# vfd voltage output



## GEORGE D

Ok guys, probably a really dumb question for most of you but this area is new for me and just wondering if the voltage on the load side of a vfd changes or just the frequency or both?


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## varmit

A VFD controls voltage, Hz, and current/torque.


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## John Valdes

varmit said:


> A VFD controls voltage, Hz, and current/torque.


And to be a bit more specific the VFD uses the voltage in direct proportion to the Hz to change motor speed.

Example: 480 Volt Motor, 1800 RPM, 60 Hz rated.

480 volts @ 60 Hz = 1800 RPM*
240 volts @ 30 Hz = 900 RPM*
120 volts @ 15 Hz = 450 RPM*
60 volts @ 7.5 Hz = 225 RPM*

VFD's can also over speed motors. Meaning you can operate the motor above at higher RPMs. Since you have 480 as the nominal input voltage you will ask, how.
The VFD bus voltage at 480 is in the 650 volt range. This gives you some wiggle room for over speed. Keep in mind that motor torque decreases as soon as you exceed base speed (1800) RPM.

* Slip not considered. + or - 10% is possible.


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## Hunter1151

John Valdes said:


> And to be a bit more specific the VFD uses the voltage in direct proportion to the Hz to change motor speed.
> 
> Example: 480 Volt Motor, 1800 RPM, 60 Hz rated.
> 
> 480 volts @ 60 Hz = 1800 RPM*
> 240 volts @ 30 Hz = 900 RPM*
> 120 volts @ 15 Hz = 450 RPM*
> 60 volts @ 7.5 Hz = 225 RPM*
> 
> VFD's can also over speed motors. Meaning you can operate the motor above at higher RPMs. Since you have 480 as the nominal input voltage you will ask, how.
> The VFD bus voltage at 480 is in the 650 volt range. This gives you some wiggle room for over speed. Keep in mind that motor torque decreases as soon as you exceed base speed (1800) RPM.
> 
> * Slip not considered. + or - 10% is possible.


If you are running volts per hertz, yes output voltage can be less than 460 under 60hz


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## etb

To add some more:

For induction motors:
The output of a VFD running equal V/Hz will follow as listed above, but on good models (that you've programmed correctly) you'll see higher V/Hz at the lower frequencies in order to maintain torque (effect of resistance is more pronounced at lower speeds).

But all that goes out the window on a vector drive. It will output whatever it needs to do based on feedback.

For servos:
Since these are synchronous, speed is controlled by adjusting frequency only and voltage is not tied to frequency but rather, is adjusted to control torque.


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## Hunter1151

etb said:


> To add some more:
> 
> For induction motors:
> The output of a VFD running equal V/Hz will follow as listed above, but on good models (that you've programmed correctly) you'll see higher V/Hz at the lower frequencies in order to maintain torque (effect of resistance is more pronounced at lower speeds).
> 
> But all that goes out the window on a vector drive. It will output whatever it needs to do based on feedback.
> 
> For servos:
> Since these are synchronous, speed is controlled by adjusting frequency only and voltage is not tied to frequency but rather, is adjusted to control torque.


AMEN, great explanation


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## John Valdes

Two trolls working in unison. Just as I expected.


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## etb

I think people's troll-detecting heuristics are pretty accurate. But aside from two noobs posting on similar threads, I can't see it here.

The OP didn't understand how a VFD works and he's asking a simple question. I think he'd have better luck with google, but who am I to judge? Maybe the replies will give him a starting point. Varmit's response answered the question, yours and mine elaborated. Some people threw out some answers to a simple question; nothing else to it in my book.


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## John Valdes

etb said:


> I think people's troll-detecting heuristics are pretty accurate. But aside from two noobs posting on similar threads, I can't see it here.
> 
> The OP didn't understand how a VFD works and he's asking a simple question. I think he'd have better luck with google, but who am I to judge? Maybe the replies will give him a starting point. Varmit's response answered the question, yours and mine elaborated. Some people threw out some answers to a simple question; nothing else to it in my book.


The point you are missing is this post is months old and the OP is long gone.
While we all welcome good advice, I think you are a little late. 
And when I see two fairly new members answering old post's and both seem to be answering in unison, I smell troll. That's all. It takes some time around here to be taken seriously. I have been here since 2007 and no one takes me seriously.


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## etb

John Valdes said:


> The point you are missing is this post is months old and the OP is long gone.


Ah...I see what you mean now. I didn't even notice that. I've just always clicked on the "New Posts" link; that would explain why the other guy & I both hit it. I do realize that replying to an old post is of little value to the OP and not relevant to the rest of us; I'll have to watch that better in the future.


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## John Valdes

etb said:


> Ah...I see what you mean now. I didn't even notice that. I've just always clicked on the "New Posts" link; that would explain why the other guy & I both hit it. I do realize that replying to an old post is of little value to the OP and not relevant to the rest of us; I'll have to watch that better in the future.


Welcome to the forum.


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## miller_elex

What model / make of drives are you referring to that adjust voltage and freq??

The only drives I am familiar with use Pulsed DC, based on a carrier frequency, to simulate AC current in the motor windings.

The drive takes AC off the L-leads, rectifies, and the control section modulates the DC sent down the T-leads.

I've only ever seen it once, there's never a reason to hook an O-scope up to the drive output and see the pulse modulated DC. Unless you're attempting a bench repair of the drive, and no one wants to do that because of liability reasons.


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## etb

miller_elex said:


> What model / make of drives are you referring to that adjust voltage and freq??
> 
> *All of them, except DC drives.*
> 
> The only drives I am familiar with use Pulsed DC, based on a carrier frequency, to simulate AC current in the motor windings.
> 
> *More than just "pulsed DC"... it's a bipolar signal (**the +&- DC buses are above and below neutral, so the drive can output +,-, and zero)**. The drive adjusts **voltage and the frequency by adjusting the duty cycle ... that's all it CAN do.*
> 
> The drive takes AC off the L-leads, rectifies, and the control section modulates the DC sent down the T-leads.
> 
> I've only ever seen it once, there's never a reason to hook an O-scope up to the drive output and see the pulse modulated DC. Unless you're attempting a bench repair of the drive, and no one wants to do that because of liability reasons.


See comments above.

Plenty of reasons to use a scope. For instance, it would show you where you're not putting 2 & 2 together. Namely, the drive is outputting this bipolar stepped waveform; if you put it through a lowpass filter (similar to a time average), it would resemble an AC sinewave. Now think about what makes a motor operate (current) and what a winding looks like electrically (inductor). The motor's inductance low-pass-filters this jittery voltage and results in something that resembles a sinewave current, adjustable in voltage, frequency, and even phase. Do you see it now?


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## Jlarson

etb said:


> See comments above.
> 
> Plenty of reasons to use a scope. For instance, it would show you where you're not putting 2 & 2 together. Namely, the drive is outputting this bipolar stepped waveform; if you put it through a lowpass filter (similar to a time average), it would resemble an AC sinewave. Now think about what makes a motor operate (current) and what a winding looks like electrically (inductor). The motor's inductance low-pass-filters this jittery voltage and results in something that resembles a sinewave current, adjustable in voltage, frequency, and even phase. Do you see it now?


Good reasons to have a multi meter with low pass filtering. A normal DMM voltage measurement might read 20-30% higher than the drive's read out because the DMM is also measuring the carrier frequency.


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## miller_elex

I get how it works, with the duty cycle, carrier freq and pwm... and how that translates to an rms voltage and the f in 120f/p.

IDK, to me a drive is all about the control loop, where the money is saved.


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## rick279

GEORGE D said:


> Ok guys, probably a really dumb question for most of you but this area is new for me and just wondering if the voltage on the load side of a vfd changes or just the frequency or both?


Both. Relationship between Voltage and Frequency on a VFD output is typically linear. 1.5 Hz = 12V, 30 Hz = 220V, 60 Hz = 480V. Roughly speaking. By the way input *power* and output *power* on a VFD is also roughly the same.


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## John Valdes

rick279 said:


> Both. Relationship between Voltage and Frequency on a VFD output is typically linear. 1.5 Hz = 12V, 30 Hz = 220V, 60 Hz = 480V. Roughly speaking. By the way input *power* and output *power* on a VFD is also roughly the same.


It is linear and directly proportional. Voltage and Cycles (Hz). You are asking about watts and not voltage, when you say *power*?

Input power and output power are most always NOT the same unless you are running the motor at base speed.
Input power is steady while out power is adjustable. Is this what you are asking about?


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## rick279

John Valdes said:


> It is linear and directly proportional. Voltage and Cycles (Hz). You are asking about watts and not voltage, when you say *power*?
> 
> Input power and output power are most always NOT the same unless you are running the motor at base speed.
> Input power is steady while out power is adjustable. Is this what you are asking about?


Yes watts as power and they are the same ( in/out ) minus losses.


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## weglobal

:thumbsup: Wall outlet voltage
Could some people tell me what is the wall outlet voltage in your area. I only need to know city and state. I need to know how much differs from place to place throughout US. Thanks


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## rick279

John Valdes said:


> It is linear and directly proportional. Voltage and Cycles (Hz). You are asking about watts and not voltage, when you say *power*?
> 
> Input power and output power are most always NOT the same unless you are running the motor at base speed.
> Input power is steady while out power is adjustable. Is this what you are asking about?


It is not necessarily linear. When a VFD is set for a Variable Torque load, which can be the default on many, voltage/HZ more closely resembles a Fan or Pump Curve.


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## Wirenuting

John Valdes said:


> The point you are missing is this post is months old and the OP is long gone.
> While we all welcome good advice, I think you are a little late.
> And when I see two fairly new members answering old post's and both seem to be answering in unison, I smell troll. That's all. It takes some time around here to be taken seriously. I have been here since 2007 and no one takes me seriously.


I understand the hitting on old posts. I have done it also. In violation of posting tip # 173.4b and I don't read the OP date before I open my mouth. 
But on the flip side of the coin,, I see a lot of great answers being written after a thread is re-opened.

Shoot,, I did it again. John posted this answer a few weeks ago. 

Ok,, I'm going for coffee.


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