Electrical Engineering Q/A 91-to-100

 91: What is Automatic Voltage regulator(AVR)?

AVR is an abbreviation for Automatic Voltage Regulator. It is important part in Synchronous Generators, it controls theoutput voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator. 

92: Why thyristor is considered as Charge controlled device?

During the triggering process of the thyristor from forward blocking state to forward conduction state through the gate signal, by applying the gate signal (voltage between gate and cathode) increases the minority carrier density in the p-layer and thereby facilitate the reverse break over of the junction J2 and thyristor starts conducting. Higher the magnitude of the gate current pulse, lesser is the time required to inject the charge and turning on the scr. By controlling the amount of charge we can control the turning on time of the scr. 

93: What is Holding current?

When scr is conducting current in forward conduction state, scr will return to forward blocking state when the anode current or forward current falls below a low level called Holding current Note: Latching current and Holding current are not same. Latching current is associated with the turn on process of the scr whereas holding current is associated with the turn. off process. In general holding current will be slightly lesser than the latching current.

94: What is the effect of positive feedback on stability of the system?

Positive feedback is not used generally in the control system because it increases the error signal and drives the system to instability. But positive feedbacks are used in minor loop control systems to amplify certain internal signals and parameters. 

95: What is Control System?

In a System the output and inputs are interrelated in such a manner that the output quantity or variable is controlled by input quantity, then such a system is called Control System. The output quantity is called controlled variable or response and the input quantity is called command signal or excitation.
or A control system manages, commands, directs or regulates the behaviour of other devices or systems. It can range from a home heating controller using a thermostat controlling a domestic boiler to large Industrial control systems which are used for controlling processes or machines

96: What System?

When a number of elements or components are connected in a sequence to perform a specific function, the group of elements that all constitute a System

97: What is the difference between sensor and a transducer?

Transducer is a device which converts one form of energy to any other form of energy where as sensors are devices which detects a physical quantity like temperature, pressure and converts it into electrical signal.
This is the reason why it is said that "Every sensor is a transducer where as not every transducer is a sensor

98: Why is generation voltage normally between 11 to 33kv?

Normally generation voltage is between 11 to 33kv.different countries is generating at different levels like 11kv, 18kv. Generation voltage is not lower then 11kv because by decreasing voltage current will rise and ultimately cost and heat losses will increases. This is why it is not kept below 11kv. Where as it is not kept above 33kv because generator has certain insulation limit. And insulation depends on voltage. More the voltage, more should be the insulation. 

99: Why ammeter is connected in series and voltmeter in parallel?

Ammeter is a device which is used to measure current. It has very low resistance so it can measure current properly. If we connect ammeter is parallel than, in parallel configuration, current follows least resistive path and least resistive path here will be ammeter's coil. So maximum current will pass through it, which will damage its coil. That is why ammeter is not connected in parallel.
Voltmeter is used to measure voltage which is potential difference BETWEEN two points. Only way to measure it is in parallel. Because voltmeter has very high resistance and if we connect it is series than no current will pass through it (Forming an open circuit). So, we won't be able to measure voltage properly. This is why voltmeter is connected in parallel and ammeter in series.

100: What type of cables are used for transmissions?

As you know that cables are required for transmitting power, give a brief on the various types namely the low tension cables use for transmitting up to 1000V, high tension cables for up to 23000V, and super tension cables that transmit from 66KV to 132KV.