Capicitor Application Issues
Capacitors must be built to tolerate voltages and currents in excess of their ratings according to standards. The applicable standard for power capacitors is IEEE Std 18-2002, IEEE Standard for Shunt Power Capacitors.
Heat as one of most common cause of motor failure
This slide speaks about that how motor operation fails due to heat. how heat affect motors?
Monday 12 May 2014
Explain the working principal of the circuit breaker?
Circuit Breaker is one which makes or breaks the circuit. It
has two contacts namely fixed contact & moving contact. Under
normal condition the moving contact comes in contact with fixed contact thereby
forming the closed contact for the flow of current. During abnormal &
faulty conditions (when current exceeds the rated value) an
arc is produced between the fixed & moving contacts &
thereby it forms the open ckt. Arc is extinguished by the Arc Quenching media like air, oil, vacuum etc.
What happens if i connect a capacitor to a generator load?
Connecting a capacitor across a generator always improves
power factor, but it will help depends up on the engine capacity
of the alternator, otherwise the alternator will be over loaded due to the
extra watts consumed due to the improvement on pf.
Secondly, don't connect a capacitor across an alternator while it is picking up or without
any other load.
what is power factor? Whether it should be high or low? why?
Power factor should be high in order to get smooth operation
of the system. Low power factor means losses will be more. it is
the ratio of true power to apparent power. it has to
be ideally 1. if it is too low then cable over heating &equipment overloading will occur. if it is greater than 1 then load will
act as capacitor and starts feeding the source and will cause
tripping.(if pf is poor ex:0.17 to meet actual power
load has to draw more current(V constant),result in more losses if pf is good ex: 0.95 to meet actual power load has to draw
less current(V constant),result in less losses).
There are a Transformer and an induction machine. Those two have the same supply. For which device the load current will be maximum? And why?
The motor has max load current compare to that of transformer
bcoz the motor consumes real power.. and the transformer is only
producing the working flux and its not consuming.. hence the load current
in the transformer is because of core loss so it is minimum.
what is meant by insulation voltage in cables?explain it?
It is the property of a cable by virtue of it can withstand
the applied voltage without rupturing it is known as insulation level of
the cable.
what is boucholz relay and the significance of it into the transformer?
Boucholz relay is a device which is used for the protection of transformer from its internal faults; it is a gas based relay. whenever
any internal fault occurs in a transformer, the boucholz relay at once
gives a horn for some time, if the transformer is isolated from the circuit then
it stop its sound itself otherwise it trips the circuit by
its own tripping mechanism.
What is the difference between Isolator and Circuit Breaker?
Isolator is a off load device which is used for isolating the
down stream circuits from upstream circuits for the reason of any
maintenance on down stream circuits. It is manually
operated and does not contain any solenoid unlike circuit breaker. it should not be operated while it is having load. first the
load on it must be made zero and then it can safely operated. Its
specification only rated current is given. But circuit
breaker is on load automatic device used for breaking the circuit in case of abnormal conditions like short-circuit, overload
etc., it is having three specification 1 is rated current and 2 is short
circuit breaking capacity and 3is instantaneous tripping
current.
What are the transformer losses?
TRANSFORMER LOSSES - Transformer losses have two sources-copper loss and magnetic loss. Copper losses are caused by the resistance
of the wire (I2R). Magnetic losses are caused by eddy currents
and hysteresis in the core. Copper loss is a constant after the coil has been
wound and therefore a measureable loss. Hysteresis loss is constant for a
particular voltage and current.Eddy-current loss, however,
is different for each frequency passed through the transformer
Define IDMT relay?
It is an inverse definite minimum time relay. In IDMT relay
its operating is inversely proportional and also a characteristic of
minimum time after which this relay operates. It is
inverse in the sense ,the tripping time will decrease as the magnitude of fault current increase
Where should the lighting arrestor be placed in distribution lines?
Near distribution transformers and out going feeders of 11kv
and incoming feeder of 33kv and near power transformers in
sub-stations.
What is the difference between MCB & MCCB,Where it can be used?
MCB is miniature circuit breaker which is thermal operated
and use for short circuit protection in small current rating circuit.
MCCB moulded case circuit breaker and is thermal
operated for over load current and magnetic operation for instant trip in short circuit condition. under voltage and under
frequency may be in built. Normally it is used where normal current is more
than 100A.
Why we do 2 types of earthing on transformer ie: body earthing & neutral earthing, what is function.
The two types of earthing are Familiar as Equipment earthing
and System earthing. In Equipment earthing: body (non
conducting part) of the equipment should be earthed
to safeguard the human beings. System Earthing: In this neutral
of the supply source (Transformer or Generator) should be grounded. With this, in case of unbalanced loading neutral will not be
shifted .so that unbalanced voltages will not arise. We can protect the
equipment also. With size of the equipment ( transformer or alternator)and
selection of relying system earthing
will be further classified into directly earthed, Impedance earthing,resistive
(NGRs) earthing
Why ELCB can't work if N input of ELCB do not connect to ground?
ELCB is used to detect earth leakage fault. Once the phase
and neutral are connected in an ELCB, the current will flow through
phase and that much current will have to return neutral so resultant current is
zero. Once there is aground fault in the load side, current from phase will
directly pass through earth and it will not return
through neutral through ELCB. That means once side current is going and not returning and hence because of this difference
in current ELCB will trip and it will safe guard the other circuits
from faulty loads. If the neutral is not grounded,
fault current will definitely high and that full fault current will come back through ELCB, and there will be no difference in current.
Why series motor cannot be started on no-load?
Series
motor cannot be started without load because of high startingtorque. Series
motor are used in Trains, Crane etc
Why in a three pin plug the earth pin is thicker and longer than the other pins?
It depends upon R=rho l/a where area(a) is inversely
proportional to resistance (R), so if (a) increases, R decreases & if R
is less the leakage current will take low resistance
path so the earth pin should be thicker. It is longer because the First to make the connection and
last to disconnect should be earth Pin. This assures Safety for the person who
uses the electrical instrument.
Why Delta Star Transformers are used for Lighting Loads?
For lighting loads, neutral conductor is must and hence the
secondary must be star winding. And this lighting load is always
unbalanced in all three phases. To minimize the
current unbalance in the primary we use delta winding in the primary. So delta / star transformer is used for lighting loads.
What are the advantages of star-delta starter with induction motor?
The main advantage of using the star delta starter is
reduction of current during the starting of the motor. Starting current
is reduced to 3-4 times Of current of Direct online
starting.(2). Hence the starting current is reduced; the voltage drops during the starting of motor in systems are reduced.
Why is the starting current high in a DC motor?
In DC motors, Voltage equation is V=Eb-IaRa (V = Terminal
voltage,Eb= Back emf in Motor,Ia = Armature current,Ra = Armature
resistance).At starting,Eb is zero. Therefore, V=IaRa,
Ia = V/Ra , where Ra is very less like 0.01ohm.i.e,Ia will become enormously increased
What is meant by regenerative braking?
When the supply is cut off for a running motor, it still
continue runningdue to inertia. In order to stop it quickly we place a load
(resistor) across thearmature winding and the motor should have maintained
continuous field supply.so that back e.m.f voltage is
made to apply across the resistor and due to loadthe motor stops
quickly. This type of breaking is called as "RegenerativeBreaking
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