Motor control via VFD using V/F ratio in Squared mode 1

[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

 

[LittleInch]

The key word is below the field weakening point.

At synchronous speed you haven't reached that point.

So I read it as that this speed , 1770, is the end point, I.e.10V/ Htz.

Below that the squared rule operates.

 

[waross]

Clearly I'm missing the point here?

Think of it as voltage root mode.
Start with rated voltage at rated frequency and work backwards down from there.
Eg: 50% Hertz = 25% Volts.

 

[edison123]

The key word is below the field weakening point.

At synchronous speed you haven't reached that point.

So I read it as that this speed , 1770, is the end point, I.e.10V/ Htz.

Below that the squared rule operates.

At around 25 Hz, you already hit the field weakening point but achieved only at 42% synch speed.

Hope jeff weighs in. Never heard of this squared mode.

lps for OP for starting this thread.

 

[LionelHutz]

Your math isn't correct.

Make 0-60Hz = 0-100% frequency or 0-1 PU frequency. The voltage is the square of this frequency range.

The field weakening point happens when the VFD runs out of voltage. This typically occurs around 100% speed.

 

[Gr8blu]

Almost all motor-driven process applications fall into one of three possible torque profiles.
1) CP (constant power). The motor produces the SAME power at all speeds.
2) CT (constant torque). The motor produces power that is LINEARLY proportional to the speed.
3) VT (variable torque). The motor produces power that in NONLINEAR with respect to speed (e.g. affinity laws: k = HP/RPM^2 or k = HP^3/RPM^2)

A motor may have multiple profiles, depending on the operating speed.
One of the most common is to have a constant (or variable) torque range followed by a constant power range.
Typically, the constant (or variable) torque range is from zero to some frequency; most cases limit the upper end to the prevailing AC line frequency (50 or 60 Hz).
The constant power range then extends into the "field weakened" range, which corresponds to frequencies above the nominal line (> 50 or 60 Hz).

Most drives are essentially current limiting devices. This means that the voltage will be proportional to the power output required. For a "square law" relationship such as a pump, the PER UNIT voltage will be proportional to the PER UNIT power and the SQUARE of the PER UNIT speed.

All ABB is telling you is that they pre-programmed a curve in their drive (e.g. "squared mode") to mimic the specific affinity law where:
k = per unit HP/ per unit RPM^2 = per unit V/ per unit HZ^2