Using components other than how they're rated - Page 1
Forum section: electronics / beginners

author: michael.hill
author_rank: Unknown
timestamp: March 30, 2014, 11:06:50 pm
content: I'm looking at using a slip ring for a project I'm working on. It says it's rated for 30A at 250V. However, I want to use it at only 12-13 Volts. Can I pull more than 30A at the lower voltage? I'm not confident in saying it's rated for 7500 Watts and therefore can pull over 550 Amps at 13V. I'm looking at pulling around 40 Amps with spikes up to 140 Amps. Any thoughts? If it matters, it's a Mercotac 230 Slip Ring.

author: sleemanj
author_rank: Super Contributor
timestamp: March 30, 2014, 11:15:22 pm
content: The slip ring has resistance, this is fixed.Power can be defined as W=I2RDo you now see the problem :-)

author: michael.hill
author_rank: Unknown
timestamp: March 30, 2014, 11:25:08 pm
content: Quote from: sleemanj on March 30, 2014, 11:15:22 pmNot really, can you walk me through it?

author: michael.hill
author_rank: Unknown
timestamp: March 30, 2014, 11:26:23 pm
content: The slip ring has a resistance of <1 milliohm btw

author: c4757p
author_rank: Super Contributor
timestamp: March 30, 2014, 11:39:48 pm
content: Quote from: michael.hill on March 30, 2014, 11:25:08 pmThe powerdissipated in the ringis not the same as the power running through it (thank goodness). The power dissipated is related only to the current and the ring's resistance, not to the voltage between the conductors, and this power is (part of) what determines the maximum current rating. Therefore, changing the voltage does not affect the current rating.

author: miguelvp
author_rank: Super Contributor
timestamp: March 30, 2014, 11:43:04 pm
content: Quote from: michael.hill on March 30, 2014, 11:25:08 pmp=I*VI=V/RSo it should be:p=V^2/REdit, nevermind did the math on my head and it's the same thing

author: michael.hill
author_rank: Unknown
timestamp: March 30, 2014, 11:45:20 pm
content: Thanks for the replies. What about momentary spikes of high current? These shouldn't be running more than a few minutes total of up to 40 Amp operation for any given amount of time, but there may be a couple of seconds of spiking of up to 133 Amps. Would this be an issue?

author: c4757p
author_rank: Super Contributor
timestamp: March 30, 2014, 11:57:06 pm
content: Jesus Christ, 133A spikes? The hell are you doing with this?I wouldnottrust 30A contacts with a > 4x overcurrent for a "few seconds". Maybe a fewmilliseconds. But maybe someone more familiar with welders can comment...

author: michael.hill
author_rank: Unknown
timestamp: March 31, 2014, 12:01:52 am
content: Quote from: c4757p on March 30, 2014, 11:57:06 pmRoboticsThese guys apparently use the exact slip rings on their drive base (the same application I'm using them for). They're pulling the same currents I'm describing. They use the slip ring to rotate their wheels in all directions continuously while driving.

author: Fank1
author_rank: Regular Contributor
timestamp: March 31, 2014, 01:25:40 am
content: The contact area sets the current rating.The spacing sets the voltage rating.Amps is amps.

author: fcb
author_rank: Super Contributor
timestamp: March 31, 2014, 08:51:35 am
content: I don't think you'll have problem with 40A through something rated at 30A. However the spikes will be another matter altogether as you'll heat-up the finger wire and could damage it (it will be have a certain thermal resistance to the ring) - I would also expect the surface of the ring to pit much quicker (significantly shorter life) with that current.Can you parallel up several slip-ring circuits? I've done that in the past and it has worked very well (less noise also).

author: richard.cs
author_rank: Super Contributor
timestamp: March 31, 2014, 11:44:09 am
content: It all depends on the construction, you need to look at the design. Big lumps of copper like the rings will be fine with 130A for a few seconds (possibly tens), thin braid will have a lower heat capacity and "a few seconds" might be too long or might not (but it wouldn't notice a 10 ms spike). Small contact areas might actually weld if they're metal on metal but graphite brushes would just get hot."30A continuous" really means "the equilibrium temperature that is eventually reached is survivable". If you can make some educated guesses about the thermal time constants involved, the dissipation in different components and the temperatures that might damage them then you are in a place to decide if 40A for 5 minutes or 130A for one second is ok (as defined by "the temperature rise of all components is within allowable limits").