Electrical Engineering Interview Questions Part 6

Q:What is inrush current?
A:Inrush current is the current drawn by a piece of electrically operated equipment when power is first applied. It can occur with AC or DC powered equipment, and can happen even with low supply voltages.

Q:In a Tap changing transformer where is the tap connected, is it connected in the primary side or secondary side? 
A:Tapings are connected to high voltage winding side, because of low current. If we connect tapings to low voltage side, sparks will produce while tap changing operation due to high current.

Q:Why transformer ratings are in kva? 
A: Since the power factor of transformer is dependent on load we only define VA rating and does not include power factor .In case of motors, power factor depend on construction and hence rating of motors is in KWatts and include power factor.

Q: What is difference between fuse and breaker?
A: Fuses are burned at the time of over current flows in the circuit but breakers are just open(not burn) at the time of over current flow. Fuses are used in only one time but breakers are used by multiple number of times.

Q:What is the difference between delta-delta, delta-star transformer?
A:Delta-delta transformer is used at generating station or a receiving station for Change of Voltage (i,e) generally it is used where the Voltage is high & Current is low.Delta-star is a distribution kind of transformer where from secondary star neutral is taken as a return path and this configuration is used for Step down voltage phenomena.

Q: Capacitor is load free component but why ampere meter shows current when capacitor bank breaker close?
A: As we know that Electrical is having two type of load, Active and Reactive .Capacitor is a reactive load which is not considering as a load,& its factor is Isin@ .Meter is design based on Current RMS value because of it meter is showing the current RMS value.

Q:How to determine alternating current frequency?
A:Zero crossings of the sine wave to trigger a monostable (pulse generator) is a way to determine alternating current frequency. A fixed width pulse is generated for each cycle. Thus there are “n” pulses per second, each with with a constant energy. The more pulses there are per second, the more the energy. The pulses are integrated (filtered or averaged) to get a steady DC voltage which is proportional to frequency. This voltage can then be displayed on an analogue or digital voltmeter, indicating frequency. This method is more suitable than a direct counter, as it can get good accuracy in a second or so.

Q:Why electricity in India is in the multiples of 11 like 11kv, 22kv, 33kv  ?
A:Transformer Induced voltage equation contains 4.44 factor.
E=4.44*f*T*phi
E -Induced emf per phase
T -number of turns
f -frequency
phi -maximum flux per pole

From the equation we see that E is proportional to 4.4 and it is in turn multiple of 11.So always transmission voltage is multiple of 11