Our everyday domestic experience is likely to lead us to believe that there is nothing more to starting a motor than closing a switch, and indeed for most low-power machines (say up to a few kW) – of whatever type – that is indeed the case. Although starting the motor directly has advantages, it also has disadvantages.
Advantages of DOL starter
The benefits of a DOL (direct on line) starter are:
- DOL starter has the shortest starting time compared to other starting methods.
- It is a cheap solution. Because it does not require so many components.
- It is very easy to implement.
- Troubleshooting a DOL starter is very easy.
- It provides 100% torque at starting. If the high starting current does not affect the power supply system and the machinery will stand the high starting torque, then full-voltage starting may be acceptable.
- It is compact in size.
- Smaller sizes of DOL starters are manually operated; larger sizes use an electromechanical contactor to switch the motor circuit.
- Only one set of cables is required from starter to motor.
- The on-off switches can be installed away from the motor.
- It reduces the time to reach a steady state.
Disadvantages of DOL starter
Drawbacks of a DOL (direct on line) starter are:
- The starting current is very large, normally 6 to 8 times the rated current.
- A high starting (inrush) current causes a voltage drop in the power supply, which can cause interference in other equipment connected to the same installation.
- It has a high starting torque. It could cause heavy wear & tear on couplings, gearboxes, etc.
- The only possible way to stop the motor is to make a direct stop. The direct stop may not be preferred in pump and conveyor systems.
- Magnetization peaks can occur. (up to 20 times the rated current)
- It could oversize cables and contactors compared to other starting methods.
- Limitation in the number of starts/hours.
- Only suitable for motors of rating less than 5…10 kW