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Old April 16th, 2018, 10:38 AM   #1
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einnh is offline
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Measuring braking resistor for future sizing.

I have been working developing winder systems for the past several years. They operate using winders running on AC induction motors and PowerFlex 755's with braking resistors sized for 110% power -- due to what I consider an overhauling layout.

I am not convinced we need braking resistors this large. On every unit I have built the resistors never get much above ambient temperature.

In an effort to optimize the system I wanted to see if there was a good way to measure how much of the braking resistor I am using in order to optimize but due to the nature of how they are used I'm not sure the best way of measuring this.

I know the DC bus is pulsed to the resistor, so I could attempt to measure voltage in those leads but I am not sure if I could capture and log quick enough for a significant amount of time.

Any ideas or suggestions would be greatly appreciated.
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Old April 16th, 2018, 10:45 AM   #2
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Does your system utilize a controlled e-stop? If so, be sure to consider stopping the full load quickly in an e-stop situation before reducing the braking capability.
It's not all the variables I am most concerned with, it's the undiscovered constants.
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Old April 16th, 2018, 12:10 PM   #3
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You already answered your own question - just measure the temperature.
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Old April 16th, 2018, 12:39 PM   #4
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I asked AB's Tech Connect about monitoring DB usage a year or so ago and this is what they had to say.

"Parameter 935 bit 20 will tell you if the brake is currently on or off
As far as power use testpoint 676
dynamic brake resistor calculated power (i.e. volts * amps for a given interrupt period)
When you put the value 676 in parameter 970 [test point sel 1] the calculated value be put in Parameter 971 and 972 _ "

It worked well for what I needed.

Just an addon thought: Once you have data you want out at the parameter level you can use several methods to record the data. I've used the method above with 3rd party trending software as well as AB's own built in trend tools. I've also tied it to one of the analog outputs of an I/O option board and then set up a volt meter so that I could see what was happening while I was out testing the equipment.

Last edited by Lynx777; April 16th, 2018 at 01:38 PM.
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Old April 16th, 2018, 01:00 PM   #5
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Why not use a line regeneration drive and eliminate the resistor altogether.
It will give you 100% regen 100% of the time and the energy can be used by other equipment.
E-Stops are a little different they require the drive to be in safe mode or disconnect the motor leads either way the motor will coast to stop.
It is except able to have a short time delay to E-Stop the motor at a fast rate using the drive and then when the motor is stopped put the drive in safe mode only a few seconds some safety relays have a built in timer for that purpose.
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