A three-phase asynchronous motor is a type of induction motor that relies on being powered by 380V three-phase AC current (with a phase difference of 120 degrees). The rotor and stator rotating magnetic fields of the three-phase asynchronous motor rotate in the same direction but at different speeds, resulting in slip, hence its name.
The rotor speed of the three-phase asynchronous motor is lower than that of the rotating magnetic field. As a result, an electromotive force and current are generated between the rotor winding and magnetic field due to their relative motion. This interaction produces electromagnetic torque, enabling energy conversion.
Compared with single-phase asynchronous motors, three-phase asynchronous motors have better operating performance and can save various materials.
According to the different rotor structures, three-phase asynchronous motors can be divided into two types: squirrel-cage type and wound type.
The squirrel-cage rotor of the asynchronous motor has a simple structure, reliable operation, light weight, and low price. It has been widely used. Its main disadvantage is difficult speed regulation.
The rotor and stator of the wound-type three-phase asynchronous motor also have three-phase windings and are connected to external variable resistors through slip rings and brushes. Adjusting the resistance of the variable resistor can improve the starting performance of the motor and adjust its speed.
Working principle of three-phase asynchronous motor
When symmetrical three-phase AC power is supplied to the stator winding of a three-phase asynchronous motor, a rotating magnetic field that rotates clockwise along the inner circular space of the stator and rotor at synchronous speed n1 is generated.
Since the rotating magnetic field rotates at a speed of n1, the rotor conductor is initially stationary. Therefore, the rotor conductor will cut through the rotating magnetic field of the stator and generate an induced electromotive force (the direction of which is determined by using the right-hand rule).
Due to short-circuit rings shorting both ends of the rotor conductor, an induced current that is basically consistent with the direction of induced electromotive force will be produced in it under its effect. The current-carrying conductors in the rotor are subjected to electromagnetic forces in the stator's magnetic field (the direction being determined by using left-hand rule). Electromagnetic force produces electromagnetic torque on shafts driving rotors to rotate along with rotational magnetic fields.
Through analysis above, we can summarize that when symmetrical three-phase AC power is supplied to each phase difference 120 degrees electrical angle stator winding of an electric motor, a rotating magnetic field will be generated. This rotating magnetic field cuts through rotor windings and generates induced currents within them (rotor windings form closed circuits).
Current-carrying conductors in rotors produce electromagnetic forces under rotation effects from stators' rotational magnetic fields; thus forming electromagnetic torque on electric motors' axes driving them to rotate. Moreover, electric motors' rotation directions are consistent with those of their rotational magnetic fields.
Wiring diagram for three-phase asynchronous motor
Basic wiring of three-phase asynchronous motor:
The six wires coming out of the winding of a three-phase asynchronous motor can be divided into two basic connection methods: delta (△) connection and star (Y) connection.
Six wires = Three motor windings = Three start terminals + Three end terminals. When measuring with a multimeter, the same winding start and end terminals are connected, that is: U1-U2, V1-V2, W1-W2.
Delta Connection △ of Three-Phase Asynchronous Motor
The delta connection △ method connects the three winding ends in sequence to form a triangle, as shown in the figure:
Star connection of three-phase asynchronous motor
Star connection means connecting the tails or heads of the three windings, and using the other three wires as power supply connections. The wiring diagram is shown below:
Text and graphic explanation of the wiring diagram for a three-phase asynchronous motor:
Three-phase motor junction box
When connecting a three-phase asynchronous motor in Y configuration, the connection method of the connecting pieces inside the junction box is as follows:
When connecting a three-phase asynchronous motor, the connection method of the connection piece in the junction box is:
There are two types of connections for three-phase asynchronous motors: star connection and delta connection.
Delta Connection
In a winding coil that withstands the same voltage and wire diameter, the number of turns per phase in delta connection is square root of 3 times (1.732 times) less than that in star connection, and the power is also square root of 3 times less. The connection method for finished motors has been fixed to withstand a voltage of 380V and generally should not be changed.
The connection method only changes when the three-phase voltage level is different from normal 380V. For example, when the three-phase voltage level is 220V, changing from star to delta connection can be used; when it's at 660V level, changing from delta to star connection can be used without affecting its power. Generally speaking, small-power motors use star connections while large-power ones use delta connections.
Under rated voltage, motors should be connected in delta shape. If they are changed to star shape, they will operate at reduced voltage with decreased power and starting current. The starting current for high-power motors (delta-connected) is very large. In order to reduce the impact on the line caused by starting current, step-down starting is generally adopted where running in original delta-connection mode switches back after being started using a star-connection.
Wiring diagram for three-phase asynchronous motor
Real-life wiring diagram for forward and reverse rotation of three-phase asynchronous motor:
To achieve forward and reverse control of the motor, any two phases in its power phase sequence can be interchanged (referred to as commutation) by adjusting the U-phase and W-phase regulators while keeping the V-phase unchanged. In order to ensure reliable switching of the motor's phase sequence when both contactors are activated, it is necessary to keep the upper terminals of the contactors consistent during wiring and adjust the phase at their lower terminals. Since two phases are swapped, it is necessary to ensure that both KM coils do not receive power simultaneously; otherwise, a serious interphase short circuit fault may occur, so interlocking must be adopted.
For safety reasons, a dual-locking forward and reverse control circuit with button interlock (mechanical) and contactor interlock (electrical) is commonly used. With button interlocking in place, even if both forward and reverse buttons are pressed simultaneously, it is impossible for both contactors used for commutation to receive power at the same time. This mechanically prevents an interphase short circuit.
In addition, due to the use of contactor interlocking, as long as one contactor receives power supply from either direction or reverse direction command signal input terminal , its normally closed contacts will not close. Therefore under mechanical-electrical double locking application mode，the motor's power supply system cannot cause an interphase short-circuit fault effectively protecting the motor while avoiding accidents caused by an interphase short-circuit during commutation which could burn out the contacts.
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