1. Manual control circuit
This is a manual control circuit that uses knife switches and circuit breakers to control the on-off operation of three-phase asynchronous motorManual control circuit
The circuit has a simple structure and is only suitable for small-capacity motors that start infrequently. The motor cannot be automatically controlled, nor can it be protected against zero voltage and voltage loss. Install a set of fuses FU to make the motor have overload and short circuit protection.
2. The jog control circuit
The start and stop of the motor is controlled by the button switch, and the contactor is used to realize the on-off operation of the motor.
Defect: If the motor in the jog control circuit is to run continuously, the start button SB must always be held down by hand.
3. Continuous operation control circuit (long motion control)
The start and stop of the motor is controlled by the button switch, and the contactor is used to realize the on-off operation of the motor.
4. The jog and long-motion control circuit
Some production machinery requires the motor to be able to move both jog and long. For example, when a general machine tool is in normal processing, the motor rotates continuously, that is, long-running, while it is often necessary to jog during commissioning and adjustment.
1. Jog and long-motion control circuit controlled by transfer switch
2. Jog and long-motion control circuits controlled by composite buttons
To sum up, the key to realizing long-running and jogging control of the line is whether it can ensure that the self-locking branch is connected after the KM coil is energized. If the self-locking branch can be connected, long movement can be achieved, otherwise only jog movement can be achieved.
5. Forward and reverse control circuit
Forward and reverse control is also called reversible control, which can realize the movement of production parts in both positive and negative directions during production. For a three-phase asynchronous motor, to realize the forward and reverse control, it only needs to change the phase sequence of its power supply, that is, to adjust any two phases of the three-phase power lines in the main circuit.
There are two commonly used control methods: one is to use the combination switch to change the phase sequence, and the other is to use the main contact of the contactor to change the phase sequence. The former is mainly suitable for motors that require frequent forward and reverse rotations, while the latter is mainly suitable for motors that require frequent forward and reverse rotations.
1. Positive-stop-reverse control circuit
The main problem of the electrical interlocking forward and reverse control circuits is that when transitioning from one steering to another, the stop button SB1 must be pressed first, and the transition cannot be made directly, which is obviously very inconvenient.
2. Forward-reverse-stop control circuit
This circuit combines the advantages of electrical interlocking and button interlocking, and is a relatively complete circuit that can not only meet the requirements of direct start of forward and reverse rotation, but also has high safety and reliability.
Line protection link
(1) Short-circuit protection The main circuit is cut off by the melt of the fuse in the event of a short-circuit.
(2) Overload protection is realized by thermal relay. Because the thermal inertia of the thermal relay is relatively large, even if a current several times the rated current flows through the thermal element, the thermal relay will not act immediately. Therefore, when the starting time of the motor is not too long, the thermal relay can withstand the impact of the starting current of the motor and will not act. Only when the motor is overloaded for a long time, it will act, disconnect the control circuit, the contactor coil will lose power, cut off the main circuit of the motor, and realize overload protection.
(3) Undervoltage and undervoltage protection Undervoltage and undervoltage protection are realized through the self-locking contacts of the contactor KM. In the normal operation of the motor, the grid voltage disappears or decreases for some reason. When the voltage is lower than the release voltage of the contactor coil, the contactor is released, the self-locking contact is disconnected, and the main contact is disconnected, cutting off the motor power. , the motor stops. If the power supply voltage returns to normal, due to the self-lock release, the motor will not start by itself, avoiding accidents.
• The above circuit start-up methods are full-voltage start-up.
When the capacity of the transformer allows, the squirrel-cage asynchronous motor should be directly started at full voltage as much as possible, which can not only improve the reliability of the control circuit, but also reduce the maintenance workload of electrical appliances.
6. Step-down starting circuit of asynchronous motor
• The full-voltage starting current of the asynchronous motor can generally reach 4-7 times the rated current. Excessive starting current will reduce the life of the motor, cause the secondary voltage of the transformer to drop significantly, reduce the starting torque of the motor itself, and even make the motor unable to start at all, and also affect the normal operation of other equipment in the same power supply network. How to judge whether a motor can start with full voltage?
• Generally, those with motor capacity below 10kW can be started directly. Whether the asynchronous motor above 10kW is allowed to start directly depends on the ratio of the motor capacity and the power transformer capacity.
• For a motor of a given capacity, generally use the following empirical formula to estimate.
•Iq/Ie≤3/4+power transformer capacity (kVA)/[4×motor capacity (kVA)]
• In the formula, Iq—motor full voltage starting current (A); Ie—motor rated current (A).
• If the calculation result satisfies the above empirical formula, it is generally possible to start at full pressure, otherwise, it is not allowed to start at full pressure, and a reduced voltage start should be considered.
•Sometimes, in order to limit and reduce the impact of the starting torque on the mechanical equipment, the motor that allows full-voltage starting also adopts the reduced-voltage starting method.
• There are several methods for step-down starting of squirrel-cage asynchronous motors: stator circuit series resistance (or reactance) step-down starting, auto-transformer step-down starting, Y-△ step-down starting, △-△ step-down starting, etc. These methods are used to limit the starting current (generally, the starting current after reducing the voltage is 2-3 times the rated current of the motor), reduce the voltage drop of the power supply mains, and ensure the normal operation of the electrical equipment of each user.
1. Series resistance (or reactance) step-down starting control circuit
During the starting process of the motor, the resistance (or reactance) is often connected in series in the three-phase stator circuit to reduce the voltage on the stator winding, so that the motor can be started at the reduced voltage to achieve the purpose of limiting the starting current. Once the motor speed is close to the rated value, cut off the series resistance (or reactance), so that the motor enters the normal operation of full voltage. The design idea of this kind of circuit is usually to use the time principle to cut off the resistance (or reactance) in series when starting to complete the starting process.
Stator string resistance step-down starting control circuit
•The advantage of series resistance starting is that the control circuit has a simple structure, low cost, reliable action, improved power factor, and is conducive to ensuring the quality of the power grid. However, due to the voltage reduction of the stator string resistance, the starting current decreases in proportion to the stator voltage, and the starting torque decreases according to the square times of the voltage drop ratio. At the same time, each start consumes a lot of power. Therefore, the three-phase squirrel-cage asynchronous motor adopts the starting method of resistance step-down, which is only suitable for small and medium-capacity motors that require smooth starting and occasions where starting is not frequent. Large-capacity motors mostly use series reactance step-down starting.
2. String autotransformer step-down starting control circuit
• In the control circuit of auto-transformer step-down starting, limiting the starting current of the motor is realized by the step-down action of the auto-transformer. The primary of the autotransformer is connected to the power supply, and the secondary of the autotransformer is connected to the motor. The secondary of the autotransformer generally has 3 taps, and 3 kinds of voltages of different values can be obtained. When used, it can be flexibly selected according to the requirements of starting current and starting torque. When the motor starts, the voltage obtained by the stator winding is the secondary voltage of the autotransformer. Once the start is completed, the autotransformer is cut off, and the motor is directly connected to the power supply, that is, the primary voltage of the autotransformer is obtained, and the motor enters full voltage operation. This type of autotransformer is often referred to as a starting compensator.
• During the step-down starting process of the autotransformer, the ratio of the starting current to the starting torque is reduced by the square of the transformation ratio. Under the condition of obtaining the same starting torque, the current obtained from the power grid by the autotransformer step-down starting is much smaller than that with the resistance step-down starting, the impact on the grid current is small, and the power loss is small. Therefore, the autotransformer is called a starting compensator. In other words, if the starting current of the same magnitude is obtained from the power grid, the step-down starting with the autotransformer will generate a larger starting torque. This starting method is often used for motors with large capacity and normal operation in star connection. The disadvantage is that the autotransformer is expensive, the relative resistance structure is complex, the volume is large, and it is designed and manufactured according to the discontinuous working system, so frequent operation is not allowed.
3. Y-△ step-down starting control circuit
• The advantage of three-phase squirrel-cage asynchronous motor with Y-△ step-down starting is: when the stator winding is connected in star, the starting voltage is 1/3 of that when the delta connection is directly used, and the starting current is 1/3 of that when the delta connection is used. /3, so the starting current characteristics are good, the circuit is simpler, and the investment is less. The disadvantage is that the starting torque is also reduced to 1/3 of the delta connection method, and the torque characteristics are poor. So this line is suitable for light load or no-load starting occasions. In addition, it should be noted that the consistency of the rotation direction should be paid attention to when connecting Y-
Post time: Jun-30-2022