What is direct online starter (DOL Starter) work, Control wiring diagram

 Direct online starter  (DOL Starter)pdf:

So discuss about what is direct online starter and how dol starter works with control diagram:

This is by far the most common starting 

method available on the market since it is a very compact and a very cheap starting 

solution. The starting equipment consists of only a main contactor and a thermal or 

electronic overload relay. The disadvan-

tage with this method is that it gives the 

highest possible starting current. A normal value is between 6 to 8 times the rated motor current but values of up to 14 times the rated current do exist. There is also a magnetization peak that can be over 20 times the rated current since the motor is not energised from the first moment when starting. 

The exact values are dependent on the 

design of the motor. In general, modern 

motors have a higher inrush current than 

older ones because of the lower resis-

tance in the windings.

During a direct on line start, the starting 

torque is also very high, and for most ap-

plications it is higher than necessary. This will cause unnecessary high stress on 

driving belts, couplings and the driven ap-

plication. Naturally, there are cases where this starting method works perfectly fine 

and there is no need to use any other 

starting method.

If starting DOL, the only possible way to 

stop the motor is to make a direct stop.

The disadvantage with DOL starter 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.

  Direct online starter (DOL) Control digram:

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: 

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