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The circuit is a Single phase motor(120V runs 4.7amps running amps), and a relay to trigger the motor on/off.

I'm looking to use a snubber circuit to protect the relay contacts for prolonged use. The motor shouldn't be switching on/off very often but to be safe.

According to this page: http://www.farnell.com/datasheets/303558.pdf

It says .5 to 1 ohm per volt so 120 v = 120ohms For amps it says about 1uf per amp. The motor runs 4.7 after startup.

For running current I would be looking for a 120ohm resistor and a 5uf capacitor.

Should the snubber circuit be valued for the running current? I also have a varistor rated for 120V, would this be a equal alternative? The varistor is rated for 1 watt of current.

Matt
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  • According to this question: https://electronics.stackexchange.com/questions/42131/how-to-design-an-rc-snubber-for-a-solenoid-relay-driving-an-inductive-load?rq=1 Values of 0.1 cap and 120 ohm resistor will be fine. Is this true? – Matt Jun 20 '17 at 17:00

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The best practice for selecting a relay to start and stop a motor is to select a relay or contractor that is designed for that purpose. For areas of the world where motors are rated in horsepower, relays and contactors that have a horsepower rating are available. For areas where IEC standards are used, relays and contactors have IEC utilization categories. Such devices are designed to start and stop motor safely without using either a snubber circuit or varistor to protect the contacts.

Since using a general purpose relay or contactor for motor control is not the generally accepted practice, it may be difficult to find guidelines for doing that. However, very small motors, smaller than about 1/4 Hp or 200 watts, might be suitably controlled by a general purpose relay. Applicable electrical codes are likely to prohibit using devices for other than their intended purpose.

Here are some similar questions:

Max allowed inductive load (pump) for a given relay

switching device for motor

Switching a 220VAC/20A Load with a 5VDC Coil Voltage?

  • I see. going through your answer I found several links and information but I'm not seeing the FLA or LRA on any of the datasheets of relays i'm looking at. I did find this one: https://www.aliexpress.com/item/5V-30A-High-Power-1-Channel-Relay-Module-with-Optocoupler-H-L-Level-Triger-for-Arduino/32642667729.html?spm=2114.40010208.4.16.4tuCCZ Which looks promising, except the wait time is very long. Any suggestions? – Matt Jun 20 '17 at 17:50
  • Is there a horsepower rating or any other ratings marked on the motor? –  Jun 20 '17 at 17:52
  • I'm trying to find the horsepower now. The tag is very dirty and old so hard to tell. The MDL# is 2Z848B, made by Dayton. Its a single phase, 5.8 Full load amps @ 60hz – Matt Jun 20 '17 at 17:58
  • Found it: https://www.grainger.com/product/DAYTON-AC-Gearmotor-Nameplate-RPM-2Z848 – Matt Jun 20 '17 at 18:00
  • So it is a 1/3 Hp motor. You should be able to find a relay rated for 1/3 or 1/2 Hp at 120 VAC. What coil voltage do you need? –  Jun 20 '17 at 18:11
  • My control circuit is 5VDC and is very much preferred, but I do have 24VAC available inside the box. – Matt Jun 20 '17 at 18:19
  • Here is a relay with a 5 volt coil rated 1 Hp at 120V. I am sure I have seen a 1/2 Hp one, but I can't find it right now. http://www.ebay.com/itm/2pcs-T90-Coil-Relay-5VDC-30A-250VAC-30VDC-6-Pin-1NO-1NC-SLA-05VDC-SL-C-PCB-Type-/391687477415?hash=item5b326498a7:g:hIAAAOSwnHZYiaHo –  Jun 20 '17 at 18:55
  • "not good practice" is not a good explanation for why not to do something. What about it makes it risky, dangerous, expensive, or otherwise undesirable to use a snubber to protect a relay? – Dave Mar 04 '19 at 20:17
  • @Dave The best answer that I can offer is that there is a "best practice" that is not the practice that I termed "not good." I have revised my question to avoid saying "not good." –  Mar 04 '19 at 22:19