Dynamic Braking Resistor 3

[rockman7892]

I have a 60hp 480V motor which is being driven by a 75hp Siemens MicroMaster 440 VFD. The application for this motor is a grizzly feeder (crusher shaker) which presents a high inertia load to the motor. Whenever the motor is VFD is stopped or ramped down, the VFD is tripping on a bus Overvoltage condition. This tripping has become a persistant problem and I need to find a solution.

I wanted to go ahead and install a braking resistor on this drive to dissipate this excess voltage when the motor is being ramped down and regenerating voltage back onto the drive. Can anyone help me determine what size braking resistor I will need for this drive. Is there a formula or some other means of calculating the size of the resistor need for this application?

 

[jraef]

Does it only trip when you shut down? Does it ever trip when you are just running?

 

[rockman7892]

jraef

I believe it trips for both. It trips when it is shut down, and becasue it is controlled by a PID loop it trips when the drive is commanded to ramp down in speed. Its hard to verify which ramp-down rates cause this trip without actually sitting there and watching it.

I have tried to extend the ramp down time function on the dirve, however this had no effect.

 

[jraef]

Grizzlies are a more complex application than most people realize. Traditionally you needed to double the VFD size to have enough capacitance to absorb the regen that comes along with the down strokes of the shaker weight. Think of every 1/2 rotation as a load: 1/2 of the rotation it is motoring, the other 1/2 it is generating. Over time, depending on the mass and positioning of the rock in the hopper, the regen portion of the stroke can be more than the motoring and the DC bus voltage builds up. It isn't much, but it can end up being somewhat cumulative. More modern drives now come with specialized software marcos that avoid the necessity to over size.

I am not an expert with the Micromaster, but I know several and called one to confirm that the Micromaster has that built-in feature. It's called "Kinetic Buffering" and controls this over voltage condition by manipulating how the transistors are fired. It essentially turns off the output during portions of the down strokes so that the motor is not excited enough to regenerate (it's actually much more complex than that, but that is the boiled down version). Instructions for implementing it should be in your manual, or call your local Siemens Drives Specialist for help.

 

[LionelHutz]

Even if you use the "Kinetic Buffering" I'd still install some type of braking resistor. Limiting the regen just means a smaller resistor.

On most drives you can just use a torque or current limit in regen operation to achieve the same thing. The drive will have to allow the motor to accelerate in the regen state if it is limiting the amount of regen energy.

It doesn't sound like you have any issues running steady state so the resistor should mostly just be required when lowering the operating speed (stopping).

 

[controlsdude]

this would the siemens micromaster 440 braking resistor lit. just download the pdf and you can order want you need.

http://support.automation.siemens.c...tandard&viewreg=WW&objid=10804929&treeLang=en

 

[rockman7892]

Thanks for all of the information. I am gonig to call Siemens tech support about "Kinetic Buffering" and also look at Siemens literature for specifying the correct braking resistor.

Jraef

I am having a difficult time explaining to folks here at the plant why this regen overfault condition if happening. You mentioned that for 1/2 the shaker cycle the motor is motoring and for the other half it is regenerating. Can you help me visualize why this shaker is regenerating on the other half cycle?

The way I understand it is that when a high inertia load (such as a shaker) forces a motor to go faster than the output of the drive then the magnetic field built up in the rotor of the motor induces a voltage on the stator of the motor and therfore senda a regenerative voltage back onto the VFD DC bus? Am I understanding this correctly or is there more to it than that? Why would this only be happening during shut off of the drive or when it is ramping down and not during the 1/2 cycles when the drive is in normal operation? Any explanations or resources would help me better explain this to production here at the plant.

Thanks

 

[LionelHutz]

An AC induction motor always tries to run close to synchronous speed. Regen occurs when the motor tries to slow down a load. The load is trying to force the motor to go faster than synchronous speed and the motor resists by taking energy from the load and putting it back into the grid (or VFD in this case).

If a motor is rated at 1740rpm then synchronous speed is 1800rpm. Running at rated hp while regening the motor would run at 1860rpm.

If a motor is driving a load then it's called motoring.

 

[blacksea]

Decreasing of deceleration ramp or adding capacitor to DC bus (if this's possible) for lower regen energy maybe suggested also.

 

[controlsdude]

rockman

sorry forgot to give you the background. I believe this link and at the bottom they give an explanation for regen energy and why the need for braking resistors.

http://www.postglover.com/dynamicbraking.html

look at the bottom and there is a pdf called this
DYNAMIC BRAKING RESISTORS TECH SHEET DB108-06

 

[rockman7892]

So if I understand this correctly, when a motor is motoring (turning a conveyor for example) them motor is turning at a speed of 1740rpm (trying to turn at 1800rpm with slip) and driving the particular load in this case the conveyor at that speed. However when the motor stops, if the load happends to be a high inertia load such as a loaded conveyor the load will then try to spin the motor at faster than 1800rmp and therefore create energy which will need to be dissipated. This energy will be dissipated in the form of regeneration back to the grid or VFD? Does this sound like a correct understanding?

I guess the one stipulation to this is that a residual magnetic field exists in the motor still. Without this magnetic field built up in the motor there will be no regeneration? So if the magnetic field has had time to collapse (after power off of say 10s) then even if a high inertia load tries to spin the motor faster than 1800rpm it wont regen becasue there is no magnetic field. Is this correct?

 

[controlsdude]

that sounds like you have the idea. The braking resistors would usually kick in during the ramp down cycle of the VFD.


the Time of ramp down does effect how much energy is put on the dc bus.

where dc braking resistors are not needed.
In the material handling we never put braking resistors on sorters, since the ramp down is between 10-15 seconds. The DC bus does not go to overvoltage since the voltage does not rise to maybe 800volts, just a guess. where the overvoltage fault would kick in.

But if you had a short ramp down on the VFD the dc bus would rise very fast since there is voltage coming back from the motor into the dc bus in the vfd. This is what causes your overvoltage trip.

But now you will have your braking resistors which bleed off this excess voltage from the DC bus and therefore keeps the dc voltage at an acceptable level.

 

[itsmoked]

Yes rockman you have it correct!

The only correction would be that the residual field can collapse in a second not 10 seconds depending on various issues.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ozmosis]

Rockman
Before purchasing additional options such as brake resistors, I would try setting the MM440 sensorless Vector Control (SVC) up as described in the manual that should be on the CD (DVD) with the drive. It shows a step-step guide on how to do this and requires you to input all motor data off the nameplate. Do this first and get the optimum performance out of the drive then see how it operates.
I'm not saying you won't need a DB resistor but worth making sure you are getting the most out of the drive before adding other options.

 

[jraef]

I agree with ozmosis. Try it with the built-in functionality first. By the way, I looked into it a little further and the Kinetic Bufferring is actually the opposite, it attempts to maintain the output if the DC bus level drops. What you want is called the "Over Voltage Control" macro, which may be called "Vdc Control" depending on the version of firmware you have. This is a feature that is already built in to that drive, you need only activate it. If you don't have the manual, post the VFD nameplate info and I'll link you up to it (Siemens' website navigation can be a little daunting for the uninitiated).

I have a lot of experience with grizzly drives (grizzly as a noun, not an adjective). DB resistors add a very well known point of failure in that application; the environment is usually too rough for them. High vibration and/or the dust builds up, they over heat and short out in relatively short order. That's why in the "olden days" (circa 5 - 7 years ago), we just doubled the size of the VFD rather than rely upon the DB resistors. Now, most time the over voltage control macro will work without having to up-size or add the DB resistors.

Try it, you'll like it!

 

[LionelHutz]

One more thing to add about the regen.

The VFD is always supplying a voltage and frequency to the motor when it is in the running state. So, when the VFD is running, the motor always has a frequency applied. This frequency is what sets the synchronous speed of the motor. When the VFD decelerates, the frequency ramps down which in turn ramps down the synchronous speed of the motor.

The magnetic field decaying only applies when the motor is stopped (disconnected from power). No regen can occur in this case because there is no place for the energy to go. So, with a contactor starter you never get regen when stopping a motor.

 

[ScumPunk]

Hi, Rockman,
For the MM440, the Vdc Max controller macro is enabled by default, P1240 should be set to 1 or 3. It sounds like it needs tuning however, so you can check the following parameters:
r1242 displays the dc link level at which the controller kicks in - it cannot be changed by the user, it simply looks at the link on power-up and sets the level to a proportion of the link.
The tuning parameters are P1243 to P1257. It's a PID system, P1243 is a straight % of the PID times so you could start by reducing this from 100% to try and get a faster reaction in the dc link controller. P1254 means you can disable the automatic setting of levels, so presumably once you have your drive working correctly, you can set this to zero and lock in the levels.
The Vdc max controller works by slowing down the ramp down rates, or increasing speed as an alternative.
Hope this helps,
Cheers,
Mort

 

[rockman7892]

I took the advice from many of these posts and tried to use the built in functionality of the drive before going out and getting a DB resistor. I spoke with Siemens tech support and they pushed the issue of setting up the sensorless vector control (SVC) as was mentioned above. I followed the procedure for setting up this control however half way through the setup my drive tripped on an bus overvoltage condition. Tech support said the the load may have been causing to much inertia during the setup procedure and recommended un-coupling the load. With the load un-coupled I again went through the setup proceudre but this time when going through the procedure I got a drive fault indicating a "motor identification failure". I tried this setup several more times and got the same trip warning each time. Siemens tech support said that they werent exactly sure why this fault was appearing, however they speculated that there could be a breakdown somewhere in the motor insulation causing this failure. He mentioned that breakdown could be a pinhole sized leasion that would allow the motor to run fine, and not even be picked up with a megger, but is picked up by the drive because of the output waveform of the drive, and the sensitivity of the drive. I was therefore unable to complete the sensorless vector control setup for this drive and am now back to square one.

In setting up this control I dont completely understand what exactly is SVC. I was hoping someone could maybe explain the theory of this control to me or site some resources so that I may gain a better understanding of what I am trying to setup.

Other than that, tech support said that I could not change the Vdc_max parameter because this was a factory setting, and that I could try turing on the kinetic buffering and it may help. I would like to exaust all of my optins before I go out and purchase a DB resistor. Thanks for the help.

 

[controlsdude]

I know in the past I had this magnetek drive where you had to have all the nameplate data entered but also how many poles there was on the motor. Do you think it needs this info?

On the AB powerflex drives there was this feature called static tune. you could use it with the load coupled but it would put max fla on the motor and it would calculate other parameters. But it also could find failue with the motor or drive real quick since the current was fla. It must of found 3 bad drives for me on this one site during this one startup. Do you have this feature on this drive, so that you could determine if the motor or drive is bad?

 

[ScumPunk]

You need to put in some of the data at the very minimum:
Assuming P100 = 1 (I'm guessing you are in USA by your use of HP and 480V):
set P10 = 1 (enters quick commisioning)
enter plate data for motor volts, FLA, power, and speed.
Then set P3900 = 3. this will calculate all the other motor data from the information supplied. I.e if you have put a number close to 1500rpm in P311 it knows it is 4 pole motor.
This calculation is fairly accurate, but not good enough to do SLVC. Failure of the auto-tune (known as motor ID in Siemens speak), is usually caused by an imbalance in windings (due to partial discharge over time - a known problem with VFDs). If you run the motor are all 3 phases showing the same current? The other cause could be an insulation failure as mentioned by tech support.
Is your supply voltage 480V? if so you are at the top end of the suply range for MM440 so you have less headroom before you hit a trip level. Tech support are correct when they say you cannot change the Vdc max level, but you can tune the PID that controls it (see my previous post). It may be reacting too slowly to the changes in the link to catch it...
Gotta go for now,
cheers,
Mort