What is the Starting Current of the Motor?

There are different opinions on how many times the rated current the starting current of a motor is, many of which are based on specific circumstances. For example, tens of times, 6-8 times, 5-8 times, 5-7 times, etc.

One theory is that at the moment of starting (i.e. the initial moment of the starting process), when the motor's speed is zero, the current value at this time should be its locked rotor current value.

The Y-series three-phase asynchronous motors that are most commonly used are clearly defined in the JB/T 10391-2002 "Y-series three-phase asynchronous motors" standard. The specified value for the ratio of the locked rotor current to the rated current of a 5.5kW motor is as follows:

At a synchronous speed of 3000, the ratio of locked rotor current to rated current is 7.0;

At a synchronous speed of 1500, the ratio of locked rotor current to rated current is 7.0;

At a synchronous speed of 1000, the ratio of locked rotor current to rated current is 6.5;

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

The power of a 5.5kW motor is relatively high, and the starting current to rated current ratio of a smaller motor is smaller. Therefore, electrical textbooks and many places say that the starting current of an asynchronous motor is 4-7 times the rated working current.

Why is the starting current of the motor high? Does the current decrease again after starting?

It is necessary for us to understand from the perspective of the starting principle and rotation principle of the motor:

When an induction motor is in a stopped state, from an electromagnetic perspective, it is like a transformer. The stator winding connected to the power supply is equivalent to the primary coil of the transformer, and the closed rotor winding is equivalent to the secondary coil of the transformer that has been short circuited; There is no electrical connection between the stator winding and the rotor winding, only a magnetic connection, and the magnetic flux forms a closed circuit through the stator, air gap, and rotor core. At the moment of closing, the rotor has not yet turned up due to 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, and this current generates magnetic energy to offset the stator magnetic field, just as the secondary magnetic flux of the transformer offsets the primary magnetic flux.

In order to maintain the original magnetic flux that is suitable for the current supply voltage, the stator automatically increases the current. Because the current of the rotor is high at this time, the stator current also increases significantly, even reaching 4-7 times the rated current, which is the reason for the high starting current. Why is the current low after starting: As the motor speed increases, the speed at which the stator magnetic field cuts the rotor conductor decreases, the induced electromotive force in the rotor conductor decreases, and the current in the rotor conductor also decreases. Therefore, 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 increases from high to low until normal.

Our main products are Brushless AC Electric Motor,3 Phase AC Gear Motor,Three Phase AC Induction Motor,1 Phase AC Motor,Single Phase Commutator Motor.