What’s the reason of motor starting current is higher than after-starting?
What is the starting current of the motor?
There are different opinions on how many times the starting current of the motor is the rated current, many of which are based on the specific situation.
For example, 10 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 time of starting process), when the speed of the motor is 0, the current value at this time should be its locked rotor current value.
For the most frequently used Y series three-phase asynchronous motor, there are clear provisions in JB / T 10391-2002 "Y series three-phase asynchronous motor" standard.
The specified value of the ratio of locked rotor current to rated current of 5.5kW motor is as follows:
When the synchronous speed is 3000, the ratio of locked rotor current to rated current is 7.0;
When the synchronous speed is 1500, the ratio of locked rotor current to rated current is 7.0;
When the synchronous speed is 1000, the ratio of locked rotor current to rated current is 6.5;
When the synchronous speed is 750, the ratio of locked rotor current to rated current is 6.0.
5.5kW motor has higher power, and the ratio of starting current and rated current of motor with lower power is smaller. Therefore, electrical teaching materials and many places say that starting current of asynchronous motor is 4-7 times of rated working current.
Why is the starting current of the motor higher, and current lower after starting? It is necessary for us to understand the starting principle and rotating principle of motor:
When the induction motor is in the stop state, from the electromagnetic point of view, just like the transformer, the stator winding connected to the power supply is equivalent to the primary coil of the transformer, and the closed-circuit rotor winding is equivalent to the short-circuit secondary coil of the transformer;
there is no electric connection between the stator winding and the rotor winding, only the magnetic connection, and the magnetic flux is closed through the stator, air gap and rotor core.
At the moment of closing, the rotor has not yet turned because of inertia, and the rotating magnetic field cuts the rotor winding at the maximum cutting speed synchronous speed, so that the rotor winding can sense the highest possible potential.
Therefore, a large current flows through the rotor conductor, which generates the magnetic energy to counteract the stator magnetic field, just as the secondary magnetic flux of the transformer needs to counteract the primary magnetic flux.
In order to maintain the original flux corresponding to the supply voltage, the stator automatically increases the current.
Because the current of the rotor is very large at this time, the stator current is also increased greatly, even up to 4-7 times of the rated current, which is the reason for the large starting current.
Why the current is small after starting:
with the increase of motor speed, the speed of stator magnetic field cutting rotor conductor decreases, the induced potential in rotor conductor decreases, and the current in rotor conductor also decreases, so the part of current in stator current used to offset the influence of magnetic flux generated by rotor current also decreases, so the stator current is from large to small until normal.
What are the methods to reduce the starting current of the motor?
The common starting methods to reduce the starting current of the motor are
l direct starting,
l series resistance starting,
l auto coupling transformer starting,
l star delta decompression starting
l frequency converter starting
Direct Starting
Direct starting is to connect the stator winding of the motor directly to the power supply and start at rated voltage.
Its has the characteristics of large starting torque and short starting time.
It is also the simplest, most economical and most reliable starting mode.
The current is large, the starting torque is small, the operation is convenient and the starting is fast.
However, this starting mode requires a lot of power grid capacity and load, which is mainly suitable for motor starting under 1W.
series resistance starting
Motor series resistance starting is a method of step-down starting.
In the starting process, when the starting current passes through the series resistance in the stator winding circuit, the voltage drop will be generated on the resistance, reducing the voltage added on the stator winding, so that the starting current can be reduced.
auto coupling transformer starting
The use of multi tap voltage reduction of autotransformer can not only meet the needs of starting with different loads, but also get more starting torque, which is often used to start large capacity motors.
Its biggest advantage is that the starting torque is large. When the winding tap is at 80%, the starting torque can reach 64% of that of direct starting, and the starting torque can be adjusted by tap
star delta decompression starting
For the squirrel cage asynchronous motor whose stator winding is delta connected in normal operation, if the stator winding is connected into a star when starting and then into a triangle after starting, the starting current can be reduced and its impact on the power grid can be reduced.
Such a starting mode is called star delta reduced pressure starting, or simply called star delta starting (Y - & starting). When star delta starting is used, the starting current is only 1 / 3 of the original direct starting by delta connection.
When the star delta starts, the starting current is only 2-2.3 times.
That is to say, when star delta starting is adopted, the starting torque is also reduced to 1 / 3 of the original direct starting according to the delta connection method. It is suitable for no load or light load starting.
Compared with any other starter, it has the simplest structure and the cheapest price. In addition, star delta starting mode has another advantage, that is, when the load is light, the motor can be operated under star connection.
At this time, the rated torque and load can be matched, which can improve the efficiency of the motor and save power consumption.
frequency converter starting
Frequency converter is the motor control device with the highest technical content, the most complete control function and the best control effect in the field of modern motor control.
Because it involves power electronic technology and microcomputer technology, the cost is high, and the requirements for maintenance technicians are also high, so it is mainly used in areas requiring speed regulation and high requirements for speed control
