Normal start of variable-frequency motor

Yes, it can be started directly as long as the capacity of the power distribution system is large enough

1) Minimize the stator and rotor resistances as much as possible. Reducing the stator resistance can lower the fundamental wave copper loss, thereby compensating for the increase in copper loss caused by higher harmonics. 2) To suppress the higher harmonics in the current, it is necessary to appropriately increase the inductance of the motor. However, due to the large leakage reactance in the rotor slots, the skin effect is also significant, resulting in increased copper loss at higher harmonics. Therefore, the magnitude of the motor leakage reactance must take into account the rationality of impedance matching across the entire speed control range. 3) The main magnetic circuit of variable-frequency motors is generally designed to be in a non-saturated state; this is done first to prevent high-order harmonics from exacerbating magnetic circuit saturation, and secondly to increase the output voltage of the inverter appropriately at low frequencies in order to boost the output torque.

In addition to the feature of forced cooling, variable-frequency motors generally have a low rotor impedance. This is because during variable-frequency speed control, acceleration takes place in small steps with a gradual increase in slip, and the starting current is controlled; a low impedance helps to enhance the stiffness of the mechanical characteristics. If used directly, problems may arise due to excessive starting current. Requirements for variable-frequency motors: In order to reduce current harmonics and improve the power factor, the leakage reactance of variable-frequency motors is higher than that of ordinary motors. However, this leakage reactance cannot be too high, as it would otherwise lead to a decrease in output torque. To increase the leakage reactance, the stator slot design tends to be narrower and deeper; therefore, the slot dimensions must be adjusted to meet the requirements of supply from a variable frequency drive. 　　Number of poles in variable-frequency motors: At the same power level, the higher the number of poles, the lower the leakage reactance. When operating under the same load, motors with lower leakage reactance exhibit higher peak current values. With a fixed capacity of the inverter, motors with low leakage reactance are more likely to trip due to these peak currents. Therefore, it is generally recommended to use four-pole or two-pole motors in conjunction with inverters, which differs from the traditional design of cranes. 　　Compared to motors powered by a standard sine wave, the flux in variable-frequency motors increases by about 10% due to the effect of harmonics; this leads to magnetic saturation of the motor as well as a reduction in its leakage reactance. At low frequencies, the inverter must increase the voltage applied in order to counteract the effects of stator resistance, which further increases the likelihood of saturation. Therefore, the core cross-sectional area of variable-frequency motors must be enlarged. The above is a description of variable-frequency motors compared to conventional motors; they can be used directly without a frequency converter, and this will not cause any substantial damage to the motor!

The drawback of direct motor starting is that it shortens the service life of the motor and related components; it can only be used for temporary emergency situations. If the operation lasts for a long time, it is recommended to use star-delta starting instead.

I have seen 75-kilowatt motors start up directly on site, and after 20 years of use, no problems have occurred.

Everyone, please note: the discussion is about variable-frequency motors, not ordinary motors. Based on their design principles, variable-frequency motors are likely to be different from ordinary motors.

What a high current will there be when a 75-kilowatt motor is started directly? 1

The variable-frequency motor can be started directly using a contactor. Note: 1. The power supply for the motor’s cooling fan; 2. Pay attention to the type of load. If the start-up time is long (over 15 seconds), an electronic thermal relay is required to handle the starting current

A 22KW unit can be started directly without any problems.