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Satisfactory commutation of D.C. machines requires brushes should be of proper grade and size, brushes should smoothly run in the holders, smooth, concentric commutator properly undercut.

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Brushes of D.C. machines are made of carbon.

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The purpose of retardation test on D.C. shunt machines is to find out stray losses.

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Hopkinson's test on D.C. machines is conducted at full-load.

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In D.C. machines the residual magnetism is of the order of 2 to 3 percent.

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In D.C. machines fractional pitch winding is used to reduce the sparking.

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In dc machines, the armature windings are placed on the rotor because of the necessity for  (1) electromechanical energy conversion (2) generation of voltage (3) commutation (4) development of torque

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3  Commutation

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For a D.C. machines laboratory following type of D.C. supply will be suitable induction motor D.C. generator set.

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In a thyristor D.C. chopper, which type of commutation results in best performance ?

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voltage commutation

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Draw the connection diagram of long shunt differential D.C. compound generator.

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For a class D commutation, answer the following: i) Explain the operation with circuit diagram. ii) Interpret with waveforms.

Answer :

Class D commutation: This is also called as auxiliary commutation because it uses an auxiliary SCR to switch the charged capacitor across conducting SCR to turn it off. In this scheme, the main ... make SCR1 forward biased and keep ready for next triggering. The above cyclic process is repeated.

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Description :

Class D commutation

Answer :

Class D commutation: This is also called as auxiliary commutation because it uses an auxiliary SCR to switch the charged capacitor across conducting SCR to turn it off. In this scheme, the main ... make SCR1 forward biased and keep ready for next triggering. The above cyclic process is repeated.

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Draw circuit diagram of Class B commutation.

Answer :

Class B: Resonant Pulse Commutation:

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Class A Load Commutation

Answer :

Class A: Load Commutation: The class A or load commutation (also called self-commutation or resonant commutation) is employed in thyristor circuits supplied from DC source. The commutating components include ... biased. Thus the zero-current and reverse-bias cause the thyristor to turn-off. 

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Differentiate between Natural and Forced commutation

Answer :

Natural commutation Forced commutation Source is AC Source is DC External commutating componants are not required  External commutating componants are required SCR turns off ... in controlled rectifiers, AC voltage controllers etc.  Used in choppers & inverters etc

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Draw a neat circuit diagram of class F commutation

Answer :

class F commutation

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Explain auxiliary commutation with a neat diagram. Also draw its waveform.

Answer :

Auxiliary Commutation At start, the Ta is triggered and turned on to carry the load current. Due to the resonant circuit R-L-C, the current initially rises, attains peak and then falls to ... , the firing of auxiliary SCR commutates the main SCR, hence name is auxiliary commutation.

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Define commutation. Give the types of commutation.

Answer :

Commutation: The process of turning-off a conducting thyristor is called commutation . Types of Commutation: i) Line commutation (Natural commutation) ii) Load commutation iii) Forced commutation iv) ... Auxiliary commutation v) Class E: External pulse commutation vi) Class F: AC line commutation

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In a 3-phase converter circuit during commutation when one SCR in one phase is turned on to turn-off an SCR in another phase, results in (A) voltage notching (B) harmonic distortion (C) voltage sag (D) voltage swell

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In a 3-phase converter circuit during commutation when one SCR in one phase is turned on to turn-off an SCR in another phase, results in voltage notching

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By using some circuit the forward DC current of thyristor is forced to zero is called forced commutation.

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Natural commutation is also called as line Communication. The turning off of thyristor due to AC supply is called natural commutation or line commutation. If the thyristor is connected to AC supply ... connected to positive and anode terminal get connected to negative, this turns off the thyristor.

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Thyristor commutation means turning off thyristor.

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State any two applications of single phase autotransformer.

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State any two conditions for parallel operation of 3-phase transformer.

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Define transformation ratio in terms of current and voltage.

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(i) Transformation Ratio (K) is defined as the ratio of the primary current to the secondary current.  (ii) Transformation Ratio (K) is defined as the ratio of the secondary voltage to the primary voltage.

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Define: (i) Commercial efficiency of a transformer and  (ii) All day efficiency of a transformer.

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(i) Commercial Efficiency : It is the ratio of output power in watt to the input power in watt. (ii) All day efficiency of a transformer: It is the ratio of output energy in kWh to the input energy in kWh in the 24 hours of the day

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A 50 KVA transformer has iron loss 2 kW on full load. Calculate its iron loss at 75% of full load.

Answer :

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universal motor also used in Handheld power tools, blowers, blenders, vacuum cleaners, polishers and kitchen appliances.universal motor is defined as a motor which may be operated either on direct or single phase AC supply at approximately the same speed and output

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For protection of rotating machines against lightning surges _______ is used.  (1) lightning arrester (2) capacitor (3) combination of lightning arrester and capacitor (4) lightning conductor and arrester

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Silicon steel is used in electrical machines because it has?

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The speed control of universal motor used for sewing machines is by varying the resistance.

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Exciters of synchronous machines are d.c. shunt machines.

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Losses which occur in rotating electric machines and do not occur in transformers are?

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If  equalising bar is connected to the 2 DC generators  the current will be equal in 2 generators irrespective of parallel connection so that we connect equaliser bar

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The critical resistance of the D.C. generator is the resistance of field.

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If residual magnetism is present in a D.C. generator, the induced e.m.f. at zero speed will be?

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If residual magnetism is present in a D.C. generator, the induced e.m.f. at zero speed will be zero.

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A 220 V D.C. generator is run at full speed without any excitation. The open circuit voltage will be about 2 V.

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In a level compounded D.C. generator, full load terminal voltage is equal to no-load terminal voltage.

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In a D.C. generator in case the resistance of the field winding is increased, then output voltage will decrease.

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In a D.C. machine stray loss is the sum of iron loss and mechanical loss.

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A D.C. generator can be considered as rotating amplifier.

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To avoid formation of grooves in the commutator of a D.C. machine the brushes of opposite polarity should track each other.

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In a D.C. generator, the critical resistance refers to the resistance of field.

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Flashing the field of D.C. generator means creating residual magnetism by a D.C. source.

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If a D.C. generator fails to build up the probable cause could not be field resistance less than the critical resistance.

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In D.C. generators on no-load, the air gap flux distribution in space is flat topped.

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