Induction motor starters
Starters; as name suggests; not only start the motor (Induction motor). But it provides protection from overload conditions. Starters connects the power supply to the motor either in a full form or either in a proportion.
Two starters, “DOL “and “START-DELTA” ; most widely used starters for Induction motors.
DOL starters is utilized for low rating Induction motor. While star-delta starters are used for high rating Induction motors. This post is specially dedicated to the differences between DOL starter and Star Delta starter.
For better understanding, Before starter, We should discuss the winding configurations first.
Winding configuration
3 Phase winding is a combination of 3 coils. Each coil has 2 node ends. By connecting these 3 coils with having total 6 nodes; 2 nodes each; A winding configuration can get made. Configuration can be a “STAR “or can be a “DELTA“.
For better understanding about star and delta, A GIF is discussed below :
- By connecting all the three coils in a series manner, A Delta connection get made. In these connections, Each coil get a line voltage.
- By connecting one node of each coil together, A junction is get created. That overall Junction formation actually a Star winding configuration. The mid point of that star winding is actually a “Neutral “.
POST CONTAINS :
DOL Starter
DOL stands for “direct online starter“. In this starter, Motor get directly connected to rated voltage without any change in voltage, frequency and winding connections.
Just one push on a button, A rated voltage get applied to the motor and motor starts with high starting torque at high starting current.
DOL starter actually works on a latch circuit, just one click and than it get Hold. Starters stops in 2 conditions either it gets trips or either it’s stop button is get pressed.
DOL can be implemented on both the windings “star“ and “delta“.
Components of DOL starter
- Thermal overload relay-1Nos.
- Contactor– 1Nos.
- Push button assembly- 1Set
An image of DOL starter fitted inside the panel is shown below. Which contains A contactor, Thermal overload relay and Back end MCB. Starter rating is 2.2KW.
Advantages of DOL starter
- Compact
- Easy circuit
- Minimum components
- Easy and quick maintenance
Applications of DOL starter
- Small blower
- Pumps
- Motors less the 5Hp
- Submersible pumps less than 5hp
Star delta starter
In star-delta starter, Winding configuration changes from star connection to delta connection. At the time of staring, Motor runs in “star connection” While after some time when motor achieves its 75% speed, connection get shifted to “delta“.
In star connection, winding of a motor get per phase voltage across each set of coil. While in delta connection, Each set of coil get line voltage. Due to low voltage in star winding connection, Motor takes current 1/3rd current in comparison to delta connections.
Components of Star delta starter
- Contactors- 3Nos.
- Thermal overload relay- 1Nos.
- Timer-1Nos.
- Push button assembly- 1Set
A image shows the total electrical components connected for 37KW 3 phase Induction motor. Starter contains 3 contactors ML-4, 1 MCCB 125Amps, 1Star delta timer by GIC, 1 control MCB.
Advantages of Star delta starter
- Applicable for High rated Induction motors
- Low starting current
- Low starting torque
Why using Star delta starter
- Induction motor takes high current at the time of starting. Because at the time of starting, There is no back emf.
- Also at the time of starting there is no moment of inertia.
- If a load is directly connected to motor shaft, than situation get worse more.
All above situations makes the starting current much higher than the rated current of Induction motor. Which can be multiple times the rated current of motor.
High current produces excess stress on the winding. Which further reduces the overall life of a motor. High starting current and torque not only harmful for the motor, But in along; it is harmful for overall connected mechanical arrangement.
To reduce the high starting current and high starting torque, Star-delta starters are used. As discussed, Star winding works at starting which takes 1/3rd current compare to delta winding connections. Current and torque both get reduced.
Applications of Star delta starter
- For Large pumps and submersibles
- Heavy Machines
- Blowers
Circuit Diagram for DOL and Star delta starter
Circuit diagram for DOL
In an image below, left side of an image contains control circuit. Which contains control mcb at start, than nc contact of thermal overload relay, than nc contact of stop button and then there is no contact of start button. For holding, no contact of contactor is connected parallel to no contact of start button.
Right side of an image contains a power diagram. Which contains mcb at start and than there is contactor and at last motor terminals are connected. Contactor contains one side 3 phase power while another side contains motor terminals.
Circuit diagram for Star delta starter
If circuit coil is 230V AC than X1 should be connected to neutral. If circuit coil is 440V AC than x1 should be connected to one of the phase. While another X2 terminal is used in controlling.
- X2 phase passes through the NC of stop button after that it is connected to NO of start button.
- When start button is pressed, supply passes through the NC of Delta then NC of timer & then connected to the timer. In parallel with timer, Star contactor in series with NC of main contactor is connected.
- Timer & Star contactor get energized. Star contactor get a HOLD from its 43-44 NO which is connected in series with NC of Main contactor.
- After the star contactor, 23-24 NO of Star get closed which results main contactor(H) get energized. Main contactor get a Hold from its 43-44 NO .
- According to the set time on timer; When timer relay operates its 4-5 NC get changed & Star contactor & timer both cut out from the supply. In the mean time, Main contactor still energized.
- After the cut out of star contactor & timer, Delta contactor get a supply through NC of Star & NC of timer.
- This circuit; Main contactor & Delta contactor hold until 95-96 NC stop button is pressed or motor trips or power supply get cut out. Actually, the overload relay also using the same 95-96 NC in tripping.
Comparison between DOL and Star delta starter
| PARTICULARS | DOL STARTER | STAR DELTA STARTER |
| CONNECTION CHANGE | No | Yes |
| NUMBER OF WIRES FROM MOTOR | 3 | 6 |
| WINDING TYPE | Can be a Star or Can be a Delta | Both works; Starting with Star and than Delta |
| VOLTAGE CHANGE | No | No |
| POWER RATING | For low rating | For high power rating |
| TIMER FUNCTION | No | Yes |
| NUMBER OF CONTACTORS | 1 | 3 |
| COST | Comparatively lower | Comparatively Higher |
| TORQUE (STARTING) | High | Low |
| STARTING CURRENT | High | Low |
| PUSH BUTTON | 1 push button assembly (On-OFF) | 1 push button assembly (On-OFF) |
| ADVANTAGE | Easy, Cheaper | Bit complicated, Costly |
| APPLICATIONS | For motors; less then 3.7KW; Generally | For motors; Above than 3.7KW; Generally |
| MAINTENANCE | Easy, Cheaper | Bit complicated, Costly |
| STARTER SIZE | Compact | larger |
Which Starter is Better: DOL or Star Delta starters?
This section often ranks well because people search:
“Which starter is better?”
- Up to 5 HP → Usually DOL
- Above 5–10 HP → Consider Star Delta
- Depends on supply capacity and starting current limitations
Frequently Asked Questions (FAQs) related to DOL and Star Delta starters
What is the main difference between a DOL starter and a Star Delta starter?
A DOL starter applies full line voltage directly to the motor, while a Star Delta starter initially reduces the voltage and current during starting by connecting the motor in star configuration before switching to delta.
Which starter has a higher starting current, DOL or Star Delta?
A DOL starter has a much higher starting current because the motor receives full supply voltage immediately. A Star Delta starter reduces the starting current significantly.
Which starter provides higher starting torque?
A DOL starter provides higher starting torque since full voltage is applied to the motor at startup. A Star Delta starter provides lower starting torque.
Why is a Star Delta starter used instead of a DOL starter?
A Star Delta starter is used to limit starting current, reduce voltage drop in the power system, and minimize mechanical stress on the motor and connected equipment.
Can a Star Delta starter be used for all induction motors?
No. A Star Delta starter can only be used with motors designed to operate in delta connection at the rated supply voltage and having six motor terminals available.
Which starter is more economical?
A DOL starter is more economical because it requires fewer components, simpler wiring, and lower installation costs.
Which starter is suitable for small motors?
A DOL starter is generally preferred for small motors where the high starting current does not cause significant voltage drop or system disturbances.
Which starter is suitable for large motors?
A Star Delta starter is commonly used for medium and large motors to reduce the starting current drawn from the supply.
Does a Star Delta starter reduce power consumption?
No. A Star Delta starter only reduces the starting current during motor startup. It does not significantly reduce power consumption during normal operation.
What are the disadvantages of a Star Delta starter?
The main disadvantages include lower starting torque, more complex wiring, additional components, higher cost, and the possibility of current surges during the star-to-delta transition.
What happens if a large motor is started with a DOL starter?
The motor may draw a very high inrush current, causing voltage dips, tripping of protective devices, or stress on the electrical system.
Which starter is better, DOL or Star Delta?
Neither is universally better. DOL starters are ideal for small motors requiring high starting torque, while Star Delta starters are preferred for larger motors where starting current must be limited.