Understanding the Direct-On-Line (DOL) Starter in Induction Motor Control
A DOL starter is the simplest and most commonly used method for starting either three-phase or single-phase induction motors in industries and homes. The single-phase motors referred to here include water pumps commonly used in households. They all operate using a DOL starter. DOL starter is use
The DOL starter directly connects the full supply voltage to the motor causing high inrush current to flow and a high voltage drop on the supply lines, which is why it’s called a Direct-On-Line Starter.
Main Components of a DOL Starter
- Contactor
- Overload relay
- Single start push button
- Single stop push button
- Protective devices, such as a three-pole breaker and a control breaker, to safeguard the circuit
- Indicator lights (optional) for monitoring:
- Red: Stop / Not running
- Green: Start / Running
- Amber: Tripped overload or fault
Operation of a DOL Starter
A DOL starter is made up of two main circuits. The control circuit and the power circuit. The control circuit consists of the stop push button, start push button, the coil and the auxiliary or hold on contact of the main contactor (KM), the overload relay contact, and sometimes an indicator lamp for monitoring. The power circuit on the other hand, consists of the main contactor, the overload relay, and the motor terminals. When the main contactor is energized, the three phase supply flows straight to the motor, thereby giving the motor full voltage to start.
Principles of Operation:
When you press the start push button, the normally open contact of the start push button closes. Current now flows from the control live line through the start button, through the stop button which is already normally closed, through the overload relay contact which is also normally closed, then to A1 of the contactor coil. Since A2 is already on neutral, the coil becomes energized.
Once the coil energizes, the hold on contact of the contactor closes and creates another path for current to flow to the coil. So even when you take your hand from the start push button, the contactor will still remain energized. As the contactor is now energized, the power contacts close to apply the full three phase supply through the overload power contacts which are normally closed to the motor, and the motor starts running.
When the stop push button is pressed, its contact opens, cutting the current flow to the coil and the coil de-energizes. As the coil is de-energized, the contactor power contacts open to cut the supply to the motor and the motor stops running. If the overload relay also trips, whiles the motor is running, its normally closed contact in series with the control circuit opens to cut the current flow to coil to de-energize it.
Motor Terminal Block Connection
Direct online starter can be applied to a three phase induction motor having its six leads connected either in star or delta configuration on the motor terminal block. This means that, for you to be able to run the motor directly from the supply, the motor should first be connected in star or delta on the terminal block, then the supply connect to terminals U1, V1 & W1 or U, V, W, depending on the terminal block markings.
On six lead terminal blocks, you will see either (U1, V1, W1 and U2, V2, W2) or (U, V, W and X, Y, Z).
Why the Overload Relay?
The overload relay protects the motor from excessive current caused by:
- Motor overload
- Seized bearings
- Misalignment on couplings
When excess heat is detected due to overcurrent, the relay deenergizes the contactor, stopping the motor to prevent damage.
Why the Circuit Breakers?
Circuit breakers are installed to cut the supply voltage to the motor in case of a short circuit, protecting both the motor and the circuit.
Advantages of a DOL Starter
- Simple design and easy to operate
- Less costly to design compared to other starters
- Provides full starting torque
Disadvantages of a DOL Starter
- High inrush current
- High voltage drop, which can affect other loads on the same supply line
Applications
DOL starter can be applied to a motor from 0.37kW to over 15kW and above depending on the desired application requirement such as:
- Water pumps
- Fans and blowers
- Conveyors
- Other small to medium industrial machines
Learn How to Wire a DOL Starter
Do you want to know how to wire this DOL starter with Indicator Lamps? Click the link below to watch the full tutorial on my YouTube channel:
Do well to subscribe for more videos. There are plenty of other useful tutorials to look out for on the channel!
Related Links
- Understanding the Star/Delta Starter in Induction Motor Control
- Visit my Training page for more learning
Contact Chris Control Automation
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