What is over excitation?

What is over excitation?
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1 Answer

Answer :

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

What is critical excitation?

Description : What is critical excitation?

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

What is under excitation?

Description : What is under excitation?

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

What is normal excitation?

Description : What is normal excitation?

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

Hunting of a synchronous motor:

Description : Hunting of a synchronous motor:

Last Answer : When the mechanical load is constant, the rotor settles down to an absolutely constant speed with the torque angle fixed by the particular delivered power. When there is a change in load the rotor speed ... are short circuited; copper bars are embedded in the faces of the field poles of the motor.

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

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

Last Answer : 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

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

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

Last Answer : 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.

In what way synchronous motor is different from other motors?

Description : In what way synchronous motor is different from other motors?

Last Answer : 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.

State the characteristic features of synchronous motor.

Description : State the characteristic features of synchronous motor.

Last Answer : 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.

Write the applications of synchronous motor.

Description : Write the applications of synchronous motor.

Last Answer : a. Used for power factor improvement in sub-stations and in industries. b. As synchronous condenser c. Used for constant speed applications

What are V and inverted V curves of synchronous motor ?

Description : What are V and inverted V curves of synchronous motor ?

Last Answer : 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.

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

Description : How the synchronous motor can be used as s synchronous condenser?

Last Answer : 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 ... to become unity. The motor does the job of capacitors and hence called as synchronous condenser.

Distinguish between synchronous phase modifier and synchronous condenser

Description : Distinguish between synchronous phase modifier and synchronous condenser

Last Answer : 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.

What is synchronous condenser?

Description : What is synchronous condenser?

Last Answer : 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.

Why a synchronous motor is a constant speed motor?

Description : Why a synchronous motor is a constant speed motor?

Last Answer : 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 ... out of synchronism. Hence the motor rotates at synchronous speed otherwise it comes out of synchronism.

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

Description : What is the effect on speed if the load is increased on a 3 phase synchronous motor?

Last Answer : The speed of operation remains constant from no load to maximum load in the motor operating at constant frequency bus bars.

What is pull out torque?

Description : What is pull out torque?

Last Answer : 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.

What is pull in torque?

Description : What is pull in torque?

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

What are the effects of hunting?

Description : What are the effects of hunting?

Last Answer : Loses synchronism Develops mechanical stress on rotor shaft Increase machine losses Increase temperature of machine

What is hunting?

Description : What is hunting?

Last Answer : 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.

What is the function of damper winding?

Description : What is the function of damper winding?

Last Answer : To provide starting torque To prevent or minimize hunting

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

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

Last Answer : 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 ... magnetic field and damper winding is zero. Hence there will be no emf induced in damper winding.

What is pony motor starting?

Description : What is pony motor starting?

Last Answer : 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.

What are the methods of starting synchronous motor?

Description : What are the methods of starting synchronous motor?

Last Answer : 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.

Why synchronous motor is not self starting?

Description : Why synchronous motor is not self starting?

Last Answer : 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 ... due to attractive or repulsive force and remain in standstill condition. Hence it is not self starting.

Which terminology deals with the excitation or stimulus applied to the system from an external source for the generation of an output? a. Input signal b. Output signal c. Error signal d. Feedback signal

Description : Which terminology deals with the excitation or stimulus applied to the system from an external source for the generation of an output? a. Input signal b. Output signal c. Error signal d. Feedback signal

Last Answer : Ans: A

Coast Guard Regulations (46 CFR) require that each alternating current generator arranged for parallel operation shall be provided with _____________. A. more than one means of excitation B. individual circuit fuses C. a high back-pressure trip D. a reverse-power trip

Description : Coast Guard Regulations (46 CFR) require that each alternating current generator arranged for parallel operation shall be provided with _____________. A. more than one means of excitation B. individual circuit fuses C. a high back-pressure trip D. a reverse-power trip

Last Answer : Answer: D

An AC generator operating in parallel loses its excitation without tripping the circuit breaker. This will __________. A. not affect the faulty generator due to the compensation of the other generators B. cause the slip rings to melt C. cause a low voltage differential to develop between the armature and the bus D. cause high currents to be induced in the field windings

Description : An AC generator operating in parallel loses its excitation without tripping the circuit breaker. This will __________. A. not affect the faulty generator due to the compensation of the other generators B. ... between the armature and the bus D. cause high currents to be induced in the field windings

Last Answer : Answer: D

A loss of field excitation to an AC generator while operating in parallel will cause it to ____________. A. absorb more and more load due to decreased armature reaction B. lose its load due to the inherent speed droop built into the governor C. smoke and overload due to field flashover as residual field flux changes polarity D. lose its load, begin to motorize, trip out on reverse power relay, and possibly overspeed

Description : A loss of field excitation to an AC generator while operating in parallel will cause it to ____________. A. absorb more and more load due to decreased armature reaction B. lose its load due to ... polarity D. lose its load, begin to motorize, trip out on reverse power relay, and possibly overspeed

Last Answer : Answer: D

If the excitation of an alternator operating in parallel is decreased below normal, its _____________. A. power factor will change in the lagging direction B. power factor will change in the leading direction C. ampere load will be greatly increased D. kilowatt load will be greatly decreased

Description : If the excitation of an alternator operating in parallel is decreased below normal, its _____________. A. power factor will change in the lagging direction B. power factor will change in the leading direction C. ampere load will be greatly increased D. kilowatt load will be greatly decreased

Last Answer : Answer: B

The speed of a squirrel-cage, multi-speed, induction motor, as used aboard ship, is varied by changing the __________. A. number of connected poles in the stator B. frequency to the motor C. excitation voltage D. resistance of the rotor circuit

Description : The speed of a squirrel-cage, multi-speed, induction motor, as used aboard ship, is varied by changing the __________. A. number of connected poles in the stator B. frequency to the motor C. excitation voltage D. resistance of the rotor circuit

Last Answer : Answer: A

If field excitation is suddenly lost to an alternator operating in parallel, that alternator will _____________. A. supply excessive current to the bus B. operate at the same load, but with reduced voltage C. lose its load and tend to overspeed D. become overloaded and slow down

Description : If field excitation is suddenly lost to an alternator operating in parallel, that alternator will _____________. A. supply excessive current to the bus B. operate at the same load, but with reduced voltage C. lose its load and tend to overspeed D. become overloaded and slow down

Last Answer : Answer: C

The effect of carrying field excitation excessively high on a synchronous motor will result in a _____________. A. tendency for the motor to fall out of step B. tendency for the motor to overspeed C. tendency for the motor to overheat D. loss of motor speed regulation

Description : The effect of carrying field excitation excessively high on a synchronous motor will result in a _____________. A. tendency for the motor to fall out of step B. tendency for the motor to overspeed C. tendency for the motor to overheat D. loss of motor speed regulation

Last Answer : Answer: C

A change in field excitation of an alternator operating in parallel will cause a change in its _____________. A. alternator frequency B. kilowatt load C. voltage output D. all of the above

Description : A change in field excitation of an alternator operating in parallel will cause a change in its _____________. A. alternator frequency B. kilowatt load C. voltage output D. all of the above

Last Answer : Answer: C

The KW load is evenly distributed between two alternators just placed in parallel by adjusting _____________. A. a balance coil B. the governor settings C. the field excitation D. a rheostat

Description : The KW load is evenly distributed between two alternators just placed in parallel by adjusting _____________. A. a balance coil B. the governor settings C. the field excitation D. a rheostat

Last Answer : Answer: B

If two AC generators have just been placed in parallel, the true power load is initially distributed evenly by _____________. A. a balance coil B. changing field excitation C. adjusting the governor control settings D. a rheostat

Description : If two AC generators have just been placed in parallel, the true power load is initially distributed evenly by _____________. A. a balance coil B. changing field excitation C. adjusting the governor control settings D. a rheostat

Last Answer : Answer: C

The speed of a synchronous motor is varied by _____________. A. interchanging any two of the three live leads B. changing the voltage of the system C. changing the input frequency D. increasing the field excitation

Description : The speed of a synchronous motor is varied by _____________. A. interchanging any two of the three live leads B. changing the voltage of the system C. changing the input frequency D. increasing the field excitation

Last Answer : Answer: C

The voltage developed by an AC generator is controlled by varying the ____________. A. speed of the prime mover B. AC excitation to the field C. DC excitation to the field D. DC excitation of the voltage regulator

Description : The voltage developed by an AC generator is controlled by varying the ____________. A. speed of the prime mover B. AC excitation to the field C. DC excitation to the field D. DC excitation of the voltage regulator

Last Answer : Answer: C

Automatic voltage regulators provided on switchboards function to _____________. A. regulate the AC load on the generator B. protect the switchboard from high voltage C. govern prime mover speed to control voltage D. vary the field excitation to the generators

Description : Automatic voltage regulators provided on switchboards function to _____________. A. regulate the AC load on the generator B. protect the switchboard from high voltage C. govern prime mover speed to control voltage D. vary the field excitation to the generators

Last Answer : Answer: D

Routine maintenance of a ship's service alternator should include ______________. A. changing the pedestal bearing insulation yearly B. megger testing of all rectifying diodes C. lubricating excitation slip rings D. periodic cleaning of the air filters or screens

Description : Routine maintenance of a ship's service alternator should include ______________. A. changing the pedestal bearing insulation yearly B. megger testing of all rectifying diodes C. lubricating excitation slip rings D. periodic cleaning of the air filters or screens

Last Answer : Answer: D

The standard method of controlling the output voltage of a 440 volt, 60 Hz, AC generator is accomplished by adjusting the ____________. A. prime mover speed droop B. number of poles C. alternator field excitation D. load on the alternator

Description : The standard method of controlling the output voltage of a 440 volt, 60 Hz, AC generator is accomplished by adjusting the ____________. A. prime mover speed droop B. number of poles C. alternator field excitation D. load on the alternator

Last Answer : Answer: C

Why is it desirable to operate paralleled AC generators at the same power factor? A. Circulating currents are kept to a minimum. B. Field excitation losses are kept to a minimum. C. Generator rotors will have a lesser tendency to hunt. D. Because a power factor increase will decrease kilowatt output.

Description : Why is it desirable to operate paralleled AC generators at the same power factor? A. Circulating currents are kept to a minimum. B. Field excitation losses are kept to a minimum. C. Generator ... will have a lesser tendency to hunt. D. Because a power factor increase will decrease kilowatt output.

Last Answer : Answer: A

The power factor of an AC generator operating singularly is determined by the __________. A. connected load B. prime mover speed C. field excitation D. generator's rated voltage

Description : The power factor of an AC generator operating singularly is determined by the __________. A. connected load B. prime mover speed C. field excitation D. generator's rated voltage

Last Answer : Answer: A

If the excitation is increased to one of two alternators operating in parallel, the _____________. A. power factor will change in the lagging direction B. power factor will change in the leading direction C. kilowatt load will be greatly increased D. ampere load will be greatly decreased

Description : If the excitation is increased to one of two alternators operating in parallel, the _____________. A. power factor will change in the lagging direction B. power factor will change in the leading direction C. kilowatt load will be greatly increased D. ampere load will be greatly decreased

Last Answer : Answer: A

The power factor at which a paralleled AC generator operates is usually adjusted by the _____________. A. connected load B. prime mover speed C. field excitation D. generator's rated voltage

Description : The power factor at which a paralleled AC generator operates is usually adjusted by the _____________. A. connected load B. prime mover speed C. field excitation D. generator's rated voltage

Last Answer : Answer: C

To equalize the power factor of two alternators operating in parallel, the ____________. A. field excitation of both units is adjusted B. governors of both units are adjusted C. phase sequence is altered D. kilowatt load is evenly divided

Description : To equalize the power factor of two alternators operating in parallel, the ____________. A. field excitation of both units is adjusted B. governors of both units are adjusted C. phase sequence is altered D. kilowatt load is evenly divided

Last Answer : Answer: A

To increase the frequency of an operating AC generator, you should ____________. A. increase the field excitation B. decrease the field excitation C. increase the number of magnetic poles D. increase the speed of the prime mover

Description : To increase the frequency of an operating AC generator, you should ____________. A. increase the field excitation B. decrease the field excitation C. increase the number of magnetic poles D. increase the speed of the prime mover

Last Answer : Answer: D

When securing an AC generator, you should FIRST ___________. A. open the generator circuit breaker B. switch the voltage regulator to 'manual' C. decrease the field excitation to minimum D. reduce the load on the unit

Description : When securing an AC generator, you should FIRST ___________. A. open the generator circuit breaker B. switch the voltage regulator to 'manual' C. decrease the field excitation to minimum D. reduce the load on the unit

Last Answer : Answer: D

As load is added to an AC generator provided with constant field excitation, the prime mover slows down with the effect of ____________. A. lowering frequency and lowering generated voltage B. increasing frequency and increasing generated voltage C. increasing frequency and lowering generated voltage D. lowering frequency and increasing generated voltage

Description : As load is added to an AC generator provided with constant field excitation, the prime mover slows down with the effect of ____________. A. lowering frequency and lowering generated ... C. increasing frequency and lowering generated voltage D. lowering frequency and increasing generated voltage

Last Answer : Answer: A

The division of kilowatt load between two paralleled alternators is determined by the _____________. A. amount of field excitation of the leading machine B. load-speed characteristics of the governors C. amount of field excitation to the lagging machine D. type of alternator

Description : The division of kilowatt load between two paralleled alternators is determined by the _____________. A. amount of field excitation of the leading machine B. load-speed characteristics of the governors C. amount of field excitation to the lagging machine D. type of alternator

Last Answer : Answer: B

Kilowatt load is divided between two AC generators operating in parallel by _____________. A. adjusting the governor controls B. varying the excitation voltage C. increasing both prime mover speeds simultaneously D. decreasing both prime mover speeds simultaneously

Description : Kilowatt load is divided between two AC generators operating in parallel by _____________. A. adjusting the governor controls B. varying the excitation voltage C. increasing both prime mover speeds simultaneously D. decreasing both prime mover speeds simultaneously

Last Answer : Answer: A