DC Bus Compensation
DC Bus Compensation
(OP)
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.
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





RE: DC Bus Compensation
RE: DC Bus Compensation
DC Bus compensation is incorporated into drive algorithms so that the output modulation is varied depending on the DC Bus voltage so that the motor voltage is less dependent on the incoming supply voltage.If the DC bus voltage increases, the output voltage in an open loop environment would also increase, likewise if the DC bus voltage reduces the output voltage would also reduce. By applying some control based on the DC bus voltage, the output voltage can be regulated by adjusting the output modulation.
When a passive harmonic filter is applied to the input of a VFD on a weak supply, instability can result if the response time of the DC bus compensation is too fast for the time constant of the input circuit including the filter. The solution is to either alter the time constant of the DC bus compensation, or to turn it off.
Best regards,
Mark.
Mark Empson
Advanced Motor Control Ltd
RE: DC Bus Compensation
RE: DC Bus Compensation
RE: DC Bus Compensation
I don't think you can do it on the ATV 61, other than switching the drive off.
RE: DC Bus Compensation
Let's attack each individually....
Complex first.....
If you say this algorithm will artificially or any other way INCREASE output voltage OVER the input voltage, then you jut doubled - plus - the price of the drive. Other than sophisticated Siemens special drives I know of no other that will make a higher output voltage than available from the input. I am not all knowing of course so I can be full of it.
That all said, to have higher output ac voltage than input requires an active front end rather than diodes, in addition to input reactors: the idea is to SHORT out the input choke each of say (as in siemens case) 2khz rate so the inductive KICK is HIGH and gets rectified. result is a higher dc bus voltage than just diode 1.4x input so you can have higher output ac voltage. I really really doubt either brand you mention has this.
so forget making dc bus higher to get higher output ac than input on your models identified.
Now to the simper rEducE output..... ok, so what cause dc bus to go higher than nominal? REGEN. Ie., deceling or overrun motor. If it goes too high then we have possiblility of OVErVOLTS fault. How to reduce dont do the thing that caused the high decel energy, or reduce the decel ramp. AH...... now your onto something... What if we watch the dc bus voltage, and if it starts to go too high and threaten to fault, we instead REDUCE THE DECEL RAMP? Can we call this "dc bus compensation?" Sure, if it helps sell more drives. OK, so let's do it... So any parameters in the equation Sure, we can pick the START point of dc bus BEFORE overvolts fault, we can pick response time like PID stuff.... Enter more modern drives.... Hitachi for one has this. You can adjust the volt sense point as well as the time before response. If you need the B085() etc paramter nos I can look it up for u. Call if "dc bus comp?' sure if it helps sell more drives....
Say it will "adjust the pwm output freq to adjust for it" sure, if it sells more drives....I'll take your order sir!
So is this a function of 'vector drives?' 'space comp drives?' 'v/hz drives?' YES. IT DOES NOT MATTER! this is a totally separate algorithm and has nothing whatso ever to do with motor speed control.
RE: DC Bus Compensation
It is not specific to regenerating motors or active front ends. Basically, the duty cycle of the PWM output is varied depending on the bus voltage change from ideal or nominal.
RE: DC Bus Compensation
The questions about the function itself, can be answered quite simply by saying other drives on the market can do it. We manufacture one of them.
However, in the interest of not promoting ones own product, this is better left where it is.
RE: DC Bus Compensation
Thank you for your long explanation, however as LionelHutz suggested, the issue is not about regeneration or any thing, but it is a pretty standard algorithm built into the waveform generator to keep the V/Hz closer to the ideal when practical, especially at reduced speeds, and reduce the dependence of the V/Hz on the incoming supply voltage. At full speed, overmodulation can be used to increase the average voltage, but this is at the expense of increased harmonic currents to the motor as the waveform becomes squarer than it should be.
As Osmosis has suggested, some drives provide the ability to turn this algorithm off, or to change the time constant to minimize instability. In some cases, the only way to reduce the problem is to reduce the rated motor voltage, reducing the flux in the iron across the whole speed range. Not ideal, but it can help with instability, but increases motor current.
Mark Empson
Advanced Motor Control Ltd
RE: DC Bus Compensation
It was unstable when I had a problem with a simple process had to disable Bus regulation mode but other settings too.
try these other settings if you have them in your drive
you dont have this or maybe they need to update the firmware? >>>set to disable Bus Reg Mode
Slip Hertz @ FLA = disable Compensation
Compensates for the inherent slip in an induction motor.
This is a mechanical or electrical compensation setting.
Enables/disables correction options that may improve problems with motor instability.
set your dynamic brake setting to 100% (I did not have a dynamic brake but still changed the setting.
NoProtection” (100% Duty Cycle) Maybe think more about this if you do have dynamic brake!
All else fails set to V/hz
v/hz last chance
RE: DC Bus Compensation
RE: DC Bus Compensation
Here you go
http://mcliterature.danfoss.com/WebPublish/doc_MG8...
Page 56.
However, it is just highlighting that there is a parameter that can be disabled.
RE: DC Bus Compensation
The reality is that 6 pulse drives create harmonic distortion. As a drives manufacturer, we need to obviously extoll the virtues and benefits of having a drive in the first place but also be conscious of the impact of fitting these devices too. This is why drives have EMC filters, harmonic filters, high IP ratings, coated boards etc. Hopefully, the benefits outweigh the issues. They will generally outweigh when you know most of the facts behind the installation and the application.
Sometimes, you don't. One of these, and quite understandably, is the fact that purchasers of drives will not always know how 'strong or weak' the supply impedance is. They will know drives create harmonic distortion and we will tell them that there are various ways of mitigating this distortion (active, passive etc). Sometimes, customers will buy a drive and buy a harmonic filter and we have no idea (as they typically don't) about the supply conditions. There is no "correct" solution, it depends completely on the specific installation. Unfortunately, we cannot advise on every sale.
So, in situations where drives and passive harmonic filters are installed on very weak supplies, you can get additional issues with a drive that does not behave as it should do and one of these is the effect described by Marke above. This is why we have to introduce functions to support an installation that respond to problems 'after the event'. The farmers in NZ generally don't take too kindly if you say to them after they have invested in drives, filters etc "why didn't you tell us before, what the supply impedance was..?" Usually the second word response is "..off!"
Hence the need to have functions that can correct certain issues we find out afterwards. We call it a hindsight parameter.
RE: DC Bus Compensation
http://mcliterature.danfoss.com/WebPublish/doc_MG8...
I search the site for a model MG80 but do not find one.
Please list maybe a drive part no. (& maybe that parameter no) so I can download its manual maybe?
thanks!
RE: DC Bus Compensation
RE: DC Bus Compensation
I recently came across a situation where the magentising current is increased by about 35% due to incorrectly calibrated DC Bus monitoring. I suspect that the VFD was calibrated without a DC Bus Choke and then this was added without recalibration.
Mark Empson
Advanced Motor Control Ltd
RE: DC Bus Compensation
how much does dc bus comp adjust the output voltage? 2%? 5%? 50%?
Has anyone ever seen it on a drive running in vector mode (i.e., not v/hz)? If so, what brand & model?
Is it more common on very large (say >500hp) drives?
Marke, you seem to see it a lot; what brands & sizes of drives was it on?
RE: DC Bus Compensation
Most of the well known drives that I see, have this feature in one form or another when running in V/Hz mode. It is often hidden with no way to control it and if you do not have problems with it, you would not know that it is there.
Ideally, if the input voltage is low, then the DC bus voltage will be low and under true open loop conditions, the output voltage will also be low. Adjusting the modulation level enables the output voltage to follow the correct V/Hz curve up to the supply voltage. Over modulation (adding output harmonics) can be applied to further boost the output voltage when the supply voltage is low. Likewise, if the supply voltage is high, the output waveform can be under modulated to reduce the output voltage so that the correct output voltage is met.
The major issues occur if the DC Bus measurement calibration is incorrect, or if the control loop time constant is too fast. Incorrect calibration will alter the V/Hz and can, as I have recently seen, overflux the motor, or if the loop is not slow enough, the output voltage can go unstable on a weak reactive supply.
There is little if anything in the manuals describing the feature. The waveform generation, (commonly space vector modulation) is able to set both the frequency and the output voltage
Mark Empson
Advanced Motor Control Ltd
RE: DC Bus Compensation
Most of the well known drives that I see, have this feature in one form or another when running in V/Hz mode. It is often hidden with no way to control it and if you do not have problems with it, you would not know that it is there.
Ideally, if the input voltage is low, then the DC bus voltage will be low and under true open loop conditions, the output voltage will also be low. Adjusting the modulation level enables the output voltage to follow the correct V/Hz curve up to the supply voltage. Over modulation (adding output harmonics) can be applied to further boost the output voltage when the supply voltage is low. Likewise, if the supply voltage is high, the output waveform can be under modulated to reduce the output voltage so that the correct output voltage is met.
The major issues occur if the DC Bus measurement calibration is incorrect, or if the control loop time constant is too fast. Incorrect calibration will alter the V/Hz and can, as I have recently seen, overflux the motor, or if the loop is not slow enough, the output voltage can go unstable on a weak reactive supply.
There is little if anything in the manuals describing the feature. The waveform generation, (commonly space vector modulation) is able to set both the frequency and the output voltage. The output voltage is set to the motor voltage at rated frequency and reduced from there as the frequency reduces with the measured DC bus voltage being part of the equation.
Mark Empson
Advanced Motor Control Ltd
RE: DC Bus Compensation
RE: DC Bus Compensation
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.
RE: DC Bus Compensation
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.
RE: DC Bus Compensation
RE: DC Bus Compensation
RE: DC Bus Compensation
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
RE: DC Bus Compensation
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.
RE: DC Bus Compensation
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.
RE: DC Bus Compensation
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
RE: DC Bus Compensation
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.
RE: DC Bus Compensation
Mark Empson
Advanced Motor Control Ltd
RE: DC Bus Compensation
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.
RE: DC Bus Compensation
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.