Well, most of the engineers, who use the motors find sometimes get quizzed that how they should find right trip point on the overload relay in the starter for the motor.
For this, one must find the current rating provided on the motor. This is usually the safe current which the winding of the motor can work with out failure. Usually the winding of motor can withstand high current also based on the time. If the motor curve is available it is easy to find out the required trip current based on the trip time.
or else, one has to consider the 1.5 times of the rated current of motor. When we set the trip point on the relay at the required current, the next step is to ensure the trip time. Trip time could be find out by using equivalent load.
The final step to ensure that you have used the proper relay is to perform rotor lock test
of the motor.
What is rotor lock test?
The name itself gives and idea that it is locking of the shaft or rotor. One has to lock the rotor and switch on the motor.
This will lead to high current passage through winding. The relay should get tripped within prescribed time by manufacturer
to stop damage to the winding. This is the worse condition of failure and thus, suitable use of relay and trip set point, will always protect the motor.
Any queries are welcome.
How to set trip point in overload relay of starter?
Polar Dielectric in Uniform Electric Field
i) There are permanent dipoles present in Polar Dielectric which are randomly aligned in such a way that there exists permanent dipole moment Pp.
ii) When a dipole is present in an uniform electric field the dipole tries to align itself in the direction of electric field.
iii) Because of this all dipoles in polar dielectric are partially aligned in the direction of the field. This partial alignment is responsible for the induced dipole moment Pi.
Therefore the electric dipole moment is increasing.
P = Pp + Pi
iv) The electric dipole moment of a polar dielectric increases
a) by increasing the applied electric field
b) by decreasing the temperature
Thermodynamics - important points to be noted for competitive exams
➔ Tephigram is the name of temperature entropy diagram
➔ PV graph in a adiabatic change is called Isentropic.
➔ Entropy of a system is an index of “unavailable energy”.
➔ When gas is expanded, work is done by the gas on surroundings.
➔ The size of “Kelvin degree” is equal to “Centigrade”.
➔ The efficiency of a carnot engine increases by raising the temperature of the source.
➔ Work done per cycle is given by the area enclosed in the indicator diagram.
➔ Conversion of heat energy into electrical energy can be made by “Thermocouple”.
➔ f(P,V,T) =0 exists for an equilibrium state and is called equation of state.
➔ The area of cycle of T-S diagram gives the “available thermal energy for useful work” in a reversible process.
➔ Uses of TS diagram:
a) used in meteorology b) check efficiency of heat engine c) useful in predicting defects of performance of engine d) to obtain work value of fuel used.
➔ Change in entropy of universe due to free expansion is
∆S = nR log e(Vf/Vi)
➔ Loss of available of energy = To.dS where ‘To’ is lowest available temperature in system.
➔ In order to maintain a body in an isothermal condition, heat has to be either supplied or withdrawn.
➔ When a gas expands adiabatically, the temperature decreases.
➔ When a gas is compressed, the temperature increases because work is done on the gas.
➔ The work done in an adiabatic change in a particular gas depends upon only change in temperature.
➔ In an adiabatic compression, the decrease in volume is associated with increase in temperature & increase in pressure.
➔ For an isothermal expansion of a perfect gas, the value of dP/P is equal to -dV/V.
➔ Change in entropy of universe due to free expansion is
∆S = nR log e(Vf/Vi)
➔ Loss of available of energy = To.dS where ‘To’ is lowest available temperature in system.
➔ In order to maintain a body in an isothermal condition, heat has to be either supplied or withdrawn.
➔ When a gas expands adiabatically, the temperature decreases.
➔ When a gas is compressed, the temperature increases because work is done on the gas.
➔ The work done in an adiabatic change in a particular gas depends upon only change in temperature.
➔ In an adiabatic compression, the decrease in volume is associated with increase in temperature & increase in pressure.
➔ For an isothermal expansion of a perfect gas, the value of dP/P is equal to -dV/V.
➔For an adiabatic expansion of a perfect gas, the value dP/P is equal to -𝛾dV/V.
➔ A reversible process is always “quasi-static”, but every quasi-static process need not be a reversible process.
➔ For reversible cycle: ∆P = ∆V = ∆T = ∆U = ∆H
➔ dW = PdV is only applicable to reversible process.
➔ In case of “irreversible processes”, dW is not equal to PdV;
➔ For reversible cycle: ∆P = ∆V = ∆T = ∆U = ∆H
➔ dW = PdV is only applicable to reversible process.
➔ In case of “irreversible processes”, dW is not equal to PdV;
➔For free expansion, dW=0
➔For free expansion, dV=0, the work may be zero (in case of PV work)
➔For free expansion, dV=0, the work may be zero (in case of PV work)
➔ Work and heat are path functions.
➔ Work is not a thermodynamic property as it is not a state function and it is not a exact differential.
➔ Both thermodynamic and temperature scales use a single reference temperature i.e triple point of water.
➔ dW = PdV is only applicable to reversible process.
➔ Work is not a thermodynamic property as it is not a state function and it is not a exact differential.
➔ Both thermodynamic and temperature scales use a single reference temperature i.e triple point of water.
➔ dW = PdV is only applicable to reversible process.
➔ In case of “irreversible processes”, dW is not equal to PdV;
Heat Transfer due to conduction
In this mechanism, heat transfer is due to vibration amplitudes of molecules & atoms present in solids.
Consider a cubicle of solid. Let us maintain one face of cube at high temperature (TH) and other opposite face at low temperature(Tc).
Due to temperature difference an amount of heat energy (Q) passes from hot face to cold face in time 't'.
Conduction rate Pcond (amount of energy transferred for uni time) is
Pcond = Q/t = K*A*(TH-Tc)/d;
where 'K' is coefficient of thermal conductivity, a constant for given material.
'd' is thickness of slab
'A' Area of slab
't' is time of conduction
Therefore, Q= K*A(TH-Tc)*t/d
Note: i) 'K' depend on nature of material of which slab is made
ii) A good thermal conductor has 'K' greater value.
Thermal Resistance to conduction(R-value):
This explains resisting of thermal conductivity. The R-value(thermal resistance) of a slab of thickness 'd' is defined as R=d/k. Thus material having less value of 'K' will have higher R-value and thus acts as a good thermal insulator.
Note:
i) 'R' is properly assigned to specified thickness of slab but not to material of slab.
ii) In steady state, conducting rates thru any no. of materials must be equal.
Therefore, Pcond = A*(TH-Tc) / Σ(d/K)
iii) Heat is transferred from molecule to molecule by conduction. In this case molecules do not bodily move but simply vibrate.
Consider a cubicle of solid. Let us maintain one face of cube at high temperature (TH) and other opposite face at low temperature(Tc).
Due to temperature difference an amount of heat energy (Q) passes from hot face to cold face in time 't'.
Conduction rate Pcond (amount of energy transferred for uni time) is
Pcond = Q/t = K*A*(TH-Tc)/d;
where 'K' is coefficient of thermal conductivity, a constant for given material.
'd' is thickness of slab
'A' Area of slab
't' is time of conduction
Therefore, Q= K*A(TH-Tc)*t/d
Note: i) 'K' depend on nature of material of which slab is made
ii) A good thermal conductor has 'K' greater value.
Thermal Resistance to conduction(R-value):
This explains resisting of thermal conductivity. The R-value(thermal resistance) of a slab of thickness 'd' is defined as R=d/k. Thus material having less value of 'K' will have higher R-value and thus acts as a good thermal insulator.
Note:
i) 'R' is properly assigned to specified thickness of slab but not to material of slab.
ii) In steady state, conducting rates thru any no. of materials must be equal.
Therefore, Pcond = A*(TH-Tc) / Σ(d/K)
iii) Heat is transferred from molecule to molecule by conduction. In this case molecules do not bodily move but simply vibrate.
X-Rays discovery
Wilhelm Roentgen was professor of physics at university of Wurzburg, Germany when he discovered X-rays in 1985. The discovery was entirely serendipitous; Roentgen was merely studying a beam of electrons in a highly evacuated glass vessel. When the electrons, moving at great speed slammed into glass wall, they produced a very high penetrating radiation - a wholly unexpected occurrence. Roentgen first noticed the radiation when it caused a paper coated with Barium Platino-cyanide to glow. The chemical compound was a standard detector of UV light which causes the chemical to fluorescence i.e. to emit visible light after it has absorbed UV light. But Roentgen's evacuated vessel was tightly covered with black cardboard and so no UV light could emerge from it. The glow must be some other kind of radiation.
When he announced the discovery of the new radiation, Roentgen wrote:
"I posesss, for instance, photographs of ............the shadow of bones of hand, the shadow of a covered wire enclosed in a box.........."
Earlier in the paper, he noted that "the darker shadow of bones is seen with in the slightly dark shadow image of hand itself.
The new radiation quickly became a diagnostic tool in hospitals all over the world. Roentgen could not determine what the rays are made of and thus rays are named as X-rays.
When he announced the discovery of the new radiation, Roentgen wrote:
"I posesss, for instance, photographs of ............the shadow of bones of hand, the shadow of a covered wire enclosed in a box.........."
Earlier in the paper, he noted that "the darker shadow of bones is seen with in the slightly dark shadow image of hand itself.
The new radiation quickly became a diagnostic tool in hospitals all over the world. Roentgen could not determine what the rays are made of and thus rays are named as X-rays.
PHYSICS DICTIONARY
Zeeman
Effect
In a magnetic
field, the energy of a particular atomic state depends on value ‘m’, the
magnetic quantum number. A state of total quantum number ‘n’ breaks up into
several sub-states when the atom in the magnetic field and the energies are
slightly more or less than energy of state in the absence of magnetic field.
This phenomenon leads to splitting of individual spectral lines when atoms
radiate in magnetic field. The spacing of lines depends on magnitude of
fields.
Zero
Point Energy
Energy possessed by atoms or molecules even
at absolute zero.
Zeroeth
Law of Thermodynamics
This law was
first enunciated by R H Fowler in 1831. According to this law, when two systems
A and B are in thermodynamic equilibrium with another system C, then A & B
will also be in thermal equilibrium.
Zone
Plate
The optical
device which verifies rectilinear propagation of light approximately by wave
theory.
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