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I am using UK mains (230 V phase, 400 V line) to drive a Japanese motor expecting 115 V phase, 200 V line), presently using 2:1 transformers. I would like to control speed between 30 and 70 Hz, ie +/- 20 Hz away from the current fixed speed of 50 Hz. The motor is running at 3.5 kW from a standard 3 phase mains supply.

I now want to add speed control to this system using a 3-phase in / 3-phase out inverter. I would really like to dispense with the transformers for all sorts of reasons. Is this possible, or is the output phase voltage of an inverter always equal to the input phase voltage.

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  • \$\begingroup\$ Go talk to an inverter supplier. There are dozens; they'll fix you up with what you want to know. \$\endgroup\$ – Andy aka Nov 21 '18 at 18:04
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Phase voltage generally means phase-to-neutral voltage. However 3-phase motors have their supply voltage applied line-to-line, not phase to neutral. Their is no such thing as "standard 3 phase mains supply." Various countries have standards for residential, commercial / light industrial and industrial use.

Variable frequency drive inverters (VFDs) generally have a maximum output voltage that is equal to the input voltage or no more than 5% below the input voltage. That is generally sufficient to operate a motor rated for the rated input voltage of the VFD.

Most VFDs can be configured to produce as little as half the input voltage with the output frequency equal to the mains frequency. If the VFD is configured that way, the VFD would need to be rated for twice the motor power to operate the motor at full power at the mains frequency.

Re clarification fo question

A 400/230 volt system allows for a 400 V, 3-phase load connection and three 230 V single phase connections. A VFD could be connected for 400 V and adjusted to provide 322 V at 70 Hz. That would allow the motor to provide rated torque at the increased speed. The power would increase in proportion to the speed, so the motor would provide 4.9 kW at 70 Hz. The VFD would need to be rated for a 7 kW motor since it is rated based on 400 V output.

Most motors are mechanically rated for up to 25% above their design speed. Most induction motors are capable of producing close to their rated torque with proportionally increased voltage at 25% above the rated frequency. Motors should not be operated above their rated current, but rated current should be sufficient to produce rated torque at the increased speed.

Before you buy a VFD, you should download the manual and read it carefully. Don't buy a VFD if you can not get the manual first. Pay close attention to information about voltage stress on the motor windings. Most 230-volt motors have insulation that can withstand operation with a 400 V VFD, but that cold be an issue.

Note that I have assumed that you have an induction motor. If that is not the case, some of the above may not apply.

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  • \$\begingroup\$ I am using UK mains (230 V phase, 400 V line) to drive a Japanese motor expecting 115 V phase, 200 V line), presently using 2:1 transformers. I would like to control speed between 30 and 70 Hz, ie +/- 20 Hz away from the current fixed speed of 50 Hz \$\endgroup\$ – Alex R Nov 22 '18 at 9:21
  • \$\begingroup\$ I have copied the above comment to the question. That is where it belongs. The above comment can be deleted. \$\endgroup\$ – Charles Cowie Nov 22 '18 at 16:08

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