# How to calculate the DC rail voltage for Variable frequency drive (VFD) for single phase induction motor?

I am working on VDF for Sigle phase induction motor. I just want to compute the total voltage of single-phase induction motor that will need to create from DC link. I am not sure about the calculation. As we need SPWM based inverter for VFD of the Single induction motor. The input of the inverter is DC link voltage. how can I calculate the required voltage for DC link for Single phase induction motor? The voltage rating for induction motor is 220V,

• If this VFD is line powered, then the way this really works is that you work with the input voltage you have. If you have 120V input, rectify it so that it is doubled. You can look up how to do this. If you have 240 coming in, just use a bridge rectifier. You may use some scheme to improve power factor and reduce harmonic distortion. But I don't think you want to add a whole separate AC to DC converter inside your VFD to supply a regulated voltage to the DC link. But I am not an expert. May 27, 2017 at 20:22
• @mkeith you can always make a boosting PFC and get higher voltage. May 27, 2017 at 20:24
• Dear @mkeith , i will not rectify DC from AC for VFD. I will get DC voltage from Solar panels. so there will not need for PFC . May 27, 2017 at 20:43

The DC voltage is calculated for the maximum AC voltage you want the converter to output. For a single phase 220VAC the DC would be:

Vdc = sqrt(2) * Vac + x + y + z = 1.41 * 220 + x + y + z = 310.2V + x + y + z


Where x is the maximum voltage drop of all the components through which the current flows and y should be added to ensure a low THD of the waveform due to the switching deadtimes and limitations of the controller. Here z stands for the half voltage ripple of the DC link capacitor. I would let:

x = 10-15V
y = 10% * 310 = 31V
z = 10%/2 * (310+x+y) = 18V


So total DC voltage you need to get a nice and clean 220VAC sinewave would be:

Vdc = 310.2V + x + y + z= 380V DC.


Of course you should verify x, y and z components for your design, but this is the basic route you should follow.

• Thank you, soo much @Todor Simeonov for replying. can you refer me any book or research paper for deep understanding . ? May 27, 2017 at 20:19
• @Fame313 Sorry, I can't. The text above is mostly based on my experience :) May 27, 2017 at 20:22
• Just a quick note. There is a lot of variation in voltages in different countries. In the US, we even sometimes have 208 and call it 220 anyway. And most of what we call 220 is probably more like 230 or 240. Just an FYI. Make sure you know what your target output voltage actually is. May 27, 2017 at 20:24
• our country allow 220VAC voltage May 27, 2017 at 20:30
• If you put the line voltage (line-to-line, like 380V) in the equation - yes. The line voltage is sqrt(3) times the phase voltage, so total DC should be nearly doubled. May 27, 2017 at 20:49

As another aside, how well did you vet this concept in the first place? There are many different types of single phase motors, only two of them are suitable for use on a VFD; Permanent Split Capacitor (PSC) type and Shaded Pole. Shaded Pole motors can also be controlled with simple voltage control, no need for the complexity of a VFD, so effectively you would only use a VFD is the motor was PSC type. Any other single phase motor type will result in failure of the motor, the VFD or both.

But to answer your question, AC V = DCV/1.41, so if you want 220VAC, the DC V must be at least 310V. But here again are some potential pitfalls. "220V" is not a standard motor design voltage, it is usually 230V, so the DC voltage would need to be 324V minimum. Since you will be doing SPWM, you are going to be in control of the output RMS voltage, so it's best to start out a little high on the DC bus; most VFDs run the DC bus at around 330V.

• "220V is not a standard motor design voltage" ... Pre-harmonization continental Europe begs to differ.
– mlp
May 9, 2018 at 12:47