How Does a Variable Frequency Drive (VFD) Affect Motor Starting Current?

How Much Current Does a Motor Draw When Starting with a Variable Frequency Drive?

Taking a three-phase asynchronous motor as an example, when the motor starts directly, the starting current can reach 5 to 7 times the rated current, while the starting torque can reach 1.2 to 1.3 times the rated torque. The high inrush current can cause overheating of the motor windings, and excessive torque may negatively impact both the motor itself and the driven equipment.

However, when a variable frequency drive (VFD) is used in combination with the motor, the starting characteristics are regulated by the VFD. The starting torque, starting current, and acceleration time all change accordingly. Typically, the current limit during startup is set at 1.2 to 1.5 times the rated current, resulting in a lower acceleration torque compared to direct starting. As a result, the startup time is relatively longer. During VFD-controlled starting, the synchronous speed gradually increases, ensuring a smoother startup process. In contrast, motors that start directly on line (DOL) experience significant vibrations at the moment of startup. This effect is especially noticeable in small, high-speed motors, which may even exhibit bouncing movements.

The selection of an appropriately sized VFD is crucial when pairing it with a motor. If the VFD is undersized, the motor may fail to start. Conversely, an oversized VFD may not provide optimal motor protection. Most VFDs come with built-in protective features, including surge protection, overload protection, regenerative overvoltage protection, undervoltage protection, ground fault current detection, cooling fan failure detection, overheating protection, and short-circuit protection. Additionally, they provide overload and overspeed protection for the motor, as well as stall overcurrent and stall regenerative voltage protection for the overall control system.

Different motor types and load characteristics require appropriately matched VFDs, and the performance level of VFDs varies by quality. Low-quality VFDs can negatively impact motor performance, leading to issues such as electromagnetic noise and vibration caused by higher-order harmonics, as well as potential shaft current problems. Therefore, selecting a high-quality VFD that is well-suited for the application is essential to ensure the reliable and efficient operation of the motor system.