A DOL motor starter also contains protection devices, and in some cases, condition monitoring. Smaller sizes of direct on-line starters are manually operated;. As the name suggests, direct-on-line starting means that the motor is started by often in the form of a hand-operated motor protection circuit breaker. Star-delta starter curve – Synchronous speed / Full-load torque. The simplest form of motor starter for the induction motor is the Direct On Line starter. The Direct On Line Motor Starter (DOL) consist a MCCB or Circuit Breaker, Contactor and an overload relay for protection. Electromagnetic contactor which can be opened by the thermal overload relay under fault conditions.
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The value of back emf E depends upon speed Ni.
E is directly proportional to N. At starting, the value of E is zero. So starting current is very high.
Direct Online Starter or DOL Starter
In a small rating motor, the rotor has more considerable axial length and small diameter. So it gets accelerated fastly. Provided the torque developed by the motor exceeds the load torque at all speeds during the start cycle, the motor will direct online starter operation full speed.
If the torque delivered by the motor is less than the torque of the load at any speed during the start cycle, the motor will stops accelerating.
How do direct-on-line (DOL) and star-delta motor starting compare? – Quantum Controls' Blog
The motor will draw a very high inrush current for a very short time, the magnetic field in the iron, and then the current will be limited to the Locked Rotor Current of the motor.
The motor will develop Locked Rotor Torque and begin to accelerate direct online starter operation full speed. The actual starting current curve is a function of the motor design, and the terminal voltage, and is totally independent of the motor load.
If the direct online starter operation delivered by the motor is less than the torque of the load at any speed during the start cycle, the motor will stops accelerating.
If the starting torque with a DOL starter is insufficient for the load, the motor must be replaced with a motor which can develop a higher starting torque.
How does a DOL starter work? - Quora
Direct online starter operation acceleration torque is the torque developed by the motor minus the load torque, and will change as the motor accelerates due to the motor speed torque curve and the load speed torque curve.
The start time is dependent on the acceleration torque and the load inertia. DOL starting have a maximum start current and maximum start torque.
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This may cause an electrical problem with the supply, or it may cause a mechanical problem with the driven load. So this will be inconvenient for the users of the supply line, always experience a voltage drop when starting a motor.
But if this motor is not a high power one it does not direct online starter operation much. Parts of DOL Starters: Magnetic contactors are electromagnetically operated switches that provide a safe and convenient means direct online starter operation connecting and interrupting branch circuits.
The three main contacts M and the auxiliary contact A are closed. The terminals a and b are short-circuited. The motor is then connected to the supply mains.
Direct Online Starter or DOL Starter
Overload relays are renewable and can withstand repeated trip and reset cycles without need of replacement. Overload relays cannot, however, take the place of over current protection equipment.
The overload direct online starter operation consists of a current-sensing unit connected in the line to the motor, plus a mechanism, actuated by the sensing unit, which serves, directly or indirectly, to break the circuit. Overload relays can be classified as being thermal, magnetic, or electronic: As the name implies, thermal overload relays rely on the rising temperatures caused by the overload current to trip the overload direct online starter operation.
Thermal overload relays can be further subdivided into two types: Magnetic overload relays react only to current excesses and are not affected by temperature.