Motor control via VFD using V/F ratio in Squared mode 5

[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.