Determining regenerative kW on VFD bus

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philly

Senior Member
I am trying to set up the duty cycle on a brake resistor on a 480V VFD. The brake resistor has a maximum continuous rating of 4000W.

I am trying to verify what the regenerative energy to verify weather or not it is under the 4000W rating of the resistor to see if the chopper duty cycle can be set at 100%.

How can I verify, the kW being dumped across this resistor? Can I simply measure the DC voltage across the resistor when the chopper is on, and use V^2 / R to calculate this power?

If the power is less than 4000W then can I safely set the duty cycle to 100%?
 

philly

Senior Member
I am trying to determine a way to measure this power value regenerated from the drive and this is the best way I can think of doing it.

Unless the drives usually have a parameter itself to display this regenerated power? Would this show up on the drives kW paramater as a negative vaule in this case?
 

Besoeker

Senior Member
Location
UK
I am trying to set up the duty cycle on a brake resistor on a 480V VFD. The brake resistor has a maximum continuous rating of 4000W.

I am trying to verify what the regenerative energy to verify weather or not it is under the 4000W rating of the resistor to see if the chopper duty cycle can be set at 100%.

How can I verify, the kW being dumped across this resistor? Can I simply measure the DC voltage across the resistor when the chopper is on, and use V^2 / R to calculate this power?

If the power is less than 4000W then can I safely set the duty cycle to 100%?
Well, V^2/R will give you the power dissipation when the brake chopper is on.
But what you are trying to do seems a bit unusual. Normally the brake resistor would be sized for operational requirements not the other way round.
And 100% duty cycle? Do you really expect the load to continuously drive the motor?
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
What is the time constant of the duty cycle setting that you are adjusting? What is the chopper time base?

The brake chopper modules that we have on our experimental inverters are simple hysteretic units; if the voltage rises above a threshold then they turn on the brake resistor, when the voltage drops below a different threshold then they turn off the brake resistor. The 'chopping' frequency is set by the time it takes the resistor to discharge the capacitor bank and the time it takes the regen source to charge the bank. Sufficient regen would turn the choppers on, and they would stay on until something failed. We try to arrange our experiments so that there is little chance of regen.

I could imagine that the higher level controls could do two different things, both called 'duty cycle'. 1) They could only let the chopper operate (both on and off, but chopping) for so many seconds before letting the chopper cool for so many seconds, possibly shutting down the entire inverter or 2) They could directly control the chopping frequency and only permit a certain fraction of 'on' time.

1) would be used to control the duration of an overload on the resistor; we are talking time periods that are of the same size as the thermal time constant of the resistor. 2) would control the maximum loading of the resistor, with time periods much shorter than the thermal time constant of the resistor.

When the choper is 'on', then the full DC bus voltage is placed across the brake resistor. For a 480V VSD, the 'on' state will occur somewhere in the neighborhood of 825V; you can just use Ohm's law to figure out the instantaneous dissipation in the resistor. It is almost certain that the power dissipation in the 'on' state will greatly exceed the continuous rating of the resistor. However with chopping the resistor power dissipation may be well within its continuous range, in which case the long term duty cycle could be 100%.

Measurement of the voltage across the resistor will tell you power dissipation, but I'd want to make this measurement with an oscilloscope, though I suppose that a good true RMS meter would give you a valid measurement. If you don't have control of the chopping duty cycle, then it will depend upon the actual regen load.

-Jon
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I am trying to set up the duty cycle on a brake resistor on a 480V VFD. The brake resistor has a maximum continuous rating of 4000W.

I am trying to verify what the regenerative energy to verify weather or not it is under the 4000W rating of the resistor to see if the chopper duty cycle can be set at 100%.

How can I verify, the kW being dumped across this resistor? Can I simply measure the DC voltage across the resistor when the chopper is on, and use V^2 / R to calculate this power?

If the power is less than 4000W then can I safely set the duty cycle to 100%?

It is very rare that a braking resistor needs to have a 100% duty cycle. Most times there is only regenerative energy generated for a few seconds at a time.

The resistor can take far more power for a short time than it can continuously. You could also "help" it out by putting a fan on it.

The thing is that you can't really 'set" the duty cycle of the resistor in the VFD. its a calculation based on how hot it is estimating the resistor is getting. Most times you can just buy a braking resistor that has a temperature switch built in that you can run back to the VFD to trip it if the resistor overheats.

if you are unable to dissipate the regenerative energy quick enough, the bus voltage will rise to the point where the VFD will trip.
 
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philly

Senior Member
The drive I am referring to is a 75hp Siemens MM440VFD. We have always had issues with this drive tripping on a DC bus overvoltage.

At first we thought is was coming from the line side of the drive but we have since installed a line reactor to hopefully eliminate this.

We then installed a braking resistor which has a continuous rating of 4000W and I believe has a max rating of 8000W or so. I believe this 8000W is only rated for a particular on time which I believe is 5% of the duty cycle. The drive uses a duty cycle parameter to set the chopper on time during braking.

We have tried a 5% setting and have even raised it to 20% and are still having issues with the drive tripping. I am somewhat hesitant to just keep raising the duty cycle with fear of burning up the resistor.

However I was thinking that if I could somehow verify how much kW was being dumped across this resistor then I may be able to set the duty cycle higer to allow the resistor to dissipate heat longer. For example if I see that only 6000W is being dumped across resistor during regen, then I could find out how long the resistor can handle this amount for and possibly increase the on-time. Or if lets say I verify that only 3000W is being dumped across the resistor then I know this is less than the 4000W continuous rating of the resistor, so I could change the duty cycle to 100% and leave the resistor on longer to dissipate this energy?

Am I on the right track here? This drive keeps tripping and I can't seem to solve it.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
If it is tripping on DC bus over voltage, you may well be having an issue with being able to get rid of the energy fast enough on the DB resistor.

4kw is not very much compared to a 75HP drive. You can verify how much energy is being dumped to the DBR by measuring the voltage with a true RMS voltmeter.

I would look at a couple of things.

1. How was the DB sized in the first place? Does it have an over temp switch?

2. Take a look at the temperature rise on the DBR when it is active. If it is starting to get to where it cannot get any warmer without being damaged you may have to take some expensive steps. If it is not getting too hot, just increase the duty cycle setting. The DBR manufacturer might be able to help you with some of these issues.

3. Consider a fan on the DBR. That will increase its ability to dissipate heat. Some drives can turn on an output when the braking transistor is on that could be used to run the fan.

In the end, it is about getting rid of the energy in the system. If you can't get rid of it across the DBR, you may need more capability to remove energy. One solution is a DBR that can take it, another is some kind of active system that puts the energy back on the line.
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
What is the resistance of the DB resistor?

From page 147 of the manual found here http://support.automation.siemens.c...lib.csinfo&lang=en&objid=23708204&caller=view,

the duty cycle setting for the DB resistor should correspond to the long term power dissipation capability of the resistor.

The bus voltage when the DB resistor is active depends upon some other variables.

V^2/R gives you your continuous power dissipation; adjust the duty cycle so that V^2/R * duty cycle is < 4KW.

I've not delved too deeply into the manual, but it looks as though you can adjust the DB voltage threshold; I wonder if it is too low, and you are triggering the DB resistor simply using line voltage.

-Jon
 

philly

Senior Member
What is the resistance of the DB resistor?

From page 147 of the manual found here http://support.automation.siemens.c...lib.csinfo&lang=en&objid=23708204&caller=view,

the duty cycle setting for the DB resistor should correspond to the long term power dissipation capability of the resistor.

The bus voltage when the DB resistor is active depends upon some other variables.

V^2/R gives you your continuous power dissipation; adjust the duty cycle so that V^2/R * duty cycle is < 4KW.

I've not delved too deeply into the manual, but it looks as though you can adjust the DB voltage threshold; I wonder if it is too low, and you are triggering the DB resistor simply using line voltage.

-Jon

The resistor is 8.2Ohms +/- 10%

So are you suggesting measuring the V^2/R with the meter and multiply this value by the duty cycle (.05%, .10%, etc..) and ensure this value is below the 4000W continuous rating?

I know that V^2/R will give the kW across the resistor, but how does multiplying the duty cycle by this value give you a continous kW dissipation as you suggest? The kW is a unit of power while the duty cycle is a measure of time. Wouldn't multiplying these give you an energy value as opposed to a power dissipation.

I will look into the DB voltage threshold adjustment.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I know that V^2/R will give the kW across the resistor, but how does multiplying the duty cycle by this value give you a continous kW dissipation as you suggest? The kW is a unit of power while the duty cycle is a measure of time. Wouldn't multiplying these give you an energy value as opposed to a power dissipation.

Duty cycle is not a total time period, it is a percentage time.

So duty cycle times the instantaneous dissipation in kW will give you the average dissipation in kW.

Say that the PWM period is 1 second, and the duty cycle is 5%, and the DC bus voltage when the brake chopper is 'on' is 810V.

During the on period, the power dissipation in the resistor is 810^2/8.2 = 80KW. Because of the duty cycle, the 'on' state will last 0.05 seconds, so 4KJ will be dissipated (as you note, power times a time period gives you energy). But because this is a duty cycle, in the _next_ second you will have another 'on' period, with another 4KJ. Every second you dissipate 4KJ, for an _average_ power of 4KW.

The above is my complex way of saying that 80KW on for 5% of the time is an average power of 4KW.

-Jon
 

philly

Senior Member
Duty cycle is not a total time period, it is a percentage time.

So duty cycle times the instantaneous dissipation in kW will give you the average dissipation in kW.

Say that the PWM period is 1 second, and the duty cycle is 5%, and the DC bus voltage when the brake chopper is 'on' is 810V.

During the on period, the power dissipation in the resistor is 810^2/8.2 = 80KW. Because of the duty cycle, the 'on' state will last 0.05 seconds, so 4KJ will be dissipated (as you note, power times a time period gives you energy). But because this is a duty cycle, in the _next_ second you will have another 'on' period, with another 4KJ. Every second you dissipate 4KJ, for an _average_ power of 4KW.

The above is my complex way of saying that 80KW on for 5% of the time is an average power of 4KW.

-Jon


O.k. this is starting to make sense now.

So if I put a meter on the DC bus or watch DC bus voltage on the display and see 810V then I know I cant go higher than 5%. But if I see someting lower then I know I can increase the duty cycle as long at the voltage I'm seeing times the duty cycle is less than 4000W? What if the voltage is fluctuating?

As a test can I go all the way to 100% just to see if this lets the drive stop without tripping then adjust based on voltage levels, or am I risking burning the resistor by doing this even just one time.

In your example above, what if the PWM time is greater than 1s? Does the calculations then change as I think they would? How do you find what the PWM time is for a given drive, does it state it.

I also want to verify that the brake is indeed coming on and the chopper is working. Is the best way to do this by measuring the voltage across the actual resistor during braking?
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
So if I put a meter on the DC bus or watch DC bus voltage on the display and see 810V then I know I cant go higher than 5%. But if I see someting lower then I know I can increase the duty cycle as long at the voltage I'm seeing times the duty cycle is less than 4000W? What if the voltage is fluctuating?

V^2/R * duty cycle = average power dissipated in the resistor

As a test can I go all the way to 100% just to see if this lets the drive stop without tripping then adjust based on voltage levels, or am I risking burning the resistor by doing this even just one time.

I'm a bad one to ask. A couple of weeks ago I (unintentionally) dumped 80KW into a test motor during a locked rotor test. The smoke came out _very_ quickly.

Seriously, such a test is plausible, but not with considerably more design work. For example, do you have a datasheet for the resistor, and does it give the resistor's overload capacity or thermal time constant? The resistor should be rated for both its maximum _continuous_ dissipation (4KW) and also rated for different power levels for different time periods. If the resistor is rated for a _peak_ dissipation of 20KW, then I would not attempt a test at 100%, where the dissipation could easily be 80KW. On the other hand, if your resistor is rated for 100KW for 10s (or some such) then I'd be much more comfortable with such a test...just be ready with an emergency power off.

In your example above, what if the PWM time is greater than 1s? Does the calculations then change as I think they would? How do you find what the PWM time is for a given drive, does it state it.

The amount of energy dissipated in a single pulse is proportional to the PWM period. In my example, the PWM period was 1 second, and the 'on' time 0.05 seconds, so you got 4KJ per pulse, 1 pulse per second. Say we keep the same % duty cycle, but make the PWM period 10 seconds. The 'on' time is now 5% of 10 seconds, or 0.5 seconds. You dissipate 0.5s * 80KW = 40KJ per pulse, one pulse every 10 seconds. Note that the _average_ power dissipation stays the same, but energy in a single pulse changes.

During the 'on' period, the resistor is dissipating power and its temperature is rising. During the 'off' period the resistor is cooling down. If the 'on' period is too long, then the resistor will overheat and fail before it has a chance to cool down. If the PWM period is very long, the resistor 'sees' the full power. If the PWM period is very short, then the temperature change during one pulse will be very small, and (at least thermally) the resistor 'sees' the average power. 'Short' and 'long' are relative terms that depend upon the thermal time constant of the resistor, how long it takes to heat and cool.

I also want to verify that the brake is indeed coming on and the chopper is working. Is the best way to do this by measuring the voltage across the actual resistor during braking?

That seems like a simple approach. Remember that you will need a meter that can handle >800V DC, switched at 2 KHz.

Question: have you evaluated the amount of kinetic energy stored in the system that you are stopping?

-Jon
 
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