Why should the dc series motor never be run without any load ?

The dc series motor develops a very heavy torque during start-up. The motor relies on the connected load to restrain the speed.

Hence, if the dc series motor is run without any load. it may result in overspeeding which can cause serious damage to the motor .

Comparison of Single Phase vs Three Phase Motors

Single Phase Motors	Three Phase Motors
Can be operated anywhere as Single phase supply is Widely Available	Can be used only when three phase supply is available.
Efficiency is lower as a single winding has to carry all the current.	Efficiency is Higher as Three windings carry the current
Torque is not uniform	Torque is uniform
Larger in Size for the same kW rating	Smaller in Size
Cost is cheaper at the fractional kW range due to high volume of production	Cost is higher at low kW range.
Cost is higher at high kW ratings	Cost is cheaper at lower kW range.

What is the permisible amount of current which can pass through the human body

When a current passes through a human body, it can cause damage in the form of burns.  The burns can also happen in the internal organs as the current passes through the body.  The resistance offered by the body is only by the skin.  Blood, being an electrolyte,  is a good conductor of electricity.

The normal limits of current and the effects they produce are as follows.

1mA - Slight Tingling Sensation

1mA -6mA- This is the let-go current when it is possible to let going of a live object.

9mA to 25 mA - Loss of Muscular control.  In this range, it is not possible to let go of the live object

>25mA - Muscles controlling breathing are affected.

50 - 100mA - Fibrillation.  The electric rhythm of the heart is disturbed. The heart stops.  This is a life threatening condition.

Semi Conductor Fuse - an overview

Semiconductor fuses serve to product semiconductor components such as diodes and triacs. These fuses are made specially to act quickly within milliseconds to isolate the electronic component.  

Semiconductor fuses are made of silver strips.  The cross section of these strips determines the fuse limits.  A number of strips may be connected in parallel to provide the desired fuse rating. The strips are enclosed in a tubing containing quartz which serves to quench the arc. 

Semiconductor fuses are designed to act at overloads of 5 to six times the rated capacity.   These fuses are available at MV ratings with a capacity of several thousand amperes.  

What is Arc Blow in Welding

Arc Blow in Welding refers to the phenomenon in which the arc does not trace the shortest path between the electrode and the workpiece.  The arc gets deflected away from the desired path.  This is due to the interaction of the magnetic field of the welding current with residual magnetic field which may be present in the metal.

Arc Blow occurs only in DC welding.  In AC welding, since the current and the magnetic field of the arc reverse direction many times a second, there is no net deflection of the arc.

What is meant by Stacking Factor of the Core in Electric Machines ?

The Stacking factor of the core refers to the thickness of the laminations in the core.  In Electric Machines, particularly AC machines, the core of the machine is made of a number of laminated steel sheets.  This is to prevent losses due to eddy current.

The stacking factor depends on the thickness of the lamination of the steel sheets which comprise the core.

The stacking factor is usually 0.9.  The stacking factor is always less than 1.

The Net Area of the core which forms the magnetic path can be calculated from the gross area by the following relation

The Net Area of the Core = Stacking factor x Gross Area of the Core

Why is the LV winding of the Transformer placed near the core in core type Transformers ?

In core type transformers, The LV winding of the Transformer is placed near the core in order to reduce the cost of insulation and the size of the Transformer.  If the HV winding of the Transformer is placed near the core, the insulation would have to be thicker leading to higher cost.

Placing the HV winding after the LV winding enables a much lesser thickness of insulation for the HV winding.

In Shell type transformers, the winding is interleaved.  That is, the LV and the HV windings are placed alternately to reduce the leakage flux.