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

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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):

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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:

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

Thanks for your attention.

  • \$\begingroup\$ Switch must be rated for 300% of Motor rating or derated to 33% \$\endgroup\$ – Tony Stewart EE75 Jan 16 '20 at 19:05
  • 2
    \$\begingroup\$ 5 € too expensive for Dentists ?? . Why using such a cheap vacuum motor? Poor design quality \$\endgroup\$ – Tony Stewart EE75 Jan 16 '20 at 19:24
  • \$\begingroup\$ Looks like your universal motor is living a hard life: electronics.stackexchange.com/questions/473290/… \$\endgroup\$ – winny Jan 16 '20 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 Jan 16 '20 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\$ – Tony Stewart EE75 Jan 16 '20 at 20:37

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

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Understand failure modes for all switches


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.

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use electrical Insulated package or add Mica then add silicon grease

  • \$\begingroup\$ Thanks @Tony, so i will test it. \$\endgroup\$ – modern Jan 16 '20 at 21:11
  • \$\begingroup\$ So is This Triac IC proper (BTA25)? \$\endgroup\$ – modern Jan 16 '20 at 21:20
  • \$\begingroup\$ good choice or BTA16-600BW with thermal grease and plastic HV rated caps \$\endgroup\$ – Tony Stewart EE75 Jan 16 '20 at 21:30

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