Voltage Suppression for 19H 50A DC Motor Field
Voltage Suppression for 19H 50A DC Motor Field
(OP)
I have a project involving large 7000Hp DC motors with 19H fields with a resistance of about 5ohm. They normally need around 20-50A of field current for most all operating scenarios. Currently, the system has selenium suppressors installed for overvoltage suppression if the exciter suddenly trips offline. There is also a discharge resistor in parallel of 350ohm and 500W. The selenium suppressors may have failed in some cases and we are looking for logical replacements.
When I take the 50A and 19H for energy stored in a motor field, I get ~24000J. Some of the largest MOV's I see from TDK, Epcos, etc list maybe 10000J but then also have a higher let through or clamping voltage than I'd like. The joule rating of these are for 1.2/100us - 8/50us type transients with really high current spikes. Not sure if the manufacturers would even be able to say what the joule rating for such devices would be in this completely different (current-limited) suppression application.
I also started to look into power avalanche/zener diodes or something similar. But, even in the largest sizes, they didn't seem to have the thermal ratings I might need.
Please don't tell me that the below Siemens product would be the normal solution for the motor fields I'm dealing with(?!):
https://mall.industry.siemens.com/mall/en/ca/Catal...
Thanks!
When I take the 50A and 19H for energy stored in a motor field, I get ~24000J. Some of the largest MOV's I see from TDK, Epcos, etc list maybe 10000J but then also have a higher let through or clamping voltage than I'd like. The joule rating of these are for 1.2/100us - 8/50us type transients with really high current spikes. Not sure if the manufacturers would even be able to say what the joule rating for such devices would be in this completely different (current-limited) suppression application.
I also started to look into power avalanche/zener diodes or something similar. But, even in the largest sizes, they didn't seem to have the thermal ratings I might need.
Please don't tell me that the below Siemens product would be the normal solution for the motor fields I'm dealing with(?!):
https://mall.industry.siemens.com/mall/en/ca/Catal...
Thanks!





RE: Voltage Suppression for 19H 50A DC Motor Field
Choosing R=10 ohms will give you a modest peak voltage of around 250 V and a 500 or 1000 W wire-wound resistor should be able to absorb and dissipate the 25 kJ without glowing too brightly.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Voltage Suppression for 19H 50A DC Motor Field
In my initial search, most of the power avalanche diodes did not pass this much current, at least at a typical non-conducting temperature I'd expect for the rarely used application. Related to this, they appeared to have a severe temperature dependence where at the same revere breakdown voltage they might go from 1A to 100A depending on whether they were at 25C or 150C. But, I didn't spend much time looking here and will go down this road further.
Any continuing thoughts along these line or otherwise would be greatly appreciated from anyone....
RE: Voltage Suppression for 19H 50A DC Motor Field
The excitation contactor has two N.O. main poles and two N.C. auxiliary poles adjusted for a make-before-break action. The main poles connect the rectifier to the field winding of the machine, and the auxiliary poles connect a field discharge resistor across the field winding. There's a momentary overlap where the discharge resistor is connected across the rectifier output but this state exists only for milliseconds. Normally a Metrosil voltage-dependent resistor is connected across the rectifier side of the main poles to clamp any switching transients to a non-damaging level, althoug a large MOV might suffice in your application. I might even have a typical schematic somewhere.
RE: Voltage Suppression for 19H 50A DC Motor Field
RE: Voltage Suppression for 19H 50A DC Motor Field
RE: Voltage Suppression for 19H 50A DC Motor Field
That's a pretty fascinating (and large!) application. Thanks for sharing that. I wish I could implement something like that at this juncture. But, the selenium suppressors are needing replaced fairly quickly so I obviously was hoping for a less involved solution. Still, it might come into play. For now, we may have to leave the selenium in place and replace with identical that are still (amazingly) on the market.
LionelHutz,
Thanks for the info. The below image file is what I came up with. Please let me know if it looks about right?
Thanks!
RE: Voltage Suppression for 19H 50A DC Motor Field