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

[nima_kheradhoosh]

TL;DR

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

 

[russ_watters]

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

 

[nima_kheradhoosh]

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

No. Input and output phase difference is my concern

 

[gneill]

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.

 

[Baluncore]

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.

 

[Svein]

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.

 

[russ_watters]

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?

 

[essenmein]

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.

 

[essenmein]

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.

 

[anorlunda]

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?