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 .

Posted by: Electrotechnik

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.

Posted by: Electrotechnik

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.

Posted by: Electrotechnik

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.  

Posted by: Electrotechnik

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