I have already built a three-phase Permanent Magnet Synchronous Motor(PMSM) and wanted to use a VFD in sensorless vector control mode to run it. Now I want to figure out the characteristics of my motor to enter them into the VFD. By characteristics I mean Motor Nominal Voltage, Nominal Frequency, Nominal Current, and Nominal Power.

The information that I have measured from my motor are the following:

  • Each phase winding inductance
  • Each phase winding resistance
  • Voltage/Frequency ratio

Do I need to get more information from my motor to find the parameters mentioned above? If not, how can I calculate those parameters?


  • \$\begingroup\$ You need to know the power you can safely dissipate in the winding resistance without exceeding the temperature ratings of the materials you built it from; that determines the rated current. \$\endgroup\$
    – user16324
    Commented Dec 21, 2016 at 19:39
  • \$\begingroup\$ I'm not worried about heat dissipation of wires. I have picked thick gauge wires rated for the maximum current the VFD can pump current into them. I believe the rated current is different from Nominal Current. I'm more concerned about Nominal Current. \$\endgroup\$ Commented Dec 21, 2016 at 20:18
  • \$\begingroup\$ Well, doesn't the VFD documentation tell you what you need to enter to control a PM motor? What is the make and model number of the VFD? \$\endgroup\$
    – user57037
    Commented Dec 22, 2016 at 4:08
  • \$\begingroup\$ I know what parameters I need to enter to control a PM motor. I don't know how to calculate or figure out the value of those parameters. \$\endgroup\$ Commented Dec 23, 2016 at 0:43
  • 1
    \$\begingroup\$ Can you please list the things you need to enter but do not know how to calculate? Also the make and model of the VFD might be helpful. \$\endgroup\$
    – user57037
    Commented Dec 23, 2016 at 4:14

1 Answer 1


OK, I only have limited experience with these things. Not an expert. But here is what I think. What you are missing is some measure of torque, or any relationship between current and torque (this is just a linear constant). The VFD will need that.

Basically the VFD needs to know a voltage and frequency so it can figure out the V/F relationship. So pick an RPM level you consider to be the maximum normal RPM level. Call that nominal RPM. Calculate nominal voltage and nominal frequency at nominal RPM. This is the electrical frequency, in Hz, required to obtain the nominal RPM. Nominal voltage will be based on back EMF at that frequency. Nominal voltage should be RMS, not peak.

In the two or three VFD's I have looked at, the nominal frequency is not necessarily the maximum frequency the VFD will allow. But it is the one used to calculate V/F.

Nominal current and power are more tricky. Just start with a nominal current level you know is safe. Calculate nominal power using torque and motor speed (rpm). The torque you use will be based on the nominal current and the motor torque constant (which you will have to figure out somehow).

Power in kW = Torque (N.m) x Speed (RPM) / 9.5488

Later, if you increase the nominal current, you should also increase the nominal power.

The VFD probably has a way to allow temporary current increases (a torque boost function) to improve output power at low speed. But the VFD will assume that 100% torque corresponds to 100% of nominal current. And it will assume that nominal power occurs at nominal current and nominal frequency.

This should hopefully get you started without blowing anything up (I hope).

You may be able to run the motor in straight V/F mode long enough to get some torque and current measurements. Good luck!

  • \$\begingroup\$ Sorry mkeith, you didn't answer my question. You can't just "pick an RPM level you consider to be the maximum normal RPM level [and] call that nominal RPM". Plus you don't calculate V/F, you need to measure it. I also don't know my motor torque constant and don't know how to find it. You said "Just start with a nominal current level you know is safe". This doesn't make sense. nominal current is one single value. There is no "safe" or "unsafe" nominal current. \$\endgroup\$ Commented Dec 27, 2016 at 21:00
  • 2
    \$\begingroup\$ Sorry my answer didn't help you. If you are expecting someone to tell you the nominal values for a motor you built, I don't think you will get an answer. I am trying to help you get started so the motor is at least spinning and you can begin to characterize it. But it seems there are quite a few things you may not know about BLDC motors, which is causing some miscommunication. Not sure I want to invest any time in clearing it up. Maybe someone else will provide a better answer that is more helpful to you. \$\endgroup\$
    – user57037
    Commented Dec 28, 2016 at 0:23
  • \$\begingroup\$ I am not waiting for someone to tell me the nominal values for my motor, but rather how to find them or calculate them. BTW, I am already spinning my motor, but I am not sure whether I am using the correct Nominal values. I have searched alot but haven't found a procedure to find the nominal values of a home made motor. \$\endgroup\$ Commented Dec 28, 2016 at 17:57
  • \$\begingroup\$ I think you need the torque constant. You can look online for methods of measuring torque. Also, it would be good to get an idea of maximum continuous (RMS) current the motor can handle without over-heating. Do you have an idea of how fast you want it to spin for whatever you plan to use it for? \$\endgroup\$
    – user57037
    Commented Dec 29, 2016 at 8:00
  • \$\begingroup\$ I have picked gauge 8 Litz wire, which should be able to carry around 35 Amps of continuous current without over-heating. I am planning to spin it at 70Hz(4200RPM). I'll search online to learn about figuring out torque constant. Thanks. \$\endgroup\$ Commented Dec 29, 2016 at 18:05

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