Attempting to find an equivalent thermostatic switch - Page 1
Forum section: electronics / projects

author: Krumm
author_rank: Contributor
timestamp: November 17, 2019, 02:05:58 pm
content: Hello all,I am attempting to find a thermostatic switch that is equivalent to:https://www.sensata.com/sites/default/files/a/sensata-3mp-motor%20protector-datasheet.pdfThis is for a 1700W AC motor, but beyond that I know very little of the motor (it's pulled from a Miele vacuum cleaner so has no faceplate). The thermostatic switch in question has the following contact ratings:27.5 A @cos 1 / 250Vac / 500 cycles18 A @cos 0.6 / 250Vac / 1.000 cycles18 A @cos 0.6 / 120Vac / 15.000 cycleThe motor is ran at 230V, however I just cannot find a thermostatic switch rated as high as this - 27.5A, or even 18A!I can find a number of thermostatic switches here:https://uk.rs-online.com/web/c/automation-control-gear/industrial-switches/thermostatic-switchesBut after filtering for a decent contact rating, I find ones such as the following:https://uk.rs-online.com/web/p/thermostatic-switches/1117739/Which for an inductive load is only rated at240VAC, 5FLA, 30LRA, 60Hz, InductiveThere must be something I am missing here. Could anybody shed any light on this or if you're feeling rather generous, make a suggestion of a suitable part from the above website?Thank you!

author: Krumm
author_rank: Contributor
timestamp: November 18, 2019, 10:36:44 pm
content: Anybody, please?

author: Gladius
author_rank: Newbie
timestamp: November 19, 2019, 03:19:29 pm
content: So if I see this correctley the switch is RATED for 28A but that not what you are using it at. If you do some calculations you will find that I=P/U.So I=P/U => 1700W/230VAC=7,4A so nowhere near your ratet 28A.Of cource we are ignoring power factor here but this is fine since your switch will have to handle the full apparent power.Now becauase motors are usualy ratet in delivered mechanical power and not in consumed electrical id generousley rount the 7A up to 10A.So now i think it would be time to search for a switch that can handle about 15A or more at a cos phi of around 0.8(Guestimation based on this link:https://www.engineeringtoolbox.com/power-factor-electrical-motor-d_654.html)Hope that helps.

author: Krumm
author_rank: Contributor
timestamp: November 20, 2019, 08:36:47 am
content: Hello Gladius,Thanks very much. I've had a look at a few datasheets and none mention anything of "cos phi" - I can only find figures for FLA and LRA. Could I simply use the FLA figure?Thanks!

author: Gladius
author_rank: Newbie
timestamp: November 20, 2019, 06:02:10 pm
content: Yes FLA figures should pretty much be fine. Keep that at about 15A and also check your specific switch for its temp. rating (at what temp it trips) also i would suggest the following filters:Contact rating:>= 15AVoltage:>=240V ACType: BimetalicTemp: whatever your particular switch is ratet for. when in doubt runn the motor and check its operating temp. Then add some saftey margain on top of that and you should be good.

author: Krumm
author_rank: Contributor
timestamp: November 22, 2019, 10:07:19 am
content: Thanks ever so much - now to find one that is around the same dimensions!

author: Gladius
author_rank: Newbie
timestamp: November 22, 2019, 10:13:37 am
content: Have you tried E-Bay ? If you are a bit carefull you can usualy find some decent stuff.Something like this maybe?https://www.ebay.ch/itm/17AM-Normally-Closed-Temperature-Switch-Thermostat-For-Motor-from-70deg/264271596262?hash=item3d87d08ae6:m:mKScO5h-LVBNtbm_3ZXvPXg