1.1. Is it necessary to put a voltage stabilizer in front of the frequency converter?
Only in case of extreme necessity, because a good stabilizer for such power can cost more than a frequency converter. It is extremely undesirable to use stepped relay stabilizers. The frequency converter is a capacitive load, the relay contacts in the stabilizer will fail very quickly. It is better to use just a frequency converter with a KCM, it works in the range of 170-260 Volts without deterioration of traction characteristics.
1.2. I have several engines, I want to work with them in turn from one frequency converter. Can I just put a switch between the drive and the motors?
Yes, but you can switch only when the frequency converter is turned off. It is strictly forbidden to connect and disconnect the motors during rotation. It is also necessary to pay attention to the quality of the switches, they must withstand the rated current of the motor for a long time.
1.3. Can I adjust the frequency of rotation of a single-phase motor using your frequency converter?
No. Our frequency converters work only with three-phase motors and check the health of all three phases before starting. There are many types of single-phase motors (capacitor, with a starting winding, etc.). Each type requires its own control algorithm for safe operation in the entire range. There are specialized frequency converters for single-phase motors. Consult your engine manufacturer.