Contactor draws 225mA from 24V supply

[treez]

Hello,

The following contactor requires 24V control connection in order for the contacts to close.
The page says it draws 5.4W of control power, so does this mean that the contactor draws 225mA of control current from 24V control voltage?

Contactor
https://uk.farnell.com/schneider-el...elay-control-5no-24vdc/dp/3621819?Ntt=3621819

 

[RCinFLA]

Yes that is what it means.

Do you really need five pole switch? They are only 10 amp contacts but with five contacts the relay coil current is high to mechancially pull all switches.

You can do a lot better if you only need two or three contacts.

 

[jrwebsterco]

One other detail that comes to mind is to be aware of pull-in versus holding current.
Most designs approaches just over-margin for the larger pull-in current.
Anyway, I'm only mentioning this in case you are really concerned down to that careful degree.
You might need to contact the manufacturer or look for a datasheet spec to get such details.

 

[treez]

We have 1.9 Amps rms (sinusoidal) current flowing at 50KHz in a 180 ohm load (its a fixed current HFAC current source). This contactor is used to suddenly short circuit the whole 180 Ohms....i think the 5 contacts will be needed for this, and i will put all five in series.

(does it draw more control current if i use all five of the contacts, as opposed to if i used just say three of the contacts?)

The datasheet didnt mention pull in current or holding current, do you know what they are liely to be?

 

[FvM]

does it draw more control current if i use all five of the contacts, as opposed to if i used just say three of the contacts?

How? It's just a coil (RL series circuit).

The datasheet didnt mention pull in current or holding current, do you know what they are liely to be?

The data sheet specifies control voltage limits and drop-out voltage. This should answer your question.

 

[treez]

i certainly see the details you speak of on page 5 (CAD50BD contactor)

there is no way of calculating the holding current from that information
Clearly the coil current is 5.4W/24VDC = 225mA.
This also tells us that the coil resistance is 106 Ohms.
Goodness knows whether this 225mA is the pull in current or the holding current. Since the datasheet says the control circuit draws 5.4W, then one assumes that this 5.4W is the "Holding" power.....and thus the holding current is therefore 225mA.



Also, 225mA is the highest that the current can get to (=24V/106R)......one would then presume that that is therefore the "pull in" current, since "Pull in" involves the contactors actually being physically moved, and thus should involve the highest current......and as we know, the current maximum is limited by the coil resistance and the coil voltage.

So i have calaculated that pull in current and holding current are the same which doesnt make sense.

I think the datasheet is not providing the info needed......"pull in" current is what you get with 50Hz AC operated solonoids, where the AC sees a different coil impedance as the plunger gets pulled into the coil.......thus you get a higher initial current as the plunger is initially out of the coil and has lower inductance...therefter its inductance is increased as the plunger is pulled further into the coil.
In this case our control voltage is 24VDC , so these pull in currents dont apply?

 

[jrwebsterco]

I may have unintentionally confused you by referring to more general relay concepts - apologies if so.
If you are fixed at 24V, then what you say is true - the current is going to be what is implied by the resistance, wattage and so on for a fixed 24V applied coil voltage.

The difference between pull-in and holding applies if you are able to change the voltage to reduce power consumption while holding the relay active.
(that is what I meant by "in case you are really concerned")
Maybe I misunderstood that you were trying to minimize the current/power, via system design flexibility.

BTW, holding current is lower because the plunger/plate/etc. is then much closer to the solenoid, no longer needs to additionally overcome inertia to swing closed, and so on.

- - - Updated - - -

i think the 5 contacts will be needed for this, and i will put all five in series.

Almost forgot to also point out that this sounds like maybe a typo?
Why put multiple relay circuits in SERIES if it seems you are wanting to minimize their effective resistance (to maximize the short-circuiting effectiveness)?
Are you sure you didn't mean in PARALLEL?

 

[FvM]

I think the datasheet is not providing the info needed......"pull in" current is what you get with 50Hz AC operated solonoids, where the AC sees a different coil impedance as the plunger gets pulled into the coil.......thus you get a higher initial current as the plunger is initially out of the coil and has lower inductance...therefter its inductance is increased as the plunger is pulled further into the coil.
In this case our control voltage is 24VDC , so these pull in currents dont apply?

The said parameter is usually named inrush current of AC driven contactors, I think. But whatever you name is, there's no inrush current with basic DC coils (not considering dual coil or contactors with built-in pwm electronics). Coildcurrent is just linear increasing with coil voltage. This means pull-in current, the current which causes the contactor to engage, can be calculated from minimal operation voltage and coil resistance.

Why put multiple relay circuits in SERIES?

Apparently to increase switching voltage.