Qbeecs2
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The document discusses transformers and DC machines. It provides definitions, explanations of principles of operation, parts, types and equations for transformers, DC generators and DC motors. Key points include: - Transformers transfer power from one circuit to another via electromagnetic induction without changing frequency. - DC generators convert mechanical energy to electrical energy via the principle of dynamically induced EMF. DC motors operate inversely. - Transformers, generators and motors each have windings, cores/frames and produce/use magnetic fields in their operation. - Equations provided relate induced EMF, current, voltage, speed and other variables.
Qbeecs2
- 1. EE 255 ELECTRICAL ENGG. & CONTROL SYSTEMS (CSE III SEMESTER) UNIT II – DC MACHINES & TRANSFORMERS 2 MARKS QUESTION & ANSWERS 1. What is a transformer? Transformer is a static device which, a) Transfers electric power from one circuit to another, b) It does so without a change in frequency, c) It accomplishes this by electromagnetic induction, d) Where the two electric circuits are in mutual inductive influence of each other. 2. Briefly explain the principle of operation of transformers. A transformer consists of two coils which are in mutual inductance. When AC supply is given to one of the coils, an alternating flux is set up, which is linked with the second coil. Due to this alternating flux there is a mutually induced emf produced in the second coil. If the second coil is closed, current flows in it and so electric energy is transferred magnetically from the first coil to the second coil. 3. What are the parts of a transformer? The transformer has mainly following parts. a) Primary winding – the coil to which the AC supply is given b) Secondary winding – the coil from which output is taken and given to load. c) Laminated Core – this acts as a mechanical support to the coils as well as provides magnetic path for the flux. 4. What are the types of core in transformer? There are two types of core available. They are, a) Core type – This has two legs. The primary winding is wounded on one leg and the secondary winding is wounded on the second leg. This is mainly used in single phase transformers. b) Shell type – This has three legs. Both primary and secondary windings are wound on the same leg. This is mainly used in three phase transformers where three primary and three secondary windings are present. 5. What is an ideal transformer? An ideal transformer is the one which has no losses i.e., its windings have no ohmic resistance, there is no magnetic leakage and hence which has no I2 R and core losses. The efficiency of an ideal transformer is 100%. 6. Give the emf equation of the transformer. The emf induced in the transformer is given by mfNE φ21 44.4= V
- 2. mfNE φ12 44.4= V where, E1 is the emf induced in the primary winding. E2 is the emf induced in the secondary winding. N1 is the number of turns in the primary winding. N2 is the number of turns in the secondary winding. φm is the maximum flux produced. f is the frequency in Hz. 7. What are the losses occurring in a transformer. The losses occurring in a transformer are a) Core loss – loss occurring in the core of the transformer. This has two components. o Hysteresis loss – The loss occurring due to the magnetization characteristics of the core. o Eddy current loss – The loss occurring due to the eddy current produced in the core. b) Copper loss (I2 R loss) – loss occurring in the windings of the transformer. This has two components. o Primary copper loss – The loss occurring due to the current flowing through the primary winding. o Secondary copper loss – The loss occurring due to the current flowing through the secondary winding. 8. What is the purpose of the magnetizing current in transformer? The magnetizing current is the component of the primary current which is responsible for the production of flux in the core. 9. What are the components of primary no-load current? The primary no-load current consists of mainly two components. They are, a) Magnetizing current ( Im ) – Produces flux in the core and hence magnetizes the core. b) Core loss component current ( Ic ) – compensates for the core losses. 10.What are Generators and motors? An electrical generator is a machine which converts mechanical energy into electrical energy. An electrical motor is a machine which converts electrical energy into mechanical energy. 11.What is the principle of working of a DC generator? The generator working is based on the principle of the production of dynamically induced emf. Whenever a conductor cuts magnetic flux, dynamically induced emf is produced in it according to Faradays’ laws of electromagnetic induction, which states that the emf induced is equal to the rate of change of flux.
- 3. 12.What are the main parts of a DC generator? The main parts if a DC generator are, a) Yoke or frame – Provides mechanical support to the poles. b) Pole core – The solid portion of the pole over which field coils are wound. c) Pole shoe – The base of pole which has projected structure to spread the flux in the air gap. d) Field coils – The coils which carry the field current responsible for producing the necessary magnetic field in the generator. e) Armature core – The solid portion of the armature which has slots on the periphery to carry the armature windings. f) Armature windings – The coil carrying the induced emf produced in the generator. g) Commutator – The mechanical rotating switch which converts the AC voltage produced in the armature to a DC voltage in the external circuit. 13.What are the types of DC generators? There are two main categories of DC generators. They are, a) Separately excited DC generator. b) Self-excited DC generator. 14.What is a separately excited DC generator? In a separately excited DC generator the field winding is excited separately using a variable DC voltage supply. 15.How does a self-excited DC generator work? In a self-excited DC generator the poles will have residual magnetism which produces small amount of flux in the air gap in which the armature rotates. Now due to this flux a very small value of emf is induced in the armature. The field windings are connected across or in series with the armature windings. The small emf produced drives some current through the field coils which increases the net flux in the air gap. This again increases the induced emf. This process continues until the voltage builds up to the rated voltage. 16.Explain the different types of self-excited DC generators. There are three types of self-excited DC generators. They are, a) Shunt Generator – The field winding is connected parallel to the armature winding. b) Series Generator – The field winding is connected in series with the armature winding. c) Compound Generator – Both series and shunt field windings are present. If the shunt field winding is connected across both the armature winding and the series field winding, then it is called long shunt compound generator. If the shunt field winding is connected only across the armature winding, then it is called the short shunt compound generator. If the shunt field and the series field windings are wound in a such a way that they aid each other ( i.e., in same direction ), then we say
- 4. it is cumulatively compounded. If the shunt field and the series field windings are wound in a such a way that they oppose each other (i.e., in opposite direction ), then we say, that it is differentially compounded. 17.Give the emf equation of DC generator. The emf produced in the armature of a DC generator is given by, A PNZ E 60 φ = V where, φ is the flux per pole (Wb) P is the number of poles N is the speed of armature in rpm (rotations per minute) Z is the total number of conductors A is the number of parallel paths 18.Explain the principle of operation of DC motor. The DC motor is based on the principle that, when a current carrying conductor is placed in a magnetic field, it experiences a mechanical force. In DC motor the field windings when passed current produces the necessary magnetic field. The armature carrying current is present in this field and hence experiences force. Since the armature conductors are present on the periphery perpendicular to the force, they experience a turning force or torque. Torque is the product of force and the radius at which the force acts. So overall armature experiences a torque and starts rotating. 19.What is back emf in a DC motor? In a DC motor when the armature starts rotating, it cuts the flux and hence there is an emf induced in the armature called the back emf. This is called back emf as it is opposite to the applied voltage according to Lenz’s law. 20.Give the back emf equation of DC motor. The back emf equation of DC motor is given by, A PNZ Eb 60 φ = V where, φ is the flux per pole (Wb) P is the number of poles N is the speed of the armature in rpm (rotations per minute) Z is the total number of conductors A is the number of parallel paths 21.Give the voltage and Torque equation of DC motor. The voltage equation: aab RIEV += V
- 5. Torque equation: A PZI T a π φ 2 = Nm where, V is applied voltage, Eb is the back emf (V) Ia is the armature current (A) Ra is the armature resistance (Ω ) φ is the flux per pole (Wb) P is the number of poles Z is the total number of conductors A is the number of parallel paths. A=2 for wave winding A=P for lap winding 22.What are the types of DC motor? The types of DC motors are, a) Shunt motor – the field winding is connected parallel to the armature winding. b) Series motor – the field winding is connected in series with the armature winding. c) Compound motor – both shunt and series field windings are present. If the shunt field winding is connected across both the armature winding and the series field winding, then it is called long shunt compound motor. If the shunt field winding is connected only across the armature winding, then it is called the short shunt compound motor. If the shunt field and the series field windings are wound in a such a way that they aid each other ( i.e., in same direction ), then we say it is cumulatively compounded. If the shunt field and the series field windings are wound in a such a way that they oppose each other (i.e., in opposite direction ), then we say, that it is differentially compounded.
- 6. Torque equation: A PZI T a π φ 2 = Nm where, V is applied voltage, Eb is the back emf (V) Ia is the armature current (A) Ra is the armature resistance (Ω ) φ is the flux per pole (Wb) P is the number of poles Z is the total number of conductors A is the number of parallel paths. A=2 for wave winding A=P for lap winding 22.What are the types of DC motor? The types of DC motors are, a) Shunt motor – the field winding is connected parallel to the armature winding. b) Series motor – the field winding is connected in series with the armature winding. c) Compound motor – both shunt and series field windings are present. If the shunt field winding is connected across both the armature winding and the series field winding, then it is called long shunt compound motor. If the shunt field winding is connected only across the armature winding, then it is called the short shunt compound motor. If the shunt field and the series field windings are wound in a such a way that they aid each other ( i.e., in same direction ), then we say it is cumulatively compounded. If the shunt field and the series field windings are wound in a such a way that they oppose each other (i.e., in opposite direction ), then we say, that it is differentially compounded.