Star-delta starters are widely used for starting three-phase induction motors. It can reduce current inrush to 1/3 of direct-on-line starting current. Here is an advanced Star-Delta starter design tool that will help you select the optimum current rating of contactors, fuses, and circuit breakers for a star-delta starter circuit.
Online star-delta starter design tool
Motor data input
|Enter the motor rated voltage|
|Enter the motor rated power|
|Enter the motor Efficiency||95%|
|Enter the motor Power factor||0.85|
|Motor Starting Current||A|
Star-Delta Stater rating
|Required fuse rating|
|Required Thermal Circuit Breaker rating|
|Main contactor current rating|
|Delta contactor current rating|
|Star contactor current rating|
|*Overload relay is located between the main contactor and the motor.|
How to use the Star-Delta starter design tool?
The above tool requires following data for calculation:
- Motor rated power in HP or kW. (Mandatory)
- Input supply voltage. (Mandatory)
- Motor Efficiency as per nameplate – (if known).
- Rated power factor (if known).
After entering the required data, click on the “Calculate” button to find out the required fuse rating, Circuit Breaker rating, contactor current rating, and overload setting.
Large induction motors, when started directly on line, draw very high currents. Therefore star-delta starters are widely used for motors rated over 10hp or 7.5kW (even small motors can be started used the star-delta method). This can reduce current inrush to 1/3 of direct-on-line starting current.
The power circuit of a star delta starter consists of three contactors, an overload relay, fuse or circuit breaker.
Let’s calculate the size of contactor, fuses, circuit breaker, and an overload relay for 7.5kW, 415 Volt Three Phase squirrel cage induction motor, motor efficiency 98%, 1480 RPM, Power Factor 0.8.
Motor full load Line-current = Motor power in watts / (1.732*Volts*Eff*pf)
Main & Star-delta contactor calculation
A star delta starter consists of three contactors – Main contactor, Delta contactor and star contactor.
- Main contactor rating = 58%X Full Load Line Current = 7.96A
- Delta contactor rating = 58%X Full Load Line Current = 7.96A
- Star contactor rating = 33%X Full Load Line Current = 4.53A
Making/breaking capacity of contactor = 10 x IFLC= 137A
The contactor must be selected such that its rating is higher than the calculated values.
Overload relay setting
The thermal overload relays are installed next to the main contactor before the motor terminals.
Thermal overload relay setting = 58% of IFLC = 7.96 Amp
Fuse rating calculation
Time delay fuses are more preferred and the rating should not exceed 175% of the full load line current.
Time delay fuse rating for star-delta starter = 1.75 * IFLC = 24A (max)
Circuit Breaker calculation
Inverse current circuit breakers are used to protect motors from short circuits. The circuit breaker rating should not exceed 250% of the full load line current.
Circuit breaker rating for star-delta starter = 2.5 * IFLC = 34.33A