Direct on line starter
What is DOL starter or Direct on line starter?
Around the world it famous with name DOL starter or direct on line starter.This is the most common starting method available on the market and at basic stater for all motor application.The starting equipment consists of only a main contactor and thermal or electronic overload relay.The disadvantage with this method is that it gives the highest possible starting current.
A normal value is between 6 to 7 times the rated motor current but values of up to 9 or 10 times the rated current exist.Besides then starting current there also exists a current peak that can rise up to 14 times the rated current since the motor is not energized from the the first moment when starting.
The values are dependent on the design and size of the motor, but in general, a smaller motor gives higher values than a larger one.
During a direct-on-line start, the starting torque is also very high, and is higher than necessary for most applications.The torque is the same as the force, and an unnecessary high force gives unnecessary high stresses on couplings and the driven application.
Naturally, there are cases where this starting method works perfectly and in some cases also the only starting method that works.
For more detail,please refer the MOTOR CONTROL HANDBOOK :-
A 15kW compressor motor use DOL starter and MCB rating is 40Amp. Is it OK?
Dear Yushar,use this formula to calculated the ampere,
kW = I x V x 1.732
I = 15kW / 415 x 1.732
I = 20.86 @ 21
MCB rating = 21 x 0.25 + 21 = 26.5 amp ( safety factor )
u can used 40 ampere.it ok..no problem..u also can use 32 ampere of MCB.
sir if we use 40A and 32A mcb in the dol. so what is the meaning of breaking. and in the over load motor does not trip…that is wrong sir
if i m wrong pls clearify
i ask u apersonal question u r indian @admin
Amazing info. I’ve bookmarked this web site.
Thanks thella :D
What is the type of MCB (B,C,D etc) recommended in the above calculation?. What is the reference (book)used to select In+0.25xIn as the rating of MCB?. A MCB manufacturer says you should refer the tripping characteristics of MCB(type B,C,D) and make sure that for the starting current (say 6In)and duration of starting(say 4secs) the MCB will not trip. Does your formula satisfies this principle.
Dear sir,please refer to my last post :
1) miniature circuit breaker MCB
2) MCB sizing
I’m young and fresh graduate of electrical engineering who wish to know more and promote techology inthe nworld.
Please reply on how I can get a copy of this text-books