Typical circuit diagram of Direct On Line starter A) Main Circuit B)Control Circuit with momentary contact control (Push button) C)Control circuit for maintained contact
Typical circuit diagram of Direct On Line starter
S0 : ‘OFF’ Push button
S1 : ‘ON’ Push button
S : Maintained command switch
K1 : Main contactor
F1 : Main circuit fuse
F2 : Overload relay
F3 : Control circuit fuse
This is by far 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 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.
