# How can I apply a variable phase shift to an AC signal generated by a VFD?

In summary: No. Input and output phase difference is my concernThe conversation is about how to introduce a variable phase shift in an AC signal generated by a VFD. The speaker is looking for a solution that does not require a delay line. Some suggestions are given, such as using a Thyristor's firing angle or using transformers, but the speaker needs to clarify their ultimate goal and the problem they are trying to solve. It is also mentioned that a variable phase shift may not make sense unless there is a reference point, such as the motor shaft for a synchronous AC machine. The speaker also needs to consider current feedback if using an induction machine. Ultimately, more clarification is needed to provide a clear solution.
TL;DR Summary
How to introduce a variable phase shift in AC signal generated from a VFD.
Hey fellas,
I’m trying to apply a variable phase shift (0 to 180 degrees) on an AC signal generated from a Variable Frequency Drive (VFD). The frequency is variable and ranged between 10 and 50 Hz. the voltage amplitude is also variable between 10-50Volts (this shouldn’t be a problem I suppose).
One solution I could find was using Thyristor’s firing angle, but it also needs a delay line on its way, which makes me confused.
Any help would be greatly appreciated.
Thanks

Are you talking about the phase angle between voltage and current?

russ_watters said:
Are you talking about the phase angle between voltage and current?
No. Input and output phase difference is my concern

No. Input and output phase difference is my concern
Um. What's the input and what's the output? Voltage? Current? How is the "phase" defined?

You need to clarify what it is you're looking to resolve.

russ_watters
Input and output phase difference is my concern
Do you have single or three phase power?

I guess you want the same output frequency as the input frequency, but with a specified phase shift of the voltage.

The VFD will have an internal clock at a much higher frequency than the output. You will need to continuously trim the frequency of the drive through an external connection, to maintain the required phase shift. That may require an external phase locked loop.

There are other ways of generating phase shifts of three phase voltages by using transformers.

Maybe if you explained why you needed the phase shift we could better identify a technique.

1. Sample the input continuously using an analog-to digital converter.
2. Do a digital delay of the digital signal using either shift registers or a microcontroller with a circular buffer
3. Run the delayed digital output through a digital-to-analog converter.

No. Input and output phase difference is my concern
If the input and output are a different frequency, can they even be said to have a phase difference?

Can you tell us more about what the problem and ultimate goal are here?

Summary:: How to introduce a variable phase shift in AC signal generated from a VFD.

Hey fellas,
I’m trying to apply a variable phase shift (0 to 180 degrees) on an AC signal generated from a Variable Frequency Drive (VFD). The frequency is variable and ranged between 10 and 50 Hz. the voltage amplitude is also variable between 10-50Volts (this shouldn’t be a problem I suppose).
One solution I could find was using Thyristor’s firing angle, but it also needs a delay line on its way, which makes me confused.
Any help would be greatly appreciated.
Thanks

So generally a VFD uses motor shaft position (or a sensor-less algorithm) to determine the correct applied voltage on the phases. When operating at a constant motor speed (constant VFD frequency) torque is controlled by changing the angle of applied voltage relative to shaft position such that the currents generated produces torque (ie the purpose of a VFD and E machine).

A variable phase shift on a generated voltage does not make much sense unless you have a reference from which you are trying to shift from, in the case of the VFD, the angle reference is the motor shaft. As note above, phase angle does not make sense if the frequency is not the same. Eg 50Hz 3 phase is made up of three voltages separated by 120deg, a one million Hz 3 phase voltage, is also made of up three voltages separated by 120 deg. But a phase angle between 50Hz and 1MHz cannot be defined, its constantly changing.

If you want to convince your VFD to make variable frequency 3 phase voltages you need to spoof the position sense signal it would be getting from the motor shaft. Even then it may get upset if its not getting current feedback, as this is generally also controlled for, ie without it the VFD will likely saturate its current controller and depending on how its coded, generate a square wave (ie heavily saturated, or "clipping") not a sine wave.

Actually my post is relevant to a VFD running a synchronous AC machine, for an asynchronous machine (ie induction motor) it may just output a variable frequency fixed voltage with no position feedback since induction machines do not need this. If you have a single phase VFD to run an induction machine, and you want a phase angle, you have to ask your self, a phase angle relative to what.

russ_watters
essenmein said:
Actually my post is relevant to a VFD running a synchronous AC machine,
The OP needs to clarify. If he means phase relative to an external load like a synchronous motor, then the phase difference between VFD and load is proportional to the power transferred. Change the power and you change the angle.

IMO there is no way to make sense of this question without clarification. Phase with respect to what?

eq1

## 1. What is AC Variable Phase Shift?

AC Variable Phase Shift is a technique used in electrical engineering to control the phase angle of an alternating current (AC) signal. This allows for precise manipulation of the timing and amplitude of the signal, which is useful in a variety of applications such as power distribution and signal processing.

## 2. How does AC Variable Phase Shift work?

AC Variable Phase Shift works by using electronic components such as capacitors, inductors, and resistors to alter the phase angle of an AC signal. By adjusting the values of these components, the phase angle can be changed to the desired amount.

## 3. What are the benefits of using AC Variable Phase Shift?

The main benefit of using AC Variable Phase Shift is the ability to control the timing and amplitude of an AC signal, which can be useful in a variety of applications. For example, it can be used to improve power efficiency in electrical systems or to manipulate the frequency response of a signal in audio equipment.

## 4. Are there any limitations to AC Variable Phase Shift?

One limitation of AC Variable Phase Shift is that it can only be used with AC signals, not direct current (DC) signals. Additionally, the components used in AC Variable Phase Shift circuits can have tolerances that may affect the accuracy of the phase shift.

## 5. How is AC Variable Phase Shift different from DC Variable Phase Shift?

AC Variable Phase Shift and DC Variable Phase Shift are similar techniques, but they differ in the type of signal they are used with. AC Variable Phase Shift is used with AC signals, while DC Variable Phase Shift is used with DC signals. Additionally, the components used in each technique may differ due to the different properties of AC and DC signals.

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