I have a 1.5kW three phase electric motor that is in a delta configuration. This motor has a motor voltage of 230/400V and a rated current of 5.9/3.4A. What sort of VFD should i be looking for that will be suitable for the above mentioned electric motor? It can be assumed that the power supply is from a single phase 230V power outlet.

Furthermore, I would like to know if only a VFD is necessary or should i be looking to add a contactor or relays for this circuit? The application of the motor is to drive one roller of a metal powder rolling mill.

  • 2
    \$\begingroup\$ Is powder rolling speed sensitive? Or would you like to be able to adjust the speed? If so you need a VFD. Is it low speed operation? If so you need a VFD. If it is a high-torque startup operation (which doesn't sound like powder rolling is), then you *might& need a VFD but you might also be able to get away with cheaper soft-start circuitry. Otherwise you don't. \$\endgroup\$
    – DKNguyen
    Commented Jun 3, 2021 at 23:06
  • \$\begingroup\$ Have you already tried a run+start capacitor circuit? \$\endgroup\$
    – Janka
    Commented Jun 4, 2021 at 0:27
  • \$\begingroup\$ You need a 1500 W VFD that accepts single-phase input power. Should be many choices available. You need some form of disconnect between the line and the VFD that is readily accessible from the VFD. If the VFD is close to the motor, then you do not need a three-phase contactor unless a building or electrical inspector tells you that you do. If the motor is far from the VFD, then a three phase contactor close to the motor would be recommended. Generally speaking, you never want to open the contactor while the motor is spinning. It is just for electrical safety if you have open the Jbox. \$\endgroup\$
    – user57037
    Commented Jun 4, 2021 at 2:32

2 Answers 2


The first requirement is a VFD that is rated for single-phase, 230 V input and output current and power that is adequate for your motor. The VFD must also be rated for a constant torque load. Some are intended only for fans and centrifugal pumps. Some manufacturers may call VFDs for constant torque loads "heavy duty." They are usually rated to provide 150% of rated output current for one minute.

You may want to consider buying from a local supplier that may be willing and able to answer questions about the use of the product. Consider the local reputation of both the supplier and the manufacturer.

If you absolutely need to disconnect the motor instantly for some safety reason, you need a contactor. Using the VFD to start and stop the motor is best except for that kind of circumstance.

You don't need relays unless there is some interface with the control of the overall machine that requires that.

You do need branch circuit protection. The VFD manual should provide guidance about selecting that. There are other installation details that you should learn about before purchasing the VFD.

Before purchasing the VFD, you should download the manufacturer's instruction material and study it. If that is not available, you probably look for products of other manufacturers.


The key to minimizing cost in a VFD is to increase the acceleration time to reach full speed and thus lower the current demand on the drivers.

At a minimum you need a VFD rated for your 200V class 3ph output with 1 ph 200V class input for 2Hp motors. The advantage of VFD’s is the output ramps up voltage and frequency for smoooth acceleration and possible constant current if inertia is the major load in starting.

But there are many options for carrier frequency to minimize motor noise or inverter noise as tradeoffs and smart sensors with analog and digital remote controls.

I recommend Hitachi brand and that you educate yourself on this low cost option that needs a line filter for EMI EC compliance. Cost goes up 10 fold with overcurrent capacity and surge torque demands but again with smooth acceleration , I think this would be one of your best quality/$ ( or bang for the buck) options. There is a learning curve on interfacing so skip to that section and it’s easy once you learn the lingo of the technology these units have to offer. Or go search for some YouTube videos on this unit.



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