The most simple and basic type of starter for motors (induction motor) is the Direct Online starter. The Direct on line starter contains an ON/OFF switch and an OLR. Here, OLR stands for overload relay for protection against excessive load current. The selection of switch depends upon the operator’s requirement.
Generally, the switch which is used with Direct online starter is coil based electromagnetic switch which may be controlled (ON/OFF) by a thermal OLR. However, one can also use manual switch with Direct online starter. If you see the below mentioned figure, here we have used two separate start and stop push buttons for controlling the contactor coil, which further controls the contactors.
Overload protection relay is also provided for protection of the motor against over current. Thus It may be said that we can safely start a small induction motor with this direct online starter. The starting current will not cause damage to the motor unless the motor is frequently started and stopped in quick time intervals.
But if we take the case of large induction motors, we can not use direct online starter to start this. This is because the comparatively high starting current of large induction motors will cause a voltage drop which may further affect the other electrical components which are feeding on the same supply.
Advantages of Direct online Starter:
low cost starter
easy to operate
Easy to trouble‐shoot.
Disadvantages of Direct online Starter:
Unnecessary high starting torque
Does not controls (limits) the starting current .
The motor is subjected to thermal Stress which affects the life of motor.
Whenever the motor is direct on line started, there is always a voltage drop in start due to the starting current. This fluctuation in voltage may harm other electrical equipment feeding on the same supply.