1. Heat dissipation issue (most critical)
The fan of a regular AC asynchronous motor is coaxial with the rotor:
High speed → fast fan rotation → good heat dissipation
Extremely low speed → almost no fan rotation → extremely poor heat dissipation
When running at low speeds, the motor current is often high, causing severe heating and making it easy to overheat and burn out.
2. Decreased torque and easy shaking
The torque formula for asynchronous motors is roughly: T ∝ sU
two
High slip rate at low speeds
But if the voltage reduction/frequency conversion is not coordinated, the torque will sharply decrease if the voltage is directly low and the speed is low
If the load is slightly large, it will not move, shake, or even stall

3. Ordinary motors are not designed for low speeds
Ordinary Y series three-phase asynchronous motor:
The rated speed is generally 1400+rpm
Designed to achieve the highest efficiency and power factor at rated points
At low speeds:
extremely inefficient
Poor power factor
High current, high heat generation
The lubrication and vibration of the bearings have also deteriorated
4.Why can variable frequency motors operate at low speeds?
Variable frequency motors solve these problems:
Independent external cooling fan, capable of strong heat dissipation even at low speeds
Stronger insulation and resistance to high-frequency harmonics
Optimization of magnetic circuit design, high low-speed torque
Cooperate with the frequency converter to achieve constant torque and low-speed operation