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Relays for dome automation

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#1 fulhair

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Posted 19 May 2020 - 07:37 PM

Hi All,

Our astro club is in the process of automating our observatory that has a 5 meter dome.

We've bought the MaxDome 2 control cards and need to integrate it with the existing motors which were previously manually driven off a switch.

Given the relays on the Max dome cards are rate at 12V, 5 Amps but the rotation motors pull around 48Amps so we need a relays to isolate the card.

I'm okay, but not great with electronics and it's not clear what/how the previous person wired this I'm looking to simplify this set up.

 

I wondering about winch motors solenoid (Double Pole Reversing Solenoid) and came across this one: https://www.jaycar.c...lenoid/p/SY4202

It's needs 12v, 5 Amp to drive the switch but looks simple to wire in the card in place of the switch and should take the motor current.

 

Does this look like a good option or does anyone else have experience or recommendations for this kind of set up?

Cheers

Andrew



#2 kathyastro

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Posted 19 May 2020 - 07:54 PM

Does the dome really need a motor that big?  Just asking - maybe it does.  The solenoid is overkill for the motor, but that's good.

 

If it takes 5 amps to drive the solenoid, and the Max Dome relay only handles 5 amps, you are operating that relay at its limit.  That is asking for trouble.  It will work, but the relay's life may be reduced.



#3 fulhair

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Posted 19 May 2020 - 09:50 PM

Hi kathyastro,

Yes, the dome is that big/heavy and we've replace the support wheels/bearings to remove resistance.

It was helicoptered into place due to the location and weight.

 

The 48 Amp draw is the peak current to get the motion started but I think it drops once the dome gets started.

I was also concern about the high current draw, but with a bit more searching, I've found another winch solenoid with lower current draw of under 2 Amp (https://www.4wdbits....?productID=5053).

 

So my question is more, is this a good option or do others have a better option on alternative relay set up?



#4 gregj888

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Posted 19 May 2020 - 10:14 PM

Fulhair,

 

Google "motor starter"...  You need a relay that is rated for inductive loads.  What does the motor actually need, Volts, Amps AC/DC?

 

If you can, find a Solid State Relay.  No noise, no arching, longer life with less maintenance.   

 

You can use your existing relay to drive either.



#5 mark77

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Posted 20 May 2020 - 01:44 PM

This is what I use, rated at 40 amps

 

They can be controlled by as little as 4 volts

 

https://www.amazon.c...0?ie=UTF8&psc=1



#6 Christopher Erickson

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Posted 21 May 2020 - 06:32 PM

The MaxDome-II cards are designed for 12VDC motors. You can adapt them to just about any kind and size of DC or even reversible-AC motor with external control relays and some diodes.

 

Be warned that the motor output on the MaxDome-II cards is always +12VDC on both motor terminals. When it wants to drive a motor, the card switches one of the motor terminals to -12VDC. This means that if you are using an automotive DC motor with a grounded frame, the motor will almost always have it's frame sitting at +12VDC and that could lead to an electrical short risk. So be careful.

 

Here are some example schematics I created that will show how to interface to bigger AC and DC motors, along with incorporating AC and DC soft start modules, which are VERY nice additions for most observatories that help everything last longer.

 

These schematics also isolate the card from the motors and also isolate the card's "always+12VDC-out" problem from the rest of the observatory.

 

Ping me with more details and I will create a schematic specifically for your application.

 

 

  

Attached Thumbnails

  • MaxDome-II - ProDome interface - Generic.gif
  • MaxDome-ACmotor.gif

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#7 fulhair

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Posted 27 May 2020 - 03:30 AM

The MaxDome-II cards are designed for 12VDC motors. You can adapt them to just about any kind and size of DC or even reversible-AC motor with external control relays and some diodes.

 

Be warned that the motor output on the MaxDome-II cards is always +12VDC on both motor terminals. When it wants to drive a motor, the card switches one of the motor terminals to -12VDC. This means that if you are using an automotive DC motor with a grounded frame, the motor will almost always have it's frame sitting at +12VDC and that could lead to an electrical short risk. So be careful.

 

Here are some example schematics I created that will show how to interface to bigger AC and DC motors, along with incorporating AC and DC soft start modules, which are VERY nice additions for most observatories that help everything last longer.

 

These schematics also isolate the card from the motors and also isolate the card's "always+12VDC-out" problem from the rest of the observatory.

 

Ping me with more details and I will create a schematic specifically for your application.

Thanks Chris, that's really interesting, I've never hear of the pwm controllers to soft start the movement. I think the instant on/off for the relay has been tough on the motor gearbox so this would be something we will definitely consider. I'm going to the observatory this weekend so will see how things are currently wired and get back to you.

Cheers

Andrew

I'm going to head on over there this



#8 MCinAZ

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Posted 29 May 2020 - 12:16 AM

This is what I use, rated at 40 amps

 

They can be controlled by as little as 4 volts

 

https://www.amazon.c...0?ie=UTF8&psc=1

Two things to note about these solid state relays. First, they are designed to switch AC loads, and most likely incorporate circuitry to switch only at zero crossings. Using them with DC loads will likely shorten their lives considerably and it may not be possible to switch them off once turned on. Also, there is a minimum load voltage. Operation at less than that value may result in unreliable behavior.

 

SSRs are a good choice for many power management applications, but it's important to get the right one for the job.



#9 mark77

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Posted 29 May 2020 - 08:09 AM

MCinAZ is exactly correct.  Sorry I forgot to mention that.  The solid state relays ONLY work with AC.  They can be driven with DC, but will only switch AC.

 

In my setup I have a large control box mounted to the side of my pier.  I do not want to have any 120VAC inside of that box but I do have 12VDC which is regulated down to 5VDC to run the Raspberry Pi's.  The 12DC is used to drive my stepper motors.  I use a relay on the R-Pi to send 12vdc to the relay which is outside of the box.  It turns on 120VAC.  It works very well.




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