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Motor Control


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7 replies to this topic

#1 Aro

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Posted 20 February 2015 - 10:45 AM

So I am trying to control a 12VDC, 3Amp motor (I don't need variable speed) using the U3-LV and a solid state relay <http://www.jameco.co...001_176655_-1>. I have +12V wired to the output 1 terminal, a connection to the + motor terminal at output 2, +5V wired to the + control, and a connection to the FIO channel at the - control.  In the same device we have an AC solid state relay controlling a heater that is working fine, but the DC relay controlling the motor is not working (both require a 3-32VDC control voltage).  Whenever the device is powered the motor runs regardless of the FIO state.  I know the U3 is functioning properly because switching from high to low in the control panel changes the FIO voltage accordingly (also, the AC relay is wired the same way and works fine).  I read somewhere else on the forum that the relay needs to be able to handle inductive loads but I cannot find that information on the relay data sheets.  Any advice?

 

Thanks



#2 LabJack Support

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Posted 20 February 2015 - 02:49 PM

If you remove the control connections to the SSR, does it still stay on all the time?



#3 Aro

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Posted 20 February 2015 - 02:56 PM

Yes.  So I suppose that means the relay will not work for our use.  But why?  What should I look for when ordering a new SSR?



#4 LabJack Support

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Posted 20 February 2015 - 03:38 PM

The part you linked is a DC-DC relay, so should be fine for controlling DC.  Perhaps the SSR is broken?



#5 Aro

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Posted 20 February 2015 - 04:03 PM

I don't think that is the problem. We ordered 2 so we would have a backup and neither one works.  Also, just to see what would happen I took a DC relay <http://www.digikey.com/product-search/en?KeyWords=gh3040-nd&WT.z_header=search_go> fromanother project that used a U9 with an RB12 board and wired it up the same way and it worked just fine.  So I guess I need to find an equivalent relay but with screw terminals.  Although I still don't know why our original SSRs won't work.



#6 LabJack Support

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Posted 20 February 2015 - 04:43 PM

It's possible that your 3A motor exceeded the max (50A) peak surge current of the SSR, and broke both of them instantly.  DC motors have a very very high inrush current, due to the low armature resistance when they first turn on, and this can sometimes damage things. 

 

You could try 1 or 2 below:

  1. (the least expensive) Add a 1-5Ω resistor in series with the motor.  Note that this resistor will get hot, so you need to make sure it doesn't get too hot(that it'll start a fire).  The resistor should also have a power rating to handle the current.  If it's a 1Ω resistor, then the power rating should be 9W or greater.  3A * 1Ω = 3V drop, 3V * 3A = 9W.  You could also just use an inrush current limiter, like this one http://www.digikey.c...-2079-ND/652156
  2. Get a relay with a higher surge current rating, such as the KG1050D

There is some more information on armature resistance in Appendix D of the PS12DC datasheet.

http://labjack.com/s...heet/appendix-d



#7 Aro

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Posted 20 February 2015 - 05:22 PM

First of all, thanks for all your help on this.  The motor is cheap, but why would the Grayhill relay I pulled off an old RB12 work?  According to its specs it only has a 5Amp peak surge current.  Could the Kyotto SSR not working have something to do with the fact that it uses an opto-transistor instead of an opto-triac which is what the AC SSR we have uses and works just fine?  



#8 LabJack Support

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Posted 23 February 2015 - 05:00 PM

I agree, it does not make sense that the small Grayhill SSR would work fine but those big external SSRs would have been blown.

 

As I recall, a TRIAC does not work with DC because it relies on the zero crossing of AC current to turn off.  If you use a TRIAC to control DC, once you turn it on it will not turn off until current stops flowing.




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