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We have one dental equipment with one universal motor  in 2000W and 220V range shown below:

enter image description here

And we gave two corruption in selled device to customers which have inrush current problem by switching the universal motor, so one of the switch poles has been burned like this ( 220V 16A):

enter image description here

And 

enter image description here

So i searched the internet for solution and seen the ntc in this inrush current :

2200÷220=10A >> 3×10A ~ its inrush current!!

Like MS35 NTC with this shape:

enter image description here

But it is expensive (~5 €), so what is any other way with ...?

Thanks for your attention.

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  • \$\begingroup\$ Switch must be rated for 300% of Motor rating or derated to 33% \$\endgroup\$
    – D.A.S.
    Commented Jan 16, 2020 at 19:05
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    \$\begingroup\$ 5 € too expensive for Dentists ?? . Why using such a cheap vacuum motor? Poor design quality \$\endgroup\$
    – D.A.S.
    Commented Jan 16, 2020 at 19:24
  • \$\begingroup\$ Looks like your universal motor is living a hard life: electronics.stackexchange.com/questions/473290/… \$\endgroup\$
    – winny
    Commented Jan 16, 2020 at 19:48
  • \$\begingroup\$ Your link is for a relay, not the motor. Having found it at aliexpress.com/i/32960700752.html there's no way that's a 2000W motor, so its running current is likely far less than you think, but your issue is with the inrush. \$\endgroup\$
    – Phil G
    Commented Jan 16, 2020 at 20:36
  • \$\begingroup\$ @PhilG it uses same motor but poor failure analysis and no good solution given yet same problem with 25A switch !! Why? because arc burns out contacts on open ,.. not surge start \$\endgroup\$
    – D.A.S.
    Commented Jan 16, 2020 at 20:37

1 Answer 1

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Your problem is not just inrush but turn-off arc quench melting plastic from dry contacts reaching >20kV then arcing in < 1us.

Motor = 2kW @ 220Vac 50Hz

Mechanical switch: two issues

  • contact bounce turning on with 5x inrush while speed up, arc power
  • contact arc turning off with full RPM back EMF and inertia time.

Inrush= 500% typical of rated current, Ir=2000/220=9.1A
..... = 46A (rms)

Turn off arc= No load current is about 10% of rated so 200W with inertia time in seconds to reduce speed for arc quench , assume 1 sec thus 200J snubber

  • needs arc absorbing snubber RC filter. 220V / 110 Ohm = 2A
    • est. triangle power=1/3 of peak = 1/3 of 440W = 150W with AC cap
      - but this is expensive using brute force snubbers

Best solution is turn on at Zero cross voltage and turn off at zero-crossing current unless also driven by speed control triac.

Ideal Use a 25A Snubberless Triac with 1A switch

enter image description here

Important:

Understand failure modes for all switches

https://www.omron.com.tw/data_pdf/mnu/z385-e1-01_ssr_tg.pdf

Output triac short circuit occurs with 80% of failures, so a series safety switch is needed.

So you see there is no cheap solution for Dentists.

Simplest is not always cheapest.

enter image description here

enter image description here

use electrical Insulated package or add Mica then add silicon grease

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  • \$\begingroup\$ Thanks @Tony, so i will test it. \$\endgroup\$
    – Soheil Paper
    Commented Jan 16, 2020 at 21:11
  • \$\begingroup\$ So is This Triac IC proper (BTA25)? \$\endgroup\$
    – Soheil Paper
    Commented Jan 16, 2020 at 21:20
  • \$\begingroup\$ good choice or BTA16-600BW with thermal grease and plastic HV rated caps \$\endgroup\$
    – D.A.S.
    Commented Jan 16, 2020 at 21:30

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