(2A) For a similar size, why a synchronous motor has a higher fault current cont
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Question
(2A) For a similar size, why a synchronous motor has a higher fault current contribution than arn induction motor? (3pts) (2B) A single phase transformer is rated 20KVA and 1200v/100V. Please (a) configure this transformer into a st output voltages and currents (7pts) (b) why the autotransformer in (a) has a much higher efficiency than the original conventional transformer(3pts) ep-up transformer 1100V/1200v, listing the input and (2C) Why is it important to establish a national standard in the positive and negative sequence phase shift of a Y or Y transformer? (3pts) (2D) What are the differences between using LTC and capacitor to control the magnitude of the voltages (4pts)Explanation / Answer
2A) Synchronous motors operate by energising windings with suitable electric source. At the other hand,induction motor works on the principle that a current carrying conductor placed in a magnetic field experiences a force which rotates the conductor. Here from both cases, the fault current contribution is more for synchronous machines because when load connected to a motor misbehaves or if any fault occurs in the motor,it starts drawing fault current or high amount of current. While in case of induction motors,mostly they are connected to one another for power factor compensation and if any fault occurs,the particular motor is disconnected and the operation is restored using the remaining connected motors.
2B)a)Given the rating=20kVA
The initial primary and secondary voltages are V1=1200V and V2=100V.
Now the primary current is given as, I1=rating/V1
=(20×10^3)/1200=16.67A
The secondary current I2=(20×10^-3)/100=200A
Now the output voltages are 1100/1200 V of that of a step up transformer and for the given power rating,the currents are,
I1=(20×10^3)/1100=18.18A
The current I2 is given as,
I2=(20×10^-3)/1200=16.66A
b)Autotransformer has higher efficiency when comoared to two winding transformer because,in autotransformer the primary and secondary windings are composed as single winding and hence the amount of copper required for forming conductor core is minimised and considerably the core loss is minimised. Hence the efficiency is high.
2C)It is important to establish national standard in positive and negative phase shift in star delta and delta star transformers because,in star and delta connections,the line voltage and currents vary depending on the type of grounding and due to variations in phase shifting. These values do not remain constant and hence the power rating calculations become difficult,and at the same time the positive and negative sequence values may also vary. So a national standard must be followed by acquiring constant or appropriate voltage values for positive and negative sequence transformers.
2D)LTC or load tap changer is used to vary the turns ratio in discrete steps to attain different voltage values while capacitor is used to make voltage low or zero at no load and increase voltage at full load. Using capacitors,variable voltage cannot be obtained. Only prescribed voltage can be reached on increase or decrease of voltage. While ltc is used to obtain variety of voltage ratings.
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