What is the starting current of a motor?
There are various opinions on how many times the starting current of a motor is compared to its rated current, and these opinions often depend on the specific circumstances. For example, some say it is more than ten times, others say 6 to 8 times, 5 to 8 times, or 5 to 7 times, etc.
One theory suggests that at the instant of startup (i.e., the initial moment of the startup process), when the motor speed is zero, the current value at this time should be its stall current value.
For the most commonly used Y-series three-phase asynchronous motors, the JB/T10391—2002 standard "Y-series three-phase asynchronous motors" provides clear specifications. The specified value for the locked-rotor current to rated current ratio of a 5.5kW motor is as follows:
At a synchronous speed of 3000 rpm, the ratio of stall current to rated current is 7.0;
At a synchronous speed of 1500 rpm, the ratio of stall current to rated current is 7.0.
At a synchronous speed of 1000 RPM, the ratio of stall current to rated current is 6.5.
At a synchronous speed of 750 rpm, the ratio of stall current to rated current is 6.0.
A 5.5kW motor has a relatively high power rating. Smaller motors have a lower ratio of starting current to rated current. Therefore, electrical engineering textbooks and many other sources state that the starting current of an asynchronous motor is 4 to 7 times its rated operating current.
Why is the starting current of the motor high, but the current decreases after starting?
Here we need to understand it from the perspective of the motor's starting principle and rotation principle:
When an induction motor is stationary, from an electromagnetic perspective, it resembles a transformer. The stator winding connected to the power source is equivalent to the primary coil of the transformer, while the closed-circuit rotor winding is equivalent to the short-circuited secondary coil. There is no electrical connection between the stator and rotor windings, only a magnetic connection; the magnetic flux forms a closed circuit through the stator, air gap, and rotor core. At the instant the circuit is closed, before the rotor starts rotating due to inertia, the rotating magnetic field cuts the rotor windings at its maximum cutting speed—synchronous speed—inducing the highest possible electromotive force in the rotor windings. Consequently, a large current flows through the rotor conductors. This current generates magnetic energy that cancels out the stator magnetic field, just as the secondary magnetic flux in a transformer cancels out the primary magnetic flux.
To maintain the original magnetic flux compatible with the power supply voltage at that time, the stator automatically increases the current. Because the rotor current is very large at this time, the stator current also increases significantly, even reaching 4 to 7 times the rated current. This is the reason for the large starting current. Why is the current smaller after starting? As the motor speed increases, the speed at which the stator magnetic field cuts the rotor conductors decreases, the induced electromotive force in the rotor conductors decreases, and the current in the rotor conductors also decreases. Therefore, the portion of the stator current used to counteract the magnetic flux generated by the rotor current also decreases, so the stator current decreases from large to small until it returns to normal.
What are some methods to reduce the starting current of an electric motor?
Common methods to reduce the starting current of motors include direct starting, series resistor starting, autotransformer starting, star-delta reduced voltage starting, and frequency converter starting to reduce the impact on the power grid.
Direct Start
Direct starting involves directly connecting the motor's stator windings to the power supply, starting the motor at its rated voltage. It features high starting torque and short starting time, and is the simplest, most economical, and most reliable starting method. Full-voltage starting involves high current but low starting torque, offering convenient operation and rapid start-up. However, this method requires a relatively large power grid capacity and load, and is mainly suitable for starting motors under 1W.
Series resistor start-up
Motor starting with a series resistor is 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 flows, a voltage drop is generated across the resistor, reducing the voltage applied to the stator winding, thus reducing the starting current.
Autotransformer start-up
Using a multi-tap autotransformer for reduced voltage can adapt to the starting needs of different loads and obtain a larger starting torque. It is a frequently used reduced voltage starting method for starting large-capacity motors. Its biggest advantage is the large starting torque. 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 tapping.
Star-delta decompression start
For a squirrel-cage induction motor with a delta-connected stator winding during normal operation, connecting the stator winding in a star configuration during startup and then reconnecting it in a delta configuration after startup reduces the starting current and lessens its impact on the power grid. This starting method is called star-delta reduced-voltage starting, or simply star-delta starting (Y-Δ starting). When using star-delta starting, the starting current is only 1/3 of that when starting directly with a delta connection. The starting current is only 2-2.3 times that of direct starting with a delta connection. This means that the starting torque is also reduced to 1/3 of that when starting directly with a delta connection. It is suitable for no-load or light-load starting applications. Furthermore, compared to any other reduced-voltage starter, it has the simplest structure and is the cheapest. In addition, star-delta starting has another advantage: when the load is light, the motor can run in a star connection. In this case, the rated torque can match the load, which improves the motor's efficiency and saves power consumption.
Inverter start
Variable frequency drives (VFDs) are the most technologically advanced, feature-rich, and effective motor control devices in modern motor control. They regulate the speed and torque of a motor by changing the frequency of the power grid. Because they involve power electronics and microcomputer technology, they are expensive and require highly skilled maintenance technicians. Therefore, they are mainly used in fields that require speed regulation and have high speed control requirements.
