Relay with resistive rated contacts switching inductive load - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: June 12, 2019, 04:53:47 pm
content: Can anyone hazard a guess as to how many operations could be expected from this relay:https://www.mouser.com/datasheet/2/418/NG_DS_RT2_1014-735741.pdfWhen powering a 120V, 2A nominal refrigerator?The relay will provide power from one of two sources, common to the refrigerator load.I would think when the main source fails, that on power-up the refrigerator would not instantaneously start the compressor, and that the refrigerators control contact would actually switch the start current a moment (or later if it happens to be off).On transfer back to the main source this little relay could be breaking the inductive compressor current.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: June 12, 2019, 05:17:34 pm
content: Between 0 and 1.

author: Jester
author_rank: Frequent Contributor
timestamp: June 12, 2019, 06:13:40 pm
content: That seems overly pessimistic?The data sheet for this particular relay only has a resistive rating, however looking at another relay with similar resistive rating that does provide an inductive switching rating, see below and appears to be capable of switching about 3A @125Vac Cos Φ = 0.4, 500,000 operations.I looked at several contact curves and the rule of thumb seems to be about 1/2 rated (voltage and current) for Cos Φ = 0.4 or in some cases just 1/2 rated voltage for inductive load.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 12, 2019, 06:32:09 pm
content: The problem with inductive loads is voltage spikes when switching off, so add a MOV to the output or across the contacts to suppress that.

author: Jester
author_rank: Frequent Contributor
timestamp: June 12, 2019, 06:47:19 pm
content: Quote from: NiHaoMike on June 12, 2019, 06:32:09 pmWe do have a MOV.

author: soldar
author_rank: Super Contributor
timestamp: June 12, 2019, 06:53:31 pm
content: Quote from: Jester on June 12, 2019, 04:53:47 pmI am trying to parse this and have no idea what it might mean.At any rate, although I would prefer to use a bigger one, I would not be afraid of using that relay. You can add a snubber and/or mov.And if you want to get really fancy, hold it closed until the compressor shuts off by itself and then release the relay. That way the problem disappears.

author: Jester
author_rank: Frequent Contributor
timestamp: June 12, 2019, 07:06:27 pm
content: Quote from: soldar on June 12, 2019, 06:53:31 pmThe relay has 2 form C contacts, the load (refrigerator) is connected to the common of the contacts and the NO and NC contacts are connected to the two sources (main and backup).The relay only operates when the main power fails (which is rarely).The relay contacts should be fully closed prior to inrush, and hopefully the MOV will help when the relay switches back, again this only occurs rarely

author: jmelson
author_rank: Super Contributor
timestamp: June 12, 2019, 07:56:57 pm
content: Quote from: Jester on June 12, 2019, 04:53:47 pmThe inductive part may be less of a problem than the starting surge. A 2 A refrigerator is pretty small. Is it a Peltier-type or a compressor type?Anyway, assuming a compressor, the starting surge will likely be 12 - 15 A for a fraction of a second, and the refrigerator, unless it has digital controls, WILL start immediately when power is applied, if the thermostat requests cooling at that time.The inductive kick can be taken car of with a series RC supressor or MOV across the contacts.Jon