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How to Wire Contactor and Motor Protection Switch – Direct OnLine Starter (DOL)
The direct online motor starter is the cheapest and simple construction to start the induction motor. DOL motor starter is not limiting the starting current and its directly start the motor. The Direct On Line Motor Starter (DOL) consist of start and stop control switches , contactor, motor protection switch and miniature circuit breaker.
Connecting of Direct OnLine starter
- Power supply is connected to (NO) Normally open contacts (1,3,5) of motor protection switch,
- Contacts (2,4,6) of motor protection switch are connected to (NO) Normally open contacts (1,3,5) of contactor, also START push button is connected to a A1 terminal of magnetic coil of contactor. Normally open contact (NO) (13) of contactor is also connected to A1 terminal of magnetic coil of the same contactor.
- Contacts (2,4,6) of contactor are connected to electric motor. Stop push button (1) is connected on contacts of START
push button (2) and contact (14) of contractor.
- Control circuit is connected in a way that power supply is connected on Miniature Circuit Breaker (MCB) (1) from Miniature Circuit Breaker (2)to a STOP push Button (2) and from that (1) to a START push button (2).
Working principle of Direct OnLine Starter (DOL)
Most important is control circuit, control circuit works on a principle of self-reliance. Self-reliance is way that current is always present on contact (2) of START push button and contact (14) of contactor. So when we press START push button current goes toward terminal A1 of contractor. Magnetic coil is energized in contactor and contactor closes all contacts.
Also current is coming on terminal A1, from another way, from contact (1) of STOP push button over contacts (13 and 14) ,which are now closed, of contactor. So, when we release START push button current constantly coming to a terminal A1 from contact (1) of STOP push button over contacts (13 and 14) ,which are now closed, of contactor. A2 terminal of contactor is connected on null wire. In this way electric motor starts to work.
If we want to stop electric motor to work, then we must press STOP push button, current will no longer flow over contacts (13 and 14) of contactor, magnetic coil of contactor will not be energized, contactor will release contacts (1/2, 3/4,5/6) and electric motor will stop to work.
Advantages and disadvantages
Direct OnLine Starter is the most common starting method available on the market. 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 the
starting current there also exists a current peak that can rise up to 14 times the rated current since the motor is not energised 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.
REFERNCE for advantages and disadvantages
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