Direct online starter or DOL Starter working principle Control wiring diagram

 Direct On-Line starter (DOL Starter)pdf:

Different starting methods are used for starting induction motors because Induction Motor draws more starting current during starting. Direct On-Line (DOL) starter is simplest starting method for induction motor. Because it's circuit wiring is easy and cheap compare to other motor starting methods. Direct online starter DOL can be used up to 5HP motor
Direct online starter is used for 3 phase and single phase induction motor. Three phase dol motor starter power diagram and control circuit is shown in figure. 
A small motor can be started by simply plugging it into an electrical a switch . A larger motor required a specialized switching unit called a motor starter. When energized, a direct on line (DOL) starter immediately connects the motor terminals directly to the power supply. 

Reduced-voltage,dol starters  connect the motor to the power supply through a voltage reduction device and increases the applied voltage gradually or in steps.

A direct on line (DOL) applies the full line voltage to the motor terminals, This is the simplest type of motor starter. 

A DOL motor starters also contains protection devices, and in some cases, condition monitoring. Smaller sizes of direct on-line starters are manually operated; larger sizes use an electromechanical contactor (relay) to switch the motor circuit.

A direct on line starter can be used if the high inrush current of the motor does not cause excessive voltage drop in the supply circuit.

The maximum size of a motor allowed on a direct on line starter may be limited by the supply utility for this reason.

DOL starting is sometimes used to start small water pumps, compressors, fans and conveyor belts. In the case of an asynchronous motor, such as the 3-phase squirrel-cage motor, the motor will draw a high starting current until it has run up to full speed.

This starting current is typically 6-7 times greater than the full load current. To reduce the inrush current, larger motors will have reduced-voltage starters or variable speed drives in order to minimise voltage dips to the power supply.

A reversing starter can connect the motor for rotation in either direction. Such a starter contains two DOL circuits—one for clockwise operation and the other for counter-clockwise operation, with mechanical and electrical interlocks to prevent simultaneous closure.

For three phase motors, this is achieved by changing  the wires connecting any two phases. Single phase AC motors and direct-current motors require additional devices for reversing rotation.

The disadvantage with DOL starters 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 then DOL starter draw 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.
So, here question arises what is dol motor starter and how dol starter work. Explanation of direct online motor starter with control and power circuit diagram given below. 

  Direct online starter (DOL) Control digram:

DOL control circuit diagram shown below

1) when start button ( normally open NO ) is pressed supply is connected to the motor through power contactor  C1

2) Power contactor coil  get energized (i.e A1,A2) due to this normally open contact C1 (NO) is closed

3) Power supply continue through stop button (NC) - C1 (NO) - power contactor coil (A1 A2)

3) When stop button pressed  supply get disconnected and the contactor coil get de energized  and it open C1 NO  contact i.e 13-14

Power diagram of direct online starter: 

Direct online starter power circuit as follows:
direct online starter power diagram
Figure a) : direct online starter power diagram

  K1 = Main contactor
  F1 = Main circuit fuse
  F2 = Overload relay
  F3 = Control circuit fuse         

Principle of Direct On Line Starter (DOL):

1) To start, the contactor is closed, applying full line voltage to the motor windings. 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 towards full speed.
2) As the motor accelerates, the current will begin to drop, but will not drop significantly until the motor is at a high speed, typically about 85% of synchronous 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.
3) The motor load will affect the time taken for the motor to accelerate to full speed and therefore the duration of the high starting current, but not the magnitude of the starting current.
4) Provided the torque developed by the motor exceeds the load torque at all speeds during the start cycle, the motor will reach 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. 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.
5) The 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.

Component of  DOL starter :

1) Over load relay (OL):
Electric motors need overcurrent protection to prevent damage from over-loading the motor, or to protect against short circuits in connecting cables or internal faults in the motor windings.

The overload sensing devices are a form of heat operated relay where a coil heats a bimetallic strip, or where a solder pot melts, releasing a spring to operate auxiliary contacts. 

These auxiliary contacts are in series with the coil. If the overload senses excess current in the load, the coil is de-energized.
This thermal protection operates relatively slowly allowing the motor to draw higher starting currents before the protection relay will trip. Where the overload relay is exposed to the same environment as the motor, a useful though crude compensation for motor ambient temperature is provided.
The other common overload protection system uses an electromagnet coil in series with the motor circuit that directly operates contacts. This is similar to a control relay but requires a rather high fault current to operate the contacts.

To prevent short over current spikes from causing nuisance triggering the armature movement is damped with a dashpot. The thermal and magnetic overload detections are typically used together in a motor protection relay.
Electronic overload protection relays measure motor current and can estimate motor winding temperature using a "thermal model" of the motor armature system that can be set to provide more accurate motor protection.

Some motor protection relays include temperature detector inputs for direct measurement from a thermocouple or resistance thermometer sensor embedded in the winding.

2) Contactor :

A contactor is a heavy-duty relay with higher current ratings,used for switching electric motors and lighting loads. Continuous current ratings for common contactors range from 10 amps to several hundred amps.

High-current contacts are made with alloys containing silver. The unavoidable arcing causes the contacts to oxidize; however, silver oxide is still a good conductor.Contactors with overload protection devices are often used to start motors.
Contactors come in many forms with varying capacities and features. Unlike a circuit breaker, a contactor is not intended to interrupt a short circuit current.

Contactors range from those having a breaking current of several amperes to thousands of amperes and 24 V DC to many kilovolts.

The physical size of contactors ranges from a device small enough to pick up with one hand, to large devices approximately a meter (yard) on a side.
Contactors can be noisy when they operate (switch on or off), so they may be unfit for use where noise is a chief concern. In such cases solid-state relays are preferred.

3) stop button NC (normally closed type): this button is normally closed type and used to stop moter
4) Start button NO (normally open type): this button is normally open type and used to start motor

 Advantages of direct online DOL Starter

1) Most Economical and Cheapest Starter

2) Simple connect, operate and maintain

3) Simple Control Circuit 

4) It provides 100% torque at the time of starting.

Disadvantages of DOL Starter

1) It does not reduce the starting current of the motor.  

 2) High Starting Current
 3) Thermal Stress on the motor, thereby reducing motor  life.

  4) High starting Torque

  5) DOL starter used upto 5HP motor


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