Inductors 5

[Jk1996]

I have a question on inductors I’m trying to get my head round. I have a rough understanding how they work as the current through the coil produces a magnetic field and it’s the back EMF that limits the current through the inductor. I also understand with motors when you get a locked rotor the current draw goes up due to the back EMF not limiting the current. But my question is when it’s just an inductor such as a relay coil can these ever cause a similar overload to what we’d get with an induction motor when it has locked rotor?

The reason why I ask is I have seen relay contacts with the current of the contacts stated a 3amp inductive and 8amp resistive which then again got me thinking as the relay was bringing on one relay but is only being backed up by a 6amp fuse so my thinking was if it could overload the contacts would melt before the fuse?

Thanks for your help in advance guys!

 

[Jk1996]

This is me being naive now but how can you tell how much current a relay contact will be taking say for example this relay coil was being switched on by a relay contact rated for 3 amps

 

[waross]

Check the spec sheet.
What I find: "Coil Power 1.2 VA"
1.2 VA at 110 Volts x 1000 = 10.9mA
Inrush limited by coil resistance of 3830 Ohms to less than 28.7 mA.
You don't suppose that you may be overthinking this?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

Dear Mr. Jk1996 (Electrical)
1. There are numerous learned advice pertaining to AC contact rating and also pointed out to be cautious on contact rating on [DC switching].
2. Arc suppression is more difficult in d.c. than in a.c. To choose a d.c. contactor, it is necessary to know i) the current and the voltage to be broken and ii) the L/R time constant of the power circuit to be controlled.
3. Some "block type" d.c. contactor on the market designed for (d.c. switching) are fitted with arc chutes with permanent magnets [specially designed for d.c. breaking]. They can be 1,2,3 or 4-poles. The (coil voltage) can be designed for a.c. or d.c., differ from the [power circuit switching voltage].
4. e.g. a d.c. contactor rated 110Vdc,10A, switching with 1-pole on L/R < 1ms; is increased to 25A with 2-poles in series. In most cases, increase in number of poles in series increases the current rating, However, there can be cases that no current increment even connected 3 or 4-poles in series. On the other hand, the same d.c. contactor on 110Vdc is rated only 6A on 1-pole with L/R <2ms; and further lowered to 4A when L/R <7.5ms.
Che Kuan Yau (Singapore)

 

[Jk1996]

Thank you guys waross I definitely have been overthinking it. Thanks Che

 

[sunnysky]

Contacts are rated according to VA product for breaking power because the motor contact opening instantly becomes a generator and that power turns in an arc burning the air between the contacts at over 5000'K. With AC the next current zero crossing might extinguish the flash, but with DC , it is sustained with a force proportional to current. Thus the gap of the contacts for DC determines the threshold for DC power that can be extinguished, and for reliability you want to be well below that curve.

An AC Relay when used with DC must use a flyback diode. For AC an RC Snubber is a good choice somewhat looks like a plastic Run cap, but with an embedded series R.

e.g. 6A "AC relay" rated for 1200 VA ac can only handle about 300 Watts DC Resistive breaking current.