what is the slip of three-phase asynchronous motor? the driving torque exists only when the closing coil has induced current.
the torque is determined by the current of the closed coil and exists only when the magnetic flux in the ring changes. at this time, there must be a speed difference between the closed coil and the rotating magnetic field.
the difference between synchronous speed (ns) and closing coil speed (n) is called "slip", which is expressed as a percentage of synchronous speed. s = [(ns-n) / ns] x 100% (s is the subscript) during operation, the rotor current frequency is the power frequency multiplied by the slip.
when the motor starts, the rotor current frequency is at the maximum value, which is equal to the stator current frequency.
the rotor current frequency decreases gradually with the increase of motor speed. the slip rate in steady state is related to the motor load.
it is affected by the power supply voltage. if the load is low, the slip is small. if the motor supply voltage is lower than the rated value, the slip increases.
synchronous speed the synchronous speed of three-phase asynchronous motor is directly proportional to the power frequency and inversely proportional to the logarithm of stator.
eg: ns = 60 f / p, where: ns - synchronous speed (r / lmin), f - frequency (hz), p - pole logarithm
the rotating magnetic field speed or synchronous speed corresponding to different pole numbers at 50hz, 60hz and 100hz industrial frequencies are given.
in fact, even if the voltage is correct, if the power supply frequency is higher than the rated frequency of asynchronous motor, it may not be able to improve the motor speed.
its mechanical and electrical capacity must be determined first. due to the slip rate, the speed of the loaded asynchronous motor is slightly lower than the synchronous speed given in the table.
change the rotation direction of the motor and change the phase sequence of the power supply, that is, exchange the three-phase voltage connected to the motor and connect it to any two phases in the motor terminal.
the induced current in the rotor coil of three-phase asynchronous motor is generated due to the relative movement between the rotor conductor and the magnetic field. the rotor speed of three-phase asynchronous motor will not be synchronized with the rotating magnetic field, let alone exceed the speed of the rotating magnetic field.
if the speed of the rotor of the three-phase asynchronous motor is equal to the speed of the rotating magnetic field, there will be no relative movement between the magnetic field and the rotor, and the conductor cannot cut the magnetic line of force. therefore, the induced electromotive force and current will not be generated in the rotor coil, and the rotor conductor of the three-phase asynchronous motor will not be affected by the electromagnetic force in the magnetic field to make the rotor rotate.
therefore, the rotor rotation speed of three-phase asynchronous motor cannot be the same as that of rotating magnetic field, and is always less than the synchronous speed of rotating magnetic field. however, under special operation mode, the rotor speed of three-phase asynchronous motor can be greater than the synchronous speed.