Motor control via VFD using V/F ratio in Squared mode 7

[bamia]

Hello everyone,

This is from ABB ACH580 firmware manual:

In squared mode (default), the ratio of the voltage to frequency increases as the square of the frequency below the field weakening point. This is typically used in centrifugal pump or fan applications. For these applications, the torque required follows the square relationship with frequency. Therefore, if the voltage is varied using the square relationship, the motor operates at improved efficiency and lower noise levels in these applications. Thus using squared mode saves energy.

Can you please break it down for me in the simplest way possible.
Say for the following motor: 600 VAC, 20HP, 19.2FLA, 1770 RPM., 60Hz

In linear mode: 600/60 = 10v/Hz. So, at 10 Hz the drive will supply the motor with 100 volts and at 20 Hz, 200 volts, etc.
In Squared mode it is difficult to determine where the model starts, but let's say at 10 Hz the drive supplied 100 volts then at 20 Hz the drive will supply 20-10=2 squared = 4 x 100 volts = 400 volts.
Is that how it's suppose to work? Does it work that way from 1 volt and keeps getting squared every increment? We're going to run out of voltage pretty quick.
Clearly I'm missing the point here?

Thanks

 

[waross]

Wow. Another poster out of his area of expertise.

Is there some way to correlate the mechanical (HP = T x RPM/63025) with electrical (HP = 1.732 x amps x volts x power factor x etc /746 watts/HP)? Is the torque/amperage constant in this discussion?

Followed by another poster out of his area of expertise, adding to the confusion.
Oh my.
Is there even one way in a thousand to resolve this impasse?

 

[dvd]

Wow. Another poster out of his area of expertise.
Followed by another poster out of his area of expertise, adding to the confusion.
Oh my.
Is there even one way in a thousand to resolve this impasse?

If my comments offend you then remove them. As a moderator your comment seems petty.

 

[edison123]

The theory is, as Jeff referred to, that as the motor slows, the torque required reduces by the square of the speed, so at half speed, only 25% torque is required, so you could reduce the flux in the iron to reduce the iron loss, BUT :

This is essentially the subject of a number of patents back in the late 70's (don't ask me how I know) and was based on a scenario put forward by Frank Nola of NASA.
If the shaft load is reduced, then the iron flux can also be reduced. Reducing the flux in the iron will also reduce the iron loss, but it will increase the slip and increase th "work current".
In the case of a very small motor, the design results in an over fluxed motor and so the iron loss as a percentage of the rated load, is quite high. A reduction in flux at less than rated shaft load, can cause a greater reduction in iron loss, than the increase in slip loss resulting in some energy reduction.
In the variable voltage constant frequency implementation (Nola) on very small motors the actual reduction in iron loss can be high relative to the motor rating because the iron loss is high relative to the motor rating. Efficiencies of 50% and lower were found.
In more modern motors and motors of larger sizes, it is common for the iron loss to be less than 2% of the motor rating, so the real energy saving is typically trivial.

From my experience, if there are going to be stability problems with the likes of Middlebrook instability, it is best to set to a true constant V/Hz.

If the motor frame at reduced load is very hot, then this method may help to reduce the frame temperature at reduced load, particularly if the motor is designed with a high flux winding.

In my book, looks good on paper, real advantages are limited and stability issues can bite.

Thanks Mark. I agree there is no greater power saving for normal motors with V2/Hz operation. Torque stalling is a big possibility with low flux.

 

[edison123]

Bill - Delete your last post. I have flagged it.

 

[jraef]

Mechanical HP is the torque and speed delivered by the motor. Electrical HP consumed (absorbed) from the source is the mechanical power minus losses in the delivery of that power (mechanical HP / Efficiency percentage).

But... as Waross said, the ACTUAL HP values are what the LOAD demands for the task at hand and is (hopefully) always LESS than the rating of the motor. So if a pump LOAD is demanding 8HP from a motor at full speed, and the motor is sized at 10HP with a motor efficiency (at 80% load) of 86%, the actual HP absorbed from the source is 8HP/.86 = 9.03 HP or 6.74kW.

Now, if that same motor is on a VFD, and the VFD is operating it at 50% speed, the pump POWER REQUIREMENT reduces at the CUBE of the speed change. So that 8HP at full speed becomes 8 x .5 x .5 x .5 = 1HP, then assuming the pump and motor EFFICIENCY drops as well to 75%, the ACTUAL absorbed power from the source becomes 1HP / .75 = 1.33HP, or roughly 1kW.

As to VOLTAGE; IF the VFD is set for V/Hz, the voltage at 50% speed will be 50% voltage. If set for V/Hz squared, it will be 25%. But if you calculate the TORQUE required by the (now) 1HP load at 50% speed, you will see that the LOAD REQUIRES far less torque at that lower speed, so it doesn't actually USE it all. Ergo, I can use the V/Hz squared function to reduce the fixed iron losses in the motor, it order to improve that 75% efficiency. In reality, it might get you 1% back, maybe 2%, so it does save SOME, but it's not a lot to get excited about on a small motor**.

So if we apply this to the values above we can prove it out:
Tq required at full so]peed and full load of 8HP at let's say 1750 RPM (I live in 60Hz world, deal with it...) = 8 x 5250/1750 = 24 lb.-ft.
Tq. required at 50% speed = 1HP x 5250 / 875 = 6 lb.-ft.
So torque at half speed was reduced to 1/4 of what it was at full speed, ergo, reduced by the SQUARE of the speed change. Ergo if we use the V/Hz squared option, we deliver only what the motor NEEDS for a centrifugal machine under the Affinity Speed Laws.

**But an extra 1-2% savings is a lot of energy on a 1,000HP motor!

I will note however that I am, for the sake of keeping this simpler, leaving out the PUMP efficiency changes and head pressure issues as it relates to changes in speed and flow here, because that gets VERY complex, especially when you lower the speed TOO far. I barely passed my Mechanical Engineering classes, hence my going into electrical... I only bring it up because I am anticipating some ME chiming in here to show how this is all incomplete. It is... but WE get tasked with explaining the ELECTRICAL side of it all.

 

[dvd]

Thank you for the discussion, jraef.

My initial comment was me wondering what this looked like graphically, in the vein of what is shown below with the constant torque, constant power regimes.

 

[LionelHutz]

This discussion has made me laugh...

Good luck having a pump in practice have a useful flow running at 1/2 speed.

Also, every engineer should know that power out = power in x efficiency.

 

[waross]

If my comments offend you then remove them. As a moderator your comment seems petty

The moderator label is a false flag operation by the new management.
The only thing I have here is a false label. I am not a moderator in this forum.
I was a moderator in one and only one forum.
For years I was never identified as a moderator of that forum.
Then I was not a moderator.
Then I was a moderator for one forum.
Then I was identified, site wide, as a moderator.
I haven't bothered to check to see if I still have moderator tools for the one forum.
One more glitch from the new regime.

 

[waross]

If my comments offend you then remove them. As a moderator your comment seems petty.

Sorry.
I was overreacting.
There has been a rash of "out of expertise" postings in some electrical fora.I complained in one forum and was put firmly in my place by an "out of expertise" poster.

Waross, a forum isn't a place for experts to validate each other. That would be pointless. It's a place for people to say things, sometimes incorrect, and get corrected. This thread has a lot of that going on and that's a good thing.

There was a time when a post such as that would result in the poster being banned from the site.

Spoiler: Once apon a time

Dave's not here, man.