Can Variable Frequency Motors Be Started Directly? (Part 2)

Can variable frequency motors be started directly? (Part 2)

4. Heat dissipation issues (especially at low speeds)

Variable frequency motors are typically designed with independent cooling fans (forced air cooling). These motors typically require independent cooling fans when running at low speeds (because their built-in fans rotate at low speeds and produce insufficient airflow). When the inverter is used to operate at low speeds, the main motor's heat dissipation capacity decreases, but the independent fans still provide sufficient airflow to cool the motor.

If started directly and operated at the main frequency, the independent fans function normally, and heat dissipation is generally not a problem. However, if a direct start is attempted and low-speed operation is performed for an extended period (for example, by connecting a resistor in series with the main frequency power supply or using voltage regulation), the independent fan speed will also be low, resulting in a severe lack of cooling airflow and causing severe motor overheating and burnout. This is a key difference between variable frequency motors and conventional motors: conventional motors are designed with low-speed self-cooling in mind, while variable frequency motors rely on independent fans for cooling.

5. Loss of the inverter's protection features

Inverters not only provide soft start/soft stop, but also offer comprehensive protection features for overcurrent, overvoltage, undervoltage, overload, phase loss, and overheating. During direct starting, these protections rely on external thermal relays or motor protectors, which are generally less accurate and faster than the built-in protection of the VFD.

Always start or operate a VFD motor with a matching VFD; this is the primary and only recommended method for VFD motor design. VFDs provide smooth, controlled starting (limiting current and torque), protect the motor and mechanical load, enable stepless speed regulation, and energy-saving operation.

In short, for safe, reliable operation and extended life of the equipment, VFD motors must be started with a VFD. Direct starting carries many risks and should be avoided.