The frequency converter is mainly composed of rectifier (AC to DC), filter, re rectifier (DC to AC), braking unit, driving unit, detection unit microprocessor unit, etc.
1. Why can the rotation speed of the motor change freely?
1: r/min
Motor rotation speed unit: number of rotations per minute, which can also be expressed as RPM
For example: 2-pole motor 50Hz 3000 [R / min]
4-pole motor 50Hz 1500 [R / min]
Conclusion: the rotation speed of the motor is proportional to the frequency
The motors mentioned in this paper are induction AC motors, most of which are used in industry. The rotation speed of induction AC motor (hereafter referred to as motor) is approximately determined by the pole number and frequency of motor. According to the working principle of the motor, the pole number of the motor is fixed. Since the pole value is not a continuous value (a multiple of 2, for example, the pole number is 2, 4, 6), it is generally not suitable to adjust the speed of the motor by changing the value.
In addition, the frequency can be adjusted outside the motor and then supplied to the motor, so that the rotation speed of the motor can be freely controlled.
Therefore, the frequency converter for the purpose of controlling frequency is the optimal equipment for motor speed control equipment.

n = 60f/p
n: Synchronization speed
f: Power frequency
p: Pole pairs of motor
Conclusion: changing frequency and voltage is the best control method
If only the frequency is changed without changing the voltage, the motor will be out of over-voltage (overexcitation) when the frequency is reduced, which may cause the motor to burn out. Therefore, the frequency converter must change the voltage at the same time. When the output frequency is higher than the rated frequency, the voltage can not be increased continuously, and the maximum voltage can only be equal to the rated voltage of the motor.
For example: in order to halve the rotation speed of the motor, change the output frequency of the frequency converter from 50Hz to 25Hz, and then the output voltage of the frequency converter needs to be changed from 400V to about 200V
Souce:https://www.reducermotor.net/news/why-can-the-rotation-speed-of-the-motor-change-freely.html