Question { Supertech, 168421 }

Why is starter necessary for a three-phase induction motor?

Answer

Starter is used to reduce starting current.starting current
is usually large because of inertia,charecteristics of
motor winding(at starting slip=1,rotor resistance low
implies less back emf).So at starting we use starter .for
example.STAR DELTA starter.by using a starter at least the
switching is improved which is much important for safe and
longer life time use.simply a DOL can be used for lower
rated motors (up to 07 HP)in this case.for higher rated
motors (up to 15 HP)we can use a star delta connection.a
delta connected motor takes 1.7 times line current than a
star connectd motor.think ! at starting the current is
almost halved .Now a days we use drives (power electronics
products which not only takes care of starting of a motor
(improving power factor) also controls it as perrequirement.

Question { Emerson, 5309 }

What is d diff. btw Online & Offline UPS???

Answer

Besides above mentioned answer (1) I would like to explain some points--
1)Using a online UPS is advantageous for us because of interruption of line supply.As mentioned after main supply fail we use power from battery...........this is a problem :the time it takes to changeover .for example,if our system is much sensitive we may get interruption in getting supply. Normally the the change over time is in order of few millisec.

2) Online UPS is observed to be more economical for higher rated UPS.

Question { 4880 }

Why most of analog o/p devices having o/p range 4 to 20 mA and
not 0 to 20 mA ?

Answer

Thank u Mr.EKailash .I think it is required answer.