Weekly Topic: January 8th, Issue 11 – Reasons for High Starting Current in Motor Equipment

The magnitude of the induced potential in the slats inside the motor is related to the speed at which these slats cut through the rotating magnetic field. During motor startup, the speed at which the slats cut through the rotating magnetic field is at its maximum; therefore, the induced potential is also at its highest, and the rotor current becomes very large. As we know, the greater the current in the secondary coil of a transformer, the greater the current in the primary coil. The same principle applies in motors: the greater the current in the slats, the greater the current in the stator windings. The starting current of a motor can be as high as 4–7 times the rated current.

At the moment the motor starts, the supply voltage is applied across the motor coils; at this point the motor has not yet begun to rotate, it does not convert electrical energy into mechanical energy, and therefore there is no equivalent resistance. At this time, the electric motor is equivalent to applying the supply voltage to a wire; the resistance is very low and the current is high, which is why a relatively large current is generated when the machine starts up.

This post was last edited by shiwang528 on 2011-1-8 09:05. In fact, not only do the starting currents of mechanical equipment used in chemical production be high, but those of household appliances are also high as well; it’s just that no ammeters are installed at home, so we don’t notice it. The starting current is high, mainly due to the induced electromotive force. When an induction motor is at rest, from an electromagnetic perspective, it behaves like a transformer. The stator windings on the side connected to the power supply correspond to the primary windings of the transformer, while the rotor windings, which form a closed circuit, correspond to the secondary windings of the transformer that are short-circuited. There is no electrical connection between the stator windings and the rotor windings; only a magnetic connection exists. The magnetic flux forms a closed loop through the stator, the air gap, and the rotor core. At the moment of closing, the rotor has not yet started rotating due to inertia; the rotating magnetic field cuts across the rotor windings at the highest possible speed (the synchronous speed), resulting in an induced electromotive force in the rotor windings that can reach its maximum value. As a result, a large current flows through the rotor conductors, and this current generates a magnetic field that counteracts the stator’s magnetic field, just as the secondary flux in a transformer counteracts the primary flux. To maintain the original flux appropriate to the power supply voltage at that time, the stator automatically increases the current. Since the current in the rotor is very high at this time, the stator current also increases significantly, sometimes reaching 4 to 7 times the rated current; this is why the starting current is high. As the motor speed increases, the speed at which the stator magnetic field cuts through the rotor conductors decreases. As a result, the induced electromotive force in the rotor conductors decreases, and so does the current in those conductors. Consequently, the portion of the stator current that is used to counteract the magnetic flux generated by the rotor current also decreases. Thus, the stator current decreases from a high value to a normal level.

Ways to reduce starting current include variable-frequency starting, soft starting, and so on

At the moment the motor starts, the supply voltage is applied across the motor coils; at this point the motor has not yet begun to rotate, it does not convert electrical energy into mechanical energy, and therefore there is no equivalent resistance. At this time, the electric motor is equivalent to applying the supply voltage to a wire; the resistance is very low, so the current is high. This is why a relatively large current is generated when the motor starts up.

I’ve learned that during startup, the current level is relatively high and the duration is short; this is a phenomenon of hard starting. Nowadays, soft starting is used for larger motors, and the current level in the case of soft starting is much lower

Reply to 5# yaakaak: Could you introduce the soft start code?

The voltage is gradually increased from zero to the rated value, thereby turning the starting current of the motor during startup from an uncontrolled overload surge current into a controllable one. And the magnitude of the starting current can be adjusted as needed. There is no impact torque throughout the entire motor startup process; instead, it starts up and operates smoothly. This is soft start.

　Soft starting generally includes the following starting methods: (1) Ramp-up voltage soft starting. 　 (2) Ramp constant-current soft start. 　　 (3) Step start. 　 (4) Pulse impact start

Working principle: The soft starter uses three-phase parallel thyristors as voltage regulators, which are connected between the power supply and the motor stator. Such a circuit is like a three-phase fully-controlled bridge rectifier circuit. When starting a motor using a soft starter, the output voltage of the thyristors increases gradually, causing the motor to accelerate slowly until the thyristors are fully conductive. At that point, the motor operates according to its mechanical characteristics at the rated voltage, enabling a smooth start, reducing the starting current, and preventing tripping due to excessive starting current. Once the motor reaches its rated speed, the starting process is completed. The soft starter automatically replaces the thyristors that have completed their function with bypass contacts, thereby providing the motor with the rated voltage needed for normal operation. This reduces the thermal losses of the thyristors, extends the service life of the soft starter, improves its efficiency, and also helps to prevent harmonic pollution in the power grid. The soft starter also provides a soft shutdown function, which is the reverse of the soft start process: the voltage is gradually reduced and the speed drops slowly to zero, thereby avoiding the torque shock that occurs during sudden shutdown.