What does "Break Frequency" for a VFD mean?

[servospec]

I have an ABB ACS800 vfd and have a question about a parameter in group 30. It is the break point or break frequency. I have seen the same parameters in other drives also. I cannot seem to get a meaningful explanation of what exactly this means. Can anyone break it down in simple terms?

Thanks.

 

[Skogsgurra]

That is usually the same thing as the -3 dB point.

The -3 dB point (or frequency) is a simplified description of a first order system, usually a low pass system and tells where the Bode diagram is 3 dB down from DC or low frequency gain. Above that frequrncy, the Bode diagram has 'unit slope'. That means that gain decreases 20 dB per decade. Or, in other words, gain is inversely proportional to frequency.

The phase shift is -45 degrees at the break frequency.

It may be a gross simplification to model a servo system as a first order low pass. OTOH, it works quite well as long as there are no other poles or zeros close to (or below) the break frequency.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

This is a parameter that can be adjusted!?!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

Yes. You adjust it to the value needed. It depends on external factors. Like motor, inertia and such things.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

OK!
Thanks.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

Just to confuse you: It may also refer to internal filters. But then, it is more common to use the filter's time constant.

There is a simple relation between the two, break frequency (f0) = 1/(time constant*two*PI)

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[DickDV]

In ABB's Group 30, Break Frequency refers to the frequency (speed) at which the thermal overload calculation begins its derate. At all lower frequencies (speeds), the derate is applied to compensate for a shaft fan (as in TEFC motor) that is turning slower and thus is not able to provide cooling as at full speed.

Or, in other words, it is part of a set of four parameters that build a custom thermal overload curve for a motor.

Note in the Instruction Manual diagrams about overload curves that Break Frequency is the point where the curve changes from flat (horizontal) to sloping.

It has nothing to do with filters or frequency response in this context.

 

[Skogsgurra]

Sorry. Should have checked that.

I think that the term break frequency should be reserved for its original purpose.

As the OP said, he has seen it in other contexts and I cannot imagine that it is used to describe how thermal protection behaves in other texts.

I think that it is one of the more and more common 'finglish' translation problems that one can find in ABB manuals produced in Finland. I keep a list of some quite funny, but potentially dangerous translations. And it is growing.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ozmosis]

I have visions of servospec and his team, after reading gunnar's initial reply, running back into the plant shouting "f***, don't change it!"...
;-)

 

[sreid]

Just to add confusion, "Brake Frequency" is where DC Braking starts.

 

[itsmoked]

That! was what I originally thought.

The jury is in... You must read your particular VFD's manual!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[servospec]

After reading further in the manual once again,the term is "break frequency", not to be confused with Dynamic "Braking". Apparently it does have to do with what DickDV posted. It seems related to the point of 100% motor FLA, at the specified frequency (not necessarily base frequency), the motor load curve and the degree of motor cooling which in my case is an externally mounted blower. I just do not seem to comprehend this "break point". The way the ABB Firmware manual describes it is not easily understood.

 

[DickDV]

In your case with an auxiliary constant speed blower, the break frequency should be set at 0Hz. That makes the 100% torque level continue all the way down to zero speed so no thermal derating is done as the motor slows down. The zero speed load parameter need not be changed since it no longer applies.

 

[servospec]

Thanks DickDV. The break frequency is now set at 45hz and this creates a problem. I will make this change at the first opportunity.