Star Delta starter is the most widely used application especially for Air-compressor and HVAC system.This starter used for 6 or 12 wire for motor terminal depend on electric motor that chosen for the application.
Basically this starter used 3 unit contactor,1 unit timer delay,1 unit overload relay and start/stop button to control the star delta motor.Around the world,it have many types of control circuit design for star delta.
It have different design and technique for the control circuit but the main objective is same,to reduce starting inrush current.
For safety condition for the star delta control circuit,it include with interlock for star and delta sequence to avoid serious damage for electric motor.That mean during star sequence,contactor for delta cannot energize or vise versa.
This post i want share about two types typically used for star delta starter control circuit.It have a pro and contra for each design and it depend on our application and objective for installation.
Types of Star Delta control circuit
1) Star Delta Open Transition
Firstly i explain why it call OPEN transition because the electrical motor is disconnected from main line when star sequence change to delta for momentarily condition.It include with interlock system for this control circuit.
For this open transition,the advantage is it depend on timing for transition when loading the electric motor.It cause the current surge and the high torque can effected electrical and mechanical due to shock on the application system.
It also can cause the instantaneous current peaks exceed the limit even the locked rotor current for short cycle of rotation.For the electrical aspect it can happen for the instantaneous peaks and could resulted the power fluctuations or losses.
Mechanical aspect it could cause the damage for system component such as drive shaft,bearing or coupling due to torque increased from current spike.
2) Star Delta Close Transition
This CLOSE transition is an alternative type for star delta starter.The method is same for basic operation but it have additional a few devices to reduce and remove the surge in current matter that faced by OPEN transition type.
For this type,the design have the additional device and component such as a magnetic contactor and power resistors. The advantage of this method is it can eliminated the surge in current problem with high efficiency.
It can reduce a damage for motor winding and mechanical part.Theoretically it can make electric motor have a long life span and save cost for replacement part for mechanical items.For the clear comparision between OPEN and CLOSE transition,please refer to the graph below :