- What is DOL Starter?
- Connection Diagram of DOL Starter
- Working principle of DOL Starter
- 3-Phase Motor Starter with Overload Protection
- Importance of Overload Protection
- Overload Protection vs. Short-Circuit Protection
- Types of Overload Relays
- Why Overload Protection Important ?
- Advantages of DOL Starter
- Disadvantages of DOL Starter
- Applications of DOL Starter
- Where DOL Starter can be used?
- Comparison of D.O.L Starter and Star-Delta starter?
- Some useful questions and answers on DOL Starter
- What is the benefit of a DOL starter?
- What are the data required to select contractor and overload relay?
- Which type of motor doesn’t need Starters?
What is DOL Starter?
A DOL starter also known as a Direct On-Line starter, is a method of starting a 3-phase induction motor. In a DOL Starter, a three-phase Induction motor is connected directly across its 3-phase supply, and the DOL starter applies the full line voltage to the motor terminals.
Connection Diagram of DOL Starter
1. Main Contact
A Contactor is used to connect Supply Voltage, Relay Coil, and Thermal Overload Relay.
R Phase through MCCB1 of Contactor (NO)
Y Phase through MCCB3 of Contactor (NO)
B Phase through MCCB5 of Contactor (NO)
NO Contact (-||-):
53-54 is a Normally Open NO contact in the contactor (closes when the relay energizes)
NC Contact (-|/|-):
DOL part – Thermal Overload Relay
Normally closed NC contact terminal (opens when the thermal overload relay trips)
2. Relay Coil Connection
A1 of Relay Coil is connected to anyone Supply Phase and A2 is connected to Thermal
overload Relay’s NC Connection (95).
3. Thermal Overload Relay Connection:
Motor terminals (T1, T2, T3) are connected to Thermal Overload Relay
Main Contactor and Motor is connected through Overload Relay
Working principle of DOL Starter
In the DOL starter, main terminals are connected with three terminals of the three-phase power supply, the control circuit is energized.
Depending on the heating of the motor due to the load current thermal overload protection relay operates because the heating effect is directly proportional to load current. When the load current is high the heating effect is also more. Spring-loaded contact in the control circuit trips out only when the bimetallic strip used inside it expands due to heat. This all happens when the amount of load current is too much that it can heat the thermal coil. Relay operating speed depends on current adjustment. The current rating must be 3-5 times the rated current of the motor. Remotely operated DOL starters are also used to control the switching of the motor from any desired place.
3-Phase Motor Starter with Overload Protection
Ensures safe and efficient operation, especially in critical industrial applications.
Understanding Overload
Overload happens when a motor requires more current than its rated capacity to meet excessive load demands.
Prolonged overload conditions can cause the motor to overheat, leading to potential damage.
Importance of Overload Protection
Thermal Overload Protection
Automatically disconnects power to the motor when it overheats due to excessive current.
Prevents motor failure and enhances equipment safety.
Overload Relays
Designed to monitor and regulate motor current, ensuring it stays within safe operating limits.
Open the circuit to stop the motor if the current exceeds a set threshold.
Overload Protection vs. Short-Circuit Protection
Overload Protection
Responds to moderate overcurrents that exceed the motor’s rating over a sustained period.
Does not react to short-duration high currents, such as during motor startup.
Short-Circuit Protection
Provided by devices like fuses or MCBs (Miniature Circuit Breakers).
Designed to handle sudden, extreme overcurrents or short circuits.
Behavior of Overload Protection
Triggers only when overload currents persist for an extended time.
Allows high inrush currents during motor startup without tripping.
Types of Overload Relays
Solid-State Relays (Electronic Relays)
Feature adjustable trip settings for tailored protection.
Offer precise monitoring and enhanced reliability.
Thermal Relays
Use temperature sensors to detect overheating caused by excessive current.
Magnetic Relays
Operate on the principle of current flow and trip during excess current events.
Rating for Overload Protection Devices
For most motors, the overload protection device is rated at 125% of the motor’s full-load current to ensure effective protection without unnecessary tripping.
Why Overload Protection Important ?
Protects motor windings from thermal damage.
Extends the motor’s lifespan and minimizes downtime due to failures.
Advantages of DOL Starter
- Very cheap and economical Starter.
- More comfortable, simple to operate, and control.
- The control circuit is much simpler.
- Very Easy to troubleshoot and easily understandable.
- Direct on-Line starter provides 100% torque at starting.
- From starter to motor only one set of cables is required.
- It connects the supply to the delta winding of the motor.
- The size of DOL is so compact that it occupies very little space
Disadvantages of DOL Starter
- The starting current of the motor doesn’t reduce.
- Direct On-Line starter has very high starting current or too much current 6 to 8 times of full load current.
- Due to the huge thermal stress on the motor, the lifespan of the machine reduces.
- There is a significant dip in voltage in the electrical installation because of too much in-rush current and due to this other customers connected to the same lines affected, it suitable only for small motors.
- The mechanical stress on the mechanical systems is increased because of unnecessary high starting torque, even when not required by the load and this is very dangerous for machine life.
- DOL starter is only suitable for motors of rating less than 10kw.
- DOL starter disadvantage is that it gives the highest possible starting current
Applications of DOL Starter
DOL starters are commonly used for starting small motors in applications like water pumps, compressors, fans, and conveyor belts, where the high inrush current does not lead to significant voltage drops in the supply circuit or where such voltage drops are acceptable.
In asynchronous motors, such as 3-phase squirrel-cage motors, a high starting current is drawn until the motor reaches its full operational speed. Direct-on-line starters are particularly suited for these scenarios, as they effectively manage the starting process for smaller motors in these applications.
Where DOL Starter can be used?
If the high inrush current of the motor does not cause an excessive voltage drop in the supply circuit a direct Online starter can be used.
Comparison of D.O.L Starter and Star-Delta starter?
| S no | D.O.L Starter | Star-Delta Starter |
| 1 | It is used up to 5HP motor only | It is used from 5HP to 20HP motors |
| 2 | It doesn’t decrease the starting current | It decreases the starting current to 1/3 times. |
| 3 | It is cheap | It is costly |
| 4 | The motor is directly connected with DOL Starter with supply for starting as well as for running | It connects the motor first in star connection at the time of starting then it changes to delta connection for running |
Some useful questions and answers on DOL Starter
What is the benefit of a DOL starter?
The benefit of a DOL starter is to avoid phase failure with the help of an overload relay, and it is also used to remote control the motor automatically; the circuit breaker must be connected before – the DOL Starter is connected to the power supply to avoid phase failure.
What are the data required to select contractor and overload relay?
A motor having a nameplate that includes
KW rating of the motor
Operating voltage
Application, KVA code (Available in NEMA code)
Parameters to be considered during the selection
- The full load current of the motor P =√3 V.I. Cosφ kW
- Operational voltage and coil supply voltage
- Type of load and duty cycle
- Breaking capacity of contactor required.
- Overload Relay setting = 100% x FLC (Full load current)
Which type of motor doesn’t need Starters?
Motors below 0.746kw are directly connected without a starter because their armature resistance is very high. They can afford the high current, due to high resistance. So, the Armature winding is safe from the high starting current.