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Old October 27th, 2008, 07:44 AM   #1

Plc_User is offline
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Braking resistors for ac drives

I was wondering how braking resistors for ac-drives are usually dimensioned. I know it is dependent on the type of application but f.i. for a 5,5 Kw motor, how many Kw is the resistor dimensioned (standalone drive with dc-bus not coupled)?
If the several drives are coupled, how do you dimension the resistor(s) then? If the dc-bus is coupled do you activate the brake function on eacht drive?
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Old October 27th, 2008, 08:25 AM   #2
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Sizing brake resistors for stand alone drives has to be done according to the load and application i.e. will the drive be in regen permanently or only during emergency stop? in what time do want it to stop? etc. For a short time period you can dump more current through a brake resistor than it's power rating would suggest.

If you're going to couple drives together on a common bus then the first thing I would suggest is that you choose a drive that is designed to do that e.g. Siemens Masterdrive/Sinamics. You can then use a common rectifier/brake module or a fully controlled "active" bridge for the bus.

If you have to couple multiple drives not really designed that way, it can be done. I have done it with Telemecanique Altivar drives using a Eurotherm 890 power supply and brake chopper for the DC bus and then fuses to supply each drive from the DC bus.

I'm currently doing a job with Eurotherm 690 drives that each have an AC supply and the DC busses are coupled through dc contactors and fuses. In this case you have to be very careful with brake resistors and install a motor starter in series with the resistor; the starter rated at the current that the resistor can take permanently. The system I'm building (actually, re-building) has one 22KW drive, four 11KW drives and two 4kw drives; i've not fitted brake resistors to the 4KW drives so they don't end up trying to brake the 22KW drive.

The best advice I can offer to speak to the drive manufacturer about your application and see what they recommend.

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Old October 27th, 2008, 11:09 AM   #3

jouellet is offline
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braking resistor selection

Check on Allen-Bradley's site; there is a free tool called "Engineering assistant"

it will guide you in that selection. you can use it to calculate the load and the resistor needed, based on duty cycle, braking requirements, etc ...
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Old October 27th, 2008, 07:47 PM   #4
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Each drive will have a minimum acceptable ohm value for its braking function. This sets the maximum current thru the drive and thru the voltage-sensing or "chopper" circuit. You can use higher resistance and get less braking torque but that spec is the minimum.

The braking wattage can be different due to a lot of things. The maximum wattage would be the drive hp rating converted to watts. That would be the rare situation when the drive is braking at full rated hp continuously or for more than, say, one-half hour.

You can get by with lower wattage ratings on the resistor if the energy being wasted thru the resistor is less than continuous. The lower the duty cycle the lower the wattage can be for the same full braking torque. This is due to the fact that a resistor is a pretty forgiving device and will take heavy over-wattage hits if allowed to cool between hits. The maximum over-wattage ratio that I use is 10/1 and that only if the resistor is cold (room temp) every time the braking is done and the braking doesn't last more than about 20 seconds.

Us can also get away with a lower wattage resistor if you are only doing a little braking. There is no rule that says that, for example, on a 100hp drive, you have to do all your braking at 100hp! If all you need is a 10hp brake on a 100hp drive, size the resistor accordingly.
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Old October 27th, 2008, 08:43 PM   #5
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Here's an earlier thread.
Just because you can doesn't mean that you should...
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