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VVVF Drive overheats Dynamic Braking Resistors

06/02/2008 4:12 PM

I am looking for anyone else that has had a problem with there VVVF drive burning up the dynamic braking resistors. Some drive manufacturers recommend circuit protection for these resistors and others do not. I know of two incidents were the drive has caused a fire and we have had to replace the drive and resistors.

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Pathfinder Tags: VVVF Drive Motor Control
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#1

Re: VVVF Drive overheats Dynamic Braking Resistors

06/03/2008 4:41 AM

Are you exceeding the rating of the resistors by switching the motor off more often than recommended within a certain amount of time?

Is the environment too warm to allow proper cooling?

Have you enough cooling air in the vicinity of the motors and resistors?

Are the resistors themselves of the correct wattage for the motors installed?

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#2

Re: VVVF Drive overheats Dynamic Braking Resistors

06/03/2008 4:55 AM

G'day Travis,

We had trouble once with a brake chopper not sized large enough and the transistors would fail, that is they failed shorted and this did make the resistors glow. There was a rush to the mainswitch to turn it off.

I would suggest maybe going back to maths, if possible, and calculate the resistor sizes need. Also look at the drive capacity and check if it large enough to handle your application with regeneration. Rockwell has a good manual for calculating the braking requirements with step by step approach, and we have found it handy for non Rockwell drives.

With our drive applications, we use the recommended 'semiconductor' fuses suggested by the manufacturers, with either common dc bus or brake connections.

Cheers,

Trevor.

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#5
In reply to #2

Re: VVVF Drive overheats Dynamic Braking Resistors

06/03/2008 1:37 PM

Hi Trevor

The motor and drives are matched together by a elevator company and are rated for the use. I have seen that some drive maufacturers recommend overload protection for the resistors. This company "Baldor" does not recommend protection but these are the two drives we have seen burn up the resistors. The controllers are engineered for the elevators that they run.

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#3

Re: VVVF Drive overheats Dynamic Braking Resistors

06/03/2008 8:02 AM

I would also check the motors and drives and make sure they are sized properly to one another. Make sure the motor is inverter duty rated and check the settings in the drive, See if it has vector setting (for fans) and turn the vector setting off for drive motors. I am also curious as to the application, I am wondering how you are getting your regeneration. I have a process where a motor is driving a press and it has to drive the springs open first 50% of operation and the springs drive the other half on the operation. This causes a good bit of regeneration for us because the motor is also has encoder feed back to keep the speed with in +/- 2 rpm. So our breaking chopper resistors get a bit hot. Go thru the inverter settings a match them to your motor and double check the motor if it is an older motor it may not be inverter rated.

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#4
In reply to #3

Re: VVVF Drive overheats Dynamic Braking Resistors

06/03/2008 1:27 PM

The motor and drives are rated for the use. They are baldor drives with dynamic braking for a elevator in a large biulding the drives are set up to use the resistors when traveling in the up direction empty or the down direction full load this is when the regeneration is required. These are controllers that are engineered by elevator companies and they do have fans for the resistors. Both elevators ran fine for a long time one for 7 years the other for one year.The motors run on a closed loop with encoder feedback.The drives are set up with speed stepping and the elevators run at 350 feet per minute.

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#6
In reply to #4

Re: VVVF Drive overheats Dynamic Braking Resistors

06/03/2008 2:13 PM

I am now full of questions. I am by no means an elevator person, however. Has this started from the beginning or is this a "new" meaning has just started intermittently every few months? I am thinking it may not be in the drive system. If you are using the dynamic breaking you are throwing a bunch of DC on the motor to maintain elevator speed.Theres you heat so... If this new, could there be any outside influences, additional breaking or holding/safety system or counter weights that could be stressing the system?

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#9
In reply to #6

Re: VVVF Drive overheats Dynamic Braking Resistors

06/03/2008 5:38 PM

Hi Logix101

Yes there is a counter wieght and that is why the dynamic braking is used when stopping in the up direction with empty car. We have had two controllers burn up the resistor pac. There has only been two incidents and there are alot of these controllers running for many years but can not find out the root cause for the problem. I will look closer at the systems in question to see if I missed any details. Thanks

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#7
In reply to #4

Re: VVVF Drive overheats Dynamic Braking Resistors

06/03/2008 3:20 PM

"The motor and drives are rated for the use."

You keep repeating that, but I think the point is if they are burning up, that is obviously NOT true! Circuit protection devices are there to prevent a fire in the case of some abnormal situation. If the resistors are overheating on a regular basis, something is not right. So what the others are saying is supportable in my opinion, someone, NOT Baldor, should evaluate this application for suitability for the conditions, both environmental and operating. To chase the problem with additional hardware is a recipe for continued failure.

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#8
In reply to #7

Re: VVVF Drive overheats Dynamic Braking Resistors

06/03/2008 5:22 PM

Hi JRaef

I agree with you that there may be other problems with the design of the system but what I am getting from the elevator controller manufacturer is to install fuses between the resistors and the drive. Other elevator controller manufactures use the same drive and install fuses with a relay to shut down the drive if the resistors fail. Thanks

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#11
In reply to #8

Re: VVVF Drive overheats Dynamic Braking Resistors

06/03/2008 11:29 PM

I don't think the fuses are a bad idea, I just don't like the idea of "solving" a problem with fuses. Like I said, fuses should be the LAST resort in a circuit, not a limiting device.

Many more sophisticated VFDs use a thermal monitoring algorithm for the DB resistors. After all, the VFD is controlling all of the power going to the resistors, it's not too difficult to keep track of it. Then if you know the resistor parameters, you know what they can take so you can take action when a problem arises, even hopefully before it becomes a problem. That algorithm is similar to what is used for the motor thermal overload protection. I have used drives with that capability without fuses to the resistors quite a bit, never had a problem. Using a fuse instead of that is a "cheap cheat" in my opinion. Kind of like saying you don't need an engine temperature monitor on your car because you have a radiator cap that will blow if it overheats. By the time that happens, it's too late. Think of it; you are going to let an elevator full of people stop completely because the braking resistor is about to catch on fire, and the only way to get it going again is to replace the fuse? What if the fuse is out of stock? Dumb plan if you ask me. A good elevator drive would detect the overheating resistor and go into a creep mode to the next floor.

There was a day, back when Baldor originally bought out Sweo Drives in Seattle, that they had good quality equipment and very knowledgeable people. I fear those days are long gone.

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#10
In reply to #7

Re: VVVF Drive overheats Dynamic Braking Resistors

06/03/2008 7:00 PM

GA

He seemingly does not want to hear such comments or to act on them.....but I'm with you completely.

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#12

Re: VVVF Drive overheats Dynamic Braking Resistors

06/06/2008 6:51 AM

I don't remember if Baldor drives require this but some VFD's have a parameter to set the DBR resistance and wattage so the drive can calculate power dissipation. It sounds as if the resistors are undersized for the application as they are failing and your brake chopper is not. It may also be that the deisign was only for a certain duty cycle which is now being exceeded. One would think that it would be designed for "continuous" elevator operation but maybe not if its a specialized application.

Lastly I don't think circuit protection is going to help as you are not grossly exceeding the ratings of the resistor but I do think some sort of thermal sense may be in order. A lof of DBR's incorporate a thermal cutout on them so that if they exceed a certain temperature they disable the drive or prevent further operation until the DBR has had a chance to cool.

Shawn

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#13
In reply to #12

Re: VVVF Drive overheats Dynamic Braking Resistors

06/07/2008 9:41 AM

Don't get lost in the forest for the trees,If your system ran with no problems for a long time,then its not a engineer problem.You have a component failure.Its a closed loop controlled system,look at you encoder is its coupling slipping? Not knowing just how smart your controller is,if your getting bad speed and location feed back,could be a problem,could be in the drives braking control board,or an SCRs braking down under load,had that problem with a 150hp DC motor on a waste wood chipper,would run fine,but when asked to Regen to stop would trip drive.Its almost always something simple,just not so simple to find.Good luck!

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#14

Re: VVVF Drive overheats Dynamic Braking Resistors

06/06/2013 2:18 AM

hi Travis

I also had similar experience in one our EOT crane Long Travel motion DBR .The DBR(8ohm ,4.5kw) is connected to ABB ACS 800(40KVA) drive .The drive was frequently tripping .when we opened the resistor ,it was found to be red hot .Even in idle condition ,the drive was dissipating energy to the resistor .We contacted the OEM but of no use . after running the drive for half an hour ,a flashing occured resulting in damage of the rectifier and IGBTs . At last we had to change the drive . Now there is no overheating or any abnormality in DBR .

regards

suraj

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Andy Germany (2); dg1258 (1); JRaef (2); logix101 (2); Shawn33 (1); surajkant tiwari (1); tooz (1); Travis (4)

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