Analysis of three-phase asynchronous motor control circuit

Operation of direct online starting circuit

A direct on-line starter or DOL is a simple electromechanical system designed for the switching and protection of induction motors.

As we all know, electric motors consume a lot of power, and this high power consumption is the result of the current absorbed by the motor windings. Therefore, the higher the current the motor consumes, the higher the power it consumes and the higher the heat it generates. This heat is usually dissipated to the environment through radiation or conduction from direct contact. However, in some cases without proper ventilation or in hot environments, the armature windings may burn due to overheating.

Therefore, it is necessary to closely monitor the motor winding current to prevent high current from flowing for a long time. Therefore, to prevent high current from flowing for a long time, the motor is usually equipped with various types of protection systems.

Typically, three-phase industrial motors driving high power loads require these protection systems. A direct-on-line starter is a mechanism that provides overload protection for three-phase squirrel cage induction motors.

The main functions provided by the direct online starter for three-phase asynchronous motors are:

Overcurrent protection or short circuit protection.

Overload protection.

Isolate motor switch setting.

Over-current protection or short-circuit protection: DOL starter consists of MCCB (circuit breaker) and fuse arrangement to disconnect the motor from the supply in case of short circuit.

Overload Protection: DOL starter consists of an electromechanical device that disconnects the motor from the power supply if the motor is overloaded or the current drawn by the motor exceeds the rated value.

Isolation Motor Switch Setup: Since high power motors are dangerous, DOL starters are designed in a way that allows the customer to switch the motor on and off indirectly.

The above three characteristics are important for low and medium power induction motors used in industry. So DOL starters are popular and widely used.

Start working directly online

To avoid confusion, we will disassemble the original DOL launcher and discuss each of its parts.

The internals of the direct-on-line starting circuit we discuss below are just to give an idea of ​​how things work, your starter’s original design may vary.

MCCB (molded case circuit breaker) and FUSE parts:

The diagram above shows the circuit connections between the molded case circuit breaker, fuses and the motor. The basic function of this part of the DOL starter is to protect the motor from faults and short circuits.

The MCCB here will be selected based on the motor rating and if there is any fault in the connection or motor windings, this MCCB will immediately trip and disconnect the entire system from the main supply line. MCCB is usually the first layer of protection for the entire system as shown in the above figure. These are also installed in our homes for safety purposes.

Fuses in the circuit are present here to protect the motor and other equipment from short circuits. If any short circuit occurs, these fuses melt immediately and disconnect the motor from the power line. Also, the fuse rating must be chosen accurately to avoid irregular fusing during operation. This can happen during the start-up of the motor when there is a large inrush current, so it is important to choose a fuse with the proper rating.

Electromagnetic contactor part:

In the above figure, the internal structure of a contactor setup is shown, as it is present in a three-phase direct on-line starter and connected to an induction motor.

Here the three phase supply is connected to the motor through three normally open metal contacts "C1", "C2" and "C3". So in static condition, no current flows in the circuit and the motor remains off. Also at this time, the "ON BUTTON" will be open and no current flows through the coil.

Now, if we press the “ON BUTTON” then here the coil gets magnetized due to the current as shown below.

Since the coil creates a magnetic field here, the metal block suspended by the spring will be attracted to the coil and move towards it. Now that the metal block is moving, the entire contactor device will also move with it as shown in the figure.

Due to this movement, the metal contacts C1, C2 and C3 will short-circuit the open terminals between the power line and the stator terminals, thereby starting the motor. In simpler terms, when the button is pressed, the motor will receive power from the power supply due to the action of the three-phase contactor. At the same time, as the three-phase contactor moves, the spring will be stretched, exerting a force on the metal block to return it to its original position.

After you momentarily press the ON button and release it, the current in the coil, which should be zero, will still flow because the current will have another path to flow after the three-phase contactor moves to its final position. You can see in the figure the closed circuit formed for the current to flow through the "SW" metal contacts.

Therefore, just press the "ON BUTTON" and the three-phase contactor will self-lock with the help of the "SW" metal contacts and maintain the connection between the three-phase power supply and the motor.

Now, to stop the motor, we have to add another push button to the above circuit as shown below.

The "OFF BUTTON" here will act as a short circuit in the rest position, so the operation of the circuit we discussed above will not change. But once the "OFF BUTTON" is pressed, the loop formed between the power line and the coil will be broken, causing the current flowing through the coil to become zero. Now that the current passing through the coil is zero, the coil will start to demagnetize itself, and once the coil completely loses its magnetization, the three-phase contactor will move back to its initial position due to the force applied by the extension spring. Obviously, since the three-phase contactor moves back to the rest position, the power supply voltage to the motor will be disconnected, causing the rotor to stop moving.

Even after releasing the stop button, the three-phase contactor will remain stationary until the start button is pressed again to magnetize the coil. So we can conclude that with this setup, we can turn the motor on forever by pressing one button, and stop the motor forever by pressing another button.

Overload protection part:

The key part of the overload protection section is the three coils G1, G2, and G3 in the figure. These three coils are connected in series with the three-phase induction motor and carry the same current as the armature winding. Therefore, whenever the motor draws power from the power line, these three windings are magnetized. Whenever they are magnetized, the metal ring fixed to the shaft is attracted by the coil. Normally, this will not be a problem, but once the motor is overloaded, it will become prominent.

So, to understand the functioning of this part, let's consider that the motor was switched on some time ago and was overloaded. Now that the motor is heavily loaded, the armature winding will draw a large current from the supply, thereby indirectly heavily magnetizing the G1, G2 and G3 coils. In the presence of this strong magnetic field, the metal ring will overcome the resistance of the springs and align itself with the respective coils. Once the metal ring moves to its final position, the "OL contact" will also move with it to open the "COIL-L" loop.

Therefore, the end result of overloading the motor is that the current loop formed between the power line and "COIL-L" is broken. We can see here that this is essentially the same function as pressing the stop button we mentioned above. The end result of both situations is to shut down the motor forever.

Therefore, overloading the motor will cause the power line to disconnect and the motor to shut down.

Direct online start control circuit

So far, we have looked at these three parts, each of which provides a specific function. We need to connect these parts together to form a DOL launcher.

Here you can see the final internal structure of Direct Online Starter.

The final conclusion is:

The MCCB-FUSE section provides short circuit and fault protection for the motor.

A three-phase contactor setup will provide simple and safe bistable switching of the motor.

The OL contactor setting will protect the motor from overload and burnout.

Advantages of Direct Online Launcher

Most Economical and Cheapest Starter: Of all the starters currently used for three-phase induction motors, the DOL starter is the cheapest and most economical.

Simple operation: The starter has only two buttons, ON and OFF, and a knob for setting overload safety, making it easy to operate.

Easy maintenance: Due to the simple internal structure of the starter, engineers can easily find and troubleshoot faults.

Since there is no starting protection, motors with fixed DOL starters provide 100% starting torque.

The small size of the DOL makes it compact and reliable.

Disadvantages of Direct Online Launcher

Since there is no starting protection, DOL starters do not limit the starting current.

Unnecessarily high starting torque during motor start-up.

Only suitable for low-end and medium power motors.

Since there is no starting protection, the power line connected to the motor will experience a voltage sag during the motor startup. This voltage fluctuation may damage other electrical equipment powered by the same power supply.

Motors are subject to thermal stresses that affect the life of the motor.

The mechanical stress on the motor increases due to the unnecessarily high starting torque during motor start-up.