This is one of the simplest method of starting motors by connecting them directly to the supply using a single contactor.
Note that this method is limited in starting motors with power less than 10KW because of the high starting current which is about 4 to 6 times the nominal or rated current. In larger motors the Starting current is so high that it may destroy destroy the windings of the motor. So this starting method should not be used to start motors whose power is greater than 10KW. Other starting methods such as the Star-Delta starting, stator resistance starter, rotor resistance starting, star-delta resistance starting and auto-transformer starting can be used to reduce this high starting current which we shall later see.
Now let us look at the direct on line starting.
Now let us look at the direct on line starting.
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| View of a direct on line starter |
Connection of Direct On Line starter
Semi automatic starting method for One direction of rotation of the motor
Materials needs:
So if there is no hold on contact, when the push button is pressed and released, the contactor will just energize and dis-energize immediately the push button is released. Hence, the motor will not run (instant ON and OFf).
- 01 Contactor (KM1)
- 01 Thermal Relay (F)
- 01 Auxiliary contact ( this would not be necessary if you just connect your load directly across the coil of the contactor).
- You could also wish to add indicator lamps ( H1: indicator lamp of the motor's state, H2: indicator lamp for Overload, H3: indicator lamp for presence of supply). This indicator lamps are necessary when the motor control is to be controlled at a distance at a control board. And ease work of the operator and for easy maintenance.
- Three pole Fuse Isolator (Q1).
- Two pole Fuse Isolator (Q3).
- Circuit Breaker (CB)
- 01 stop push button (S1: Normally closed-NC)
- 01 start push button (S2: Normally open-NO)
- A 3 phase motor with power less than 10KW.
- Supply (in our case 3 phase system)
Connection diagram of Direct online stater (DOL)
Now let's see how to connect a DOL starter with the materials listed above.
The circuit is divided into the power and control circuit.
The circuit is divided into the power and control circuit.
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| Power Diagram DOL starter |
Complete circuit (power +control)
Principle of operation of the Direct On line starter:
- When the fuse Isolator and circuit breaker is switched ON (closed). The lamp H3 shines indicating the presence of supply. see the figure below.
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| presence of supply |
- When S2 pressed, current flows to the coil of the contactor (KM1) and the coil energizes.
- As the contactor energizes, it closes its main contacts ( the contacts of KM1 at the power circuit) and closes the hold on contact ( The contacts of KM1 at the control circuit) and we see that the motor is supplied as the contacts are close.
- Note that, as the contactor energizes, it closes it's hold on contact so that current keeps flowing to the contactor and the contactor is continuously in the energized state.
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| start push button pressed to put on motor |
- To stop the motor when running, press the stop push button (S1) and the motor is stopped (gradually goes to rest). For instant braking at stop, a braking mechanism needs to be installed. We shall see this in our subsequent articles.
- In the case of prolonged overload on the motor, the thermal relay is activated and puts of the motor by open the its normally closed contact (F-NC) and puts on the fault indicator lamp by closing its normally opened contact to which the lamp is connected (F-NO). See in the diagram above.
If you have problems identifying the contacts of your components or switch gears.
For forward and reverse rotation direct Online starter semi automatic method.
If you have completely read this article, I believe you can now install a direct Online starter.
Booom Kudos
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Direct Online starter forward and reverse
necessity of DC motor starters
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Star delta starting diagram
Contactor, switch gears numbering and identification
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