## 07/03/2020

### DC Generator Interview Question Answer - 4

 61 What is a criterion for selecting numbers of equalizing rings in the DC Generator? The numbers of equalizing rings in the DC Generator is  =  Numbers of armature conductors / Numbers of pair of poles 62 What is meaning of winding has been developed for 100% equalization connection? If all the armature conductors are connected to the equalizer rings, it is called as 100% equalization connection. 63 Why the equalizing connection is not used in the wave wound machines? Equalization connection The equalizing connection is not used in the wave winding because there are two parallel paths in the wave winding resulting no unbalance of current in that path.  If there is inequality of flux, they will affect each parallel path equally. 64 What is happened when two DC shunt Generator is connected   parallel with incorrect polarity? When two DC Generators are paralleled with incorrect polarity, short circuit occurs. This will damages brushes and commutator and tripping of the Generators. 65 What is meaning of ‘floating’ condition in the two parallel operated DC shunt Generator? Floating When two Generators are paralleled with each other, the induced emfs of the both Generators are same.  This will result the current cannot be flow between them. This condition is known as ‘floating condition’. 66 How one Generator is taken out of service in the two parallel operated DC shunt Generator? Two parallel DC shunt generator The field of the DC Generator which is taken out of service is weakened and that of the other Generator is increased till the ammeter of the Generator shows zero.  The circuit breaker and the switch are open thus removing the DC Generator out of service. 67 Which Generator delivers more current than that of other one if the DC shunt Generator A has more drooping characteristics than that of DC shunt Generator B? The DC shunt Generator A delivers less current than that of DC shunt Generator B. 68 The parallel operation of two compound Generators is possible or not? Why? The parallel operation of the two compound Generators is unstable for constant load condition. Let it is assumed that compound Generator A carries slightly higher load than that of compound Generator B due to some reason.  Due to current passing through series field winding of compound Generator A, its field strengthens resulting raises the generated emf. The Generator A takes even more load.  The load on the Generator B drop as the system load is constant. Therefore the generated emf of the compound Generator B reduces result further drop in load.  This process is cumulative resulting Generator A will take entire load and Generator B acts as a motor.  The circuit breaker of one Generator will open. This will result the parallel operation of the Generator stop. 69 What is magnetizing characteristic of the DC Generator? The magnetizing characteristics of the DC Generator show the relation between no load armature emf and field current. 70 Which type of DC Generator does not requires equalizing ring for Parallel operation? Under compound DC Generator 71 Which characteristic of DC Generator helps to decide stability for a particular application? External characteristics of DC Generator : Terminal voltage ( V ) versus load current ( I ) 72 Explain : ‘The open circuit characteristics of the self excited Generator is straight line at low value of flux density.’ Open circuit characteristic The reluctance of the iron path is negligible at low value of flux density resulting the total reluctance is only air gap reluctance which is constant.  Therefore the generated emf is directly proportional to field current at low value of flux density. 73 ‘ The generated emf is directly proportional to field current ’ comment on above statement Generated emf – Field current The generated emf is directly proportional to the field current is true only for low value of flux density.  The value of permeability is small at high value of flux density therefore the reluctance of the iron path becomes appreciable.  Due to above reason the relation between generated emf and field current is not straight line. 74 Is there any emf is generated at zero field current in the self excited DC Generator? Some emf is always generated at zero field current in the self excited DC Generator  due to residual magnetism of poles. 75 Which parameter greatly affects the initial slope of the generated emf – field current? Air – gap width 76 Explain the term : Critical resistance Critical resistance The slope of the open circuit characteristics of the DC shunt Generator is called as critical resistance. 77 Describe the condition of building up DC Shunt Generator. Condition of building up DC Shunt Generator There must be some residual magnetism in the poles.  The shunt field coil should be correctly connected to armature coil. The shunt filed resistance should be less than the critical resistance if it is excited an open circuit. 78 How to obtain driving torque in the DC Generator? The driving torque is nothing but prime mover therefore it is diesel engine, steam turbine or water turbine. 79 What is direction of rotor and electromagnetic torque in the case of ( a ) Generator ( b ) Motor ? Generator The direction of rotor and electromagnetic torque are in the opposite direction. Motor The direction of the rotor and electromagnetic torque are in the same direction. 80 Why the electromagnetic torque in the ( a ) Generator is opposite to that of the rotor direction. ( b ) Motor is same as that of direction of the rotor? Generator The field flux in the Generator concentrated at the trailing pole tips whereas it weaken at leading pole tips so the generated electromagnetic torque is opposite direction to that of the direction of the rotor. Motor The field flux in the motor concentrated at the leading pole tips whereas it weaken at trailing pole tips so the generated electromagnetic torque is same direction as that of the direction of the rotor.

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