DC amps versus AC amps

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doctorb5
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DC amps versus AC amps

Post by doctorb5 » Mon Jun 02, 2003 11:56 am

Hi all,<p>This has probably been answered before, but I thought I'd try to get a fresh perspective. I need to switch a 24volt, 10A current. I went to my local hardware store and found lots of switches that can carry a 10 or 20A load. But they are all rated for an AC current. Actually, the ones that listed both AC and DC ratings, showed a *lower* current for DC than AC. In other words, I could get a switch rated 10A for 125 volts AC or 3 A for 12 volts DC.<p>Why is that? Isn't "current", "current" in AC or DC? I know that I am over simplifying here, but I always thought that the current rating of an AC system was calculated at RMS (an average of the sinusoidal wave). So why is an average AC current not equivalent to a fixed DC current of the same value? <p>And my follow up question is: How do you calculate the DC current rating of a switch that you only have an AC rating for?<p>Thanks for any help.

Chris Foley
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Re: DC amps versus AC amps

Post by Chris Foley » Mon Jun 02, 2003 1:30 pm

The short answer is, you don't. Inductive DC loads will tend to draw a spark as the contact is breaking (Lenz), which deteriorates the mating surfaces of the switch, making them resistive and eventually causing them to fail. This tendency is overcome with switches which are designed to switch DC by using a "snap action" to quickly pull the contacts apart to minimize the arc, bringing the contacts farther apart than they normally would for AC, and using switch contact metal alloys which are better able to tolerate the higher temperature of any arcing. Switches which are only rated for AC rely on the 50/60 Hz commutation of the line voltage to help quench an arc. You may also notice, on some switches that are AC/DC rated, they also specify the maximum horsepower motor current that can be switched.<p>As an example, a typical industrial microswitch might be rated for 15A at 125, 250, 480VAC, 2A at 600VAC, 1/8HP 125VAC, 1/4HP 250VAC, 1/2A 125VDC, and 1/4A 250VDC.<p>Worst possible case, you might be switching a motor load with a little panel-mount switch, and the first time you try to turn it off, the motor current sustains itself through the arc inside the switch until the poor switch burns up. Not a good thing. Next worse case, the switch might work just long enough to get it into service, and the contacts will become resistive enough to cause switch failure right after it goes onto the factory floor or your customer's hands.<p>Without knowing more about your application, it's hard to offer any additional advice. For instance, what load are you switching?<p>[ June 02, 2003: Message edited by: Chris Foley ]</p>

doctorb5
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Re: DC amps versus AC amps

Post by doctorb5 » Mon Jun 02, 2003 2:42 pm

Thanks for the "short" answer... ;) <p>I'll give you more details if you want: The switch is going to be used as a cutoff switch for a 24 Volt system. The system starts with two 12 Volt, sealed lead-acid batteries in series. They feed an electronic (PWM) speed controller that powers two 12 Volts DC motors in series. The whole system draws about 7 amps at most. So I was shooting for a 10 Amp DPDT switch.<p>Maybe I should go to an automotive center and buy a large 12 volt switch instead?

Chris Foley
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Re: DC amps versus AC amps

Post by Chris Foley » Mon Jun 02, 2003 4:54 pm

Sounds good -- that's pretty much what I was thinking.

rshayes
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Re: DC amps versus AC amps

Post by rshayes » Tue Jun 03, 2003 12:36 am

I would look for military surplus circuit breakers. They usually have 24 volt ratings as well as 110 VAC 400 Hz ratings. These can be manually switched off, provide short circuit protection, and are rated to break high fault currents.<p>Unless your motors are quite small, there is the potential for suprisingly high currents. When a motor starts, there is no back EMF, and the current is determined by the external circuit and the armature resistance, which may be a fraction of an ohm. The motor starting current is usually much higher than the running current. Your PWM controller may alleviate this by operating the motor in a current limited mode while it is starting.<p>If the rotation of the motor is mechanically blocked, the current can rise to the same level as the starting current, or even higher if it is forced to rotate in reverse. The worst case is probably a locked rotor at the same time as a shorted switching device in the PWM controller. With nothing to break the circuit, the current may be limited by the series resistance of the motor armature and the internal resistance of the batteries. This is the fault current that the switch may have to open.<p>The power available from lead acid batteries under short circuit conditions can be embarassing. Use a switch rated to break the worst case current you can imagine, including semiconductor device failures.<p>[ June 03, 2003: Message edited by: stephen ]</p>

Rodney
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Re: DC amps versus AC amps

Post by Rodney » Wed Jun 04, 2003 11:08 am

I have bought toggle switches at Radio Shack rated for over 20 amps DC.

doctorb5
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Re: DC amps versus AC amps

Post by doctorb5 » Wed Jun 04, 2003 12:46 pm

Yes, that's what I ended up doing. I paid about $4 for a 20 Amp DPDT switch at Radioshack. But I enjoyed learning about the reasons why I shouldn't use a strictly AC switch. :)

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