximated to a set of linear simultaneous equations using tailors series expansion and the terms are limited to first order approximation.

Q What is Jacobian matrix? How the elements of Jacobian matrix are computed. The matrix formed from the first derivatives of load flow equations is called Jacobian matrix. The elements of Jacobian matrix will change in every itration. In each iteration the elements of the jacobian matrix are obtained by partially differentiating the load flow equations w.r. t a unknown variable and then evaluating the first derivatives using the solution of previous iteration

Q.What are the adv. Of N-R method? The N-R method is faster, more reliable and the results are accurate. Requires less no. of iterations Suitable for large size system

Q What are the disadv. Of N-R method? The programming is more complex The memory requirement is more

 Q What is off nominal turns ratio? When the voltage or turns ratio of a transformer is not used to decide the ratio of base KV then its voltage ratio is called off nominal turns ratio. Usually the voltage ratio of regulating transformer will be off nominal ratios.

 Q What is meant by a fault? A fault in a circuit is any failure which interferes with the normal flow of current. The faults are associated with abnormal change in current, voltage and frequency of the power systwm. The faults may cause dan\mage to the equipments if it is allowed to persit for a long time. Hence every part of the system has been protected by means of relays and circuir breakers to sence the faults to isolate the faulty part from the healthy part in the event of fault.

Q Why fault accurs in a power system? The fault occur in a power system due to insulation failure of equipments, flashover of lines initiated by a lightning stroke, due to permanent damage to conductors and towers

Q How are faults are classified? The faults are classified as shunt and series faults. The shunt faults are due to short circuits in conductors and the series faults are due to oped conductors.

Q List the various types of series and shunt fault? Shunt faults are, Line to ground fault line to line fault, double line to ground fault, three phase fault. Series faults are, One open conductor fault Two open conductor fault

Q What is symmetrical and unsymmetrical fault? The fault is called symmetrical fault if the fault current is equal in all the phases. The fault is called unsymmetrical fault if the fault current is not equal in all the phases.

Q Name any two methods of reducing short circuit current/ By providing neutral reactance. By introducing a large value of shunt reactance between buses.

Q What is meant by fault calculations/ The fault condition of a power system can be dived into subtransient, transient and steady state periods. The currents in the various parts of the system and in the fault are different in these periods. The estimation of these currents for various types of faults at various locations in the system are commenly reffered as fault calculations.

Q.Define stability? The stability of a system is defined as the ability of power system to return to stable operation when it is subjected to a disturbance. 68. Define steady state stability The stady statenstability is defined as the ability of a system to remain stable for small disturbance.

 Q Define transient stability? The transient stabilitymis defined as the abilitymof a system to remain stable for large disturbance.

Q What is steady state state stability limit? The steady state stability limit is the max.

power that can be transferred by a machine toa receving system without loss of synchronism.

Q Define swing curve? What is the use of swing curve. The swing curve is the plot between the power angle and time. It is usually plotted for a transient state to stydy the nature of variation in angle for a sudden large disturbance.

 Q Define power angle/ The power angle is defined as the angular displacement of the rotor from synchronously rotating referance frame.

 Q Define critical clearing time and critiacal clearing angle The critical clearing angle is the max. allowable change in the power angle before clearing the fault, without loss of synchronism. The critical clearing time can be defined as thw max. time delay that can be allowed to clear a fault without loss of synchronism.

Q.Define equal area criterion? The equal area criterion for stability states that the system is stable if the area under pvs angle curve reduces to zero at some value of angle.

Q.What is transient state stability limit? The transient state stability limit is the max. power that can be transmitted by a machine to a receiving system without loss of synchronism.In steady state the power transferred by synchronous machine is always less than the steady state stability limit.

Q what is transient stability limit ? The transient stability limit is the max power that can be transmitted by a machine to a fault or a receiving system during a transient state without loss of synchronism.The transient stability limit is always less than the steady state stability limit.