Understand Star Delta Starter: Advantages and Disadvantages

Most of the machines used in the industries are three-phase induction motors. They have simple and rugged construction and their robust nature make them possible to operate in all environmental condition. Induction machines are cheaper in cost and maintenance-free, have starting torque, and are widely used in domestic and industrial applications. During starting of an induction motor, the starting current is around eight to ten times the rated current and this persists for a few cycles. This may be very much damaging for the machine, causing a disturbance of voltage on the supply lines due to large starting current surges. To limit the starting current surges, large induction motors are started at reduced voltage and have full supply voltage reconnected when they run up to near rotating speed. Star delta starter is the most common starting method that reduces the starting current and starting torque. Although the star delta starter has many advantages for the system, it has also some disadvantages. Let’s check them one by one.

Advantages of Star Delta Starter

The benefits of star delta starter are:

• Star delta starter is probably the most common reduced voltage starters. It is used in an attempt to reduce the start current applied to the motor during start as a means of reducing the disturbances and interference on the electrical supply.
• It reduces the mechanical stress on the motors hence less motor burning is caused.
• The operation of the star-delta method is simple and rugged.
• The star delta starter is manufactured from three contactors, a timer, and a thermal overload. The contactors are smaller than the single contactor used in a direct online starter as they are controlling winding currents only. The currents through the winding are 58% of the current in the line.
• The starting torque is reduced to about 33% of the torque available at a direct online start.

Starting Type	% Voltage Applied During Start	% Full Load Starting Torque	% Full Load Rated Current
Full Voltage	100	100	600
Star Delta Starting	58	33	200

• Traditionally in many supply regions, there has been a requirement to fit a reduced voltage starter on all motors greater than 5HP (4KW). The star delta (or Wye/Delta) starter is one of the lowest-cost electromechanical reduced voltage starters that can be applied.
• This starting method only works when the application is lightly loaded during the start. If the motor is too heavily loaded, there will not be enough torque to accelerate the motor up to speed before switching over to the delta position.
• Star delta starting method had the least power quality problems occur. Although these power quality events occur, it is not capable enough to trip the contactor or fuses.
• There are two contactors that are close during the run, often referred to as the main contractor and the delta contactor. These are AC3 rated at 58% of the current rating of the motor. The third contactor is the star contactor and which only carries star current while the motor is connected in star. The current in star is one-third of the current in delta, so this contactor can be AC3 rated at one-third (33%) of the motor rating. By using smaller contactors compared to other starting methods, you can reduce the initial cost.
• It generates less heat compared to other starting methods.

Disadvantages of Star Delta Starter

Drawbacks of star-delta starter are:

• The motor must be delta connected during a normal run, in order to be able to use this starting method.
• When starting up, the load torque is low at the beginning of the start and increases with the square of the speed. When reaching approximately 80-85% of the motor-rated speed the load torque is equal to the motor torque and the acceleration ceases. To reach the rated speed, a switch over to delta position is necessary, and this will very often result in high transmission and current peaks. In some cases, the current peak can reach a value that is even bigger than for a DOL starter.
• Applications with a load torque higher than 50% of the motor-rated torque will not be able to start using the star-delta starter. It does not work well in heavy-loaded conditions.
• It has a long starting time compared to the DOL starter.
• The soft stop function is not possible like soft starters or VFDs. The motor stops directly.
• The components normally consist of three contactors, an overload relay, and a timer for setting the time in the star position (starting position). That’s why so much space and wiring are needed in the electrical panel. It requires two sets of cables from the starter to the motor.
• Starting torque is reduced but it cannot be adjusted.