need 24 VDC auto transfer switch

[shipelec]

I need a 24 VDC automatic transfer switch for a shipboard application. It has to switch to an alternate power source upon loss of the normal power source. I can't use a "best voltage" diode arrangement because the normal source is not necessarily the best voltage. This switchover has to general a signal that can be used for an alarm. It would also be handy to have a "power available" indicator light for each source. The load can vary between 2 and 30 volts. If somebody could steer me to a manufacturer or website, I would appreciate it. Most of my work is in power distribution and when I talk to my normal suppliers, they say they don't work with a voltage that low.

 

[nbucska]

how much is the current (steady state and in-rush?)

<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[shipelec]

nbucska,

The 24 VDC load is for controls so it consists mostly of a number of relays and indicator lights. I don't think there is any inrush to speak of. The load variation would be caused by how many of the 7 systems fed from the 24 VDC panel are energized. The minimum load is about 2 A and the maximum about 22 A. One side is fed via a 75 A circuit breaker.

 

[nbucska]

Do you need one or for production? Can you build it?
Can you E-mail detailed description?

Up to 22A you can switch with relay or MOSFET
The relay uses current but MOSFET can be easier
damaged.



<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[buzzp]

There are a few mfgs that handle DC voltage monitoring. I can not recall any names but just search for DC voltage monitor (I dont think you will find one with an adequate current rating on its output contacts but could use another relay). The switching current (22A) is not that large so many automotive type relays may work although I am not sure about the 24VDC rating on the contacts (think most are rated for 30VDC).

Is there a seperate power source available for powering up the monitoring device? Or should it use the voltage that it is monitoring for power?

I am also curious as to how many you need.

 

[OperaHouse]

It could be as simple as a diode and a relay. Feed the load through a diode and a set of relay contacts from from the main power source. Power the relay from the main power source. When the main power drops out, the relay switches to the alternate power source. A second set of contacts operates an alarm. If one of the problems is the main source can drop very low, use a 12V relay with a 12V zener in series with the coil.

I assume in the statement, "The load can vary between 2 and 30 volts" that you ment to say amps

 

[ScottyUK]

This going to sound dumb, but what about a contactor carrying a combination of NO and NC poles? Feed the contactor coil from the preferred supply, connect the preferred supply to the load via the NO contacts and the alternate supply to the load via the NC contacts. Add auxiliary contacts for lamps, alarms, etc.

If you are fussy about voltage limits, use a voltage monitoring relay to control the contactor coil.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[shipelec]

Here is the full list of salient features:
· continuous current rating 75 A
· 24 VDC
· Open Transition
· Marine duty
· 50°C temp ambient
· Enclosure rating IP 34 (totally enclosed, dripproof)
· Withstand rating 200 A
· “Power Available” indicator lights for each source
· Alarm signal on switch from normal to alternate
· User-assigned normal and alternate source

nbucska,
I need eight and may need two or three more than that. However, they will be installed over a period of years.

OperaHouse,
Yes, I did mean to say the load can vary between 2 and 30 AMPS.

ScottyUK,
No, you don't sound dumb. What you suggest is what I will probably get. I was hoping there was something very much like what I want out there already so I don't have to have it custom-built. We like to get off-the-shelf stuff since that way we get things that are tested and have a support infrastructure.

 

[ScottyUK]

shipelec,

The contactor scheme I suggested could be easily implemented by a bar & shaft type contactor. These are available from the likes of ABB, Telemecanique, Siemens, and Hubbell. They are usually assembled to order, as they are not that popular today since the demise of heavy current DC in most applications. They are very robust, and most of the parts are available as individual items (coils, contacts, etc.)

I have some info on these devices if you can't find it locally. ABB's product is the R-Series; they are cheaper than Telemecanique and Siemens in the UK and are well-built.




----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[jraef]

Joslyn Clark is another good source in the US.

http://www.jenkins.com/jenkins/pdf/pdf_files/Controls/dc_contactors&starters.pdf

&quot;Venditori de oleum-vipera non vigere excordis populi&quot;

 

[Lakey]

In the UK, we mostly use:-

http://www.albright.co.uk

Regards,

 

[ScottyUK]

Hi Lakey,

Do we? I've never used their products - are they of a reasonable standard?

They look like they are for light traction applications - electric vehicles and the like. I couldn't find voltage ratings or making/breaking classes (DC3, DC5, etc) for these contactors.

They are possibly more suitable for shipelec's appliaction than the heavy duty contactors I had listed earlier - much more compact, and probably a lot cheaper too.




----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[Lakey]

Apologies, I did not intend to mislead anyone.

By “we” I mean our fork truck company (i.e. they are our company’s main supplier).
And yes, they do the job satisfactorily.

Shipelec wanted websites and I’ve posted one!

Regards,

P.S. Opinions expressed here are my own and may not be representative of my employer.

 

[ScottyUK]

No apology needed - they look to have some useful switching capability at low voltage. There is no such thing as too much choice, so thanks for the link. Sometimes we probably use the bar contactors when we could use something lighter. I have learned something today.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!