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Using Arduino => Project Guidance => Topic started by: subha26 on Oct 10, 2018, 03:34 pm

Title: Snubber circuit design for 30A relay.
Post by: subha26 on Oct 10, 2018, 03:34 pm
I am planning to control a 1.75HP water pump using Arduino for automatic tank pump project.
I bought a 12V 30A relay for that. Initially I thought that the relay with flyback diode, optocoupler and some transistor to control it. After googling for a while, I saw that a snubber circuit was also needed to control a inductive load. I saw some circuit designs, but not specifically for a 30A relay. Can anyone suggest me the R(esistor) and C(apacitor) values. Also, are bigger C values safe or dangerous. Thanks in advance.
Title: Re: Snubber circuit design for 30A relay.
Post by: wvmarle on Oct 10, 2018, 04:09 pm
That pump runs on 12V DC? Or 12V AC? Or some other voltage?
Title: Re: Snubber circuit design for 30A relay.
Post by: terryking228 on Oct 10, 2018, 04:58 pm
Hi,

good design document here: http://www.cde.com/resources/technical-papers/design.pdf (http://www.cde.com/resources/technical-papers/design.pdf)

"DESIGN OF SNUBBERS FOR POWER CIRCUITS"
Title: Re: Snubber circuit design for 30A relay.
Post by: subha26 on Oct 10, 2018, 05:30 pm
12V is the coil voltage.
The pump runs on 240VAC.
Thank you.
Title: Re: Snubber circuit design for 30A relay.
Post by: wvmarle on Oct 10, 2018, 06:31 pm
So what do you need that snubber for? Just put a flyback diode across that relay coil.
Title: Re: Snubber circuit design for 30A relay.
Post by: subha26 on Oct 10, 2018, 06:40 pm
To prevent the relay from voltage spikes and the welding of contacts. By the way, I don't know all the details, but I read in many articles as well as as forums(avrfreaks, edaboard etc.) that we need a snubber circuit for driving inductive load for an increased relay life. Thank you.
Title: Re: Snubber circuit design for 30A relay.
Post by: wvmarle on Oct 10, 2018, 07:18 pm
This shouldn't be an issue with an AC load, thanks to the current dropping to zero 100 times a second (or 120 times if you're on 60 Hz).
Title: Re: Snubber circuit design for 30A relay.
Post by: terryking228 on Oct 10, 2018, 07:31 pm
This shouldn't be an issue with an AC load, thanks to the current dropping to zero 100 times a second (or 120 times if you're on 60 Hz).
Usually true.. IF the relay is rated for motor type load (usually expressed in Horsepower and well as current.)  Relay contacts designed for AC motor loads can have special properties.  The voltage and current of a motor load do not go to zero at the same time, though, and when the contacts break there still may be energy that needs to go somewhere. 
Title: Re: Snubber circuit design for 30A relay.
Post by: subha26 on Oct 11, 2018, 11:02 am
Quote
This shouldn't be an issue with an AC load, thanks to the current dropping to zero 100 times a second (or 120 times if you're on 60 Hz).
Thanks wvmarle, I didn't think about it, thank you for sharing.
By the way I attached the datasheet of the relay down below. It is no where mentioned that it is for motor load. Can you please tell me if it is for motor load or do I need a snubber circuit.
Title: Re: Snubber circuit design for 30A relay.
Post by: MarkT on Oct 11, 2018, 11:49 am
This shouldn't be an issue with an AC load, thanks to the current dropping to zero 100 times a second (or 120 times if you're on 60 Hz).
No, it very much is an issue with an inductive mains load - you don't understand what happens when
breaking a highly inductive circuit - arcing damage is done on a much shorter timescale than a mains half-cycle,
and the high voltages generated can compromise the motor winding insulation or the motor wiring if not controlled too. 

A 1.75hp motor may pull many tens of amps worst-case (at start up or stalled), and if that current is interrupted kW or tens of kW are directly available to pump into the arc and damage/weld the contacts as the magnetic
energy in the motor returns to the circuit rapidly.

If you are lucky the motor has some snubbing built in, but you'd have to check for that.
Title: Re: Snubber circuit design for 30A relay.
Post by: subha26 on Oct 11, 2018, 02:16 pm
So can anyone suggest me a website or a place where I can find the information, if possible please suggest the RC values.
Title: Re: Snubber circuit design for 30A relay.
Post by: wvmarle on Oct 11, 2018, 02:40 pm
Lots of info, including ANs from major electronics companies, related to TRIACs (https://www.google.com/search?q=triac+snubber+design). Not sure to what extent those snubbers work for you.

Title: Re: Snubber circuit design for 30A relay.
Post by: Slumpert on Oct 11, 2018, 04:30 pm
A 1.75 HP 220v motor that is stall loaded (attached to a water pump) should be hooked up to mains via a contactor and starter capacitor. 

Then use a normal 5v relay to control the contactor... 

Title: Re: Snubber circuit design for 30A relay.
Post by: Slumpert on Oct 11, 2018, 05:17 pm
Also, that relay you listed is only good for 560watts which is less than half of what a 1.75hp motor needs.