Dear Readers, Welcome to Electrical Machine Design Interview Questions and Answers have been designed specially to get you acquainted with the nature of questions you may encounter during your Job interview for the subject of Electrical Machine Design. These Electrical Machine Design Questions are very important for campus placement test and job interviews. As per my experience good interviewers hardly plan to ask any particular questions during your Job interview and these model questions are asked in the online technical test and interview of many IT & Non IT Industries.
It is the property of the material enabling it to be drawn into wire, with the application of tensile force. It must be both strong and plastic. It is usually measured in terms of percentage elongation and reduction in area, (e.g.) Ni, Al, and Cu.
When a material is subjected to repeated stress, it fails at stresses below the yield point stress; such type of failure of the material is called fatigue.
The ratio between maximum stresses to working stress is known as factor of safety.
Factor of safety = Maximum stress /Working stress
Endurance limit is the maximum value of completely reversed stress that the standard specimen can sustain an infinite number (106) of cycles without failure.
If the time of load application is less than one third of the lowest natural period of vibration of the part, it is called an impact load.
The various phases of design process are:
Recognition of need.
Definition of problem
Analysis and optimization
Different loads on machine components are:
Factors affecting endurance strength are:
Load ii. Surface finish
Types of variable stresses are:
Completely reversed or cyclic stresses
Repeated stress refers to a stress varying from zero to a maximum value of same nature.
Reversed stress of cyclic stress varies from one value of tension to the same value of compression.
In brittle fracture, crack growth is up to a small depth of the material.
In ductile fracture large amount of plastic deformation is present to a higher depth.
Stress concentration is the increase in local stresses at points of rapid change in cross section or discontinuities.
Stress concentration factor is the ratio of maximum stress at critical section to the nominal stress.
Size factor is used to consider the effect of the size on endurance strength. A large size object will have more defects compared to a small one. So, endurance strength is reduced. If K is the size factor, then
Actual endurance strength = Theoretical endurance limit x K
A crack can propagate if the energy release rate of crack is greater than crack resistance.
The different the modes of fractures are:
Mode I (Opening mode) – Displacement is normal to crack surface.
Mode II (Sliding mode) – Displacement is in the plane of the plate.
Mode III (Tearing mode) – Out of plane shear.
While selecting a material for a machine element, the following factors are to be considered
Required material properties
The failure theories are:
Maximum principal stress theory.
Maximum shear stress theory.
Maximum principal strain theory.
The factors involved in arriving at factor of safety are:
Nature of loads
Presence of localized stresses
Mode of failures
Some of the methods are:
Avoiding sharp corners.
Use of multiple holes instead of single hole
Undercutting the shoulder parts.
Notch sensitivity (q) is the degree to which the theoretical effect of stress concentration is actually reached.
The relation is, Kf = 1 + q (Kt-1)
The factors effecting notch sensitivity are:
Size of component
Type of loading
They are used to solve the problems of variable stresses.
It is the property of the material, which refers to a relative ease with which a material can be cut. It is measured in a number of ways such as comparing the tool life for cutting different material.
N curve has fatigue stress on ‘Y’ axis and number of loading cycles in ‘X’ axis. It is used to find the fatigue stress value corresponding to a given number of cycles.
In curved beam the neutral axis does not coincide with the centroidal axis.
C frame, crane hook.
A plane which has no shear stress is called principle plane the corresponding stress is called principle stress.
The bending moment equation is, M/I = f/y= E/R,
M – Bending moment (M is in N-mm)
I - Moment of inertia about centroidal axis (I is in mm4)
f – Bending Stress (f is in N/mm²)
y - Distance from neutral axis (y is in mm)
E - Young’s modulus (E is in N/mm²)
R - Radius of curvature (R is in mm)
The torsional equation is, T/J = q/r = G?/L
T – Torsional moment (T is in N-mm)
J - Polar moment of inertia (J is in mm4)
q – Shear stress in the element (q is in N/mm²)
r - Distance of element from centre of shaft (r is in mm)
G- Modulus of Rigidity (G is in N/mm²)
? – Angle of twist (? is in radians)
L – Length of the shaft (L is in mm)
The profiles used for splines in machine elements are straight sided splines, triangular splines and in volute splines.
Pipe threads are used for pressure tight joints and square threads for power transmission.
Computer is able to store lot of information and same can be retrieved fast.
It performs calculations very fast. The design can be visualized on screen and any alterations made and its effects clearly seen. Thus optimum designs can be achieved much faster and these are thoroughly tested from all angles.
In Hazop (Hazards and operatibility) study each and every conceivable process deviation from normal operating condition of a system having identical design intention, is thoroughly analysed for causes and consequence and required action plan to mitigate hazard/operability problems are suggested on the basis of available protective system.
When length of the member is increased, the member may fail due to buckling even though the member is symmetrical in all respects and the load is concentric.
Allowance in limits and fits refers to maximum clearance between shaft and hole.
Number of strands x number of wires in each strand.
To prevent relative axial movement between the shaft and the bearing.
Maximum shear stress theory.
Uniform wear condition is assumed because it leads to safer design.
Mass and stiffness.
n = 0.2d + 3
By pitch circle diameter.
An increase in the speed of the outer wheel.
Rankine’s theory of failure is applicable for brittle materials. Guest’s theory of failure is applicable for ductile materials.
To vary the torque at the road vehicles.
Endurance limit or fatigue limit is the maximum stress that a member can with stand for an infinite number of load applications without failure when subjected to completely reversed loading.
NPT( National Pipe Threads)