Induction motors are the basis of industrial & commercial processes.
However, one of the most significant issues with these motors is the large inrush current during startup, which may be 6 to 8 times the FLC – full load current.
- What is a Star Delta Starter?
- Why is Star Delta Starter preferred for Induction Motors?
- 1). Reduced Starting Current
- 2). Reduced Mechanical Stress
- 3). Improved Electrical Safety
- 4). Economical & Simple Design
- 5). Typically used for High Power Motors
- Where Star Delta Starter is not Suitable?
- Conditions for using Star Delta Starter
- Comparison Table: Star Delta vs DOL Starter
This sudden surge has the potential to cause damage to electrical components, result in drop in voltage, and reduce the lifespan of the motor.
A wide-ranging selection of motor starting methods are applied in order to overcome this issue.
One method that is considered to be among the most popular is the Star Delta Starter.
Let’s have a look at the reasons why it is advantageous for induction motors.
What is a Star Delta Starter?
A Star Delta Starter is a lower voltage starter that limits the starting current of 3 phase induction motors
It initially links the motor windings in a star (Y) configuration, lowering the voltage per winding.
After a predetermined time, it converts to a delta (Δ) arrangement, allowing full voltage & torque.
Working Principle:
At the start of a star connection, the line voltage is divided by √3 (~0.577), resulting in a reduction in current.
Delta Connection After Startup: Normal operation requires full line voltage.
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Why is Star Delta Starter preferred for Induction Motors?
The star delta starter is one of the most utilized starting methods for 3-phase induction motors, particularly in industrial applications.
Its popularity arises from its capacity to decrease the initial surge of the current & mechanical stress during motor startup, thereby protecting the motor while simultaneously improving system dependability and operating efficiency.
- Reduced Starting Current
- Reduced Mechanical Stress
- Improved Electrical Safety
- Economical & Simple Design
- Typically used for High Power Motors
1). Reduced Starting Current
During startup, an induction motor draws six to eight times its nominal full-load current.
This inrush current can stress electrical components and produce voltage drops in the supply network.
The star delta starter solves this problem by initially connecting the motor windings in a star (Y) arrangement.
This design reduces the voltage provided to each phase of the motor by a factor of 1/√3 (or ~58%), considerably lowering the starting current approximately one-third that drawn in the direct-on-line (DOL) starting.
After a few seconds, the motor is shifted to a delta (Δ) mode and runs at full voltage.
- Reduces phase voltage (Vph) to around 58% on startup.
- Reduces high inrush current.
- Protects upstream electrical components.
2). Reduced Mechanical Stress
A rapid full-voltage start not only affects the electrical system, but it also puts mechanical stress on the motor & driven equipment.
The star delta starter produces smoother acceleration by eliminating torque spikes, which could otherwise cause jerking, wear, & misalignment in the mechanical components such as shafts, gears, and couplings.
This gradual start improves the mechanical life of the engine and the equipment it powers.
- Minimizes shock load on each mechanical components.
- Increases coupling & bearing life.
- Suitable for low (or) no load starting circumstances.
3). Improved Electrical Safety
The reduction in the starting current improves electrical safety by reducing voltage drops across the system, which is especially essential when several motors or sensitive equipment use the same power network.
It also decreases the possibility of the nuisance tripping of safety devices such as
- Breakers and
- Relays,
improving the overall reliability of the electrical system.
- Prevents voltage fluctuations in the electrical system.
- Prevents nuisance tripping when the motor starts.
- Protects the cables, switchgear, & relays.
4). Economical & Simple Design
When compared to more advanced starting methods like
the star delta starter is a less expensive and theoretically simpler option.
It uses only three contactors, a timer, & an overload relay, making it simple to install, operate, and maintain.
This makes it appropriate for industries that seek a dependable solution without a large initial investment.
- A low-cost alternative for motor starting.
- Simple wiring & control logic.
- Simple to debug and maintain.
5). Typically used for High Power Motors
Star delta starters are commonly used for motors with power ratings greater than 5 HP (3.75 kW) & up to 200 HP (150 kW), if the starting torque required is low.
This makes them appropriate for light to medium initial load applications such
- Air compressors,
- Centrifugal pumps,
- Blowers,
- Fans, &
- Conveyors.
These are frequent systems in which the motor starts under no or partial load conditions.
- Ideal for motors operating in light-load applications.
- Ideal for the ratings between 5 HP and 200 HP.
- Widely utilized in the HVAC, water treatment, & manufacturing industries.
Where Star Delta Starter is not Suitable?
Although star-delta is important in a number of factors, it is not appropriate in the following conditions:
- Increased starting torque is needed for the motor.
- The motor is less than 5 horsepower (use DOL instead).
- The application process requires a lot of starting and stopping again.
Conditions for using Star Delta Starter
| Parameters | Requirements |
| Motor Type | Squirrel cage, 3-phase induction motor |
| Motor Rating | Typically > 5 HP (3.75 kW) |
| Starting Torque Requirement | Low to medium |
| Load Type | Non-heavy load during start-up. |
| Supply Voltage | Usually 400V (or) 415V |
| Availability of 6 Motor Terminals | Must be accessible for the Star/Delta wiring |
Comparison Table: Star Delta vs DOL Starter
| Features | Star Delta Starter | DOL Starter |
| Starting Current | 2-to-3 times Full Load Current (FLC). | 6-to-8 times Full Load Current (FLC). |
| Starting Torque | 33% of the full torque. | 100% of the full torque. |
| Cost | The cost is moderate. | The cost is low. |
| Control Complexity | Moderate. | Simple. |
| Mechanical Stress | Very low mechanical stress. | Very high mechanical stress. |
When it comes to starting 3-phase induction motors in the industrial settings, the Star Delta Starter continues to be one of the most popular methods.
In addition to providing a dependable, risk-free, and effective method for lowering starting current & mechanical stress, it strikes a balance between performance and economy.
However, prior to choosing this starter, it is necessary to conduct an analysis of the application load, starting torque & motor rating.
When the need includes a strong torque or gradual acceleration, variable frequency drives (VFDs) or soft starts can be the preferable option.
