Today we have more methods of reducing the “pressure” of the current when starting motors with medium power as well as ways to increase the durability of the motor in one way or another. The star-delta starting method is one of the smart choices in reducing the “pressure” of the current, prolonging the life of the motor, which cannot be ignored for those of you who are working in the field. In this article, I will share all the basics of star triangle circuit. Surely this article will bring you useful information. Let’s follow the article below:
1. The concept of Star-Delta Starting
The Star/Delta Starting is a circuit that is composed and connected to each other from 3 resistors. Because the connected circuit is shaped like stars or delta, it is called star triangle. As the picture above we can also easily imagine this name. However, in reality, the circuit will be much more complicated. Requires you to have professional knowledge to operate and connect properly. However, don’t worry too much, after reading this article, you will have more confidence to manipulate it yourself.
2. The main use of the Star-Delta control circuit
+ To answer the question of which method of starting an electric motor can minimize the value of the starting current; does not affect other devices much; The relatively low cost is the use of star/delta starters. As you all know, equipment is used in production when it is connected to a power source. When starting up, the current will increase many times compared to other common electrical devices (from five to the rated current). That is also the cause of voltage drop in the power system.
+ This seriously affects the life of the engine. Along with that is the operating cost, depreciation of tools and tools that enterprises will have to spend more. Therefore, the star-delta starting method can ensure the life of the motor and minimize the voltage drop in the electrical system. Star triangle starter is widely applied in industrial field. Specifically for motors with a capacity of 11kW to 110kW.
3. Structure of Star-Delta starter circuit
A complete star delta circuit for a three-phase asynchronous motor requires additional devices such as aptomat, timer, contactor, interlock switch, overheat relay:
- APTOMAT: is a device to turn on and off electricity. We should choose aptomat that is suitable for the capacity of the engine. For example, for a motor with a large load, it is advisable to choose an aptomat with a rated current twice the motor load current. As for the motor with a small load, the aptomat current rating is close to 1.5 motor load current.
- Thermal relay: used to protect electricity from overloading when the current increases suddenly.
- Timer is used to set the time at an appropriate time to switch from star to triangle mode.
- Contactor used to switch equipment including contator with triangle mode; main contactor; contactor in star mode.
4. Differentiate between Star and Delta Wiring
+ Star Type:
As shown above, we can easily understand how to fight in star pattern. Specifically, 3-phase AC consists of three phase wires with symbols: R (red), S (yellow), T (blue). We connect in turn order with the motor’s windings: W1, V1, U1. And U2, V2, W2 will be connected together and connected to the three contacts of the motor winding.
+ Delta Type:
Contrary to the star connection type, the triangle connection type we will connect the contacts of the coil together as shown above. Then just connect the 3 phase power in the order as below:
- Connect R (red) the first fire phase to W1 and V2.
- Connect T (blue) third phase of fire to U1 and W2.
- Connect S (yellow) phase of the fire to V1 and U2.
5. Working principle of star-delta starter circuit using 3 contactor
+ The star-delta starter circuit has a relatively simple and understandable operating principle. When the MCCB is closed, three indicator lights are on, indicating that power is ready to be supplied to the switching circuit.
+ When we press the On button at contact 13-14, then A1-A2 will be energized and Contactor K at contact 13-14 will close. Star mode is set up as soon as S (A1-A2) operates control. starter circuit.
+ About 1 preset time at Timer T. T contact (55-56) opens causing S (A1-A2) to lose power. The T contact (67-68) closes and supplies power to the TG. The triangle contactor TG is closed. The star contactor S is opened. The circuit will be operated in star-delta mode.
Why let the engine operate in “Star Mode” first?
+ The main purpose is to reduce the load pressure of the current. Reduce the inrush current to one third of the rated current. After a period of time has started stably. we should switch to triangle mode (increasing U and I) to keep the device active.
+ You should choose the starting method to suit each type of engine. Because not all three-phase motors can be started by the star-delta starting method. Because it depends on many factors such as:
- Motor load characteristics: The 3-phase grid power used in Vietnam is currently 380V. Only motors with a parameter of three hundred and eighty volts (380V) above six hundred and sixty volts (660V), qualified (380/660) can use this method. In addition, they also have to depend on the quality and starting frequency of the motor.
- Motor power: applicable to motors with a capacity of less than 7kW. Motors with a capacity greater than 7kW should use other methods such as soft starters, inverters, etc.
6. Advantage and Disadvantage of Star-Delta starting
+ Advantage: The open delta-star starter is easy to implement in terms of cost and circuitry, it does not require additional voltage reducing equipment.
+ Disadvantage: The open star-delta circuit produces a surge of current and torque when the change shocks the system both electrically and mechanically. Electrically, the result of transient peak currents can be jittery or hazardous. Mechanically, the increased torque that occurs due to a surge in current can be enough to harm system components, i.e. capture the bearing shaft.
