DC Bus Compensation

[Marke]

Is there a way to disable the DC Bus compensation on the PDL Elite VFDs and the ATV 61 VFDs?
We have found this can be necessary to prevent oscillation with drives used with harmonic filters on weak supplies.

Mark Empson
Advanced Motor Control Ltd

 

[LionelHutz]

I've also seen a VFD have some torque or current instability during regeneration into a resistor and I suspect it was the bus compensation.

 

[controlsdude]

Here is an explanation on bus reg disable from a ab powerflex pdf

ATTENTION: The bus regulator function is extremely useful for
preventing nuisance overvoltage faults resulting from aggressive
decelerations, overhauling loads, and eccentric loads. However, it can
also cause either of the following two conditions to occur.
1. Fast positive changes in input voltage or imbalanced input voltages
can cause uncommanded positive speed changes;
2. Actual deceleration times can be longer than commanded
deceleration times
However, a “Stall Fault” is generated if the drive remains in this state
for 1 minute. If this condition is unacceptable, the bus regulator must be
disabled (see parameter A117). In addition, installing a properly sized
dynamic brake resistor will provide equal or better performance in most
cases.

 

[mikekilroy]

yep, like I explained in earlier post, a scheme to increase decel ramp when too much regen power is immenent. I believe what most folks call 'bus comp' is indeed just this. More descrip from powerflex manual::

This method takes advantage of the characteristic of the induction motor whereby frequencies greater
than zero (DC braking) can be applied to a spinning motor that provides more braking torque without
causing the drive to regenerate.
1. On Stop, the drive output decreases based on the motor speed, keeping the motor out of the regen
region. This is accomplished by lowering the output frequency below the motor speed where
regeneration does not occur. This causes excess energy to be lost in the motor.
2. The method uses a PI based bus regulator to regulate the bus voltage to a reference (for example
750V) by automatically decreasing output frequency at the proper rate.
3. When the frequency is decreased to a point where the motor no longer causes the bus voltage to
increase, the frequency is forced to zero. DC brake is used to complete the stop if the DC Braking
Time is non-zero, then the output is shut off.
4. Use of the current regulator ensures that over current trips don’t occur and enable an easily
adjustable and controllable level of braking torque.
5. Use of the bus voltage regulator results in a smooth, continuous control of the frequency and forces
the maximum allowable braking torque to be utilized at all times.
6. Important: For this feature to function properly the active Bus Reg Mode A or B must be set to
Adjust “Freq” and NOT be “Disabled”.

Just reconfirms this is simply a routine to change decel ramp to attempt preventing overvoltage fault.

Hitachi does same with their AVG parameter B0161:

DC Bus AVR (Automatic Voltage Regulation
This function is to achieve stable DC
bus voltage in case of deceleration. DC
bus voltage rises du e to regeneration
during deceleration. When this
function is activated ( B130=01 or 02),
inverter controls the deceleration time
so that the DC bus voltage not to go up
to the overvoltage trip level, and leads
to the trip-less operation during
deceleration.
Please note that the actual
deceleration time can be longer in this
case.

Now I accept there may be a few drives with a routine that attempts to maintain a constant output voltage on a v/hz drive at a constant speed if bus voltage changes. But it still seems a waste when a few percent voltage change will have such a small effect on the motor performance and this routine must by definition be limited in range of adjustment.

It also is of zero value in a true vector drive since we regulate current and let the voltage do anything it needs in the process.

I also question any drive trying to increase output voltage by increasing PWM freq beyond where it should be to put harmonics in the motor to attempt to raise the average vs. rms sine motor voltage in attempt to raise voltage at a given set speed: we spend a majority of our time in design and application of drives striving to reduce output harmonics, which do nothing but add unusable heat in the motor and thus reduce its capacity.

I have searched many drive manuals in addition to the ones I have used and am familiar with and have found no definitive description to anything such as referred to in the initial posts here. I am still not a believer in a magic 'dc bus comp' routine that does more.

 

[LionelHutz]

Really? Most every drive will employ a form of it coded into their V/Hz PWM routine. You often won't likely find anything at all written about it in the manual. You can test it by applying 500VAC to a VFD and checking that the motor still runs at the 460VAC or 480VAC level you programmed in the drive.

 

[mikekilroy]

really, from kiljoy.

 

[Marke]

Hi Mike and Controlsdude

There are two different algorithms often provided within the drive. These are usually transparent and the operator is not usually aware of them.
One algorithm normalizes the output voltage so that it becomes more independent of the supply voltage. This is usually referred to as DC Bus compensation. As the supply voltage reduces at a given output speed, the modulation is increased to keep the output voltage constant, and as the supply voltage increases, the modulation is reduced to keep the output voltage applied to the motor constant.

This is totally different to the algorithm which alters the speed of the motor in a manner to a) provide a ride through in the case of a supply failure and b) reduces deceleration rate in the case of increased DC bus voltage on deceleration.

One algorithm controls the modulation and is built into the PWM generator, the other controls the frequency for a different reason.
The two processes are getting confused, but do both exist for different reasons.

It is the voltage regulation that enables you to operate a lower voltage motor on a drive, i.e. a 230V motor on a 460V drive, plus it corrects for supply voltage variations.

Mark Empson
Advanced Motor Control Ltd

 

[mikekilroy]

Marke, you are correct of course that these are two completely different things; I was simply pointing out that the pwm FREQ change routine listed as the bus comp is NOT what you referred to in the beginning of your post.

Yes, there can be a routine to change PWM width and not freq as you stated. Yes, it may be there in some drives but it is NOT adjustable or even referenced in most manuals.

What IS referenced is the PWM FREQ changing for the other reasons.

Also do not mix in setting a max output freq to this discussion: keep in mind that has nothing to do with running a 230v motor on a 460v drive; that is simply setting the v/hz to stop at 230v output rather than going up to 460.

Lots of confusion in this thread.

 

[controlsdude]

example:
Well only used this feature on an overhauling load, where I just wanted the speed to be constant after long acceleration ramp up. Very simple application and nothing fancy going on with my application. So the result of it was that when I disabled the bus regulation, the speed stayed constant. Before I disabled this the load varied in speed which is not what I wanted at all. I did not even use the dynamic brake, just let the load coast to a stop.

 

[Marke]

Hello Mike

The voltage regulation algorithm I referred to is commonly used in drives but usually not referenced in manuals. In some cases, there is a publically available means of turning it off as in the drives referred to by Ozmosis, in others it is hidden in the "engineering menus" and in some cases, it is just not able to be turned OFF at all.
My reason for asking the question on the two drives referred to, is that these drives do include this algorithm and they do exhibit instability on weak supplies with tuned filters on the input. We do experience this in the field and need a solution because harmonic filters are now mandatory in some areas here. Three other major drive suppliers that are used locally do provide a means of disabling or detuning this functionality.

Mark Empson
Advanced Motor Control Ltd

 

[LionelHutz]

Also do not mix in setting a max output freq to this discussion: keep in mind that has nothing to do with running a 230v motor on a 460v drive; that is simply setting the v/hz to stop at 230v output rather than going up to 460.

But it does. How do you think the PWM generating algorithm only generates 230V with 480V applied? The PWM generating algorithm produces that 230V output by measuring the DC bus voltage and generating the correct PWM duty cycle to produce 230V.

 

[Marke]

The PWM generating algorithm produces that 230V output by measuring the DC bus voltage and generating the correct PWM duty cycle to produce 230V.

You can either measure it, or you can assume the value. With compensation ON, you measure, with compensation OFF you assume a value.

Mark Empson
Advanced Motor Control Ltd

 

[LionelHutz]

The V/Hz PWM generator has a number of things which correct or modify the PWM. The measured DC bus voltage is typically used as an input to a bit of code that modifies the PWM duty cycle in an attempt to maintain the ideal V/Hz pattern. The voltage boost routine is another bit of code that modifies the PWM duty cycle to increase the V/Hz ratio on the low end. There are likely other adjustments as well.

Posting something like "so now a small comp routine is deciding the complete PWM on/off control algorithm too?" is just being asinine.

Yes, there is a lot of misinformation. Mostly due to people refusing to believe that bus compensation could exist....


Mark - You are correct. In reality, the PWM algorithm will generate the waveform to produce 230V assuming the ideal bus voltage. Then, if the bus compensation exists (it usually does) then it will only modify the PWM duty cycle the small amount to compensate for the change in bus voltage.

 

[mikekilroy]

Mark - You are correct. In reality, the PWM algorithm will generate the waveform to produce 230V assuming the ideal bus voltage. Then, if the bus compensation exists (it usually does) then it will only modify the PWM duty cycle the small amount to compensate for the change in bus voltage.

ALL industrial VFDs measure the dc bus and use that information in various basic drive routines. There are no industrial vfd drives that do not measure & monitor the dc bus. Additionally, I don't believe any post here ever suggested dc comp does not exist although some did, and still do, question if "it usually does." Thank you for stating this to help clarify the issue. I would be very upset if I had to return all the compensation I have received over the last 30 years in successful PWM drive design consultation fees, over this type of misunderstanding of basic design facts.