Quick question about a contactor

Batteries, Chargers, and Battery Management Systems.
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mxs   10 W

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Quick question about a contactor

Post by mxs » Dec 17 2019 11:14am

What will happen if I connect 48V battery pack to the below contactor? The battery pack amp rating is well well below max cont. amps of the contactor?

https://www.alliedelec.com/m/d/3fc07518 ... 618654.pdf

Thanks

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amberwolf   100 GW

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Re: Quick question about a contactor

Post by amberwolf » Dec 18 2019 12:13am

Battery current rating hasn't anything to do with contactor current rating.

The contactor has to be able to handle the *load's* current rating.

Meaning, whatever the current draw of the controller and motor is, under max load, is what the contactor has to handle, *and* it has to be able to safely break the circuit under that max load. (if it can't, then the arcing across it's contacts could weld them shut and it would be unable to break the circuit at all).

The contactor *voltage* rating does have to match the battery voltage rating however; it must be able to handle the fully-charged voltage of the pack (and also the highest possible regen voltage the motor / controller can possibly generate in this system, so if it has to break the circuit while braking it can safely break the arc).



The battery also has to be able to handle the load's current rating; the max that the controller/motor draws shouldn't be alllowed to be higher than what the battery is designed to handle.

mxs   10 W

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Re: Quick question about a contactor

Post by mxs » Dec 18 2019 8:27am

Thanks for your reply.

So it is a bad idea to connect 48V battery to a contactor which is rated for 48V coil voltage trigger, yet only for 36V contact voltage (this is the only contactor which I found actually states a contact voltage or where it’s coil and contact voltages are not the same .... I guess the reason is that most applications just use the same battery source for the coil as well as load) .... regardless how low the load amps will be, correct? Any idea what will physically happen?

john61ct   100 GW

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Re: Quick question about a contactor

Post by john61ct » Dec 18 2019 9:01am

The signal spec is just "stay within the range".

Many many applications the signaling voltage is much lower than that of the power line being handled.

The power line rating is a maximum, going lower voltage is no problem, contactor's Amp rating means less watts of course.

Should leave headroom on those amps for better longevity




mxs   10 W

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Re: Quick question about a contactor

Post by mxs » Dec 18 2019 9:54am

I have a room in the amps department ... but I have no room on power line voltage department. They claim 36V contact voltage as per specs ... and my system operates at 48V nominal .... so that's why I am asking here ... plus I have not seen other DC contactors where the manufacturer states two voltages ... coil and contact and at that those two being different.

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TommyCat   1 kW

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Re: Quick question about a contactor

Post by TommyCat » Dec 18 2019 10:39am

Since the voltage rating of the contacts does not meet your requirements, I would pass. (hard to use terminals?)

Surely with the millions of contactors/electronic relays available in the world, something can be found.

If you would fill in your definitive specifications, and what your overall goal is, I'm sure many here will help look! :shock:

Example: I want this relay/contactor to switch my battery power to my controller using a remote switch. My controller max amp draw is XX. My battery max amp output is XX.

Coil Voltage-48vdc (some electronic relays don't need a "coil" voltage, just use a switch)...
Contactor Voltage- 48vdc
Contactor maximum Amperage- ???
Contactor Poles Required
Any other particulars that you deem important.


Regards,
T.C.
See my completed Magic Pie V5 rear hub E-Bike build Here!

john61ct   100 GW

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Re: Quick question about a contactor

Post by john61ct » Dec 18 2019 10:54am


mxs wrote:I have not seen other DC contactors where the manufacturer states two voltages ... coil and contact and at that those two being different.
Then you haven't looked much.

Use keyword "relay" as well, Newark, Mouser & Digikey are good sources.

12 or 24V coils, contactor voltages in the hundreds are very common


mxs   10 W

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Re: Quick question about a contactor

Post by mxs » Dec 18 2019 11:45am

Not sure why would say that ... Of course I have looked, even at places you recommend. The contactor I asked about was given to me ... but since I looked the specs up, I noticed something I have not seen clearly posted on any other sites. That's why I ask, how does it really works ...

Digikey ... for 48V coil voltage they show two options below ...

Coil Voltage Contact Form Contact Rating (Current) Switching Voltage Features Termination Style Operating Temperature Must Operate Voltage Must Release Voltage
48VDC SPST-NO (DM) (1 Form X) 100A 900VDC - Max Sealed - Hermetically Stud -40°C ~ 85°C 33 VDC 4.8 VDC
48VDC SPST-NO (DM) (1 Form X) 500A 900VDC - Max Sealed - Hermetically Stud -40°C ~ 85°C 38 VDC 7 VDC


What contact voltage(s) will these two according to you work with? Obviously I understand the current contact rating, but the contact voltage? ... not sure at all. Is it the switching voltage? The maximum it will accept? If that is the case, is it the same parameter as "contact voltage"? that would 900VDC vs 36V VDC .... me thinks it's not the same thing they talk about ... 36V sounds to me like "Must Operate Voltage" parameter above?

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Re: Quick question about a contactor

Post by amberwolf » Dec 18 2019 7:33pm

mxs wrote:
Dec 18 2019 9:54am
... plus I have not seen other DC contactors where the manufacturer states two voltages ... coil and contact and at that those two being different.
It is common for contactors to use a different coil voltage (often 12v) vs the contact voltage, as there is often a low-voltage control system for the high-voltage traction system.
mxs wrote:
Dec 18 2019 8:27am
Any idea what will physically happen?
As previously noted, the contactor may not be able to break the arc, if it needs to disconnect under load.

That could result in welding of the contacts, which means the contactor *can't* disconnect the circuit anymore.

It could result in arc damage to the contacts, increasing their resistance, causing voltage drop across them, and causing the contactor to heat up under load. Or the increased resistance could be so high that the system doesn't operate normally (or at all) due to the voltage drop across it and the decreased current avalable thru it.




BTW, the most common difference between between a simple "relay" and a "contactor" is that contactors are often environmentally sealed, with neutral gas filling them so the normal compounds in air wont' react during the arcing that happens inside, to minimize contact damage over time.

Some relays are also built this way, but most are not.


If it's useful, these are a couple of contactor manufacturers commonly used in EV conversions and such:
https://www.gigavac.com/catalog/power-products
https://www.te.com/usa-en/products/rela ... ctors.html
or all versions
https://www.te.com/usa-en/products/rela ... ctors.html


Other threads with contactor info
search.php?keywords=contactor*&terms=al ... mit=Search

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