220V Mains Switch for large inrush - Page 1
Forum section: electronics / general

author: metrologist
author_rank: Super Contributor
timestamp: October 03, 2016, 06:49:48 pm
content: I use a twelve dollar Leviton 30 Amp double pole industrial "motor duty" switch from Home Depot - it looks like a typical light switch - to switch mains power to a 2.2 kW VFD (Hitatchi WJ200) and 70 VDC power supply (20,000 µF).Someone is telling me the switch is going to fail due to the switching load (not sure if inrush or when the connection breaks). Is this a reasonable concern with this setup?

author: electr_peter
author_rank: Unknown
timestamp: October 04, 2016, 05:47:50 pm
content: Short answer - I guess that AC switch will be destroyed.Switch rated for 220V AC can be rated for only 24-100V DC, check datasheet for your model. This is because of arcing damage to switch contacts. AC voltage alternates and breaks arc by itself, while DC sustains arc. AC and DC switch ratings were discussed on this forum multiple times, try searching.You mentioned high power DC motor (meaning inductive load, thus more arcing) and high capacitance DC power supply. I would look for a DC rated switch for this application as high amperage rated AC switch will likely not perform.

author: metrologist
author_rank: Super Contributor
timestamp: October 04, 2016, 05:53:47 pm
content: It is switching AC mains that supply the VFD and DC power supply. There is no load when switched, other than charging the capacitors.

author: electr_peter
author_rank: Unknown
timestamp: October 04, 2016, 07:54:37 pm
content: Quote from: metrologist on October 04, 2016, 05:53:47 pmIn such case 30A rated switch will do. Just make sure inrush current is not too big - you can measure it and add NTC inrush limiting resistor, if neccesary.

author: metrologist
author_rank: Super Contributor
timestamp: October 04, 2016, 08:23:59 pm
content: I was wondering how I could measure this peak current? I have a scope and was thinking of wrapping some wire around the supply chord, maybe 10 wraps, and completing the circuit with a resistor. Then I could "calibrate" the current in this loop with a known current through the chord, and interpolate from there...I came across this, will watch later:I've been doing lots of searching, but it seems there are some conflicts in searching terms of switch, VFD, current, inrush, etc.Thanks.

author: electr_peter
author_rank: Unknown
timestamp: October 04, 2016, 08:42:42 pm
content: The are few approaches to measuring peak inrush current. You can use clamp meter with peak hold (or inrush current) capability or measure current waveform with oscilloscope. Search for term "current transformer" as few loops of wire won't generate enough voltage to pick up on scope. High voltage differential probes with resistor in series may work as well.Measuring mains current draw directly is very tricky (from wiring point of view) - ensure that working environment is safe for you and others. Oscilloscope can only be used for such task if it is isolated from mains.

author: metrologist
author_rank: Super Contributor
timestamp: October 04, 2016, 09:12:50 pm
content: I think the VFD has a bit of soft start built into it, just from reading internet musings... I skimmed the VFD documentation and there is no spec that I could find about it, other than fuse rating of 30A AIC 200kA, but that AIC rating is used for every model.I briefly saw a post here by another member that is deleted, said something along the lines of the DC supply somewhat buffers the AC line from its filtering capacitors.The most I'd do with the scope is just wrap wire around the line cord. It might be difficult to discern anything useful as there are two live lines. I was thinking that I could put one end to the probe and the other to BNC ground, maybe with a resistor in there. I'll have to review Dave's video on blowing up oscilloscopes though. Maybe my DMM has a peak voltage detector.I'll search more about measuring inrush before doing anything silly.