How much is the starting current of a motor? The starting current of a motor is how many times the rated current is said to be different, many are based on specific situations. For example, it is said to be more than ten times, 6 to 8 times, 5 to 8 times, 5 to 7 times, etc.

　　One is to say that at the moment of starting (i.e. the initial moment of the starting process) when the speed of the motor is zero, the current value at this time should be its blocking current value.

　　For the frequently used Y series three-phase asynchronous motors, there are clear regulations in the standard JB/T 10391 "Y series three-phase asynchronous motors". The ratio of the blocking current to the rated current for 5.5kW motors is specified as follows:

　　When the synchronous speed is 3000, the ratio of the blocking current to the rated current is 7.0.

　　At a synchronous speed of 1500, the ratio of the blocking current to the rated current is 7.0.

　　At synchronous speed 1000, the ratio of blocking current to rated current is 6.5.

　　At a synchronous speed of 750, the ratio of blocking current to rated current is 6.0.

　　5.5kW motor power is relatively large, the power of smaller motor starting current and rated current ratio to be smaller, so the electrician textbook and many places are said to be asynchronous motor starting current is rated operating current of 4 ~ 7 times.

　　Why is the motor starting current high? After starting the current is small again?

　　Here we need to understand from the perspective of motor starting principle and motor rotation principle:

　　When the induction motor is stopped, from the electromagnetic point of view, like a transformer, the stator winding received from the power supply is equivalent to the primary coil of the transformer, the rotor winding into a closed circuit is equivalent to the secondary coil of the transformer is short-circuited; there is no electrical connection between the stator winding and the rotor winding, only the magnetic connection, magnetic flux through the stator, air gap, rotor core into a closed circuit. At the moment of closing, the rotor has not yet turned up due to inertia, the rotating magnetic field cuts the rotor winding with a greater cutting speed - synchronous speed, so that the rotor winding induces a greater potential that may be reached, thus, a large current flows in the rotor conductor, this current produces the magnetic energy to offset the stator magnetic field, like the secondary flux of a transformer to offset This current has the effect of counteracting the magnetic energy of the stator field in the same way that the secondary flux of a transformer counteracts the primary flux.

　　The stator automatically increases the current in order to maintain the original flux which is compatible with the supply voltage at that time. Because the rotor current is very large at this time, the stator current is also very large, even up to 4~7 times of the rated current, which is the reason for the large starting current. Why is the current small after starting: As the motor speed increases, the stator magnetic field cutting rotor conductor speed decreases, the induced potential in the rotor conductor decreases, the current in the rotor conductor also decreases, so the part of the stator current used to offset the influence of the magnetic flux generated by the rotor current also decreases, so the stator current goes from large to small until normal.

　　What are the ways to reduce the starting current of a motor?

　　Common methods of reducing motor starting current include direct starting, series resistance starting, self-coupled transformer starting, star delta voltage reduction starting and frequency converter starting to reduce the impact on the grid.

　　Direct start

　　Direct starting is a simple, economical and reliable way to start a motor, as the stator winding is connected directly to the power supply and started at the rated voltage. It is also a simple, economical and reliable starting method. Full voltage starting has a high current, but not a high starting torque, easy operation and rapid starting, but this starting method requires a large grid capacity and load, and is mainly suitable for starting motors under 1W.

　　Series resistance starting

　　Series resistance starting of motors is also a method of reduced voltage starting. During the starting process, a resistor is connected in series in the stator winding circuit. When the starting current passes through, a voltage drop is generated across the resistor, reducing the voltage added to the stator winding, which reduces the starting current.

　　Autotransformer starting

　　The use of autotransformers with multiple taps to reduce the voltage can be adapted to the needs of different load starters, but also to obtain a larger starting torque, and is often used to start larger capacity motors with reduced voltage. It has the added advantage of having a larger starting torque, up to 64% of that of a direct start when the winding is tapped at 80%, and the starting torque can be adjusted by the tap.

　　Star-delta reduced voltage starting

　　For a squirrel-cage asynchronous motor with a triangular stator winding in normal operation, if the stator winding is connected to a star when starting and then to a triangle after starting, the starting current can be reduced and the impact on the grid reduced. Such a starting method is called star-delta reduced voltage starting, or simply star-delta starting (y-& starting). When using star-delta start, the starting current is only 1/3 of the original direct start according to the triangle connection. 2-2.3 times the starting current in star-delta start. This means that when using a star-delta starter, the starting torque is also reduced to 1/3 of that of the original direct triangular starter, which is suitable for no-load or light-load starting. It is also simpler and cheaper than any other reduced voltage starter. In addition to this, the star-delta method has the advantage that the motor can be operated with a star connection when the load is light. In this case, the rated torque can be matched to the load, which improves the efficiency of the motor and saves power consumption as a result.

　　Inverter start

　　The frequency converter is a modern motor control device with greater technical content, full control function and good control effect. It regulates the speed and torque of the motor by changing the frequency of the grid. Because they involve power electronics and microcomputer technology, they are costly and require a high level of maintenance technicians, so they are mainly used in areas where speed regulation is required and where high speed control is required.