DC activated AC relay

[dave-novice]

Hello right off the bat I'm a novice with a little better understanding than most(father was a electrician) but I am not an electrician.
I'm converting a metal bandsaw from 3 phase to single phase, replacing the motor(which is dead) and wiring.
I am looking to use a DC signal to the safety and manual run trigger on the handle for obvious safety reasons.
I'll be using a transformer to convert AC to DC in this circuit.
What I need to find is a relay that is DC activated that runs AC supply and don't know how to phrase my search to find one can anyone help please
Cheers Dave

 

[mainline]

Hello right off the bat I'm a novice with a little better understanding than most(father was a electrician) but I am not an electrician.
I'm converting a metal bandsaw from 3 phase to single phase, replacing the motor(which is dead) and wiring.
I am looking to use a DC signal to the safety and manual run trigger on the handle for obvious safety reasons.
I'll be using a transformer to convert AC to DC in this circuit.
What I need to find is a relay that is DC activated that runs AC supply and don't know how to phrase my search to find one can anyone help please
Cheers Dave

A transformer alone won't give DC.

You need to know the dc voltage and also the ac current that's going to be switched.

 

[dave-novice]

Once again your right I'm not an electrician(wrong word)

https://www.tro.com.au/PROeco-Switch-Mode-Power-Supply-72W-12V-6A?gad_source=1&gclid=CjwKCAiA8sauBhB3EiwAruTRJhzT2YiWkvqx2WBZLRHBhd1iJv8hVwWEXRdrViF98VA042b7MJlcARoCwxEQAvD_BwE

Hear is an example of what I may use, and to what voltage current will depend on the relay.
I will buy the power supply to suit the relay I can get, I'm looking at 12/24v DC any amo will depend on what relay I can get the AC side will have to be 20amp minimum would prefer about 25amp.
The motor is 1.5kw with 18amp peek draw,
Cheers Dave

 

[dave-novice]

Sorry 240v single phase motor

 

[dogbob]

There are relays with DC coils that are rated for AC switching. The keys are the load voltage and current (240VAC at 15A or something like that). Then you specify one with the right coil voltage.

For example, TE Connectivity Potter & Brumfield makes a few like that. Digikey.com is a distributor. Check out PRD-11DG0-12, PRD-7DY0-12, and PRD-11DY0-12.

There are also "solid state relays", which offer the same isolation advantage as mechanical relays, but have no contacts to arc. Solid state relays require less power to the input, but waste more power in the switch.

For example, Panasonic AQA221VL & Picker PCS28-12D-240A-40R. Again, you can buy these from Digikey or other electronic distributors.

I hope this helps.

 

[dave-novice]

Thank you very much that helps a lot

 

[Avo Mk8]

Good luck with the conversion of your bandsaw to single phase, Dave.
You don't mention whether you will be keeping the existing arrangements for start and emergency stop, but I hope you will, or if you make any changes, that you think about fault tolerant design in your replacement wiring.
As I'm sure you are aware, the 'Safety of Machinery' is taken very seriously, and is regulated, in most countries. There are safety standards that machinery manufacturers must meet, and a design process that must include hazard analyses, risk assessment, and mitigation of risks to an acceptable level. The application of all that should lead to a design that is robust and fault tolerant.
Within that process, electrical design usually requires specific 'safety' components, such as a 'safety relay' being used for emergency stop, and wired in a configuration such that any single failure in the wiring would not render the emergency stop inoperative.
If you are aware of the techniques normally used that's great, and I wish you well with the modifications.
But if this is a new area for you, I would encourage you to read up on the circuitry normally used to ensure a machine is safe, particularly with an emergency stop. One of many manufacturers of safety relays is PILZ, who (well, used to at least) publish useful guidance on their safety relays and components.
This is just a randomly selected example to illustrate what I'm going on about - I'm not condoning its accuracy!:

I'm no expert in this field, just aware of how important it is for users safety, and unfortunately how complicated it can get!

 

[w0z]

To add a few points
The DC supply you linked to is massively overrated for what you need to drive a relay (low milliamps for SS, higher mA for inductive (coiled) relay which would also need a diode to prevent reverse emf spike) not to mention that if you lost power to the supply (as opposed to disconnecting the output) it may well still provide enough voltage to keep a relay energised for some time (especially if SS). (small load required to prevent that).
My point being (in agreement with Avo's post), this needs to be carefully thought out.
Also islolation of the motor should be 2 pole (Live and Neutral),
I'm not sure if the emergency stop is required to be 2 pole? but I'd still make it so, as safer.
(later edit..the panasonic relay example is only 15A?)
have a look at this

 

[dave-novice]

Hello just a quick reply about the bandsaw.
Basically the DC side of the relay is for att the safety switched which I'm planning to put in it's own circuit.
The reason for the DC side of the relay was so I don't have 240v AC running to the switches (safety switches) and the AC side of the relay(mains power) is to run the 1.5kw electric motor and fluid pump.
In line there will be an isolator switch then the DC/AC relay to stop mains if the safety switches trip.
Next in line is a magnetic on off switch for manual motor start(magnetic so if the safety relay via safety switches trip the magnetic switch needs to be turned on again manually) and last but not least it then runs to the motor/fluid pump.
The motor is rated to draw 9.7amp so I am overkilling the relay specs.
Lastly this will be run on a generator 11kva which powers all my larger draw equipment, this has a 32amp RCD.
Cheers Dave

 

[dave-novice]

Just reading other reply posts, yes the DC power supply was just an example.
What supply I'll use will depend on the required amps the DC coil requires to operate but that's the part I'm still sourcing the AC load side I'd like to be 20-25amp (and this is a little overkill)not the DC coil side but until I purchase the relay I personally won't know the DC requirements.
Cheer Dave

 

[mainline]

You don't need a DC relay, for instance a 24v 2 pole contactor would be better using a simple 24v transformer.

 

[dave-novice]

I looked closer at the old 3phase relay and you are dead right the coil side is 24vac so I'd be able to re-use the relay.
Been looking at the transformers from 240vac to 24vac there was a earlier post saying about only needing milliamps that's the next question in 24vac what amp transformer should I look at for the 3phase relay?
Cheers Dave

 

[dave-novice]

Hello again everyone, sorry to keep posting/annoying for answers.
The old 3phase contactor/relay is a c-12d10 with 24vac coil could anyone tell me the amps/Millie amps needed to operate the relay?
Cheers Dave

 

[dave-novice]

You don't need a DC relay, for instance a 24v 2 pole contactor would be better using a simple 24v transformer.

Hello mainline, I looked at the contactor/relay that was for the 3phase it has a 24vac coil.
Model c-12D10 I'm thinking of using this but only utilising 2 contactors but what amp transformer would this work with?
Being AC 50-60hz I'm hoping it wouldn't need 1amp or over but I don't know I can't use existing transformer from the old setup as it rated input is 415v.
If you had any idea that would be greatly appreciated.
Cheers Dave

 

[James]

Get the same Va rating as the original.