Explosion proof motor manufacturer asked if there is no frequency converter, how can the motor realize speed regulation?

As we all know, frequency converter is a kind of power control equipment which uses frequency conversion technology microelectronics technology to control AC motor by changing the frequency of motor working power supply. In short, the motor uses frequency converter to adjust the speed. So, in the era of no frequency converter, how is the motor to achieve speed regulation? Today, explosion-proof motor manufacturers come to popularize this problem for everyone. I don't understand it!

When there was no frequency converter, it was basically the world of DC motor speed control system. If before the 1960s, when the thyristor was born, it was still driven by the generator to speed DC motor. Here are three ways to adjust speed:

Slip speed regulation: it means "slip" to regulate the speed. The actual motor speed will change after starting, but it will slip through the "slip head" between the motor the load, so as to reduce the speed on the load side.

Variable pole speed regulation: in addition to variable frequency speed regulation, there is actually a debugging method for asynchronous motor, which is to change the number of poles to achieve speed regulation. For example, when the speed of four pole motor is 1500 revolutions, the eight pole motor is only 750 revolutions.

DC motor speed control system: as long as the armature current is adjusted, it can directly achieve accurate torque control of the motor, can easily meet the control requirements of constant torque, which is also the root cause of the very good low-speed torque of DC motor speed control system.